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diff --git a/html/TLS_README.html b/html/TLS_README.html new file mode 100644 index 0000000..7f950ab --- /dev/null +++ b/html/TLS_README.html @@ -0,0 +1,3252 @@ +<!doctype html public "-//W3C//DTD HTML 4.01 Transitional//EN" + "http://www.w3.org/TR/html4/loose.dtd"> + +<html> + +<head> + +<title>Postfix TLS Support </title> + +<meta http-equiv="Content-Type" content="text/html; charset=utf-8"> + +</head> + +<body> + +<h1><img src="postfix-logo.jpg" width="203" height="98" ALT="">Postfix TLS Support +</h1> + +<hr> + +<h2> What Postfix TLS support does for you </h2> + +<p> Transport Layer Security (TLS, formerly called SSL) provides +certificate-based authentication and encrypted sessions. An +encrypted session protects the information that is transmitted with +SMTP mail or with SASL authentication. </p> + +<p> NOTE: By turning on TLS support in Postfix, you not only get +the ability to encrypt mail and to authenticate remote SMTP clients +or servers. You also turn on hundreds of thousands of lines of +OpenSSL library code. Assuming that OpenSSL is written as carefully +as Wietse's own code, every 1000 lines introduces one additional bug +into Postfix. </p> + +<p> Topics covered in this document: </p> + +<ul> + +<li><a href="#how">How Postfix TLS support works</a> + +<li><a href="#server_tls">SMTP Server specific settings</a> + +<li> <a href="#client_tls">SMTP Client specific settings</a> + +<li><a href="#tlsmgr_controls"> TLS manager specific settings </a> + +<li><a href="#build_tls">Building Postfix with TLS support</a> + +<li><a href="#problems"> Reporting problems </a> + +<li><a href="#credits"> Credits </a> + +</ul> + +<p> And last but not least, for the impatient: </p> + +<ul> + +<li><a href="#quick-start">Getting started, quick and dirty</a> + +</ul> + +<h2><a name="how">How Postfix TLS support works</a></h2> + +<p> The diagram below shows the main elements of the Postfix TLS +architecture and their relationships. Colored boxes with numbered +names represent Postfix daemon programs. Other colored boxes +represent storage elements. </p> + +<ul> + +<li> <p> The <a href="smtpd.8.html">smtpd(8)</a> server implements the SMTP over TLS server +side. </p> + +<li> <p> The <a href="smtp.8.html">smtp(8)</a> client implements the SMTP (and LMTP) over TLS +client side. </p> + +<li> <p> The <a href="tlsmgr.8.html">tlsmgr(8)</a> server maintains the pseudo-random number +generator (PRNG) that seeds the TLS engines in the <a href="smtpd.8.html">smtpd(8)</a> server +and <a href="smtp.8.html">smtp(8)</a> client processes, and maintains the TLS session key +cache files. </p> + +</ul> + +<p> Not shown in the figure are the <a href="tlsproxy.8.html">tlsproxy(8)</a> server and the +<a href="postscreen.8.html">postscreen(8)</a> server. These use TLS in the same manner as <a href="smtpd.8.html">smtpd(8)</a>. +</p> + +<table> + +<tr> <td>Network<tt>-> </tt> </td> <td align="center" +bgcolor="#f0f0ff"> <br> <a href="smtpd.8.html">smtpd(8)</a> <br> </td> <td colspan="2"> + +<tt> <---seed----<br><br><-key/cert-> </tt> </td> <td +align="center" bgcolor="#f0f0ff"> <br> <a href="tlsmgr.8.html">tlsmgr(8)</a> <br> </td> +<td colspan="3"> <tt> ----seed---><br> <br><-key/cert-> + +</tt> </td> <td align="center" bgcolor="#f0f0ff"> <br> <a href="smtp.8.html">smtp(8)</a> <br> + </td> <td> <tt> -></tt>Network </td> </tr> + +<tr> <td colspan="3"> </td> <td align="right"> <table> <tr> <td> + +</td> <td> / </td> </tr> <tr> <td> / </td> <td> </td> </tr> </table> +</td> <td align="center"> |<br> |</td> <td align="left"> <table> + +<tr> <td> \ </td> <td> </td> </tr> <tr> <td> </td> <td> \ </td> +</tr> </table> </td> <td colspan="3"> </td> </tr> + +<tr> <td colspan="2"> </td> <td align="center" bgcolor="#f0f0ff"> +smtpd<br> session<br> key cache </td> <td> </td> <td align="center" +bgcolor="#f0f0ff"> PRNG<br> state <br>file </td> <td> </td> <td +align="center" bgcolor="#f0f0ff"> smtp<br> session<br> key cache +</td> + +<td colspan="2"> </td> </tr> + +</table> + +<h2><a name="server_tls">SMTP Server specific settings</a></h2> + +<p> Topics covered in this section: </p> + +<ul> + +<li><a href="#server_cert_key">Server-side certificate and private +key configuration </a> + +<li><a href="#server_pfs">Server-side forward-secrecy configuration </a> + +<li><a href="#server_logging"> Server-side TLS activity logging +</a> + +<li><a href="#server_enable">Enabling TLS in the Postfix SMTP server </a> + +<li><a href="#server_vrfy_client">Client certificate verification</a> + +<li><a href="#server_tls_auth">Supporting AUTH over TLS only</a> + +<li><a href="#server_tls_cache">Server-side TLS session cache</a> + +<li><a href="#server_access">Server access control</a> + +<li><a href="#server_cipher">Server-side cipher controls</a> + +<li><a href="#server_misc"> Miscellaneous server controls</a> + +</ul> + +<h3><a name="server_cert_key">Server-side certificate and private +key configuration </a> </h3> + +<p> In order to use TLS, the Postfix SMTP server generally needs +a certificate and a private key. Both must be in "PEM" format. The +private key must not be encrypted, meaning: the key must be accessible +without a password. The certificate and private key may be in the same +file, in which case the certificate file should be owned by "root" and +not be readable by any other user. If the key is stored separately, +this access restriction applies to the key file only, and the +certificate file may be "world-readable". </p> + +<p> Public Internet MX hosts without certificates signed by a +well-known public CA must still generate, and be prepared to present +to most clients, a self-signed or private-CA signed certificate. +The remote SMTP client will generally not be able to verify the +self-signed certificate, but unless the client is running Postfix +or similar software, it will only negotiate TLS ciphersuites that +require a server certificate. </p> + +<p> For servers that are <b>not</b> public Internet MX hosts, Postfix +supports configurations with no certificates. This entails the use of +just the anonymous TLS ciphers, which are not supported by typical SMTP +clients. Since some clients may not fall back to plain text after a TLS +handshake failure, a certificate-less Postfix SMTP server will be unable +to receive email from some TLS-enabled clients. To avoid accidental +configurations with no certificates, Postfix enables certificate-less +operation only when the administrator explicitly sets +"<a href="postconf.5.html#smtpd_tls_cert_file">smtpd_tls_cert_file</a> = none". This ensures that new Postfix SMTP server +configurations will not accidentally enable TLS without certificates. </p> + +<p> Note that server certificates are <b>not</b> optional in TLS 1.3. To +run without certificates you'd have to disable the TLS 1.3 protocol by +including "<=TLSv1.2" (or, for Postfix < 3.6, "!TLSv1.3") in +"<a href="postconf.5.html#smtpd_tls_protocols">smtpd_tls_protocols</a>" and perhaps also "<a href="postconf.5.html#smtpd_tls_mandatory_protocols">smtpd_tls_mandatory_protocols</a>". +It is simpler instead to just configure a certificate chain. +Certificate-less operation is not recommended. <p> + +<p> RSA, DSA and ECDSA (Postfix ≥ 2.6) certificates are supported. +Most sites only have RSA certificates. You can configure all three +at the same time, in which case the ciphersuite negotiated with the +remote SMTP client determines which certificate is used. If your +DNS zone is signed, and you want to publish DANE TLSA (<a href="https://tools.ietf.org/html/rfc6698">RFC 6698</a>, +<a href="https://tools.ietf.org/html/rfc7671">RFC 7671</a>, <a href="https://tools.ietf.org/html/rfc7672">RFC 7672</a>) records, these must match all of the configured +certificate chains. Since the best practice is to publish "3 1 1" +certificate associations, create a separate TLSA record to match +each public-key certificate digest. </p> + +<h4> Creating the server certificate file </h4> + +<p> To verify the Postfix SMTP server certificate, the remote SMTP +client must receive the issuing CA certificates via the TLS handshake +or via public-key infrastructure. This means that the Postfix server +public-key certificate file must include the server certificate +first, then the issuing CA(s) (bottom-up order). The Postfix SMTP +server certificate must be usable as an SSL server certificate and +hence pass the "<tt>openssl verify -purpose sslserver ...</tt>" test. +</p> + +<p> The examples that follow show how to create a server certificate +file. We assume that the certificate for "server.example.com" was +issued by "intermediate CA" which itself has a certificate issued +by "root CA". </p> + +<ul> + +<li> <p> With legacy public CA trust verification, you can omit the +root certificate from the "server.pem" certificate file. If the +client trusts the root CA, it will already have a local copy of the +root CA certificate. Omitting the root CA certificate reduces the +size of the server TLS handshake. </p> + +<blockquote> +<pre> +% <b>cat server_cert.pem intermediate_CA.pem > server.pem</b> +</pre> +</blockquote> + +<li> <p> If you publish DANE TLSA (<a href="https://tools.ietf.org/html/rfc6698">RFC 6698</a>, <a href="https://tools.ietf.org/html/rfc7671">RFC 7671</a>, <a href="https://tools.ietf.org/html/rfc7672">RFC 7672</a>) +"2 0 1" or "2 1 1" records to specify root CA certificate digests, +you must include the corresponding root CA certificates in the +"server.pem" certificate file. </p> + +<blockquote> +<pre> +% <b>cat server_cert.pem intermediate_CA.pem root.pem > server.pem</b> +</pre> +</blockquote> + +<p> Remote SMTP clients will be able to use the TLSA record you +publish (which only contains the certificate digest) only if they +have access to the corresponding certificate. Failure to verify +certificates per the server's published TLSA records will typically +cause the SMTP client to defer mail delivery. The foregoing also +applies to "2 0 2" and "2 1 2" TLSA records or any other digest of +a CA certificate, but it is expected that SHA256 will be by far the +most common digest for TLSA. </p> + +<p> As a best practice, publish "3 1 1" TLSA associations that specify +the SHA256 digest of the server's public key. These continue to work +unmodified when a certificate is renewed with the same public/private +key pair. </p> + +</ul> + +<p> For instructions on how to compute the digest of a certificate +or its public key for use in TLSA records, see the documentation of +the <a href="postconf.5.html#smtpd_tls_fingerprint_digest">smtpd_tls_fingerprint_digest</a> <a href="postconf.5.html">main.cf</a> parameter. </p> + +<p> When a new key or certificate is generated, an additional TLSA +record with the new digest must be published in advance of the +actual deployment of the new key or certificate on the server. You +must allow sufficient time for any TLSA RRsets with only the old +digest to expire from DNS caches. The safest practice is to wait +until the DNSSEC signature on the previous TLSA RRset expires, and +only then switch the server to use new keys published in the updated +TLSA RRset. Once the new certificate trust chain and private key +are in effect, the DNS should be updated once again to remove the +old digest from the TLSA RRset. </p> + +<p> If you want the Postfix SMTP server to accept remote SMTP client +certificates issued by one or more root CAs, append the root +certificate to $<a href="postconf.5.html#smtpd_tls_CAfile">smtpd_tls_CAfile</a> or install it in the $<a href="postconf.5.html#smtpd_tls_CApath">smtpd_tls_CApath</a> +directory. </p> + +<h4> Configuring the server certificate and key files </h4> + +<p> Example: Postfix ≥ 3.4 all-in-one chain file(s). One or more +chain files that start with a key that is immediately followed by the +corresponding certificate and any additional issuer certificates. A +single file can hold multiple <i>(key, cert, [chain])</i> sequences, one +per algorithm. It is typically simpler to keep the chain for each +algorithm in its own file. Most users are likely to deploy just a +single RSA chain, but with OpenSSL 1.1.1, it is possible to deploy up to +five chains, one each for RSA, ECDSA, ED25519, ED448, and even the +obsolete DSA. </p> + +<blockquote> +<pre> + # Postfix ≥ 3.4. Preferred configuration interface. Each file + # starts with the private key, followed by the corresponding + # certificate, and any intermediate issuer certificates. The root CA + # cert may also be needed when published as a DANE trust anchor. + # + <a href="postconf.5.html#smtpd_tls_chain_files">smtpd_tls_chain_files</a> = + /etc/postfix/rsa.pem, + /etc/postfix/ecdsa.pem, + /etc/postfix/ed25519.pem, + /etc/postfix/ed448.pem +</pre> +</blockquote> + +<p> You can also store the keys separately from their certificates, again +provided each is listed before the corresponding certificate chain. Storing a +key and its associated certificate chain in separate files is not recommended, +because this is prone to race conditions during key rollover, as there is no +way to update multiple files atomically. </p> + +<blockquote> +<pre> + # Postfix ≥ 3.4. + # Storing keys separately from the associated certificates is not + # recommended. + <a href="postconf.5.html#smtpd_tls_chain_files">smtpd_tls_chain_files</a> = + /etc/postfix/rsakey.pem, + /etc/postfix/rsacerts.pem, + /etc/postfix/ecdsakey.pem, + /etc/postfix/ecdsacerts.pem +</pre> +</blockquote> + +<p> The below examples show the legacy algorithm-specific configurations +for Postfix 3.3 and older. With Postfix ≤ 3.3, even if the key is +stored in the same file as the certificate, the file is read twice and a +(brief) race condition still exists during key rollover. While Postfix +≥ 3.4 avoids the race when the key and certificate are in the same +file, you should use the new "<a href="postconf.5.html#smtpd_tls_chain_files">smtpd_tls_chain_files</a>" interface shown +above. <p> + +<p> RSA key and certificate examples: </p> + +<blockquote> +<pre> +/etc/postfix/<a href="postconf.5.html">main.cf</a>: + <a href="postconf.5.html#smtpd_tls_cert_file">smtpd_tls_cert_file</a> = /etc/postfix/server.pem + <a href="postconf.5.html#smtpd_tls_key_file">smtpd_tls_key_file</a> = $<a href="postconf.5.html#smtpd_tls_cert_file">smtpd_tls_cert_file</a> +</pre> +</blockquote> + +<p> Their DSA counterparts: </p> + +<blockquote> +<pre> +/etc/postfix/<a href="postconf.5.html">main.cf</a>: + <a href="postconf.5.html#smtpd_tls_dcert_file">smtpd_tls_dcert_file</a> = /etc/postfix/server-dsa.pem + <a href="postconf.5.html#smtpd_tls_dkey_file">smtpd_tls_dkey_file</a> = $<a href="postconf.5.html#smtpd_tls_dcert_file">smtpd_tls_dcert_file</a> +</pre> +</blockquote> + +<p> Their ECDSA counterparts (Postfix ≥ 2.6 + OpenSSL ≥ 1.0.0): </p> + +<blockquote> +<pre> +/etc/postfix/<a href="postconf.5.html">main.cf</a>: + # Some clients will not be ECDSA capable, so you will likely still need + # an RSA certificate and private key. + # + <a href="postconf.5.html#smtpd_tls_eccert_file">smtpd_tls_eccert_file</a> = /etc/postfix/server-ecdsa.pem + <a href="postconf.5.html#smtpd_tls_eckey_file">smtpd_tls_eckey_file</a> = $<a href="postconf.5.html#smtpd_tls_eccert_file">smtpd_tls_eccert_file</a> +</pre> +</blockquote> + +<p> TLS without certificates for servers serving exclusively +anonymous-cipher capable clients: </p> + +<blockquote> +<pre> +/etc/postfix/<a href="postconf.5.html">main.cf</a>: + # Not recommended: breaks TLS 1.3 and clients that don't support + # anonymous cipher suites. + <a href="postconf.5.html#smtpd_tls_cert_file">smtpd_tls_cert_file</a> = none +</pre> +</blockquote> + +<p> To verify a remote SMTP client certificate, the Postfix SMTP +server needs to trust the certificates of the issuing Certification +Authorities. These certificates in "PEM" format can be stored in a +single $<a href="postconf.5.html#smtpd_tls_CAfile">smtpd_tls_CAfile</a> or in multiple files, one CA per file in +the $<a href="postconf.5.html#smtpd_tls_CApath">smtpd_tls_CApath</a> directory. If you use a directory, don't forget +to create the necessary "hash" links with: </p> + +<blockquote> +<pre> +# <b>$OPENSSL_HOME/bin/c_rehash <i>/path/to/directory</i> </b> +</pre> +</blockquote> + +<p> The $<a href="postconf.5.html#smtpd_tls_CAfile">smtpd_tls_CAfile</a> contains the CA certificates of one or +more trusted CAs. The file is opened (with root privileges) before +Postfix enters the optional chroot jail and so need not be accessible +from inside the chroot jail. </p> + +<p> Additional trusted CAs can be specified via the $<a href="postconf.5.html#smtpd_tls_CApath">smtpd_tls_CApath</a> +directory, in which case the certificates are read (with $<a href="postconf.5.html#mail_owner">mail_owner</a> +privileges) from the files in the directory when the information +is needed. Thus, the $<a href="postconf.5.html#smtpd_tls_CApath">smtpd_tls_CApath</a> directory needs to be +accessible inside the optional chroot jail. </p> + +<p> When you configure the Postfix SMTP server to request <a +href="#server_vrfy_client">client certificates</a>, the DNs of Certification +Authorities in $<a href="postconf.5.html#smtpd_tls_CAfile">smtpd_tls_CAfile</a> are sent to the client, in order to allow +it to choose an identity signed by a CA you trust. If no $<a href="postconf.5.html#smtpd_tls_CAfile">smtpd_tls_CAfile</a> +is specified, no preferred CA list is sent, and the client is free to +choose an identity signed by any CA. Many clients use a fixed identity +regardless of the preferred CA list and you may be able to reduce TLS +negotiation overhead by installing client CA certificates mostly or +only in $<a href="postconf.5.html#smtpd_tls_CApath">smtpd_tls_CApath</a>. In the latter case you need not specify a +$<a href="postconf.5.html#smtpd_tls_CAfile">smtpd_tls_CAfile</a>. </p> + +<p> Note, that unless client certificates are used to allow greater +access to TLS authenticated clients, it is best to not ask for +client certificates at all, as in addition to increased overhead +some clients (notably in some cases qmail) are unable to complete +the TLS handshake when client certificates are requested. </p> + +<p> Example: </p> +<blockquote> +<pre> +/etc/postfix/<a href="postconf.5.html">main.cf</a>: + <a href="postconf.5.html#smtpd_tls_CAfile">smtpd_tls_CAfile</a> = /etc/postfix/CAcert.pem + <a href="postconf.5.html#smtpd_tls_CApath">smtpd_tls_CApath</a> = /etc/postfix/certs +</pre> +</blockquote> + +<h3><a name="server_pfs"> Server-side forward-secrecy configuration </a> </h3> + +<p> If you want to take maximal advantage of ciphers that offer <a +href="FORWARD_SECRECY_README.html#dfn_fs">forward secrecy</a> see +the <a href="FORWARD_SECRECY_README.html#quick-start">Getting +started</a> section of <a +href="FORWARD_SECRECY_README.html">FORWARD_SECRECY_README</a>. The +full document conveniently presents all information about Postfix +forward secrecy support in one place: what forward secrecy is, how +to tweak settings, and what you can expect to see when Postfix uses +ciphers with forward secrecy. </p> + +<h3><a name="server_logging"> Server-side TLS activity logging </a> </h3> + +<p> To get additional information about Postfix SMTP server TLS +activity you can increase the log level from 0..4. Each logging +level also includes the information that is logged at a lower +logging level. </p> + +<blockquote> + +<table border="1"> + +<tr> <th> Level </th> <th> Postfix 2.9 and later</th> <th> Earlier +releases. </th> </tr> + +<tr> <td valign="top"> 0 </td> <td valign="top" colspan="2"> Disable +logging of TLS activity. </td> </tr> + +<tr> <td valign="top"> 1 </td> <td valign="top"> Log only a summary +message on TLS handshake completion — no logging of client +certificate trust-chain verification errors if client certificate +verification is not required. </td> <td valign="top"> Log the summary +message, peer certificate summary information and unconditionally log +trust-chain verification errors. </td> </tr> + +<tr> <td valign="top"> 2 </td> <td valign="top" colspan="2"> Also +log levels during TLS negotiation. </td> </tr> + +<tr> <td valign="top"> 3 </td> <td valign="top" colspan="2"> Also +log hexadecimal and ASCII dump of TLS negotiation process. </td> +</tr> + +<tr> <td valign="top"> 4 </td> <td valign="top" colspan="2"> Also +log hexadecimal and ASCII dump of complete transmission after +STARTTLS. </td></tr> + +</table> + +</blockquote> + +<p> Use log level 3 only in case of problems. Use of log level 4 is +strongly discouraged. </p> + +<p> Example: </p> + +<blockquote> +<pre> +/etc/postfix/<a href="postconf.5.html">main.cf</a>: + <a href="postconf.5.html#smtpd_tls_loglevel">smtpd_tls_loglevel</a> = 0 +</pre> +</blockquote> + +<p> To include information about the protocol and cipher used as +well as the client and issuer CommonName into the "Received:" +message header, set the <a href="postconf.5.html#smtpd_tls_received_header">smtpd_tls_received_header</a> variable to true. +The default is no, as the information is not necessarily authentic. +Only information recorded at the final destination is reliable, +since the headers may be changed by intermediate servers. </p> + +<p> Example: </p> + +<blockquote> +<pre> +/etc/postfix/<a href="postconf.5.html">main.cf</a>: + <a href="postconf.5.html#smtpd_tls_received_header">smtpd_tls_received_header</a> = yes +</pre> +</blockquote> + +<h3><a name="server_enable">Enabling TLS in the Postfix SMTP server </a> </h3> + +<p> By default, TLS is disabled in the Postfix SMTP server, so no +difference to plain Postfix is visible. Explicitly switch it on +with "<a href="postconf.5.html#smtpd_tls_security_level">smtpd_tls_security_level</a> = may". </p> + +<p> Example: </p> + +<blockquote> +<pre> +/etc/postfix/<a href="postconf.5.html">main.cf</a>: + <a href="postconf.5.html#smtpd_tls_security_level">smtpd_tls_security_level</a> = may +</pre> +</blockquote> + +<p> With this, the Postfix SMTP server announces STARTTLS support to +remote SMTP clients, but does not require that clients use TLS encryption. +</p> + +<p> Note: when an unprivileged user invokes "sendmail -bs", STARTTLS +is never offered due to insufficient privileges to access the Postfix +SMTP server +private key. This is intended behavior. </p> + +<p> <a name="server_enforce">You can ENFORCE the use of TLS</a>, +so that the Postfix SMTP server announces STARTTLS and accepts no +mail without TLS encryption, by setting +"<a href="postconf.5.html#smtpd_tls_security_level">smtpd_tls_security_level</a> = encrypt". According to <a href="https://tools.ietf.org/html/rfc2487">RFC 2487</a> this +MUST NOT be applied in case +of a publicly-referenced Postfix SMTP server. This option is off +by default and should only seldom be used. </p> + +<p> Example: </p> + +<blockquote> +<pre> +/etc/postfix/<a href="postconf.5.html">main.cf</a>: + <a href="postconf.5.html#smtpd_tls_security_level">smtpd_tls_security_level</a> = encrypt +</pre> +</blockquote> + +<p> TLS is also used in the "wrapper" mode where +a server always uses TLS, instead of announcing STARTTLS support +and waiting for remote SMTP clients to request TLS service. Some +clients, namely +Outlook [Express] prefer the "wrapper" mode. This is true for OE +(Win32 < 5.0 and Win32 >=5.0 when run on a port<>25 +and OE (5.01 Mac on all ports). </p> + +<p> It is strictly discouraged to use this mode from <a href="postconf.5.html">main.cf</a>. If +you want to support this service, enable a special port in <a href="master.5.html">master.cf</a> +and specify "-o <a href="postconf.5.html#smtpd_tls_wrappermode">smtpd_tls_wrappermode</a>=yes" (note: no space around +the "=") as an <a href="smtpd.8.html">smtpd(8)</a> command line option. Port 465 (smtps) was +once chosen for this feature. +</p> + +<p> Example: </p> + +<blockquote> +<pre> +/etc/postfix/<a href="master.5.html">master.cf</a>: + smtps inet n - n - - smtpd + -o <a href="postconf.5.html#smtpd_tls_wrappermode">smtpd_tls_wrappermode</a>=yes -o <a href="postconf.5.html#smtpd_sasl_auth_enable">smtpd_sasl_auth_enable</a>=yes +</pre> +</blockquote> + +<h3><a name="server_vrfy_client">Client certificate verification</a> </h3> + +<p> To receive a remote SMTP client certificate, the Postfix SMTP +server must explicitly ask for one (any contents of $<a href="postconf.5.html#smtpd_tls_CAfile">smtpd_tls_CAfile</a> +are also sent to the client as a hint for choosing a certificate from +a suitable CA). Unfortunately, Netscape clients will either complain +if no matching client certificate is available or will offer the user +client a list of certificates to choose from. Additionally some MTAs +(notably some versions of qmail) are unable to complete TLS negotiation +when client certificates are requested, and abort the SMTP session. So +this option is "off" by default. You will however need the certificate +if you want to use certificate based relaying with, for example, the +<a href="postconf.5.html#permit_tls_clientcerts">permit_tls_clientcerts</a> feature. A server that wants client certificates +must first present its own certificate. While Postfix by default +offers anonymous ciphers to remote SMTP clients, these are automatically +suppressed +when the Postfix SMTP server is configured to ask for client +certificates. </p> + +<p> Example: </p> + +<blockquote> +<pre> +/etc/postfix/<a href="postconf.5.html">main.cf</a>: + <a href="postconf.5.html#smtpd_tls_ask_ccert">smtpd_tls_ask_ccert</a> = yes + <a href="postconf.5.html#smtpd_tls_security_level">smtpd_tls_security_level</a> = may +</pre> +</blockquote> + +<p> When TLS is <a href="#server_enforce">enforced</a> you may also decide +to REQUIRE a remote SMTP client certificate for all TLS connections, +by setting "<a href="postconf.5.html#smtpd_tls_req_ccert">smtpd_tls_req_ccert</a> = yes". This feature implies +"<a href="postconf.5.html#smtpd_tls_ask_ccert">smtpd_tls_ask_ccert</a> = yes". When TLS is not enforced, +"<a href="postconf.5.html#smtpd_tls_req_ccert">smtpd_tls_req_ccert</a> = yes" is ignored and a warning is +logged. </p> + +<p> Example: </p> + +<blockquote> +<pre> +/etc/postfix/<a href="postconf.5.html">main.cf</a>: + <a href="postconf.5.html#smtpd_tls_req_ccert">smtpd_tls_req_ccert</a> = yes + <a href="postconf.5.html#smtpd_tls_security_level">smtpd_tls_security_level</a> = encrypt +</pre> +</blockquote> + +<p> The client certificate verification depth is specified with the +<a href="postconf.5.html">main.cf</a> <a href="postconf.5.html#smtpd_tls_ccert_verifydepth">smtpd_tls_ccert_verifydepth</a> parameter. The default verification +depth is 9 (the OpenSSL default), for compatibility with Postfix +versions before 2.5 where <a href="postconf.5.html#smtpd_tls_ccert_verifydepth">smtpd_tls_ccert_verifydepth</a> was ignored. +When you configure trust in a +root CA, it is not necessary to explicitly trust intermediary CAs signed +by the root CA, unless $<a href="postconf.5.html#smtpd_tls_ccert_verifydepth">smtpd_tls_ccert_verifydepth</a> is less than the +number of CAs in the certificate chain for the clients of interest. With +a verify depth of 1 you can only verify certificates directly signed +by a trusted CA, and all trusted intermediary CAs need to be configured +explicitly. With a verify depth of 2 you can verify clients signed by a +root CA or a direct intermediary CA (so long as the client is correctly +configured to supply its intermediate CA certificate). </p> + +<p> Example: </p> + +<blockquote> +<pre> +/etc/postfix/<a href="postconf.5.html">main.cf</a>: + <a href="postconf.5.html#smtpd_tls_ccert_verifydepth">smtpd_tls_ccert_verifydepth</a> = 2 +</pre> +</blockquote> + +<h3><a name="server_tls_auth">Supporting AUTH over TLS only</a></h3> + +<p> Sending AUTH data over an unencrypted channel poses a security +risk. When TLS layer encryption is required +("<a href="postconf.5.html#smtpd_tls_security_level">smtpd_tls_security_level</a> = encrypt"), the Postfix SMTP server will +announce and accept AUTH only after the TLS layer has been activated +with STARTTLS. When TLS layer encryption is optional +("<a href="postconf.5.html#smtpd_tls_security_level">smtpd_tls_security_level</a> = may"), it may however still be useful +to only offer AUTH when TLS is active. To maintain compatibility +with non-TLS clients, the default is to accept AUTH without encryption. +In order to change this behavior, set +"<a href="postconf.5.html#smtpd_tls_auth_only">smtpd_tls_auth_only</a> = yes". </p> + +<p> Example: </p> + +<blockquote> +<pre> +/etc/postfix/<a href="postconf.5.html">main.cf</a>: + <a href="postconf.5.html#smtpd_tls_auth_only">smtpd_tls_auth_only</a> = no +</pre> +</blockquote> + +<h3><a name="server_tls_cache">Server-side TLS session cache</a> </h3> + +<p> The Postfix SMTP server and the remote SMTP client negotiate a +session, which takes some computer time and network bandwidth. SSL +protocol versions other than SSLv2 support resumption of cached +sessions. Not only is this more CPU and bandwidth efficient, it +also reduces latency as only one network round-trip is used to +resume a session while it takes two round-trips to create a session +from scratch. </p> + +<p> Since Postfix uses multiple <a href="smtpd.8.html">smtpd(8)</a> service processes, an +in-memory cache is not sufficient for session re-use. Clients store +at most one cached session per server and are very unlikely to +repeatedly connect to the same server process. Thus session caching +in the Postfix SMTP server generally requires a shared cache (an +alternative available with Postfix ≥ 2.11 is described below). +</p> + +<p> To share the session information between multiple +<a href="smtpd.8.html">smtpd(8)</a> processes, a session cache database is used. You +can specify any database type that can store objects of several +kbytes and that supports the sequence operator. DBM databases are +not suitable because they can only store small objects. The cache +is maintained by the <a href="tlsmgr.8.html">tlsmgr(8)</a> process, so there is no problem with +concurrent access. Session caching is highly recommended, because +the cost of repeatedly negotiating TLS session keys is high.</p> + +<p> Starting with Postfix 2.11, linked with a compatible OpenSSL +library (at least 0.9.8h, preferably 1.0.0 or later) the Postfix +SMTP server supports <a href="https://tools.ietf.org/html/rfc5077">RFC 5077</a> TLS session resumption without +server-side state when the remote SMTP client also supports <a href="https://tools.ietf.org/html/rfc5077">RFC</a> +<a href="https://tools.ietf.org/html/rfc5077">5077</a>. The session is encrypted by the server in a <i>session +ticket</i> returned to client for storage. When a client sends a +valid session ticket, the server decrypts it and resumes the session, +provided neither the ticket nor the session have expired. This +makes it possible to resume cached sessions without allocating space +for a shared database on the server. Consequently, for Postfix +≥ 2.11 the <a href="postconf.5.html#smtpd_tls_session_cache_database">smtpd_tls_session_cache_database</a> parameter should +generally be left empty. Session caching can be disabled by setting +the session cache timeout to zero, otherwise the timeout must be +at least 2 minutes and at most 100 days. </p> + +<p> Note, session tickets can only be negotiated if the client +disables SSLv2 and does not use the legacy SSLv2 compatible HELLO +message. This is true by default with the Postfix ≥ 2.6 SMTP +client. </p> + +<p> Example: </p> + +<blockquote> +<pre> +/etc/postfix/<a href="postconf.5.html">main.cf</a>: + <a href="postconf.5.html#smtpd_tls_session_cache_database">smtpd_tls_session_cache_database</a> = <a href="DATABASE_README.html#types">btree</a>:/var/lib/postfix/smtpd_scache +</pre> +</blockquote> + +<p> Note: as of version 2.5, Postfix no longer uses root privileges +when opening this file. The file should now be stored under the +Postfix-owned <a href="postconf.5.html#data_directory">data_directory</a>. As a migration aid, an attempt to +open the file under a non-Postfix directory is redirected to the +Postfix-owned <a href="postconf.5.html#data_directory">data_directory</a>, and a warning is logged. </p> + +<p> Cached Postfix SMTP server session information expires after +a certain amount of time. Postfix/TLS does not use the OpenSSL +default of 300s, but a longer time of 3600sec (=1 hour). <a href="https://tools.ietf.org/html/rfc2246">RFC 2246</a> +recommends a maximum of 24 hours. </p> + +<p> Example: </p> + +<blockquote> +<pre> +/etc/postfix/<a href="postconf.5.html">main.cf</a>: + <a href="postconf.5.html#smtpd_tls_session_cache_timeout">smtpd_tls_session_cache_timeout</a> = 3600s +</pre> +</blockquote> + +<p> As of Postfix 2.11 this setting cannot exceed 100 days. If set +≤ 0, session caching is disabled. If set to a positive value +less than 2 minutes, the minimum value of 2 minutes is used instead. </p> + +<p> When the Postfix SMTP server does not save TLS sessions to an +external cache database, client-side session caching is unlikely +to be useful. To reduce waste of client resources, the Postfix SMTP server can +be configured to not issue TLS session ids. By default the Postfix +SMTP server always issues TLS session ids. This works around known +interoperability issues with some MUAs, and prevents possible +interoperability issues with other MTAs. </p> + +<p> Example: </p> + +<blockquote> +<pre> + <a href="postconf.5.html#smtpd_tls_always_issue_session_ids">smtpd_tls_always_issue_session_ids</a> = no +</pre> +</blockquote> + +<h3><a name="server_access">Server access control</a> </h3> + +<p> Postfix TLS support introduces three additional features for +Postfix SMTP server access control: </p> + +<blockquote> + +<dl> + +<dt> <a href="postconf.5.html#permit_tls_clientcerts">permit_tls_clientcerts</a> </dt> <dd> <p> Allow the remote SMTP +client request if the client certificate fingerprint or certificate +public key fingerprint (Postfix 2.9 and later) is listed in the +client certificate table (see <a href="postconf.5.html#relay_clientcerts">relay_clientcerts</a> discussion below). +</p> </dd> + +<dt> <a href="postconf.5.html#permit_tls_all_clientcerts">permit_tls_all_clientcerts</a> </dt> <dd> <p> Allow the remote SMTP +client request if the client certificate passes trust chain verification. +Useful with private-label CAs that only issue certificates to trusted +clients (and not otherwise). </p> </dd> + +<dt> <a href="postconf.5.html#check_ccert_access">check_ccert_access</a> <a href="DATABASE_README.html">type:table</a></dt> <dd> <p> Use the remote +SMTP client certificate fingerprint or public key fingerprint +(Postfix 2.9 and later) as the lookup key for the specified <a href="access.5.html">access(5)</a> +table. </p> </dd> + +</dl> + +</blockquote> + +<p> The digest algorithm used to compute the client certificate +fingerprints is specified with the <a href="postconf.5.html">main.cf</a> <a href="postconf.5.html#smtpd_tls_fingerprint_digest">smtpd_tls_fingerprint_digest</a> +parameter. The default algorithm is <b>sha256</b> with Postfix ≥ +3.6 and the <b><a href="postconf.5.html#compatibility_level">compatibility_level</a></b> set to 3.6 or higher. With +Postfix ≤ 3.5, the default algorithm is <b>md5</b>. The +best-practice algorithm is now <b>sha256</b>. Recent advances in hash +function cryptanalysis have led to md5 and sha1 being deprecated in +favor of sha256. However, as long as there are no known "second +pre-image" attacks against the older algorithms, their use in this +context, though not recommended, is still likely safe. </p> + +<p> The <a href="postconf.5.html#permit_tls_all_clientcerts">permit_tls_all_clientcerts</a> feature must be used with caution, +because it can result in too many access permissions. Use this +feature only if a special CA issues the client certificates, and +only if this CA is listed as a trusted CA. If other CAs are trusted, +any owner of a valid client certificate would be authorized. +The <a href="postconf.5.html#permit_tls_all_clientcerts">permit_tls_all_clientcerts</a> feature can be practical for a +specially created email relay server. </p> + +<p> It is however recommended to stay with the <a href="postconf.5.html#permit_tls_clientcerts">permit_tls_clientcerts</a> +feature and list all certificates via $<a href="postconf.5.html#relay_clientcerts">relay_clientcerts</a>, as +<a href="postconf.5.html#permit_tls_all_clientcerts">permit_tls_all_clientcerts</a> does not permit any control when a +certificate must no longer be used (e.g. an employee leaving). </p> + +<p> Example: </p> + +<blockquote> +<pre> +# With Postfix 2.10 and later, the mail relay policy is +# preferably specified under <a href="postconf.5.html#smtpd_relay_restrictions">smtpd_relay_restrictions</a>. +/etc/postfix/<a href="postconf.5.html">main.cf</a>: + <a href="postconf.5.html#smtpd_relay_restrictions">smtpd_relay_restrictions</a> = + <a href="postconf.5.html#permit_mynetworks">permit_mynetworks</a> + <a href="postconf.5.html#permit_tls_clientcerts">permit_tls_clientcerts</a> + <a href="postconf.5.html#reject_unauth_destination">reject_unauth_destination</a> +</pre> + +<pre> +# Older configurations combine relay control and spam control under +# <a href="postconf.5.html#smtpd_recipient_restrictions">smtpd_recipient_restrictions</a>. To use this example with Postfix ≥ +# 2.10 specify "<a href="postconf.5.html#smtpd_relay_restrictions">smtpd_relay_restrictions</a>=". +/etc/postfix/<a href="postconf.5.html">main.cf</a>: + <a href="postconf.5.html#smtpd_recipient_restrictions">smtpd_recipient_restrictions</a> = + <a href="postconf.5.html#permit_mynetworks">permit_mynetworks</a> + <a href="postconf.5.html#permit_tls_clientcerts">permit_tls_clientcerts</a> + <a href="postconf.5.html#reject_unauth_destination">reject_unauth_destination</a> + ...other rules... +</pre> +</blockquote> + +<p> Example: Postfix lookup tables are in the form of (key, value) +pairs. Since we only need the key, the value can be chosen freely, e.g. +the name of the user or host:</p> + +<blockquote> +<pre> +/etc/postfix/<a href="postconf.5.html">main.cf</a>: + <a href="postconf.5.html#relay_clientcerts">relay_clientcerts</a> = <a href="DATABASE_README.html#types">hash</a>:/etc/postfix/relay_clientcerts + +/etc/postfix/relay_clientcerts: + D7:04:2F:A7:0B:8C:A5:21:FA:31:77:E1:41:8A:EE:80 lutzpc.at.home +</pre> +</blockquote> + +<p> To extract the public key fingerprint from an X.509 certificate, +you need to extract the public key from the certificate and compute +the appropriate digest of its DER (ASN.1) encoding. With OpenSSL +the "-pubkey" option of the "x509" command extracts the public +key always in "PEM" format. We pipe the result to another OpenSSL +command that converts the key to DER and then to the "dgst" command +to compute the fingerprint. </p> + +<p> Example: </p> +<blockquote> +<pre> +$ openssl x509 -in cert.pem -noout -pubkey | + openssl pkey -pubin -outform DER | + openssl dgst -sha256 -c +(stdin)= 64:3f:1f:f6:e5:1e:d4:2a:...:8b:fc:09:1a:61:98:b5:bc:7c:60:58 +</pre> +</blockquote> + +<h3><a name="server_cipher">Server-side cipher controls</a> </h3> + +<p> The Postfix SMTP server supports 5 distinct cipher grades as +specified by the <a href="postconf.5.html#smtpd_tls_mandatory_ciphers">smtpd_tls_mandatory_ciphers</a> configuration parameter, +which determines the minimum cipher grade with mandatory TLS +encryption. The default minimum cipher grade for mandatory TLS is +"medium" which is essentially 128-bit encryption or better. The +<a href="postconf.5.html#smtpd_tls_ciphers">smtpd_tls_ciphers</a> parameter (Postfix ≥ 2.6) controls the minimum +cipher grade used with opportunistic TLS. Here, the default minimum +cipher grade is "medium" for Postfix releases after the middle of +2015, "export" for older Postfix releases. With Postfix < 2.6, +the minimum opportunistic TLS cipher grade is always "export". </p> + +<p> By default anonymous ciphers are enabled. They are automatically +disabled when remote SMTP client certificates are requested. If +clients are expected to always verify the Postfix SMTP +server certificate you may want to disable anonymous ciphers +by setting "<a href="postconf.5.html#smtpd_tls_mandatory_exclude_ciphers">smtpd_tls_mandatory_exclude_ciphers</a> = aNULL" or +"<a href="postconf.5.html#smtpd_tls_exclude_ciphers">smtpd_tls_exclude_ciphers</a> = aNULL", as appropriate. One can't force +a remote SMTP client to check the server certificate, so excluding +anonymous ciphers is generally unnecessary. </p> + +<p> With mandatory and opportunistic TLS encryption, the Postfix +SMTP server by default disables SSLv2 and SSLv3 with Postfix releases +after the middle of 2015; older releases only disable SSLv2 for +mandatory TLS. The mandatory TLS protocol list is specified via the +<a href="postconf.5.html#smtpd_tls_mandatory_protocols">smtpd_tls_mandatory_protocols</a> configuration parameter. The +<a href="postconf.5.html#smtpd_tls_protocols">smtpd_tls_protocols</a> parameter (Postfix ≥ 2.6) +controls the TLS protocols used with opportunistic TLS. </p> + +<p> Note that the OpenSSL library only supports protocol exclusion +(not inclusion). For this reason, Postfix can exclude only protocols +that are known at the time the Postfix software is written. If new +protocols are added to the OpenSSL library, they cannot be excluded +without corresponding changes to the Postfix source code. </p> + +<p> For a server that is not a public Internet MX host, Postfix +supports configurations with no <a href="#server_cert_key">server +certificates</a> that use <b>only</b> the anonymous ciphers. This is +enabled by explicitly setting "<a href="postconf.5.html#smtpd_tls_cert_file">smtpd_tls_cert_file</a> = none" +and not specifying an <a href="postconf.5.html#smtpd_tls_dcert_file">smtpd_tls_dcert_file</a> or <a href="postconf.5.html#smtpd_tls_eccert_file">smtpd_tls_eccert_file</a>. +Such configurations may not interoperate with some clients, and require +that TLSv1.3 be explicitly disabled. Therefore, they are not +recommended, it is better and simpler to just configure a suitable +certificate. </p> + +<p> Example, MSA that requires TLSv1.2 or higher, with high grade +ciphers: </p> + +<blockquote> +<pre> +/etc/postfix/<a href="postconf.5.html">main.cf</a>: + <a href="postconf.5.html#smtpd_tls_cert_file">smtpd_tls_cert_file</a> = /etc/postfix/cert.pem + <a href="postconf.5.html#smtpd_tls_key_file">smtpd_tls_key_file</a> = /etc/postfix/key.pem + <a href="postconf.5.html#smtpd_tls_mandatory_ciphers">smtpd_tls_mandatory_ciphers</a> = high + <a href="postconf.5.html#smtpd_tls_mandatory_exclude_ciphers">smtpd_tls_mandatory_exclude_ciphers</a> = aNULL, MD5 + <a href="postconf.5.html#smtpd_tls_security_level">smtpd_tls_security_level</a> = encrypt + # Preferred syntax with Postfix ≥ 3.6: + <a href="postconf.5.html#smtpd_tls_mandatory_protocols">smtpd_tls_mandatory_protocols</a> = >=TLSv1.2 + # Legacy syntax: + <a href="postconf.5.html#smtpd_tls_mandatory_protocols">smtpd_tls_mandatory_protocols</a> = !SSLv2, !SSLv3, !TLSv1, !TLSv1.1 +</pre> +</blockquote> + +<p> With Postfix ≥ 3.4, specify instead a single file that holds the +key followed by the corresponding certificate and any associated issuing +certificates, leaving the "<a href="postconf.5.html#smtpd_tls_cert_file">smtpd_tls_cert_file</a>" and "<a href="postconf.5.html#smtpd_tls_key_file">smtpd_tls_key_file</a>" +and related DSA and ECDSA parameters empty. </p> + +<blockquote> +<pre> +/etc/postfix/<a href="postconf.5.html">main.cf</a>: + <a href="postconf.5.html#smtpd_tls_chain_files">smtpd_tls_chain_files</a> = /etc/postfix/rsachain.pem + <a href="postconf.5.html#smtpd_tls_cert_file">smtpd_tls_cert_file</a> = + <a href="postconf.5.html#smtpd_tls_key_file">smtpd_tls_key_file</a> = + ... +</pre> +</blockquote> + +<p> If you want to take maximal advantage of ciphers that offer <a +href="FORWARD_SECRECY_README.html#dfn_fs">forward secrecy</a> see +the <a href="FORWARD_SECRECY_README.html#quick-start">Getting +started</a> section of <a +href="FORWARD_SECRECY_README.html">FORWARD_SECRECY_README</a>. The +full document conveniently presents all information about Postfix +forward secrecy support in one place: what forward secrecy is, how +to tweak settings, and what you can expect to see when Postfix uses +ciphers with forward secrecy. </p> + +<p> Postfix 2.8 and later, in combination with OpenSSL 0.9.7 and later +allows TLS servers to preempt the TLS client's cipher-suite preference list. +This is possible only with SSLv3 and later, as in SSLv2 the client +chooses the cipher-suite from a list supplied by the server. </p> + +<p> By default, the OpenSSL server selects the client's most preferred +cipher-suite that the server supports. With SSLv3 and later, the server +may choose its own most preferred cipher-suite that is supported (offered) +by the client. Setting "<a href="postconf.5.html#tls_preempt_cipherlist">tls_preempt_cipherlist</a> = yes" enables server +cipher-suite preferences. The default OpenSSL behavior applies with +"<a href="postconf.5.html#tls_preempt_cipherlist">tls_preempt_cipherlist</a> = no". </p> + +<p> While server cipher-suite selection may in some cases lead to +a more secure or performant cipher-suite choice, there is some risk +of interoperability issues. In the past, some SSL clients have +listed lower priority ciphers that they did not implement correctly. +If the server chooses a cipher that the client prefers less, it may +select a cipher whose client implementation is flawed. Most notably +Windows 2003 Microsoft Exchange servers have flawed implementations +of DES-CBC3-SHA, which OpenSSL considers stronger than RC4-SHA. +Enabling server cipher-suite selection may create interoperability +issues with Windows 2003 Microsoft Exchange clients. </p> + +<h3><a name="server_misc"> Miscellaneous server controls</a> </h3> + +<p> The <a href="postconf.5.html#smtpd_starttls_timeout">smtpd_starttls_timeout</a> parameter limits the time of Postfix +SMTP server write and read operations during TLS startup and shutdown +handshake procedures. </p> + +<p> Example: </p> + +<blockquote> +<pre> +/etc/postfix/<a href="postconf.5.html">main.cf</a>: + <a href="postconf.5.html#smtpd_starttls_timeout">smtpd_starttls_timeout</a> = 300s +</pre> +</blockquote> + +<p> With Postfix 2.8 and later, the <a href="postconf.5.html#tls_disable_workarounds">tls_disable_workarounds</a> parameter +specifies a list or bit-mask of default-enabled OpenSSL bug +work-arounds to disable. This may be necessary if one of the +work-arounds enabled by default in OpenSSL proves to pose a security +risk, or introduces an unexpected interoperability issue. The list +of enabled bug work-arounds is OpenSSL-release-specific. See the +<a href="postconf.5.html#tls_disable_workarounds">tls_disable_workarounds</a> parameter documentation for the list of +supported values.</p> + +<p> Example: </p> + +<blockquote> +<pre> +/etc/postfix/<a href="postconf.5.html">main.cf</a>: + <a href="postconf.5.html#tls_disable_workarounds">tls_disable_workarounds</a> = 0xFFFFFFFF + <a href="postconf.5.html#tls_disable_workarounds">tls_disable_workarounds</a> = CVE-2010-4180 +</pre> +</blockquote> + +<p> With Postfix ≥ 2.11, the <a href="postconf.5.html#tls_ssl_options">tls_ssl_options</a> parameter specifies +a list or bit-mask of OpenSSL options to enable. Specify one or +more of the named options below, or a hexadecimal bitmask of options +found in the ssl.h file corresponding to the run-time OpenSSL +library. While it may be reasonable to turn off all bug workarounds +(see above), it is not a good idea to attempt to turn on all features. +See the <a href="postconf.5.html#tls_ssl_options">tls_ssl_options</a> parameter documentation for the list of +supported values. </p> + +<p> Example: </p> + +<blockquote> +<pre> +/etc/postfix/<a href="postconf.5.html">main.cf</a>: + <a href="postconf.5.html#tls_ssl_options">tls_ssl_options</a> = no_ticket, no_compression +</pre> +</blockquote> + +<p> You should only enable features via the hexadecimal mask when +the need to control the feature is critical (to deal with a new +vulnerability or a serious interoperability problem). Postfix DOES +NOT promise backwards compatible behavior with respect to the mask +bits. A feature enabled via the mask in one release may be enabled +by other means in a later release, and the mask bit will then be +ignored. Therefore, use of the hexadecimal mask is only a temporary +measure until a new Postfix or OpenSSL release provides a better +solution. </p> + +<h2> <a name="client_tls">SMTP Client specific settings</a> </h2> + +<p> Topics covered in this section: </p> + +<ul> + +<li><a href="#client_tls_levels"> Configuring TLS in the SMTP/LMTP client </a> + +<li><a href="#client_logging"> Client-side TLS activity logging </a> + +<li><a href="#client_cert_key">Client-side certificate and private +key configuration </a> + +<li><a href="#client_tls_reuse">Client-side TLS connection reuse</a> + +<li><a href="#client_tls_cache">Client-side TLS session cache</a> + +<li><a href="#client_tls_limits"> Client TLS limitations </a> + +<li><a href="#client_tls_policy"> Per-destination TLS policy </a> + +<li><a href="#client_tls_discover"> Discovering servers that support TLS </a> + +<li><a href="#client_vrfy_server">Server certificate verification depth</a> + +<li> <a href="#client_cipher">Client-side cipher controls </a> + +<li> <a href="#client_smtps">Client-side SMTPS support </a> + +<li> <a href="#client_misc"> Miscellaneous client controls </a> + +</ul> + +<h3><a name="client_tls_levels"> Configuring TLS in the SMTP/LMTP client </a> +</h3> + +<p> Similar to the Postfix SMTP server, the Postfix SMTP/LMTP client +implements multiple TLS security levels. These levels are described +in more detail in the sections that follow.</p> + +<dl> +<dt><b>none</b></dt> +<dd><a href="#client_tls_none">No TLS.</a></dd> +<dt><b>may</b></dt> +<dd><a href="#client_tls_may">Opportunistic TLS.</a></dd> +<dt><b>encrypt</b></dt> +<dd><a href="#client_tls_encrypt">Mandatory TLS encryption.</a> +<dt><b>dane</b></dt> +<dd><a href="#client_tls_dane">Opportunistic DANE TLS.</a> +<dt><b>dane-only</b></dt> +<dd><a href="#client_tls_dane">Mandatory DANE TLS.</a> +<dt><b>fingerprint</b></dt> +<dd><a href="#client_tls_fprint">Certificate fingerprint verification.</a> +<dt><b>verify</b></dt> +<dd><a href="#client_tls_verify">Mandatory server certificate verification.</a> +<dt><b>secure</b></dt> +<dd><a href="#client_tls_secure">Secure-channel TLS.</a> +</dl> + +<h4><a name="client_lmtp_tls"> TLS support in the LMTP delivery agent </a> </h4> + +<p> The <a href="smtp.8.html">smtp(8)</a> and <a href="lmtp.8.html">lmtp(8)</a> delivery agents are implemented by a +single dual-purpose program. Specifically, all the TLS features +described below apply +equally to SMTP and LMTP, after replacing the "smtp_" prefix of the each +parameter name with "lmtp_". + +<p> The Postfix LMTP delivery agent can communicate with LMTP servers +listening +on UNIX-domain sockets. When server certificate verification is enabled +and the server is listening on a UNIX-domain socket, the $<a href="postconf.5.html#myhostname">myhostname</a> +parameter is used to set the TLS verification <i>nexthop</i> and +<i>hostname</i>. </p> + +<p> NOTE: Opportunistic encryption of LMTP traffic over UNIX-domain +sockets or loopback TCP connections is futile. TLS is only useful +in this context when +it is mandatory, typically to allow at least one of the server or the +client to authenticate the other. The "null" cipher grade may be +appropriate in this context, when available on both client and server. +The "null" ciphers provide authentication without encryption. </p> + +<h4><a name="client_tls_none"> No TLS encryption </a> </h4> + +<p> At the "none" TLS security level, TLS encryption is +disabled. This is the default security level, and +can be configured explicitly by setting "<a href="postconf.5.html#smtp_tls_security_level">smtp_tls_security_level</a> = none". +For LMTP, use the corresponding "lmtp_" parameter. </p> + +<p> Per-destination settings may override this default setting, in which case +TLS is used selectively, only with destinations explicitly configured +for TLS. </p> + +<p> You can disable TLS for a subset of destinations, while leaving +it enabled for the rest. With the Postfix TLS <a +href="#client_tls_policy">policy table</a>, specify the "none" +security level. + +<h4><a name="client_tls_may"> Opportunistic TLS </a> </h4> + +<p> At the "may" TLS security level, TLS encryption is <i>opportunistic</i>. +The SMTP transaction is encrypted if the STARTTLS ESMTP feature +is supported by the server. Otherwise, messages are sent in the clear. +Opportunistic TLS can be configured by setting "<a href="postconf.5.html#smtp_tls_security_level">smtp_tls_security_level</a> = may". +For LMTP, use the corresponding "lmtp_" parameter. </p> + +<p> The "<a href="postconf.5.html#smtp_tls_ciphers">smtp_tls_ciphers</a>" and "<a href="postconf.5.html#smtp_tls_protocols">smtp_tls_protocols</a>" configuration +parameters (Postfix ≥ 2.6) provide control over the cipher grade +and protocols used with opportunistic TLS. With earlier Postfix +releases, opportunistic TLS always uses the cipher grade "export" +and enables all protocols. </p> + +<p> With opportunistic TLS, mail delivery continues even if the +server certificate is untrusted or bears the wrong name. +When the TLS handshake fails for an opportunistic +TLS session, rather than give up on mail delivery, the Postfix SMTP +client retries the transaction +with TLS disabled. Trying an unencrypted connection makes +it possible to deliver mail to sites with non-interoperable server +TLS implementations. </p> + +<p> Opportunistic encryption is never used for LMTP over UNIX-domain +sockets. The communications channel is already confidential without +TLS, so the only potential benefit of TLS is authentication. Do not +configure opportunistic TLS for LMTP deliveries over UNIX-domain sockets. +Only configure TLS for LMTP over UNIX-domain sockets at the +<a href="#client_tls_encrypt">encrypt</a> security level or higher. +Attempts to configure opportunistic encryption of LMTP sessions will +be ignored with a warning written to the mail logs. </p> + +<p> You can enable opportunistic TLS just for selected destinations. With +the Postfix TLS <a href="#client_tls_policy">policy table</a>, +specify the "may" security level. </p> + +<p> This is the most common security level for TLS protected SMTP +sessions, stronger security is not generally available and, if needed, +is typically only configured on a per-destination basis. See the section +on TLS <a href="#client_tls_limits">limitations</a> above. </p> + +<p> Example: </p> + +<blockquote> +<pre> +/etc/postfix/<a href="postconf.5.html">main.cf</a>: + <a href="postconf.5.html#smtp_tls_security_level">smtp_tls_security_level</a> = may +</pre> +</blockquote> + +<h4><a name="client_tls_encrypt"> Mandatory TLS encryption </a> </h4> + +<p> At the "encrypt" TLS security level, messages are sent only +over TLS encrypted sessions. The SMTP transaction is aborted unless +the STARTTLS ESMTP feature is supported by the remote SMTP server. +If no suitable +servers are found, the message will be deferred. +Mandatory TLS encryption can be configured by setting +"<a href="postconf.5.html#smtp_tls_security_level">smtp_tls_security_level</a> = encrypt". Even though TLS +encryption is always used, mail delivery continues even if the server +certificate is untrusted or bears the wrong name. +For LMTP, use the corresponding "lmtp_" parameter. </p> + +<p> At this security level and higher, the <a href="postconf.5.html#smtp_tls_mandatory_protocols">smtp_tls_mandatory_protocols</a> +and <a href="postconf.5.html#smtp_tls_mandatory_ciphers">smtp_tls_mandatory_ciphers</a> configuration parameters determine +the list of sufficiently secure SSL protocol versions and the minimum +cipher strength. If the protocol or cipher requirements are not +met, the mail transaction is aborted. The documentation for these +parameters includes useful interoperability and security guidelines. +</p> + +<p> Despite the potential for eliminating passive eavesdropping attacks, +mandatory TLS encryption is not viable as a default security level for +mail delivery to the public Internet. Some MX hosts do not support TLS at +all, and some of those that do have broken implementations. On a host +that delivers mail to the Internet, you should not configure mandatory +TLS encryption as the default security level. </p> + +<p> You can enable mandatory TLS encryption just for specific destinations. +With the Postfix TLS <a href="#client_tls_policy">policy +table</a>, specify the "encrypt" security level. +</p> + +<p> Examples: </p> + +<p> In the example below, traffic to <i>example.com</i> and its sub-domains +via the corresponding MX hosts always uses TLS. The SSLv2 protocol +will be disabled (the default setting of <a href="postconf.5.html#smtp_tls_mandatory_protocols">smtp_tls_mandatory_protocols</a> +excludes SSLv2+3). Only high- or medium-strength (i.e. 128 bit or +better) ciphers will be used by default for all "encrypt" security +level sessions. </p> + +<blockquote> +<pre> +/etc/postfix/<a href="postconf.5.html">main.cf</a>: + <a href="postconf.5.html#smtp_tls_policy_maps">smtp_tls_policy_maps</a> = <a href="DATABASE_README.html#types">hash</a>:/etc/postfix/tls_policy + +/etc/postfix/tls_policy: + example.com encrypt + .example.com encrypt +</pre> +</blockquote> + +<p> In the next example, secure message submission is configured +via the MSA "<tt>[example.net]:587</tt>". TLS sessions are encrypted +without authentication, because this MSA does not possess an acceptable +certificate. This MSA is known to be capable of "TLSv1" and "high" grade +ciphers, so these are selected via the <a href="#client_tls_policy">policy +table</a>. </p> + +<p><b>Note:</b> the policy table lookup key is the verbatim next-hop +specification from the recipient domain, <a href="transport.5.html">transport(5)</a> table or <a href="postconf.5.html#relayhost">relayhost</a> +parameter, with any enclosing square brackets and optional port. Take +care to be consistent: the suffixes ":smtp" or ":25" or no port suffix +result in different policy table lookup keys, even though they are +functionally equivalent nexthop specifications. Use at most one of these +forms for all destinations. Below, the policy table has multiple keys, +just in case the transport table entries are not specified consistently. </p> + +<blockquote> +<pre> +/etc/postfix/<a href="postconf.5.html">main.cf</a>: + <a href="postconf.5.html#smtp_tls_policy_maps">smtp_tls_policy_maps</a> = <a href="DATABASE_README.html#types">hash</a>:/etc/postfix/tls_policy + +/etc/services: + submission 587/tcp msa # mail message submission + +/etc/postfix/tls_policy: + # Postfix ≥ 3.6 "protocols" syntax + [example.net]:587 encrypt protocols=>=TLSv1.2 ciphers=high + # Legacy "protocols" syntax + [example.net]:msa encrypt protocols=!SSLv2:!SSLv3 ciphers=high +</pre> +</blockquote> + +<h4><a name="client_tls_dane">DANE TLS authentication.</a> </h4> + +<p> The Postfix SMTP client supports two TLS security levels based +on DANE TLSA (<a href="https://tools.ietf.org/html/rfc6698">RFC 6698</a>, <a href="https://tools.ietf.org/html/rfc7671">RFC 7671</a>, <a href="https://tools.ietf.org/html/rfc7672">RFC 7672</a>) records. The opportunistic +"dane" level and the mandatory "dane-only" level. </p> + +<p> The "dane" level is a stronger form of <a +href="#client_tls_may">opportunistic</a> TLS that is resistant to +man in the middle and downgrade attacks when the destination domain +uses DNSSEC to publish DANE TLSA records for its MX hosts. If a +remote SMTP server has "usable" (see section 3 of <a href="https://tools.ietf.org/html/rfc7672">RFC 7672</a>) DANE +TLSA records, the server connection will be authenticated. When +DANE authentication fails, there is no fallback to unauthenticated +or plaintext delivery. </p> + +<p> If TLSA records are published for a given remote SMTP server +(implying TLS support), but are all "unusable" due to unsupported +parameters or malformed data, the Postfix SMTP client will use <a +href="#client_tls_encrypt">mandatory</a> unauthenticated TLS. +Otherwise, when no TLSA records are published, the Postfix SMTP +client behavior is the same as with <a href="#client_tls_may">may</a>. </p> + +<p> TLSA records must be published in DNSSEC validated DNS zones. +Any TLSA records in DNS zones not protected via DNSSEC are ignored. +The Postfix SMTP client will not look for TLSA records associated +with MX hosts whose "A" or "AAAA" records lie in an "insecure" DNS +zone. Such lookups have been observed to cause interoperability +issues with poorly implemented DNS servers, and are in any case not +expected to ever yield "secure" results, since that would require +a very unlikely DLV DNS trust anchor configured between the host +record and the associated "_25._tcp" child TLSA record. </p> + +<p> The "dane-only" level is a form of <a +href="#client_tls_secure">secure-channel</a> TLS based on the DANE PKI. +If "usable" TLSA records are present these are used to authenticate the +remote SMTP server. Otherwise, or when server certificate verification +fails, delivery via the server in question tempfails. </p> + +<p> At both security levels, the TLS policy for the destination is +obtained via TLSA records validated with DNSSEC. For TLSA policy +to be in effect, the destination domain's containing DNS zone must +be signed and the Postfix SMTP client's operating system must be +configured to send its DNS queries to a recursive DNS nameserver +that is able to validate the signed records. Each MX host's DNS +zone needs to also be signed, and needs to publish DANE TLSA (see +section 3 of <a href="https://tools.ietf.org/html/rfc7672">RFC 7672</a>) records that specify how that MX host's TLS +certificate is to be verified. </p> + +<p> TLSA records do not preempt the normal SMTP MX host +selection algorithm, if some MX hosts support TLSA and others do +not, TLS security will vary from delivery to delivery. It is up +to the domain owner to configure their MX hosts and their DNS +sensibly. To configure the Postfix SMTP client for DNSSEC lookups +see the documentation for the <a href="postconf.5.html#smtp_dns_support_level">smtp_dns_support_level</a> <a href="postconf.5.html">main.cf</a> +parameter. The <a href="postconf.5.html#tls_dane_digests">tls_dane_digests</a> parameter controls the list of +supported digests. </p> + +<p> As explained in section 3 of <a href="https://tools.ietf.org/html/rfc7672">RFC 7672</a>, certificate usages "0" +and "1", which are intended to "constrain" existing Web-PKI trust, +are not supported with MTA-to-MTA SMTP. Rather, TLSA records with +usages "0" and "1" are treated as "unusable". </p> + +<p> The Postfix SMTP client supports only certificate usages "2" +and "3". Experimental support for silently mapping certificate +usage "1" to "3" has been withdrawn starting with Postfix 3.2. </p> + +<p> When usable TLSA records are obtained for the remote SMTP server +the Postfix SMTP client sends the SNI TLS extension in its SSL +client hello message. This may help the remote SMTP server live +up to its promise to provide a certificate that matches its TLSA +records. </p> + +<p> For purposes of protocol and cipher selection, the "dane" +security level is treated like a "mandatory" TLS security level, +and weak ciphers and protocols are disabled. Since DANE authenticates +server certificates the "aNULL" cipher-suites are transparently +excluded at this level, no need to configure this manually. <a href="https://tools.ietf.org/html/rfc7672">RFC</a> +<a href="https://tools.ietf.org/html/rfc7672">7672</a> (DANE) TLS authentication is available with Postfix 2.11 and +later. </p> + +<p> When a DANE TLSA record specifies a trust-anchor (TA) certificate +(that is an issuing CA), the strategy used to verify the peername +of the server certificate is unconditionally "nexthop, hostname". +Both the nexthop domain and the hostname obtained from the +DNSSEC-validated MX lookup are safe from forgery and the server +certificate must contain at least one of these names. </p> + +<p> When a DANE TLSA record specifies an end-entity (EE) certificate, +(that is the actual server certificate), as with the fingerprint +security level below, no name checks or certificate expiration checks +are applied. The server certificate (or its public key) either matches +the DANE record or not. Server administrators should publish such +EE records in preference to all other types. </p> + +<p> The pre-requisites for DANE support in the Postfix SMTP client are: </p> +<ul> +<li> A <i>compile-time</i> OpenSSL library that supports the TLS SNI +extension and "SHA-2" message digests. +<li> A <i>compile-time</i> DNS resolver library that supports DNSSEC. +Postfix binaries built on an older system will not support DNSSEC even +if deployed on a system with an updated resolver library. +<li> The "<a href="postconf.5.html#smtp_dns_support_level">smtp_dns_support_level</a>" must be set to "dnssec". +<li> The "<a href="postconf.5.html#smtp_host_lookup">smtp_host_lookup</a>" parameter must include "dns". +<li> A DNSSEC-validating recursive resolver (see note below). +</ul> +<p> The above client pre-requisites do not apply to the Postfix SMTP server. +It will support DANE provided it supports TLSv1 and its TLSA records are +published in a DNSSEC signed zone. To receive DANE secured mail for multiple +domains, use the same hostname to add the server to each domain's MX +records. The Postfix SMTP server supports SNI (Postfix 3.4 and later), +configured with <a href="postconf.5.html#tls_server_sni_maps">tls_server_sni_maps</a>. </p> + +<p> Note: The Postfix SMTP client's internal stub DNS resolver is +DNSSEC-aware, but it does not itself validate DNSSEC records, rather +it delegates DNSSEC validation to the operating system's configured +recursive DNS nameserver. The Postfix DNS client relies on a secure +channel to the resolver's cache for DNSSEC integrity, but does not +support TSIG to protect the transmission channel between itself and +the nameserver. Therefore, it is strongly recommended (DANE security +guarantee void otherwise) that each MTA run a local DNSSEC-validating +recursive resolver ("unbound" from nlnetlabs.nl is a reasonable +choice) listening on the loopback interface, and that the system +be configured to use <i>only</i> this local nameserver. The local +nameserver may forward queries to an upstream recursive resolver +on another host if desired. </p> + +<p> Note: When the operating system's recursive nameserver is not +local, enabling EDNS0 expanded DNS packet sizes and turning on the +DNSSEC "DO" bit in the DNS request and/or the new DNSSEC-specific +records returned in the nameserver's replies may cause problems +with older or buggy firewall and DNS server implementations. +Therefore, Postfix does not enable DNSSEC by default. Since MX +lookups happen before the security level is determined, DANE support +is disabled for all destinations unless you set "<a href="postconf.5.html#smtp_dns_support_level">smtp_dns_support_level</a> += dnssec". To enable DNSSEC lookups selectively, define a new +dedicated transport with a "-o <a href="postconf.5.html#smtp_dns_support_level">smtp_dns_support_level</a>=dnssec" +override in <a href="master.5.html">master.cf</a> and route selected domains to that transport. +If DNSSEC proves to be sufficiently reliable for these domains, you +can enable it for all destinations by changing the global +<a href="postconf.5.html#smtp_dns_support_level">smtp_dns_support_level</a> in <a href="postconf.5.html">main.cf</a>. </p> + +<p><b>Example</b>: "dane" security for selected destinations, with +opportunistic TLS by default. This is the recommended configuration +for early adopters. <p> +<ul> +<li> <p> The "example.com" destination uses DANE, but if TLSA records +are not present or are unusable, mail is deferred. </p> + +<li> <p> The "example.org" destination uses DANE if possible, but if no TLSA +records are found opportunistic TLS is used. </p> +</ul> + +<blockquote> +<pre> +<a href="postconf.5.html">main.cf</a>: + indexed = ${<a href="postconf.5.html#default_database_type">default_database_type</a>}:${<a href="postconf.5.html#config_directory">config_directory</a>}/ + # + # default: Opportunistic TLS with no DNSSEC lookups. + # + <a href="postconf.5.html#smtp_tls_security_level">smtp_tls_security_level</a> = may + <a href="postconf.5.html#smtp_dns_support_level">smtp_dns_support_level</a> = enabled + # + # Per-destination TLS policy + # + <a href="postconf.5.html#smtp_tls_policy_maps">smtp_tls_policy_maps</a> = ${indexed}tls_policy + # + # <a href="postconf.5.html#default_transport">default_transport</a> = smtp, but some destinations are special: + # + <a href="postconf.5.html#transport_maps">transport_maps</a> = ${indexed}transport +</pre> +</blockquote> + +<blockquote> +<pre> +transport: + example.com dane + example.org dane +</pre> +</blockquote> + +<blockquote> +<pre> +tls_policy: + example.com dane-only +</pre> +</blockquote> + +<blockquote> +<pre> +<a href="master.5.html">master.cf</a>: + dane unix - - n - - smtp + -o <a href="postconf.5.html#smtp_dns_support_level">smtp_dns_support_level</a>=dnssec + -o <a href="postconf.5.html#smtp_tls_security_level">smtp_tls_security_level</a>=dane +</pre> +</blockquote> + +<h4><a name="client_tls_fprint"> Certificate fingerprint verification </a> </h4> + +<p> At the <i>fingerprint</i> security level, no trusted Certification +Authorities are used or required. The certificate trust chain, +expiration date, etc., are not checked. Instead, the +<a href="postconf.5.html#smtp_tls_fingerprint_cert_match">smtp_tls_fingerprint_cert_match</a> parameter or the "match" attribute +in the <a href="#client_tls_policy">policy</a> table lists the +remote SMTP server certificate fingerprint or public key fingerprint. +Certificate fingerprint verification is available with Postfix 2.5 +and later, public-key fingerprint support is available with Postfix +2.9 and later. </p> + +<p> If certificate fingerprints are exchanged securely, this is the +strongest, and least scalable security level. The administrator needs +to securely collect the fingerprints of the X.509 certificates of each +peer server, store them into a local file, and update this local file +whenever the peer server's public certificate changes. If public key +fingerprints are used in place of fingerprints of the entire certificate, +the fingerprints remain valid even after the certificate is renewed, +<b>provided</b> that the same public/private keys are used to obtain +the new certificate. </p> + +<p> Fingerprint verification may be feasible for an SMTP "VPN" connecting +a small number of branch offices over the Internet, or for secure +connections to a central mail hub. It works poorly if the remote SMTP +server is managed by a third party, and its public certificate changes +periodically without prior coordination with the verifying site. </p> + +<p> The digest algorithm used to calculate the fingerprint is +selected by the <b><a href="postconf.5.html#smtp_tls_fingerprint_digest">smtp_tls_fingerprint_digest</a></b> parameter. In the <a +href="#client_tls_policy">policy</a> table multiple fingerprints can be +combined with a "|" delimiter in a single match attribute, or multiple +match attributes can be employed. The ":" character is not used as a +delimiter as it occurs between each pair of fingerprint (hexadecimal) +digits. </p> + +<p> The default algorithm is <b>sha256</b> with Postfix ≥ 3.6 +and the <b><a href="postconf.5.html#compatibility_level">compatibility_level</a></b> set to 3.6 or higher; with Postfix +≤ 3.5, the default algorithm is <b>md5</b>. The +best-practice algorithm is now <b>sha256</b>. Recent advances in hash +function cryptanalysis have led to md5 and sha1 being deprecated in +favor of sha256. However, as long as there are no known "second +pre-image" attacks against the older algorithms, their use in this +context, though not recommended, is still likely safe. </p> + +<p> Example: fingerprint TLS security with an internal mailhub. +Two matching fingerprints are listed. The <a href="postconf.5.html#relayhost">relayhost</a> may be multiple +physical hosts behind a load-balancer, each with its own private/public +key and self-signed certificate. Alternatively, a single <a href="postconf.5.html#relayhost">relayhost</a> may +be in the process of switching from one set of private/public keys to +another, and both keys are trusted just prior to the transition. </p> + +<blockquote> +<pre> + <a href="postconf.5.html#relayhost">relayhost</a> = [mailhub.example.com] + <a href="postconf.5.html#smtp_tls_security_level">smtp_tls_security_level</a> = fingerprint + <a href="postconf.5.html#smtp_tls_fingerprint_digest">smtp_tls_fingerprint_digest</a> = sha256 + <a href="postconf.5.html#smtp_tls_fingerprint_cert_match">smtp_tls_fingerprint_cert_match</a> = + 51:e9:af:2e:1e:40:1f:de:64:...:30:35:2d:09:16:31:5a:eb:82:76 + b6:b4:72:34:e2:59:cd:fb:c2:...:63:0d:4d:cc:2c:7d:84:de:e6:2f +</pre> +</blockquote> + +<p> Example: Certificate fingerprint verification with selected destinations. +As in the example above, we show two matching fingerprints: </p> +<blockquote> +<pre> +/etc/postfix/<a href="postconf.5.html">main.cf</a>: + <a href="postconf.5.html#smtp_tls_policy_maps">smtp_tls_policy_maps</a> = <a href="DATABASE_README.html#types">hash</a>:/etc/postfix/tls_policy + <a href="postconf.5.html#smtp_tls_fingerprint_digest">smtp_tls_fingerprint_digest</a> = sha256 +</pre> +</blockquote> +<blockquote> +<pre> +/etc/postfix/tls_policy: + example.com fingerprint + match=51:e9:af:2e:1e:40:1f:de:...:35:2d:09:16:31:5a:eb:82:76 + match=b6:b4:72:34:e2:59:cd:fb:...:0d:4d:cc:2c:7d:84:de:e6:2f +</pre> +</blockquote> + +<p> To extract the public key fingerprint from an X.509 certificate, +you need to extract the public key from the certificate and compute +the appropriate digest of its DER (ASN.1) encoding. With OpenSSL +the "-pubkey" option of the "x509" command extracts the public +key always in "PEM" format. We pipe the result to another OpenSSL +command that converts the key to DER and then to the "dgst" command +to compute the fingerprint. </p> + +<p> Example: </p> +<blockquote> +<pre> +$ openssl x509 -in cert.pem -noout -pubkey | + openssl pkey -pubin -outform DER | + openssl dgst -sha256 -c +(stdin)= 64:3f:1f:f6:e5:1e:d4:2a:56:...:09:1a:61:98:b5:bc:7c:60:58 +</pre> +</blockquote> + +<h4><a name="client_tls_verify"> Mandatory server certificate verification </a> </h4> + +<p> At the <i>verify</i> TLS security level, messages are sent only over +TLS encrypted sessions if the remote SMTP server certificate is +valid (not +expired or revoked, and signed by a trusted Certification Authority) +and where the server certificate name matches a known pattern. +Mandatory +server certificate verification can be configured by setting +"<a href="postconf.5.html#smtp_tls_security_level">smtp_tls_security_level</a> = verify". The +<a href="postconf.5.html#smtp_tls_verify_cert_match">smtp_tls_verify_cert_match</a> parameter can override the default +"hostname" certificate name matching strategy. Fine-tuning the +matching strategy is generally only appropriate for <a +href="#client_tls_secure">secure-channel</a> destinations. +For LMTP use the corresponding "lmtp_" parameters. </p> + +<p> If the server certificate chain is trusted (see <a href="postconf.5.html#smtp_tls_CAfile">smtp_tls_CAfile</a> +and <a href="postconf.5.html#smtp_tls_CApath">smtp_tls_CApath</a>), any DNS names in the SubjectAlternativeName +certificate extension are used to verify the remote SMTP server name. +If no +DNS names are specified, the certificate CommonName is checked. +If you want mandatory encryption without server certificate +verification, see <a href="#client_tls_encrypt">above</a>. </p> + +<p> With Postfix ≥ 2.11 the "<a href="postconf.5.html#smtp_tls_trust_anchor_file">smtp_tls_trust_anchor_file</a>" parameter +or more typically the corresponding per-destination "tafile" attribute +optionally modifies trust chain verification. If the parameter is +not empty the root CAs in CAfile and CApath are no longer trusted. +Rather, the Postfix SMTP client will only trust certificate-chains +signed by one of the trust-anchors contained in the chosen files. +The specified trust-anchor certificates and public keys are not +subject to expiration, and need not be (self-signed) root CAs. They +may, if desired, be intermediate certificates. Therefore, these +certificates also may be found "in the middle" of the trust chain +presented by the remote SMTP server, and any untrusted issuing +parent certificates will be ignored. </p> + +<p> Despite the potential for eliminating "man-in-the-middle" and other +attacks, mandatory certificate trust chain and subject name verification +is not viable as a default Internet mail delivery policy. Some MX hosts +do not support TLS at all, and a significant portion of TLS-enabled +MTAs use self-signed certificates, or certificates that are signed by +a private Certification Authority. On a machine that delivers mail to +the Internet, you should not configure mandatory server certificate +verification as a default policy. </p> + +<p> Mandatory server certificate verification as a default security +level may be appropriate if you know that you will only connect to +servers that support <a href="https://tools.ietf.org/html/rfc2487">RFC 2487</a> <i>and</i> that present verifiable +server certificates. An example would be a client that sends all +email to a central mailhub that offers the necessary STARTTLS +support. In such cases, you can often use a <a +href="#client_tls_secure">secure-channel</a> configuration instead. +</p> + +<p> You can enable mandatory server certificate verification just +for specific destinations. With the Postfix TLS <a +href="#client_tls_policy">policy table</a>, specify the "verify" +security level. </p> + +<p> Example: </p> + +<p> In this example, the Postfix SMTP client encrypts all traffic to the +<i>example.com</i> domain. The peer hostname is verified, but +verification is vulnerable to DNS response forgery. Mail transmission +to <i>example.com</i> recipients uses "high" grade ciphers. </p> + +<blockquote> +<pre> +/etc/postfix/<a href="postconf.5.html">main.cf</a>: + indexed = ${<a href="postconf.5.html#default_database_type">default_database_type</a>}:${<a href="postconf.5.html#config_directory">config_directory</a>}/ + <a href="postconf.5.html#smtp_tls_CAfile">smtp_tls_CAfile</a> = ${<a href="postconf.5.html#config_directory">config_directory</a>}/CAfile.pem + <a href="postconf.5.html#smtp_tls_policy_maps">smtp_tls_policy_maps</a> = ${indexed}tls_policy + +/etc/postfix/tls_policy: + example.com verify ciphers=high +</pre> +</blockquote> + +<h4><a name="client_tls_secure"> Secure server certificate verification </a> </h4> + +<p> At the <i>secure</i> TLS security level, messages are sent only over +<i>secure-channel</i> TLS sessions where DNS forgery resistant server +certificate verification succeeds. If no suitable servers are found, the +message will be deferred. Postfix secure-channels +can be configured by setting "<a href="postconf.5.html#smtp_tls_security_level">smtp_tls_security_level</a> = secure". +The <a href="postconf.5.html#smtp_tls_secure_cert_match">smtp_tls_secure_cert_match</a> parameter can override the default +"nexthop, dot-nexthop" certificate match strategy. +For LMTP, use the corresponding "lmtp_" parameters. </p> + +<p> If the server certificate chain is trusted (see <a href="postconf.5.html#smtp_tls_CAfile">smtp_tls_CAfile</a> and +<a href="postconf.5.html#smtp_tls_CApath">smtp_tls_CApath</a>), any DNS names in the SubjectAlternativeName certificate +extension are used to verify the remote SMTP server name. If no DNS names +are +specified, the CommonName is checked. If you want mandatory encryption +without server certificate verification, see <a +href="#client_tls_encrypt">above</a>. </p> + +<p> With Postfix ≥ 2.11 the "<a href="postconf.5.html#smtp_tls_trust_anchor_file">smtp_tls_trust_anchor_file</a>" parameter +or more typically the corresponding per-destination "tafile" attribute +optionally modifies trust chain verification. If the parameter is +not empty the root CAs in CAfile and CApath are no longer trusted. +Rather, the Postfix SMTP client will only trust certificate-chains +signed by one of the trust-anchors contained in the chosen files. +The specified trust-anchor certificates and public keys are not +subject to expiration, and need not be (self-signed) root CAs. They +may, if desired, be intermediate certificates. Therefore, these +certificates also may be found "in the middle" of the trust chain +presented by the remote SMTP server, and any untrusted issuing +parent certificates will be ignored. </p> + +<p> Despite the potential for eliminating "man-in-the-middle" and other +attacks, mandatory secure server certificate verification is not +viable as a default Internet mail delivery policy. Some MX hosts +do not support TLS at all, and a significant portion of TLS-enabled +MTAs use self-signed certificates, or certificates that are signed +by a private Certification Authority. On a machine that delivers mail +to the Internet, you should not configure secure TLS verification +as a default policy. </p> + +<p> Mandatory secure server certificate verification as a default +security level may be appropriate if you know that you will only +connect to servers that support <a href="https://tools.ietf.org/html/rfc2487">RFC 2487</a> <i>and</i> that present +verifiable server certificates. An example would be a client that +sends all email to a central mailhub that offers the necessary +STARTTLS support. </p> + +<p> You can enable secure TLS verification just for specific destinations. +With the Postfix TLS <a href="#client_tls_policy">policy table</a>, +specify the "secure" security level. </p> + +<p> Examples: </p> + +<ul> + +<li> <p> Secure-channel TLS without <a href="transport.5.html">transport(5)</a> table overrides: </p> + +<p> The Postfix SMTP client will encrypt all traffic and verify the +destination name +immune from forged DNS responses. MX lookups are still used to find +the hostnames of the SMTP servers for <i>example.com</i>, but these +hostnames are not used when +checking the names in the server certificate(s). Rather, the requirement +is that the MX hosts for <i>example.com</i> have trusted certificates +with a subject name of <i>example.com</i> or a sub-domain, see the +documentation for the <a href="postconf.5.html#smtp_tls_secure_cert_match">smtp_tls_secure_cert_match</a> parameter. </p> + +<p> The related domains <i>example.co.uk</i> and <i>example.co.jp</i> are +hosted on the same MX hosts as the primary <i>example.com</i> domain, and +traffic to these is secured by verifying the primary <i>example.com</i> +domain in the server certificates. This frees the server administrator +from needing the CA to sign certificates that list all the secondary +domains. The downside is that clients that want secure channels to the +secondary domains need explicit TLS <a href="#client_tls_policy">policy +table</a> entries. </p> + +<p> Note, there are two ways to handle related domains. The first is to +use the default routing for each domain, but add policy table entries +to override the expected certificate subject name. The second is to +override the next-hop in the transport table, and use a single policy +table entry for the common nexthop. We choose the first approach, +because it works better when domain ownership changes. With the second +approach we securely deliver mail to the wrong destination, with the +first approach, authentication fails and mail stays in the local queue, +the first approach is more appropriate in most cases. <p> + +<blockquote> +<pre> +/etc/postfix/<a href="postconf.5.html">main.cf</a>: + <a href="postconf.5.html#smtp_tls_CAfile">smtp_tls_CAfile</a> = /etc/postfix/CAfile.pem + <a href="postconf.5.html#smtp_tls_policy_maps">smtp_tls_policy_maps</a> = <a href="DATABASE_README.html#types">hash</a>:/etc/postfix/tls_policy + +/etc/postfix/transport: + +/etc/postfix/tls_policy: + example.com secure + example.co.uk secure match=example.com:.example.com + example.co.jp secure match=example.com:.example.com +</pre> +</blockquote> + +<li> <p> Secure-channel TLS with <a href="transport.5.html">transport(5)</a> table overrides: <p> + +<p> In this case traffic to <i>example.com</i> and its related domains +is sent to a single logical gateway (to avoid a single point of failure, +its name may resolve to one or more load-balancer addresses, or to the +combined addresses of multiple physical hosts). All the physical hosts +reachable via the gateway's IP addresses have the logical gateway name +listed in their certificates. </p> + +<blockquote> +<pre> +/etc/postfix/<a href="postconf.5.html">main.cf</a>: + <a href="postconf.5.html#smtp_tls_CAfile">smtp_tls_CAfile</a> = /etc/postfix/CAfile.pem + <a href="postconf.5.html#transport_maps">transport_maps</a> = <a href="DATABASE_README.html#types">hash</a>:/etc/postfix/transport + <a href="postconf.5.html#smtp_tls_policy_maps">smtp_tls_policy_maps</a> = <a href="DATABASE_README.html#types">hash</a>:/etc/postfix/tls_policy + +/etc/postfix/transport: + example.com <a href="smtp.8.html">smtp</a>:[tls.example.com] + example.co.uk <a href="smtp.8.html">smtp</a>:[tls.example.com] + example.co.jp <a href="smtp.8.html">smtp</a>:[tls.example.com] + +/etc/postfix/tls_policy: + [tls.example.com] secure match=tls.example.com +</pre> +</blockquote> + +</ul> + +<h3><a name="client_logging"> Client-side TLS activity logging </a> </h3> + +<p> To get additional information about Postfix SMTP client TLS +activity you can increase the loglevel from 0..4. Each logging +level also includes the information that is logged at a lower +logging level. </p> + +<blockquote> + +<table border="1"> + +<tr> <th> Level </th> <th> Postfix 2.9 and later</th> <th> Earlier +releases. </th> </tr> + +<tr> <td valign="top"> 0 </td> <td valign="top" colspan="2"> Disable +logging of TLS activity. </td> </tr> + +<tr> <td valign="top"> 1 </td> <td valign="top"> Log only a summary +message on TLS handshake completion — no logging of remote SMTP +server certificate trust-chain verification errors if server certificate +verification is not required. </td> <td valign="top"> Log the summary +message and unconditionally log trust-chain verification errors. +</td> </tr> + +<tr> <td valign="top"> 2 </td> <td valign="top" colspan="2"> Also +log levels during TLS negotiation. </td> </tr> + +<tr> <td valign="top"> 3 </td> <td valign="top" colspan="2"> Also +log hexadecimal and ASCII dump of TLS negotiation process. </td> +</tr> + +<tr> <td valign="top"> 4 </td> <td valign="top" colspan="2"> Also +log hexadecimal and ASCII dump of complete transmission after +STARTTLS. </td> </tr> + +</table> + +</blockquote> + +<p> Example: </p> + +<blockquote> +<pre> +/etc/postfix/<a href="postconf.5.html">main.cf</a>: + <a href="postconf.5.html#smtp_tls_loglevel">smtp_tls_loglevel</a> = 0 +</pre> +</blockquote> + +<h3><a name="client_cert_key">Client-side certificate and private +key configuration </a> </h3> + +<p> Do not configure Postfix SMTP client certificates unless you <b>must</b> +present +client TLS certificates to one or more servers. Client certificates are +not usually needed, and can cause problems in configurations that work +well without them. The recommended setting is to let the defaults stand: </p> + +<blockquote> +<pre> + <a href="postconf.5.html#smtp_tls_cert_file">smtp_tls_cert_file</a> = + <a href="postconf.5.html#smtp_tls_dcert_file">smtp_tls_dcert_file</a> = + <a href="postconf.5.html#smtp_tls_key_file">smtp_tls_key_file</a> = + <a href="postconf.5.html#smtp_tls_dkey_file">smtp_tls_dkey_file</a> = + # Postfix ≥ 2.6 + <a href="postconf.5.html#smtp_tls_eccert_file">smtp_tls_eccert_file</a> = + <a href="postconf.5.html#smtp_tls_eckey_file">smtp_tls_eckey_file</a> = + # Postfix ≥ 3.4 + <a href="postconf.5.html#smtp_tls_chain_files">smtp_tls_chain_files</a> = +</pre> +</blockquote> + +<p> The best way to use the default settings is to comment out the above +parameters in <a href="postconf.5.html">main.cf</a> if present. </p> + +<p> During TLS startup negotiation the Postfix SMTP client may present a +certificate to the remote SMTP server. Browsers typically let the user +select among the certificates that match the CA names indicated by the +remote SMTP server. The Postfix SMTP client does not yet have a mechanism +to select from multiple candidate certificates on the fly, and supports a +single set of certificates (at most one per public key algorithm). </p> + +<p> RSA, DSA and ECDSA (Postfix ≥ 2.6) certificates are supported. +You can configure all three at the same time, in which case the +cipher used determines which certificate is presented. </p> + +<p> It is possible for the Postfix SMTP client to use the same +key/certificate pair as the Postfix SMTP server. If a certificate +is to be presented, it must be in "PEM" format. The private key +must not be encrypted, meaning: it must be accessible without +a password. Both parts (certificate and private key) may be in the +same file. </p> + +<p> With OpenSSL 1.1.1 and Postfix ≥ 3.4 it is also possible to +configure Ed25519 and Ed448 certificates. Rather than add two more +pairs of key and certificate parameters, Postfix 3.4 introduces a new +"<a href="postconf.5.html#smtp_tls_chain_files">smtp_tls_chain_files</a>" parameter which specifies all the configured +certificates at once, and handles files that hold both the key and the +associated certificates in one pass, thereby avoiding potential race +conditions during key rollover. </p> + +<p> To enable remote SMTP servers to verify the Postfix SMTP client +certificate, the issuing CA certificates must be made available to the +server. You should include the required certificates in the client +certificate file, the client certificate first, then the issuing +CA(s) (bottom-up order). </p> + +<p> Example: the certificate for "client.example.com" was issued by +"intermediate CA" which itself has a certificate issued by "root CA". +As the "root" super-user create the client.pem file with: </p> + +<blockquote> +<pre> +# <b>umask 077</b> +# <b>cat client_key.pem client_cert.pem intermediate_CA.pem > chain.pem </b> +</pre> +</blockquote> + +<p> A Postfix SMTP client certificate supplied here must be usable +as an SSL client certificate and hence pass the "openssl verify -purpose +sslclient ..." test. </p> + +<p> A server that trusts the root CA has a local copy of the root +CA certificate, so it is not necessary to include the root CA +certificate here. Leaving it out of the "chain.pem" file reduces +the overhead of the TLS exchange. </p> + +<p> If you want the Postfix SMTP client to accept remote SMTP server +certificates issued by these CAs, append the root certificate to +$<a href="postconf.5.html#smtp_tls_CAfile">smtp_tls_CAfile</a> or install it in the $<a href="postconf.5.html#smtp_tls_CApath">smtp_tls_CApath</a> directory. </p> + +<p> Example: Postfix ≥ 3.4 all-in-one chain file(s). One or more +chain files that start with a key that is immediately followed by the +corresponding certificate and any additional issuer certificates. A +single file can hold multiple <i>(key, cert, [chain])</i> sequences, one +per algorithm. It is typically simpler to keep the chain for each +algorithm in its own file. Most users are likely to deploy at most a +single RSA chain, but with OpenSSL 1.1.1, it is possible to deploy up +five chains, one each for RSA, ECDSA, ED25519, ED448, and even the +obsolete DSA. </p> + +<blockquote> +<pre> + # Postfix ≥ 3.4. Preferred configuration interface. Each file + # starts with the private key, followed by the corresponding + # certificate, and any intermediate issuer certificates. + # + <a href="postconf.5.html#smtp_tls_chain_files">smtp_tls_chain_files</a> = + /etc/postfix/rsa.pem, + /etc/postfix/ecdsa.pem, + /etc/postfix/ed25519.pem, + /etc/postfix/ed448.pem +</pre> +</blockquote> + +<p> You can also store the keys separately from their certificates, again +provided each is listed before the corresponding certificate chain. Storing a +key and its associated certificate chain in separate files is not recommended, +because this is prone to race conditions during key rollover, as there is no +way to update multiple files atomically. </p> + +<blockquote> +<pre> + # Postfix ≥ 3.4. + # Storing keys separately from the associated certificates is not + # recommended. + <a href="postconf.5.html#smtp_tls_chain_files">smtp_tls_chain_files</a> = + /etc/postfix/rsakey.pem, + /etc/postfix/rsacerts.pem, + /etc/postfix/ecdsakey.pem, + /etc/postfix/ecdsacerts.pem +</pre> +</blockquote> + +<p> The below examples show the legacy algorithm-specific configurations +for Postfix 3.3 and older. With Postfix ≤ 3.3, even if the key is +stored in the same file as the certificate, the file is read twice and a +(brief) race condition still exists during key rollover. While Postfix +≥ 3.4 avoids the race when the key and certificate are in the same +file, you should use the new "<a href="postconf.5.html#smtp_tls_chain_files">smtp_tls_chain_files</a>" interface shown +above. <p> + +<p> RSA key and certificate examples: </p> + +<blockquote> +<pre> +/etc/postfix/<a href="postconf.5.html">main.cf</a>: + <a href="postconf.5.html#smtp_tls_cert_file">smtp_tls_cert_file</a> = /etc/postfix/client.pem + <a href="postconf.5.html#smtp_tls_key_file">smtp_tls_key_file</a> = $<a href="postconf.5.html#smtp_tls_cert_file">smtp_tls_cert_file</a> +</pre> +</blockquote> + +<p> Their DSA counterparts: </p> + +<blockquote> +<pre> +/etc/postfix/<a href="postconf.5.html">main.cf</a>: + <a href="postconf.5.html#smtp_tls_dcert_file">smtp_tls_dcert_file</a> = /etc/postfix/client-dsa.pem + <a href="postconf.5.html#smtp_tls_dkey_file">smtp_tls_dkey_file</a> = $<a href="postconf.5.html#smtp_tls_dcert_file">smtp_tls_dcert_file</a> +</pre> +</blockquote> + +<p> Their ECDSA counterparts (Postfix ≥ 2.6 + OpenSSL ≥ 1.0.0): </p> + +<blockquote> +<pre> +/etc/postfix/<a href="postconf.5.html">main.cf</a>: + <a href="postconf.5.html#smtp_tls_eccert_file">smtp_tls_eccert_file</a> = /etc/postfix/client-ecdsa.pem + <a href="postconf.5.html#smtp_tls_eckey_file">smtp_tls_eckey_file</a> = $<a href="postconf.5.html#smtp_tls_eccert_file">smtp_tls_eccert_file</a> +</pre> +</blockquote> + +<p> To verify a remote SMTP server certificate, the Postfix SMTP +client needs to trust the certificates of the issuing Certification +Authorities. These certificates in "pem" format can be stored in a +single $<a href="postconf.5.html#smtp_tls_CAfile">smtp_tls_CAfile</a> or in multiple files, one CA per file in +the $<a href="postconf.5.html#smtp_tls_CApath">smtp_tls_CApath</a> directory. If you use a directory, don't forget +to create the necessary "hash" links with: </p> + +<blockquote> +<pre> +# <b>$OPENSSL_HOME/bin/c_rehash <i>/path/to/directory</i> </b> +</pre> +</blockquote> + +<p> The $<a href="postconf.5.html#smtp_tls_CAfile">smtp_tls_CAfile</a> contains the CA certificates of one or more +trusted CAs. The file is opened (with root privileges) before Postfix +enters the optional chroot jail and so need not be accessible from inside the +chroot jail. </p> + +<p> Additional trusted CAs can be specified via the $<a href="postconf.5.html#smtp_tls_CApath">smtp_tls_CApath</a> +directory, in which case the certificates are read (with $<a href="postconf.5.html#mail_owner">mail_owner</a> +privileges) from the files in the directory when the information +is needed. Thus, the $<a href="postconf.5.html#smtp_tls_CApath">smtp_tls_CApath</a> directory needs to be accessible +inside the optional chroot jail. </p> + +<p> The choice between $<a href="postconf.5.html#smtp_tls_CAfile">smtp_tls_CAfile</a> and $<a href="postconf.5.html#smtp_tls_CApath">smtp_tls_CApath</a> is +a space/time tradeoff. If there are many trusted CAs, the cost of +preloading them all into memory may not pay off in reduced access time +when the certificate is needed. </p> + +<p> Example: </p> + +<blockquote> +<pre> +/etc/postfix/<a href="postconf.5.html">main.cf</a>: + <a href="postconf.5.html#smtp_tls_CAfile">smtp_tls_CAfile</a> = /etc/postfix/CAcert.pem + <a href="postconf.5.html#smtp_tls_CApath">smtp_tls_CApath</a> = /etc/postfix/certs +</pre> +</blockquote> + +<h3><a name="client_tls_reuse">Client-side TLS connection reuse</a> </h3> + +<p> Historically, the Postfix SMTP client has supported multiple +deliveries per plaintext connection. Postfix 3.4 introduces support +for multiple deliveries per TLS-encrypted connection. Multiple +deliveries per connection improve mail delivery performance, +especially for destinations that throttle clients that don't combine +deliveries. </p> + +<p> To enable multiple deliveries per TLS connection, specify:</p> + +<blockquote> +<pre> +/etc/postfix/<a href="postconf.5.html">main.cf</a>: + <a href="postconf.5.html#smtp_tls_connection_reuse">smtp_tls_connection_reuse</a> = yes +</pre> +</blockquote> + +<p> Alternatively, specify the attribute "connection_reuse=yes" in +an <a href="postconf.5.html#smtp_tls_policy_maps">smtp_tls_policy_maps</a> entry. </p> + +<p> The implementation of TLS connection reuse relies on the same +<a href="scache.8.html">scache(8)</a> service as used for delivering plaintext SMTP mail, the +same <a href="tlsproxy.8.html">tlsproxy(8)</a> daemon as used by the <a href="postscreen.8.html">postscreen(8)</a> service, and +relies on the same hints from the <a href="qmgr.8.html">qmgr(8)</a> daemon. + +See "<a href="CONNECTION_CACHE_README.html">Postfix Connection +Cache</a>" for a description of the underlying connection reuse +infrastructure. </p> + +<p> Initial SMTP handshake:</p> +<pre> <a href="smtp.8.html">smtp(8)</a> -> remote SMTP server</pre> + +<p> Reused SMTP/TLS connection, or new SMTP/TLS connection: </p> +<pre> <a href="smtp.8.html">smtp(8)</a> -> <a href="tlsproxy.8.html">tlsproxy(8)</a> -> remote SMTP server </pre> + +<p> Cached SMTP/TLS connection:</p> +<pre> <a href="scache.8.html">scache(8)</a> -> <a href="tlsproxy.8.html">tlsproxy(8)</a> -> remote SMTP server</pre> + +<p> As of Postfix 3.4, TLS connection reuse is disabled by default. +This may change once the impact on over-all performance is understood. +</p> + +<h3><a name="client_tls_cache">Client-side TLS session cache</a> </h3> + +<p> The remote SMTP server and the Postfix SMTP client negotiate a +session, which takes some computer time and network bandwidth. By +default, this session information is cached only in the <a href="smtp.8.html">smtp(8)</a> +process actually using this session and is lost when the process +terminates. To share the session information between multiple +<a href="smtp.8.html">smtp(8)</a> processes, a persistent session cache can be used. You +can specify any database type that can store objects of several +kbytes and that supports the sequence operator. DBM databases are +not suitable because they can only store small objects. The cache +is maintained by the <a href="tlsmgr.8.html">tlsmgr(8)</a> process, so there is no problem with +concurrent access. Session caching is highly recommended, because +the cost of repeatedly negotiating TLS session keys is high. Future +Postfix SMTP servers may limit the number of sessions that a client +is allowed to negotiate per unit time.</p> + + +<p> Example: </p> + +<blockquote> +<pre> +/etc/postfix/<a href="postconf.5.html">main.cf</a>: + <a href="postconf.5.html#smtp_tls_session_cache_database">smtp_tls_session_cache_database</a> = <a href="DATABASE_README.html#types">btree</a>:/var/lib/postfix/smtp_scache +</pre> +</blockquote> + +<p> Note: as of version 2.5, Postfix no longer uses root privileges +when opening this file. The file should now be stored under the +Postfix-owned <a href="postconf.5.html#data_directory">data_directory</a>. As a migration aid, an attempt to +open the file under a non-Postfix directory is redirected to the +Postfix-owned <a href="postconf.5.html#data_directory">data_directory</a>, and a warning is logged. </p> + +<p> Cached Postfix SMTP client session information expires after +a certain amount of time. Postfix/TLS does not use the OpenSSL +default of 300s, but a longer time of 3600s (=1 hour). <a href="https://tools.ietf.org/html/rfc2246">RFC 2246</a> +recommends a maximum of 24 hours. </p> + +<p> Example: </p> + +<blockquote> +<pre> +/etc/postfix/<a href="postconf.5.html">main.cf</a>: + <a href="postconf.5.html#smtp_tls_session_cache_timeout">smtp_tls_session_cache_timeout</a> = 3600s +</pre> +</blockquote> + +<p> As of Postfix 2.11 this setting cannot exceed 100 days. If set +≤ 0, session caching is disabled. If set to a positive value +less than 2 minutes, the minimum value of 2 minutes is used instead. </p> + +<h3><a name="client_tls_limits"> Client TLS limitations </a> +</h3> + +<p> The security properties of TLS communication channels are +application specific. While the TLS protocol can provide a confidential, +tamper-resistant, mutually authenticated channel between client +and server, not all of these security features are applicable to every +communication. </p> + +<p> For example, while mutual TLS authentication between browsers and web +servers is possible, it is not practical, or even useful, for web-servers +that serve the public to verify the identity of every potential user. In +practice, most HTTPS transactions are asymmetric: the browser verifies +the HTTPS server's identity, but the user remains anonymous. Much of +the security policy is up to the client. If the client chooses to not +verify the server's name, the server is not aware of this. There are many +interesting browser security topics, but we shall not dwell +on them here. Rather, our goal is to understand the security features +of TLS in conjunction with SMTP. </p> + +<p> An important SMTP-specific observation is that a public MX host is +even more at the mercy of the SMTP client than is an HTTPS server. Not only +can it not enforce due care in the client's use of TLS, but it cannot even +enforce the use of TLS, because TLS support in SMTP clients is still the +exception rather than the rule. One cannot, in practice, limit access to +one's MX hosts to just TLS-enabled clients. Such a policy would result +in a vast reduction in one's ability to communicate by email with the +world at large. </p> + +<p> One may be tempted to try enforcing TLS for mail from specific +sending organizations, but this, too, runs into obstacles. One such +obstacle is that we don't know who is (allegedly) sending mail until +we see the "MAIL FROM:" SMTP command, and at that point, if TLS +is not already in use, a potentially sensitive sender address (and +with SMTP PIPELINING one or more of the recipients) has (have) already been +leaked in the clear. Another obstacle is that mail from the sender to +the recipient may be forwarded, and the forwarding organization may not +have any security arrangements with the final destination. Bounces also +need to be protected. These can only be identified by the IP address and +HELO name of the connecting client, and it is difficult to keep track +of all the potential IP addresses or HELO names of the outbound email +servers of the sending organization. </p> + +<p> Consequently, TLS security for mail delivery to public MX hosts is +almost entirely the client's responsibility. The server is largely a +passive enabler of TLS security, the rest is up to the client. While the +server has a greater opportunity to mandate client security policy when +it is a dedicated MSA that only handles outbound mail from trusted clients, +below we focus on the client security policy. </p> + +<p> On the SMTP client, there are further complications. When +delivering mail to a given domain, in contrast to HTTPS, one rarely +uses the domain name directly as the target host of the SMTP session. +More typically, one uses MX lookups — these are usually +unauthenticated — to obtain the domain's SMTP server hostname(s). +When, as is current practice, the client verifies the insecurely +obtained MX hostname, it is subject to a DNS man-in-the-middle +attack. </p> + +<p> Adoption of DNSSEC and <a href="https://tools.ietf.org/html/rfc6698">RFC6698</a> (DANE) may gradually (as domains +implement DNSSEC and publish TLSA records for their MX hosts) address +the DNS man-in-the-middle risk and provide scalable key management +for SMTP with TLS. Postfix ≥ 2.11 supports the new <a +href="#client_tls_dane">dane</a> and <a href="#client_tls_dane">dane-only</a> +security levels that take advantage of these standards. </p> + +<p> If clients instead attempted to verify the recipient domain name, +an SMTP server for multiple domains would need to +list all its email domain names in its certificate, and generate a +new certificate each time a new domain were added. At least some CAs set +fairly low limits (20 for one prominent CA) on the number of names that +server certificates can contain. This approach is not consistent with +current practice and does not scale. </p> + +<p> It is regrettably the case that TLS <i>secure-channels</i> +(fully authenticated and immune to man-in-the-middle attacks) impose +constraints on the sending and receiving sites that preclude ubiquitous +deployment. One needs to manually configure this type of security for +each destination domain, and in many cases implement non-default TLS +<a href="#client_tls_policy">policy table</a> entries for additional +domains hosted at a common secured destination. For these reasons +secure-channel configurations +will never be the norm. For the generic domain with which you +have made no specific security arrangements, this security level is not +a good fit. </p> + +<p> Given that strong authentication is not generally possible, and that +verifiable certificates cost time and money, many servers that implement +TLS use self-signed certificates or private CAs. This further limits +the applicability of verified TLS on the public Internet. </p> + +<p> Historical note: while the documentation of these issues and many of the +related features were new with Postfix 2.3, the issue was well +understood before Postfix 1.0, when Lutz Jänicke was designing +the first unofficial Postfix TLS patch. See his original post <a +href="http://www.imc.org/ietf-apps-tls/mail-archive/msg00304.html">http://www.imc.org/ietf-apps-tls/mail-archive/msg00304.html</a> +and the first response <a +href="http://www.imc.org/ietf-apps-tls/mail-archive/msg00305.html">http://www.imc.org/ietf-apps-tls/mail-archive/msg00305.html</a>. +The problem is not even unique to SMTP or even TLS, similar issues exist +for secure connections via aliases for HTTPS and Kerberos. SMTP merely +uses indirect naming (via MX records) more frequently. </p> + +<h3> <a name="client_tls_policy"> TLS policy table </a> +</h3> + +<p> A small fraction of servers offer STARTTLS but the negotiation +consistently fails. As long as encryption is not mandatory, the +Postfix SMTP client retries the delivery immediately with TLS +disabled, without any need to explicitly disable TLS for the problem +destinations. </p> + +<p> The policy table is specified via the <a href="postconf.5.html#smtp_tls_policy_maps">smtp_tls_policy_maps</a> +parameter. This lists optional lookup tables with the Postfix SMTP client +TLS security policy by next-hop destination. </p> + +<p> The TLS policy table is indexed by the full next-hop destination, +which is either the recipient domain, or the verbatim next-hop +specified in the transport table, $<a href="postconf.5.html#local_transport">local_transport</a>, $<a href="postconf.5.html#virtual_transport">virtual_transport</a>, +$<a href="postconf.5.html#relay_transport">relay_transport</a> or $<a href="postconf.5.html#default_transport">default_transport</a>. This includes any enclosing +square brackets and any non-default destination server port suffix. The +<a href="#client_lmtp_tls">LMTP</a> socket type prefix (inet: or unix:) +is not included in the lookup key. </p> + +<p> Only the next-hop domain, or $<a href="postconf.5.html#myhostname">myhostname</a> with LMTP over UNIX-domain +sockets, is used as the nexthop name for certificate verification. The +port and any enclosing square brackets are used in the table lookup key, +but are not used for server name verification. </p> + +<p> When the lookup key is a domain name without enclosing square brackets +or any <i>:port</i> suffix (typically the recipient domain), and the full +domain is not found in the table, just as with the <a href="transport.5.html">transport(5)</a> table, +the parent domain starting with a leading "." is matched recursively. This +allows one to specify a security policy for a recipient domain and all +its sub-domains. </p> + +<p> The lookup result is a security level, followed by an optional +list of whitespace and/or comma separated name=value attributes +that override related <a href="postconf.5.html">main.cf</a> settings. The TLS security <a +href="#client_tls_levels">levels</a> are described above. Below, we +describe the corresponding table syntax: </p> + +<dl> + +<dt><b>none</b></dt> <dd><a href="#client_tls_none">No TLS</a>. No +additional attributes are supported at this level. </dd> + +<dt><b>may</b></dt> <dd><a href="#client_tls_may">Opportunistic TLS</a>. +The optional "ciphers", "exclude" and "protocols" attributes +(available for opportunistic TLS with Postfix ≥ 2.6) override the +"<a href="postconf.5.html#smtp_tls_ciphers">smtp_tls_ciphers</a>", "<a href="postconf.5.html#smtp_tls_exclude_ciphers">smtp_tls_exclude_ciphers</a>" and "<a href="postconf.5.html#smtp_tls_protocols">smtp_tls_protocols</a>" +configuration parameters. At this level and higher, the optional +"servername" attribute (available with Postfix ≥ 3.4) overrides the +global "<a href="postconf.5.html#smtp_tls_servername">smtp_tls_servername</a>" parameter, enabling per-destination +configuration of the SNI extension sent to the remote SMTP server. </dd> + +<dt><b>encrypt</b></dt> <dd><a href="#client_tls_encrypt"> Mandatory encryption</a>. +Mail is delivered only if the remote SMTP server offers STARTTLS +and the TLS handshake succeeds. At this level and higher, the optional +"protocols" attribute overrides the <a href="postconf.5.html">main.cf</a> <a href="postconf.5.html#smtp_tls_mandatory_protocols">smtp_tls_mandatory_protocols</a> +parameter, the optional "ciphers" attribute overrides the +<a href="postconf.5.html">main.cf</a> <a href="postconf.5.html#smtp_tls_mandatory_ciphers">smtp_tls_mandatory_ciphers</a> parameter, and the optional +"exclude" attribute (Postfix ≥ 2.6) overrides the <a href="postconf.5.html">main.cf</a> +<a href="postconf.5.html#smtp_tls_mandatory_exclude_ciphers">smtp_tls_mandatory_exclude_ciphers</a> parameter. </dd> + +<dt><b>dane</b></dt> <dd><a href="#client_tls_dane">Opportunistic DANE TLS</a>. +The TLS policy for the destination is obtained via TLSA records in +DNSSEC. If no TLSA records are found, the effective security level +used is <a href="#client_tls_may">may</a>. If TLSA records are +found, but none are usable, the effective security level is <a +href="#client_tls_encrypt">encrypt</a>. When usable TLSA records +are obtained for the remote SMTP server, SSLv2+3 are automatically +disabled (see <a href="postconf.5.html#smtp_tls_mandatory_protocols">smtp_tls_mandatory_protocols</a>), and the server certificate +must match the TLSA records. <a href="https://tools.ietf.org/html/rfc7672">RFC 7672</a> (DANE) TLS authentication +and DNSSEC support is available with Postfix 2.11 and later. </dd> + +<dt><b>dane-only</b></dt> <dd><a href="#client_tls_dane">Mandatory DANE TLS</a>. +The TLS policy for the destination is obtained via TLSA records in +DNSSEC. If no TLSA records are found, or none are usable, no +connection is made to the server. When usable TLSA records are +obtained for the remote SMTP server, SSLv2+3 are automatically disabled +(see <a href="postconf.5.html#smtp_tls_mandatory_protocols">smtp_tls_mandatory_protocols</a>), and the server certificate must +match the TLSA records. <a href="https://tools.ietf.org/html/rfc7672">RFC 7672</a> (DANE) TLS authentication and +DNSSEC support is available with Postfix 2.11 and later. </dd> + +<dt><b>fingerprint</b></dt> <dd><a href="#client_tls_fprint">Certificate +fingerprint verification.</a> Available with Postfix 2.5 and +later. At this security level, there are no trusted Certification +Authorities. The certificate trust chain, expiration date, ... are +not checked. Instead, the optional <b>match</b> attribute, or else +the <a href="postconf.5.html">main.cf</a> <b><a href="postconf.5.html#smtp_tls_fingerprint_cert_match">smtp_tls_fingerprint_cert_match</a></b> parameter, lists +the server certificate fingerprints or public key fingerprints +(Postfix 2.9 and later). The +digest algorithm used to calculate fingerprints is selected by the +<b><a href="postconf.5.html#smtp_tls_fingerprint_digest">smtp_tls_fingerprint_digest</a></b> parameter. Multiple fingerprints can +be combined with a "|" delimiter in a single match attribute, or multiple +match attributes can be employed. The ":" character is not used as a +delimiter as it occurs between each pair of fingerprint (hexadecimal) +digits. </dd> + +<dt><b>verify</b></dt> <dd><a href="#client_tls_verify">Mandatory +server certificate verification</a>. Mail is delivered only if the +TLS handshake succeeds, if the remote SMTP server certificate can +be validated (not expired or revoked, and signed by a trusted +Certification Authority), and if the server certificate name matches +the optional "match" attribute (or the <a href="postconf.5.html">main.cf</a> <a href="postconf.5.html#smtp_tls_verify_cert_match">smtp_tls_verify_cert_match</a> +parameter value when no optional "match" attribute is specified). +With Postfix ≥ 2.11 the "tafile" attribute optionally modifies +trust chain verification in the same manner as the +"<a href="postconf.5.html#smtp_tls_trust_anchor_file">smtp_tls_trust_anchor_file</a>" parameter. The "tafile" attribute +may be specified multiple times to load multiple trust-anchor +files. </dd> + +<dt><b>secure</b></dt> <dd><a href="#client_tls_secure">Secure certificate +verification.</a> Mail is delivered only if the TLS handshake succeeds, +and DNS forgery resistant remote SMTP certificate verification succeeds +(not expired or revoked, and signed by a trusted Certification Authority), +and if the server certificate name matches the optional "match" attribute +(or the <a href="postconf.5.html">main.cf</a> <a href="postconf.5.html#smtp_tls_secure_cert_match">smtp_tls_secure_cert_match</a> parameter value when no optional +"match" attribute is specified). With Postfix ≥ 2.11 the "tafile" +attribute optionally modifies trust chain verification in the same manner +as the "<a href="postconf.5.html#smtp_tls_trust_anchor_file">smtp_tls_trust_anchor_file</a>" parameter. The "tafile" attribute +may be specified multiple times to load multiple trust-anchor +files. </dd> + +</dl> + +<p> Notes: </p> + +<ul> + +<li> <p> The "match" attribute is especially useful to verify TLS +certificates for domains that are hosted on a shared server. In +that case, specify "match" rules for the shared server's name. +While secure verification can also be achieved with manual routing +overrides in Postfix <a href="transport.5.html">transport(5)</a> tables, that approach can deliver +mail to the wrong host when domains are assigned to new gateway +hosts. The "match" attribute approach avoids the problems of manual +routing overrides; mail is deferred if verification of a new MX +host fails. </p> + +<li> <p> When a policy table entry specifies multiple match patterns, +multiple match strategies, or multiple protocols, these must be +separated by colons. </p> + +<li> <p> The "exclude" attribute (Postfix ≥ 2.6) is used to disable +ciphers that cause handshake failures with a specific mandatory TLS +destination, without disabling the ciphers for all mandatory destinations. +Alternatively, you can exclude ciphers that cause issues with multiple +remote servers in <a href="postconf.5.html">main.cf</a>, and selectively enable them on a per-destination +basis in the policy table by setting a shorter or empty exclusion list. The +per-destination "exclude" list preempts both the opportunistic and +mandatory security level exclusions, so that all excluded ciphers +can be enabled for known-good destinations. For non-mandatory TLS +destinations that exhibit cipher-specific problems, Postfix will fall +back to plain-text delivery. If plain-text is not acceptable make TLS +mandatory and exclude the problem ciphers. </p> + +</ul> + +<p> +Example: +</p> + +<blockquote> +<pre> +/etc/postfix/<a href="postconf.5.html">main.cf</a>: + <a href="postconf.5.html#smtp_tls_policy_maps">smtp_tls_policy_maps</a> = <a href="DATABASE_README.html#types">hash</a>:/etc/postfix/tls_policy + # Postfix 2.5 and later + <a href="postconf.5.html#smtp_tls_fingerprint_digest">smtp_tls_fingerprint_digest</a> = sha256 +/etc/postfix/tls_policy: + example.edu none + example.mil may + example.gov encrypt ciphers=high + example.com verify match=hostname:dot-nexthop ciphers=high + example.net secure + .example.net secure match=.example.net:example.net + [mail.example.org]:587 secure match=nexthop + # Postfix 2.5 and later + [thumb.example.org] fingerprint + match=b6:b4:72:34:e2:59:cd:fb:...:0d:4d:cc:2c:7d:84:de:e6:2f + match=51:e9:af:2e:1e:40:1f:de:...:35:2d:09:16:31:5a:eb:82:76 + # Postfix ≥ 3.6 "protocols" syntax + example.info may protocols=>=TLSv1 ciphers=medium exclude=3DES + # Legacy protocols syntax + example.info may protocols=!SSLv2:!SSLv3 ciphers=medium exclude=3DES +</pre> +</blockquote> + +<p> <b>Note:</b> The "hostname" strategy if listed in a non-default setting +of <a href="postconf.5.html#smtp_tls_secure_cert_match">smtp_tls_secure_cert_match</a> or in the "match" attribute in the policy +table can render the "secure" level vulnerable to DNS forgery. Do not use +the "hostname" strategy for <a href="#client_tls_secure">secure-channel</a> +configurations in environments where DNS security is not assured. </p> + +<h3> <a name="client_tls_discover"> Discovering servers that support +TLS </a> </h3> + +<p> As we decide on a "per site" basis whether or not to use TLS, +it would be good to have a list of sites that offered "STARTTLS". +We can collect it ourselves with this option. </p> + +<p> If the <a href="postconf.5.html#smtp_tls_note_starttls_offer">smtp_tls_note_starttls_offer</a> feature is enabled and a +server offers STARTTLS while TLS is not already enabled for that +server, the Postfix SMTP client logs a line as follows: </p> + +<blockquote> +<pre> +postfix/smtp[pid]: Host offered STARTTLS: [hostname.example.com] +</pre> +</blockquote> + +<p> Example: </p> + +<blockquote> +<pre> +/etc/postfix/<a href="postconf.5.html">main.cf</a>: + <a href="postconf.5.html#smtp_tls_note_starttls_offer">smtp_tls_note_starttls_offer</a> = yes +</pre> +</blockquote> + +<h3><a name="client_vrfy_server">Server certificate verification depth</a> </h3> + +<p> The server certificate verification depth is specified with the +<a href="postconf.5.html">main.cf</a> <a href="postconf.5.html#smtp_tls_scert_verifydepth">smtp_tls_scert_verifydepth</a> parameter. The default verification +depth is 9 (the OpenSSL default), for compatibility with Postfix +versions before 2.5 where <a href="postconf.5.html#smtp_tls_scert_verifydepth">smtp_tls_scert_verifydepth</a> was ignored. +When you configure trust +in a root CA, it is not necessary to explicitly trust intermediary CAs +signed by the root CA, unless $<a href="postconf.5.html#smtp_tls_scert_verifydepth">smtp_tls_scert_verifydepth</a> is less than the +number of CAs in the certificate chain for the servers of interest. With +a verify depth of 1 you can only verify certificates directly signed +by a trusted CA, and all trusted intermediary CAs need to be configured +explicitly. With a verify depth of 2 you can verify servers signed by a +root CA or a direct intermediary CA (so long as the server is correctly +configured to supply its intermediate CA certificate). </p> + +<p> Example: </p> + +<blockquote> +<pre> +/etc/postfix/<a href="postconf.5.html">main.cf</a>: + <a href="postconf.5.html#smtp_tls_scert_verifydepth">smtp_tls_scert_verifydepth</a> = 2 +</pre> +</blockquote> + +<h3> <a name="client_cipher">Client-side cipher controls </a> </h3> + +<p> The Postfix SMTP client supports 5 distinct cipher grades +as specified by the <a href="postconf.5.html#smtp_tls_mandatory_ciphers">smtp_tls_mandatory_ciphers</a> configuration +parameter. This setting controls the minimum acceptable SMTP client +TLS cipher grade for use with mandatory TLS encryption. The default +value "medium" is suitable for most destinations with which you may +want to enforce TLS, and is beyond the reach of today's cryptanalytic +methods. See <a href="postconf.5.html#smtp_tls_policy_maps">smtp_tls_policy_maps</a> for information on how to configure +ciphers on a per-destination basis. </p> + +<p> By default anonymous ciphers are allowed, and automatically +disabled when remote SMTP server certificates are verified. If you +want to +disable anonymous ciphers even at the "encrypt" security level, set +"<a href="postconf.5.html#smtp_tls_mandatory_exclude_ciphers">smtp_tls_mandatory_exclude_ciphers</a> = aNULL"; and to +disable anonymous ciphers even with opportunistic TLS, set +"<a href="postconf.5.html#smtp_tls_exclude_ciphers">smtp_tls_exclude_ciphers</a> = aNULL". There is generally +no need to take these measures. Anonymous ciphers save bandwidth +and TLS session cache space, if certificates are ignored, there is +little point in requesting them. </p> + +<p> The "<a href="postconf.5.html#smtp_tls_ciphers">smtp_tls_ciphers</a>" configuration parameter (Postfix ≥ 2.6) +provides control over the minimum cipher grade for opportunistic TLS. +The default minimum cipher grade for opportunistic TLS is "medium" +for Postfix releases after the middle of 2015, and "export" for +older releases. With Postfix < 2.6, the minimum opportunistic +TLS cipher grade is always "export". </p> + +<p> With mandatory and opportunistic TLS encryption, the Postfix +SMTP client will by default disable SSLv2 and SSLv3. The mandatory +TLS protocol list is specified via the +<a href="postconf.5.html#smtp_tls_mandatory_protocols">smtp_tls_mandatory_protocols</a> configuration parameter. The corresponding +<a href="postconf.5.html#smtp_tls_protocols">smtp_tls_protocols</a> parameter (Postfix ≥ 2.6) controls +the TLS protocols used with opportunistic TLS. </p> + +<p> Example: </p> + +<blockquote> +<pre> +/etc/postfix/<a href="postconf.5.html">main.cf</a>: + <a href="postconf.5.html#smtp_tls_mandatory_ciphers">smtp_tls_mandatory_ciphers</a> = medium + <a href="postconf.5.html#smtp_tls_mandatory_exclude_ciphers">smtp_tls_mandatory_exclude_ciphers</a> = RC4, MD5 + <a href="postconf.5.html#smtp_tls_exclude_ciphers">smtp_tls_exclude_ciphers</a> = aNULL + <a href="postconf.5.html#smtp_tls_ciphers">smtp_tls_ciphers</a> = medium + # Preferred form with Postfix ≥ 3.6: + <a href="postconf.5.html#smtp_tls_mandatory_protocols">smtp_tls_mandatory_protocols</a> = >=TLSv1.2 + <a href="postconf.5.html#smtp_tls_protocols">smtp_tls_protocols</a> = >=TLSv1 + # Legacy form for Postfix < 3.6: + <a href="postconf.5.html#smtp_tls_mandatory_protocols">smtp_tls_mandatory_protocols</a> = !SSLv2, !SSLv3, !TLSv1, !TLSv1.1 + <a href="postconf.5.html#smtp_tls_protocols">smtp_tls_protocols</a> = !SSLv2,!SSLv3 +</pre> +</blockquote> + +<h3> <a name="client_smtps">Client-side SMTPS support </a> </h3> + +<p> These sections show how to send mail to a server that does not +support STARTTLS, but that provides the SMTPS service +on TCP port 465. Depending on the Postfix version, some additional +tooling may be required. </p> + +<h4> Postfix ≥ 3.0 </h4> + +<p> The Postfix SMTP client has SMTPS support built-in as of version +3.0. Use one of the following examples, to send all remote mail, +or to send only some remote mail, to an SMTPS server. </p> + +<h5> Postfix ≥ 3.0: Sending all remote mail to an SMTPS server </h5> + +<p> The first example will send all remote mail over SMTPS through +a provider's server called "mail.example.com": </p> + +<blockquote> +<pre> +/etc/postfix/<a href="postconf.5.html">main.cf</a>: + # Client-side SMTPS requires "encrypt" or stronger. + <a href="postconf.5.html#smtp_tls_security_level">smtp_tls_security_level</a> = encrypt + <a href="postconf.5.html#smtp_tls_wrappermode">smtp_tls_wrappermode</a> = yes + # The [] suppress MX lookups. + <a href="postconf.5.html#relayhost">relayhost</a> = [mail.example.com]:465 +</pre> +</blockquote> + +<p> Use "postfix reload" to make the change effective. </p> + +<p> See <a href="SOHO_README.html">SOHO_README</a> for additional information about SASL authentication. +</p> + +<h5> Postfix ≥ 3.0: Sending only mail for a specific destination +via SMTPS </h5> + +<p> The second example will send only mail for "example.com" via +SMTPS. This time, Postfix uses a transport map to deliver only +mail for "example.com" via SMTPS: </p> + +<blockquote> +<pre> +/etc/postfix/<a href="postconf.5.html">main.cf</a>: + <a href="postconf.5.html#transport_maps">transport_maps</a> = <a href="DATABASE_README.html#types">hash</a>:/etc/postfix/transport + +/etc/postfix/transport: + example.com relay-smtps:example.com:465 + +/etc/postfix/<a href="master.5.html">master.cf</a>: + relay-smtps unix - - n - - smtp + # Client-side SMTPS requires "encrypt" or stronger. + -o <a href="postconf.5.html#smtp_tls_security_level">smtp_tls_security_level</a>=encrypt + -o <a href="postconf.5.html#smtp_tls_wrappermode">smtp_tls_wrappermode</a>=yes +</pre> +</blockquote> + +<p> Use "postmap <a href="DATABASE_README.html#types">hash</a>:/etc/postfix/transport" and "postfix reload" +to make the change effective. </p> + +<p> See <a href="SOHO_README.html">SOHO_README</a> for additional information about SASL +authentication. </p> + +<h4> Postfix < 3.0 </h4> + +<p> Although older Postfix SMTP client versions do not support TLS +wrapper mode, it is relatively easy to forward a connection through +the stunnel program if Postfix needs to deliver mail to some legacy +system that doesn't support STARTTLS. </p> + +<h5> Postfix < 3.0: Sending all remote mail to an SMTPS server </h5> + +<p> The first example uses SMTPS to send all remote mail to a +provider's mail server called "mail.example.com". </p> + +<p> A minimal stunnel.conf file is sufficient to set up a tunnel +from local port 11125 to the remote destination "mail.example.com" +and port "smtps". Postfix will later use this tunnel to connect to +the remote server. </p> + +<blockquote> +<pre> +/path/to/stunnel.conf: + [smtp-tls-wrapper] + accept = 11125 + client = yes + connect = mail.example.com:smtps +</pre> +</blockquote> + +<p> To test this tunnel, use: </p> + +<blockquote> +<pre> +$ telnet localhost 11125 +</pre> +</blockquote> + +<p> This should produce the greeting from the remote SMTP server +at mail.example.com. </p> + +<p> On the Postfix side, the <a href="postconf.5.html#relayhost">relayhost</a> feature sends all remote +mail through the local stunnel listener on port 11125: </p> + +<blockquote> +<pre> +/etc/postfix/<a href="postconf.5.html">main.cf</a>: + <a href="postconf.5.html#relayhost">relayhost</a> = [127.0.0.1]:11125 +</pre> +</blockquote> + +<p> Use "postfix reload" to make the change effective. </p> + +<p> See <a href="SOHO_README.html">SOHO_README</a> for additional information about SASL +authentication. </p> + +<h4> Postfix < 3.0: Sending only mail for a specific destination via SMTPS </h4> + +<p> The second example will use SMTPS to send only mail for +"example.com" via SMTPS. It uses the same stunnel configuration +file as the first example, so it won't be repeated here. </p> + +<p> This time, the Postfix side uses a transport map to direct only +mail for "example.com" through the tunnel: </p> + +<blockquote> +<pre> +/etc/postfix/<a href="postconf.5.html">main.cf</a>: + <a href="postconf.5.html#transport_maps">transport_maps</a> = <a href="DATABASE_README.html#types">hash</a>:/etc/postfix/transport + +/etc/postfix/transport: + example.com relay:[127.0.0.1]:11125 +</pre> +</blockquote> + +<p> Use "postmap <a href="DATABASE_README.html#types">hash</a>:/etc/postfix/transport" and "postfix reload" +to make the change effective. </p> + +<p> See <a href="SOHO_README.html">SOHO_README</a> for additional information about SASL authentication. +</p> + +<h3> <a name="client_misc"> Miscellaneous client controls </a> </h3> + +<p> The <a href="postconf.5.html#smtp_starttls_timeout">smtp_starttls_timeout</a> parameter limits the time of Postfix +SMTP client write and read operations during TLS startup and shutdown +handshake procedures. In case of problems the Postfix SMTP client +tries the next network address on the mail exchanger list, and +defers delivery if no alternative server is available. </p> + +<p> Example: </p> + +<blockquote> +<pre> +/etc/postfix/<a href="postconf.5.html">main.cf</a>: + <a href="postconf.5.html#smtp_starttls_timeout">smtp_starttls_timeout</a> = 300s +</pre> +</blockquote> + +<p> With Postfix 2.8 and later, the <a href="postconf.5.html#tls_disable_workarounds">tls_disable_workarounds</a> parameter +specifies a list or bit-mask of OpenSSL bug work-arounds to disable. This +may be necessary if one of the work-arounds enabled by default in +OpenSSL proves to pose a security risk, or introduces an unexpected +interoperability issue. Some bug work-arounds known to be problematic +are disabled in the default value of the parameter when linked with +an OpenSSL library that could be vulnerable. </p> + +<p> Example: </p> + +<blockquote> +<pre> +/etc/postfix/<a href="postconf.5.html">main.cf</a>: + <a href="postconf.5.html#tls_disable_workarounds">tls_disable_workarounds</a> = 0xFFFFFFFF + <a href="postconf.5.html#tls_disable_workarounds">tls_disable_workarounds</a> = CVE-2010-4180, LEGACY_SERVER_CONNECT +</pre> +</blockquote> + +<p> Note: Disabling LEGACY_SERVER_CONNECT is not wise at this +time, lots of servers are still unpatched and Postfix is <a +href="http://www.postfix.org/wip.html#tls-renegotiation">not +significantly vulnerable</a> to the renegotiation issue in the TLS +protocol. </p> + +<p> With Postfix ≥ 2.11, the <a href="postconf.5.html#tls_ssl_options">tls_ssl_options</a> parameter specifies +a list or bit-mask of OpenSSL options to enable. Specify one or +more of the named options below, or a hexadecimal bitmask of options +found in the ssl.h file corresponding to the run-time OpenSSL +library. While it may be reasonable to turn off all bug workarounds +(see above), it is not a good idea to attempt to turn on all features. +</p> + +<p> A future version of OpenSSL may by default no longer allow +connections to servers that don't support secure renegotiation. +Since the exposure for SMTP is minimal, and some SMTP servers may +remain unpatched, you can add LEGACY_SERVER_CONNECT to the +options to restore the more permissive default of current OpenSSL +releases. </p> + +<p> Example: </p> + +<blockquote> +<pre> +/etc/postfix/<a href="postconf.5.html">main.cf</a>: + <a href="postconf.5.html#tls_ssl_options">tls_ssl_options</a> = NO_TICKET, NO_COMPRESSION, LEGACY_SERVER_CONNECT +</pre> +</blockquote> + +<p> You should only enable features via the hexadecimal mask when +the need to control the feature is critical (to deal with a new +vulnerability or a serious interoperability problem). Postfix DOES +NOT promise backwards compatible behavior with respect to the mask +bits. A feature enabled via the mask in one release may be enabled +by other means in a later release, and the mask bit will then be +ignored. Therefore, use of the hexadecimal mask is only a temporary +measure until a new Postfix or OpenSSL release provides a better +solution. </p> + +<h2><a name="tlsmgr_controls"> TLS manager specific settings </a> </h2> + +<p> The security of cryptographic software such as TLS depends +critically on the ability to generate unpredictable numbers for +keys and other information. To this end, the <a href="tlsmgr.8.html">tlsmgr(8)</a> process +maintains a Pseudo Random Number Generator (PRNG) pool. This is +queried by the <a href="smtp.8.html">smtp(8)</a> and <a href="smtpd.8.html">smtpd(8)</a> processes when they initialize. +By default, these daemons request 32 bytes, the equivalent to 256 +bits. This is more than sufficient to generate a 128bit (or 168bit) +session key. </p> + +<p> Example: </p> + +<blockquote> +<pre> +/etc/postfix/<a href="postconf.5.html">main.cf</a>: + <a href="postconf.5.html#tls_daemon_random_bytes">tls_daemon_random_bytes</a> = 32 +</pre> +</blockquote> + +<p> In order to feed its in-memory PRNG pool, the <a href="tlsmgr.8.html">tlsmgr(8)</a> reads +entropy from an external source, both at startup and during run-time. +Specify a good entropy source, like EGD or /dev/urandom; be sure +to only use non-blocking sources (on OpenBSD, use /dev/arandom +when <a href="tlsmgr.8.html">tlsmgr(8)</a> complains about /dev/urandom timeout errors). +If the entropy source is not a +regular file, you must prepend the source type to the source name: +"dev:" for a device special file, or "egd:" for a source with EGD +compatible socket interface. </p> + +<p> Examples (specify only one in <a href="postconf.5.html">main.cf</a>): </p> + +<blockquote> +<pre> +/etc/postfix/<a href="postconf.5.html">main.cf</a>: + <a href="postconf.5.html#tls_random_source">tls_random_source</a> = dev:/dev/urandom + <a href="postconf.5.html#tls_random_source">tls_random_source</a> = egd:/var/run/egd-pool +</pre> +</blockquote> + +<p> By default, <a href="tlsmgr.8.html">tlsmgr(8)</a> reads 32 bytes from the external entropy +source at each seeding event. This amount (256bits) is more than +sufficient for generating a 128bit symmetric key. With EGD and +device entropy sources, the <a href="tlsmgr.8.html">tlsmgr(8)</a> limits the amount of data +read at each step to 255 bytes. If you specify a regular file as +entropy source, a larger amount of data can be read. </p> + +<p> Example: </p> + +<blockquote> +<pre> +/etc/postfix/<a href="postconf.5.html">main.cf</a>: + <a href="postconf.5.html#tls_random_bytes">tls_random_bytes</a> = 32 +</pre> +</blockquote> + +<p> In order to update its in-memory PRNG pool, the <a href="tlsmgr.8.html">tlsmgr(8)</a> +queries the external entropy source again after a pseudo-random +amount of time. The time is calculated using the PRNG, and is +between 0 and the maximal time specified with <a href="postconf.5.html#tls_random_reseed_period">tls_random_reseed_period</a>. +The default maximal time interval is 1 hour. </p> + +<p> Example: </p> + +<blockquote> +<pre> +/etc/postfix/<a href="postconf.5.html">main.cf</a>: + <a href="postconf.5.html#tls_random_reseed_period">tls_random_reseed_period</a> = 3600s +</pre> +</blockquote> + +<p> The <a href="tlsmgr.8.html">tlsmgr(8)</a> process saves the PRNG state to a persistent +exchange file at regular times and when the process terminates, so +that it can recover the PRNG state the next time it starts up. +This file is created when it does not exist. </p> + +<p> Examples: </p> + +<blockquote> +<pre> +/etc/postfix/<a href="postconf.5.html">main.cf</a>: + <a href="postconf.5.html#tls_random_exchange_name">tls_random_exchange_name</a> = /var/lib/postfix/prng_exch + <a href="postconf.5.html#tls_random_prng_update_period">tls_random_prng_update_period</a> = 3600s +</pre> +</blockquote> + +<p> As of version 2.5, Postfix no longer uses root privileges when +opening this file. The file should now be stored under the Postfix-owned +<a href="postconf.5.html#data_directory">data_directory</a>. As a migration aid, an attempt to open the file +under a non-Postfix directory is redirected to the Postfix-owned +<a href="postconf.5.html#data_directory">data_directory</a>, and a warning is logged. If you wish to continue +using a pre-existing PRNG state file, move it to the <a href="postconf.5.html#data_directory">data_directory</a> +and change the ownership to the account specified with the <a href="postconf.5.html#mail_owner">mail_owner</a> +parameter. </p> + +<p> With earlier Postfix versions the default file location +is under the Postfix configuration directory, which is not the +proper place for information that is modified by Postfix. </p> + +<h2><a name="quick-start">Getting started, quick and dirty</a></h2> + +<p> The following steps will get you started quickly. Because you +sign your own Postfix public key certificate, you get TLS encryption +but no TLS authentication. This is sufficient for testing, and +for exchanging email with sites that you have no trust relationship +with. For real authentication you need also enable DNSSEC record +signing for your domain and publish TLSA records and/or your Postfix +public key certificate needs to be signed by a recognized Certification +Authority. To authenticate the certificates of a remote host you +need a DNSSEC-validating local resolver and to enable <a +href="#client_tls_dane">DANE</a> authentication and/or configure +the Postfix SMTP client with a list of public key certificates of +Certification Authorities, but make sure to read about the <a +href="#client_tls_limits">limitations</a> of the latter approach. +</p> + +<p> In the examples below, user input is shown in <b><tt>bold</tt></b> +font, and a "<tt>#</tt>" prompt indicates a super-user shell. </p> + +<ul> + +<li> <p> <a href="#built-in">Quick-start TLS with Postfix ≥ 3.1</a>.</p> + +<li> <p> <a href="#self-signed">Self-signed server certificate</a>.</p> + +<li> <p> <a href="#private-ca">Private Certification Authority</a>. </p> + +</ul> + +<h3><a name="built-in">Quick-start TLS with Postfix ≥ 3.1</a></h3> + +<p> Postfix 3.1 provides built-in support for enabling TLS in the +SMTP client and server and for ongoing certificate and DANE TLSA +record management. + +<ul> +<li> <p> <a href="#quick-client">Quick-start TLS in the Postfix ≥ 3.1 SMTP client</a>. </p> +<li> <p> <a href="#quick-server">Quick-start TLS in the Postfix ≥ 3.1 SMTP server</a>. </p> +</ul> + +<h4> <a name="quick-client">Quick-start TLS in the Postfix ≥ 3.1 SMTP client</a>. </h4> + +<p> If you are using Postfix 3.1 or later, and your SMTP client TLS +settings are in their default state, you can enable <a +href="#client_tls_may">opportunistic</a> TLS in the SMTP client as +follows: </p> + +<blockquote> +<pre> +# <a href="postfix-tls.1.html">postfix tls</a> enable-client +# postfix reload +</pre> +</blockquote> + +<p> If some of the Postfix SMTP client TLS settings are not in their +default state, this will not make any changes, but will instead +suggest the minimal required settings for SMTP client TLS. The +"postfix reload" command is optional, it is only needed if you want +the settings to take effect right away. Note, this does not enable +trust in any public certification authorities, and does not configure +client TLS certificates as these are largely pointless with <a +href="#client_tls_may">opportunistic</a> TLS. </p> + +<p> There is not yet a turn-key command for enabling <a +href="#client_tls_dane">DANE</a> authentication. This is because +DANE requires changes to your <b>resolv.conf</b> file and a +corresponding DNSSEC-validating resolver local to the Postfix host, +these changes are difficult to automate in a portable way. </p> + +<p> If you're willing to revert your settings to the defaults and +switch to a "stock" opportunistic TLS configuration, then you can: +erase all the SMTP client TLS settings and then enable client TLS: </p> + +<blockquote> +<pre> +# postconf -X `postconf -nH | egrep '^smtp(_|_enforce_|_use_)tls'` +# <a href="postfix-tls.1.html">postfix tls</a> enable-client +# postfix reload +</pre> +</blockquote> + +<h4><a name="quick-server">Quick-start TLS in the Postfix ≥ 3.1 SMTP server</a>.</h4> + +<p> If you are using Postfix 3.1 or later, and your SMTP server TLS +settings are in their default state, you can enable +opportunistic TLS in the SMTP server as follows: </p> + +<blockquote> +<pre> +# <a href="postfix-tls.1.html">postfix tls</a> enable-server +# postfix reload +</pre> +</blockquote> + +<p> If some of the Postfix SMTP client TLS settings are not in their +default state, this will not make any changes, but will instead +suggest the minimal required settings for SMTP client TLS. The +"postfix reload" command is optional, it is only needed if you want +the settings to take effect right away. This will generate a +self-signed private key and certificate and enable TLS in the Postfix +SMTP server. </p> + +<p> If you're willing to revert your settings to the defaults and +switch to a "stock" server TLS configuration, then you can: erase +all the SMTP server TLS settings and then enable server TLS: </p> + +<blockquote> +<pre> +# postconf -X `postconf -nH | egrep '^smtpd(_|_enforce_|_use_)tls'` +# <a href="postfix-tls.1.html">postfix tls</a> enable-server +# postfix reload +</pre> +</blockquote> + +<p> Postfix ≥ 3.1 provides additional built-in support for ongoing +management of TLS in the SMTP server, via additional "<a href="postfix-tls.1.html">postfix tls</a>" +sub-commands. These make it easy to generate certificate signing +requests, create and deploy new keys and certificates, and generate +DANE TLSA records. See the <a href="postfix-tls.1.html">postfix-tls(1)</a> documentation for details. +</p> + +<h3><a name="self-signed">Self-signed server certificate</a></h3> + +<p> The following commands (credits: Viktor Dukhovni) generate and +install a 2048-bit RSA private key and 10-year self-signed certificate +for the local Postfix system. This requires super-user privileges. +(By using date-specific filenames for the certificate and key files, +and updating <a href="postconf.5.html">main.cf</a> with new filenames, a potential race condition +in which the key and certificate might not match is avoided). +</p> + +<blockquote> +<pre> +# dir="$(postconf -h <a href="postconf.5.html#config_directory">config_directory</a>)" +# fqdn=$(postconf -h <a href="postconf.5.html#myhostname">myhostname</a>) +# case $fqdn in /*) fqdn=$(cat "$fqdn");; esac +# ymd=$(date +%Y-%m-%d) +# key="${dir}/key-${ymd}.pem"; rm -f "${key}" +# cert="${dir}/cert-${ymd}.pem"; rm -f "${cert}" +# (umask 077; openssl genrsa -out "${key}" 2048) && + openssl req -new -key "${key}" \ + -x509 -subj "/CN=${fqdn}" -days 3650 -out "${cert}" && + postconf -e \ + "<a href="postconf.5.html#smtpd_tls_cert_file">smtpd_tls_cert_file</a> = ${cert}" \ + "<a href="postconf.5.html#smtpd_tls_key_file">smtpd_tls_key_file</a> = ${key}" \ + '<a href="postconf.5.html#smtpd_tls_security_level">smtpd_tls_security_level</a> = may' \ + '<a href="postconf.5.html#smtpd_tls_received_header">smtpd_tls_received_header</a> = yes' \ + '<a href="postconf.5.html#smtpd_tls_loglevel">smtpd_tls_loglevel</a> = 1' \ + '<a href="postconf.5.html#smtp_tls_security_level">smtp_tls_security_level</a> = may' \ + '<a href="postconf.5.html#smtp_tls_loglevel">smtp_tls_loglevel</a> = 1' \ + '<a href="postconf.5.html#smtp_tls_session_cache_database">smtp_tls_session_cache_database</a> = <a href="DATABASE_README.html#types">btree</a>:${<a href="postconf.5.html#data_directory">data_directory</a>}/smtp_scache' \ + '<a href="postconf.5.html#tls_random_source">tls_random_source</a> = dev:/dev/urandom' +</pre> +</blockquote> + +<p> Note: the last command requires both single (') and double (") +quotes. </p> + +<p> The <a href="postconf.1.html">postconf(1)</a> command above enables opportunistic TLS for +receiving and sending mail. It also enables logging of TLS connections +and recording of TLS use in the "Received" header. TLS session +caching is also enabled in the Postfix SMTP client. With Postfix +≥ 2.10, the SMTP server does not need an explicit session cache +since session reuse is better handled via <a href="https://tools.ietf.org/html/rfc5077">RFC 5077</a> TLS session +tickets. </p> + +<h3><a name="private-ca">Private Certification Authority</a></h3> + +<ul> + +<li> <p> Become your own Certification Authority, so that you can +sign your own certificates, and so that your own systems can +authenticate certificates from your own CA. This example uses the +CA.pl script that ships with OpenSSL. On some systems, OpenSSL +installs this as <tt>/usr/local/openssl/misc/CA.pl</tt>. Some systems +install this as +part of a package named <tt>openssl-perl</tt> or something similar. +The script creates a private key in <tt>./demoCA/private/cakey.pem</tt> +and a public key in <tt>./demoCA/cacert.pem</tt>.</p> + +<blockquote> +<pre> +% <b>/usr/local/ssl/misc/CA.pl -newca</b> +CA certificate filename (or enter to create) + +Making CA certificate ... +Using configuration from /etc/ssl/openssl.cnf +Generating a 1024 bit RSA private key +....................++++++ +.....++++++ +writing new private key to './demoCA/private/cakey.pem' +Enter PEM pass phrase:<b>whatever</b> +</pre> +</blockquote> + +<li> <p> Create an unpassworded private key for host foo.porcupine.org and create +an unsigned public key certificate. </p> + +<blockquote> +<pre> +% <b>(umask 077; openssl req -new -newkey rsa:2048 -nodes -keyout foo-key.pem -out foo-req.pem)</b> +Using configuration from /etc/ssl/openssl.cnf +Generating a 2048 bit RSA private key +........................................++++++ +....++++++ +writing new private key to 'foo-key.pem' +----- +You are about to be asked to enter information that will be incorporated +into your certificate request. +What you are about to enter is what is called a Distinguished Name or a DN. +There are quite a few fields but you can leave some blank +For some fields there will be a default value, +If you enter '.', the field will be left blank. +----- +Country Name (2 letter code) [AU]:<b>US</b> +State or Province Name (full name) [Some-State]:<b>New York</b> +Locality Name (eg, city) []:<b>Westchester</b> +Organization Name (eg, company) [Internet Widgits Pty Ltd]:<b>Porcupine</b> +Organizational Unit Name (eg, section) []: +Common Name (eg, YOUR name) []:<b>foo.porcupine.org</b> +Email Address []:<b>wietse@porcupine.org</b> + +Please enter the following 'extra' attributes +to be sent with your certificate request +A challenge password []:<b>whatever</b> +An optional company name []: +</pre> +</blockquote> + +<li> <p> Sign the public key certificate for host foo.porcupine.org with the +Certification Authority private key that we created a few +steps ago. </p> + +<blockquote> +<pre> +% <b>openssl ca -out foo-cert.pem -days 365 -infiles foo-req.pem</b> +Using configuration from /etc/ssl/openssl.cnf +Enter PEM pass phrase:<b>whatever</b> +Check that the request matches the signature +Signature ok +The Subjects Distinguished Name is as follows +countryName :PRINTABLE:'US' +stateOrProvinceName :PRINTABLE:'New York' +localityName :PRINTABLE:'Westchester' +organizationName :PRINTABLE:'Porcupine' +commonName :PRINTABLE:'foo.porcupine.org' +emailAddress :IA5STRING:'wietse@porcupine.org' +Certificate is to be certified until Nov 21 19:40:56 2005 GMT (365 days) +Sign the certificate? [y/n]:<b>y</b> + + +1 out of 1 certificate requests certified, commit? [y/n]<b>y</b> +Write out database with 1 new entries +Data Base Updated +</pre> +</blockquote> + +<li> <p> Install the host private key, the host public key certificate, +and the Certification Authority certificate files. This requires +super-user privileges. </p> + +<p> The following commands assume that the key and certificate will +be installed for the local Postfix MTA. You will need to adjust the +commands if the Postfix MTA is on a different host. </p> + +<blockquote> +<pre> +# <b>cp demoCA/cacert.pem foo-key.pem foo-cert.pem /etc/postfix</b> +# <b>chmod 644 /etc/postfix/foo-cert.pem /etc/postfix/cacert.pem</b> +# <b>chmod 400 /etc/postfix/foo-key.pem</b> +</pre> +</blockquote> + +<li> <p> Configure Postfix, by adding the following to +<tt>/etc/postfix/<a href="postconf.5.html">main.cf</a> </tt>. It is generally best to not configure +client certificates, unless there are servers which authenticate your mail +submission via client certificates. Often servers that perform TLS client +authentication will issue the required certificates signed by their own +CA. If you configure the client certificate and key incorrectly, you +will be unable to send mail to sites that request a client certificate, +but don't require them from all clients. </p> + +<blockquote> +<pre> +/etc/postfix/<a href="postconf.5.html">main.cf</a>: + <a href="postconf.5.html#smtp_tls_CAfile">smtp_tls_CAfile</a> = /etc/postfix/cacert.pem + <a href="postconf.5.html#smtp_tls_session_cache_database">smtp_tls_session_cache_database</a> = + <a href="DATABASE_README.html#types">btree</a>:/var/lib/postfix/smtp_tls_session_cache + <a href="postconf.5.html#smtp_tls_security_level">smtp_tls_security_level</a> = may + <a href="postconf.5.html#smtp_tls_loglevel">smtp_tls_loglevel</a> = 1 + <a href="postconf.5.html#smtpd_tls_CAfile">smtpd_tls_CAfile</a> = /etc/postfix/cacert.pem + <a href="postconf.5.html#smtpd_tls_cert_file">smtpd_tls_cert_file</a> = /etc/postfix/foo-cert.pem + <a href="postconf.5.html#smtpd_tls_key_file">smtpd_tls_key_file</a> = /etc/postfix/foo-key.pem + <a href="postconf.5.html#smtpd_tls_received_header">smtpd_tls_received_header</a> = yes + <a href="postconf.5.html#smtpd_tls_session_cache_database">smtpd_tls_session_cache_database</a> = + <a href="DATABASE_README.html#types">btree</a>:/var/lib/postfix/smtpd_tls_session_cache + <a href="postconf.5.html#tls_random_source">tls_random_source</a> = dev:/dev/urandom + <a href="postconf.5.html#smtpd_tls_security_level">smtpd_tls_security_level</a> = may + <a href="postconf.5.html#smtpd_tls_loglevel">smtpd_tls_loglevel</a> = 1 +</pre> +</blockquote> + +</ul> + + +<h2><a name="build_tls">Building Postfix with TLS support</a></h2> + +<p> These instructions assume that you build Postfix from source +code as described in the <a href="INSTALL.html">INSTALL</a> document. Some modification may +be required if you build Postfix from a vendor-specific source +package. </p> + +<p> To build Postfix with TLS support, first we need to generate +the <tt>make(1)</tt> files with the necessary definitions. This is +done by invoking the command "<tt>make makefiles</tt>" in the Postfix +top-level directory and with arguments as shown next. </p> + +<p> <b> NOTE: Do not use Gnu TLS. It will spontaneously terminate +a Postfix daemon process with exit status code 2, instead of allowing +Postfix to 1) report the error to the maillog file, and to 2) provide +plaintext service where this is appropriate. </b> </p> + +<ul> + +<li> <p> If the OpenSSL include files (such as <tt>ssl.h</tt>) are +in directory <tt>/usr/include/openssl</tt>, and the OpenSSL libraries +(such as <tt>libssl.so</tt> and <tt>libcrypto.so</tt>) are in +directory <tt>/usr/lib</tt>: </p> + +<blockquote> +<pre> +% <b>make tidy</b> # if you have left-over files from a previous build +% <b>make makefiles CCARGS="-DUSE_TLS" AUXLIBS="-lssl -lcrypto"</b> +</pre> +</blockquote> + +<li> <p> If the OpenSSL include files (such as <tt>ssl.h</tt>) are +in directory <tt>/usr/local/include/openssl</tt>, and the OpenSSL +libraries (such as <tt>libssl.so</tt> and <tt>libcrypto.so</tt>) +are in directory <tt>/usr/local/lib</tt>: </p> + +<blockquote> +<pre> +% <b>make tidy</b> # if you have left-over files from a previous build +% <b>make makefiles CCARGS="-DUSE_TLS -I/usr/local/include" \ + AUXLIBS="-L/usr/local/lib -lssl -lcrypto" </b> +</pre> +</blockquote> + +<p> If your OpenSSL shared library is in a directory that the RUN-TIME +linker does not know about, add a "-Wl,-R,/path/to/directory" option after +"-lcrypto". </p> + +<p> On Solaris, specify the <tt>-R</tt> option as shown below: + +<blockquote> +<pre> +% <b>make tidy</b> # if you have left-over files from a previous build +% <b>make makefiles CCARGS="-DUSE_TLS -I/usr/local/include" \ + AUXLIBS="-R/usr/local/lib -L/usr/local/lib -lssl -lcrypto" </b> +</pre> +</blockquote> + +</ul> + +<p> If you need to apply other customizations (such as Berkeley DB +databases, MySQL, PostgreSQL, LDAP or SASL), see the respective +Postfix README documents, and combine their "<tt>make makefiles</tt>" +instructions with the instructions above: </p> + +<blockquote> +<pre> +% <b>make tidy</b> # if you have left-over files from a previous build +% <b>make makefiles CCARGS="-DUSE_TLS \ + <i>(other -D or -I options)</i>" \ + AUXLIBS="-lssl -lcrypto \ + <i>(other -l options for libraries in /usr/lib)</i> \ + <i>(-L/path/name + -l options for other libraries)</i>"</b> +</pre> +</blockquote> + +<p> To complete the build process, see the Postfix <a href="INSTALL.html">INSTALL</a> +instructions. Postfix has TLS support turned off by default, so +you can start using Postfix as soon as it is installed. </p> + +<h2> <a name="problems"> Reporting problems </a> </h2> + +<p> Problems are preferably reported via <postfix-users@postfix.org>. +See <a href="http://www.postfix.org/lists.html">http://www.postfix.org/lists.html</a> for subscription information. +When reporting a problem, please be thorough in the report. Patches, +when possible, are greatly appreciated too. </p> + +<h2><a name="credits">Credits </a> </h2> + +<ul> + +<li> TLS support for Postfix was originally developed by Lutz +Jänicke at Cottbus Technical University. + +<li> Wietse Venema adopted the code, did some restructuring, and +compiled this part of the documentation from Lutz's documents. + +<li> Victor Duchovni was instrumental with the re-implementation +of the <a href="postconf.5.html#smtp_tls_per_site">smtp_tls_per_site</a> code in terms of enforcement levels, which +simplified the implementation greatly. + +<li> Victor Duchovni implemented the fingerprint security level, +added more sanity checks, and separated TLS connection management +from security policy enforcement. The latter change simplified the +code that verifies certificate signatures, certificate names, and +certificate fingerprints. + +</ul> + +</body> + +</html> |