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|
<!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 smtpd(8) server implements the SMTP over TLS server
side. </p>
<li> <p> The smtp(8) client implements the SMTP (and LMTP) over TLS
client side. </p>
<li> <p> The tlsmgr(8) server maintains the pseudo-random number
generator (PRNG) that seeds the TLS engines in the smtpd(8) server
and smtp(8) client processes, and maintains the TLS session key
cache files. </p>
</ul>
<p> Not shown in the figure are the tlsproxy(8) server and the
postscreen(8) server. These use TLS in the same manner as smtpd(8).
</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
"smtpd_tls_cert_file = 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
"smtpd_tls_protocols" and perhaps also "smtpd_tls_mandatory_protocols".
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 (RFC 6698,
RFC 7671, RFC 7672) 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 (RFC 6698, RFC 7671, RFC 7672)
"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 smtpd_tls_fingerprint_digest main.cf 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 $smtpd_tls_CAfile or install it in the $smtpd_tls_CApath
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.
#
smtpd_tls_chain_files =
/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.
smtpd_tls_chain_files =
/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 "smtpd_tls_chain_files" interface shown
above. <p>
<p> RSA key and certificate examples: </p>
<blockquote>
<pre>
/etc/postfix/main.cf:
smtpd_tls_cert_file = /etc/postfix/server.pem
smtpd_tls_key_file = $smtpd_tls_cert_file
</pre>
</blockquote>
<p> Their DSA counterparts: </p>
<blockquote>
<pre>
/etc/postfix/main.cf:
smtpd_tls_dcert_file = /etc/postfix/server-dsa.pem
smtpd_tls_dkey_file = $smtpd_tls_dcert_file
</pre>
</blockquote>
<p> Their ECDSA counterparts (Postfix ≥ 2.6 + OpenSSL ≥ 1.0.0): </p>
<blockquote>
<pre>
/etc/postfix/main.cf:
# Some clients will not be ECDSA capable, so you will likely still need
# an RSA certificate and private key.
#
smtpd_tls_eccert_file = /etc/postfix/server-ecdsa.pem
smtpd_tls_eckey_file = $smtpd_tls_eccert_file
</pre>
</blockquote>
<p> TLS without certificates for servers serving exclusively
anonymous-cipher capable clients: </p>
<blockquote>
<pre>
/etc/postfix/main.cf:
# Not recommended: breaks TLS 1.3 and clients that don't support
# anonymous cipher suites.
smtpd_tls_cert_file = 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 $smtpd_tls_CAfile or in multiple files, one CA per file in
the $smtpd_tls_CApath directory. If you use a directory, don't forget
to create the necessary "hash" links with: </p>
<blockquote>
<pre>
# <b>$OPENSSL_HOME/bin/c_rehash <i>/path/to/directory</i> </b>
</pre>
</blockquote>
<p> The $smtpd_tls_CAfile 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 $smtpd_tls_CApath
directory, in which case the certificates are read (with $mail_owner
privileges) from the files in the directory when the information
is needed. Thus, the $smtpd_tls_CApath 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 $smtpd_tls_CAfile are sent to the client, in order to allow
it to choose an identity signed by a CA you trust. If no $smtpd_tls_CAfile
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 $smtpd_tls_CApath. In the latter case you need not specify a
$smtpd_tls_CAfile. </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/main.cf:
smtpd_tls_CAfile = /etc/postfix/CAcert.pem
smtpd_tls_CApath = /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/main.cf:
smtpd_tls_loglevel = 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 smtpd_tls_received_header 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/main.cf:
smtpd_tls_received_header = 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 "smtpd_tls_security_level = may". </p>
<p> Example: </p>
<blockquote>
<pre>
/etc/postfix/main.cf:
smtpd_tls_security_level = 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
"smtpd_tls_security_level = encrypt". According to RFC 2487 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/main.cf:
smtpd_tls_security_level = 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 main.cf. If
you want to support this service, enable a special port in master.cf
and specify "-o smtpd_tls_wrappermode=yes" (note: no space around
the "=") as an smtpd(8) command line option. Port 465 (smtps) was
once chosen for this feature.
</p>
<p> Example: </p>
<blockquote>
<pre>
/etc/postfix/master.cf:
smtps inet n - n - - smtpd
-o smtpd_tls_wrappermode=yes -o smtpd_sasl_auth_enable=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 $smtpd_tls_CAfile
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
permit_tls_clientcerts 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/main.cf:
smtpd_tls_ask_ccert = yes
smtpd_tls_security_level = 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 "smtpd_tls_req_ccert = yes". This feature implies
"smtpd_tls_ask_ccert = yes". When TLS is not enforced,
"smtpd_tls_req_ccert = yes" is ignored and a warning is
logged. </p>
<p> Example: </p>
<blockquote>
<pre>
/etc/postfix/main.cf:
smtpd_tls_req_ccert = yes
smtpd_tls_security_level = encrypt
</pre>
</blockquote>
<p> The client certificate verification depth is specified with the
main.cf smtpd_tls_ccert_verifydepth parameter. The default verification
depth is 9 (the OpenSSL default), for compatibility with Postfix
versions before 2.5 where smtpd_tls_ccert_verifydepth 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 $smtpd_tls_ccert_verifydepth 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/main.cf:
smtpd_tls_ccert_verifydepth = 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
("smtpd_tls_security_level = 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
("smtpd_tls_security_level = 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
"smtpd_tls_auth_only = yes". </p>
<p> Example: </p>
<blockquote>
<pre>
/etc/postfix/main.cf:
smtpd_tls_auth_only = 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 smtpd(8) 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
smtpd(8) 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 tlsmgr(8) 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 RFC 5077 TLS session resumption without
server-side state when the remote SMTP client also supports RFC
5077. 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 smtpd_tls_session_cache_database 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/main.cf:
smtpd_tls_session_cache_database = btree:/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 data_directory. As a migration aid, an attempt to
open the file under a non-Postfix directory is redirected to the
Postfix-owned data_directory, 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). RFC 2246
recommends a maximum of 24 hours. </p>
<p> Example: </p>
<blockquote>
<pre>
/etc/postfix/main.cf:
smtpd_tls_session_cache_timeout = 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>
smtpd_tls_always_issue_session_ids = 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> permit_tls_clientcerts </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 relay_clientcerts discussion below).
</p> </dd>
<dt> permit_tls_all_clientcerts </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> check_ccert_access type:table</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 access(5)
table. </p> </dd>
</dl>
</blockquote>
<p> The digest algorithm used to compute the client certificate
fingerprints is specified with the main.cf smtpd_tls_fingerprint_digest
parameter. The default algorithm is <b>sha256</b> with Postfix ≥
3.6 and the <b>compatibility_level</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 permit_tls_all_clientcerts 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 permit_tls_all_clientcerts feature can be practical for a
specially created email relay server. </p>
<p> It is however recommended to stay with the permit_tls_clientcerts
feature and list all certificates via $relay_clientcerts, as
permit_tls_all_clientcerts 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 smtpd_relay_restrictions.
/etc/postfix/main.cf:
smtpd_relay_restrictions =
permit_mynetworks
permit_tls_clientcerts
reject_unauth_destination
</pre>
<pre>
# Older configurations combine relay control and spam control under
# smtpd_recipient_restrictions. To use this example with Postfix ≥
# 2.10 specify "smtpd_relay_restrictions=".
/etc/postfix/main.cf:
smtpd_recipient_restrictions =
permit_mynetworks
permit_tls_clientcerts
reject_unauth_destination
...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/main.cf:
relay_clientcerts = hash:/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 smtpd_tls_mandatory_ciphers 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
smtpd_tls_ciphers 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 "smtpd_tls_mandatory_exclude_ciphers = aNULL" or
"smtpd_tls_exclude_ciphers = 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
smtpd_tls_mandatory_protocols configuration parameter. The
smtpd_tls_protocols 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 "smtpd_tls_cert_file = none"
and not specifying an smtpd_tls_dcert_file or smtpd_tls_eccert_file.
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/main.cf:
smtpd_tls_cert_file = /etc/postfix/cert.pem
smtpd_tls_key_file = /etc/postfix/key.pem
smtpd_tls_mandatory_ciphers = high
smtpd_tls_mandatory_exclude_ciphers = aNULL, MD5
smtpd_tls_security_level = encrypt
# Preferred syntax with Postfix ≥ 3.6:
smtpd_tls_mandatory_protocols = >=TLSv1.2
# Legacy syntax:
smtpd_tls_mandatory_protocols = !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 "smtpd_tls_cert_file" and "smtpd_tls_key_file"
and related DSA and ECDSA parameters empty. </p>
<blockquote>
<pre>
/etc/postfix/main.cf:
smtpd_tls_chain_files = /etc/postfix/rsachain.pem
smtpd_tls_cert_file =
smtpd_tls_key_file =
...
</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 "tls_preempt_cipherlist = yes" enables server
cipher-suite preferences. The default OpenSSL behavior applies with
"tls_preempt_cipherlist = 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 smtpd_starttls_timeout 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/main.cf:
smtpd_starttls_timeout = 300s
</pre>
</blockquote>
<p> With Postfix 2.8 and later, the tls_disable_workarounds 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
tls_disable_workarounds parameter documentation for the list of
supported values.</p>
<p> Example: </p>
<blockquote>
<pre>
/etc/postfix/main.cf:
tls_disable_workarounds = 0xFFFFFFFF
tls_disable_workarounds = CVE-2010-4180
</pre>
</blockquote>
<p> With Postfix ≥ 2.11, the tls_ssl_options 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 tls_ssl_options parameter documentation for the list of
supported values. </p>
<p> Example: </p>
<blockquote>
<pre>
/etc/postfix/main.cf:
tls_ssl_options = 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 smtp(8) and lmtp(8) 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 $myhostname
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 "smtp_tls_security_level = 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 "smtp_tls_security_level = may".
For LMTP, use the corresponding "lmtp_" parameter. </p>
<p> The "smtp_tls_ciphers" and "smtp_tls_protocols" 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/main.cf:
smtp_tls_security_level = 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
"smtp_tls_security_level = 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 smtp_tls_mandatory_protocols
and smtp_tls_mandatory_ciphers 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 smtp_tls_mandatory_protocols
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/main.cf:
smtp_tls_policy_maps = hash:/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, transport(5) table or relayhost
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/main.cf:
smtp_tls_policy_maps = hash:/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 (RFC 6698, RFC 7671, RFC 7672) 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 RFC 7672) 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 RFC 7672) 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 smtp_dns_support_level main.cf
parameter. The tls_dane_digests parameter controls the list of
supported digests. </p>
<p> As explained in section 3 of RFC 7672, 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. RFC
7672 (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 "smtp_dns_support_level" must be set to "dnssec".
<li> The "smtp_host_lookup" 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 tls_server_sni_maps. </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 "smtp_dns_support_level
= dnssec". To enable DNSSEC lookups selectively, define a new
dedicated transport with a "-o smtp_dns_support_level=dnssec"
override in master.cf 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
smtp_dns_support_level in main.cf. </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>
main.cf:
indexed = ${default_database_type}:${config_directory}/
#
# default: Opportunistic TLS with no DNSSEC lookups.
#
smtp_tls_security_level = may
smtp_dns_support_level = enabled
#
# Per-destination TLS policy
#
smtp_tls_policy_maps = ${indexed}tls_policy
#
# default_transport = smtp, but some destinations are special:
#
transport_maps = ${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>
master.cf:
dane unix - - n - - smtp
-o smtp_dns_support_level=dnssec
-o smtp_tls_security_level=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
smtp_tls_fingerprint_cert_match 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>smtp_tls_fingerprint_digest</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>compatibility_level</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 relayhost may be multiple
physical hosts behind a load-balancer, each with its own private/public
key and self-signed certificate. Alternatively, a single relayhost 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>
relayhost = [mailhub.example.com]
smtp_tls_security_level = fingerprint
smtp_tls_fingerprint_digest = sha256
smtp_tls_fingerprint_cert_match =
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/main.cf:
smtp_tls_policy_maps = hash:/etc/postfix/tls_policy
smtp_tls_fingerprint_digest = 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
"smtp_tls_security_level = verify". The
smtp_tls_verify_cert_match 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 smtp_tls_CAfile
and smtp_tls_CApath), 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 "smtp_tls_trust_anchor_file" 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 RFC 2487 <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/main.cf:
indexed = ${default_database_type}:${config_directory}/
smtp_tls_CAfile = ${config_directory}/CAfile.pem
smtp_tls_policy_maps = ${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 "smtp_tls_security_level = secure".
The smtp_tls_secure_cert_match 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 smtp_tls_CAfile and
smtp_tls_CApath), 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 "smtp_tls_trust_anchor_file" 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 RFC 2487 <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 transport(5) 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 smtp_tls_secure_cert_match 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/main.cf:
smtp_tls_CAfile = /etc/postfix/CAfile.pem
smtp_tls_policy_maps = hash:/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 transport(5) 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/main.cf:
smtp_tls_CAfile = /etc/postfix/CAfile.pem
transport_maps = hash:/etc/postfix/transport
smtp_tls_policy_maps = hash:/etc/postfix/tls_policy
/etc/postfix/transport:
example.com smtp:[tls.example.com]
example.co.uk smtp:[tls.example.com]
example.co.jp smtp:[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/main.cf:
smtp_tls_loglevel = 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>
smtp_tls_cert_file =
smtp_tls_dcert_file =
smtp_tls_key_file =
smtp_tls_dkey_file =
# Postfix ≥ 2.6
smtp_tls_eccert_file =
smtp_tls_eckey_file =
# Postfix ≥ 3.4
smtp_tls_chain_files =
</pre>
</blockquote>
<p> The best way to use the default settings is to comment out the above
parameters in main.cf 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
"smtp_tls_chain_files" 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
$smtp_tls_CAfile or install it in the $smtp_tls_CApath 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.
#
smtp_tls_chain_files =
/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.
smtp_tls_chain_files =
/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 "smtp_tls_chain_files" interface shown
above. <p>
<p> RSA key and certificate examples: </p>
<blockquote>
<pre>
/etc/postfix/main.cf:
smtp_tls_cert_file = /etc/postfix/client.pem
smtp_tls_key_file = $smtp_tls_cert_file
</pre>
</blockquote>
<p> Their DSA counterparts: </p>
<blockquote>
<pre>
/etc/postfix/main.cf:
smtp_tls_dcert_file = /etc/postfix/client-dsa.pem
smtp_tls_dkey_file = $smtp_tls_dcert_file
</pre>
</blockquote>
<p> Their ECDSA counterparts (Postfix ≥ 2.6 + OpenSSL ≥ 1.0.0): </p>
<blockquote>
<pre>
/etc/postfix/main.cf:
smtp_tls_eccert_file = /etc/postfix/client-ecdsa.pem
smtp_tls_eckey_file = $smtp_tls_eccert_file
</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 $smtp_tls_CAfile or in multiple files, one CA per file in
the $smtp_tls_CApath directory. If you use a directory, don't forget
to create the necessary "hash" links with: </p>
<blockquote>
<pre>
# <b>$OPENSSL_HOME/bin/c_rehash <i>/path/to/directory</i> </b>
</pre>
</blockquote>
<p> The $smtp_tls_CAfile 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 $smtp_tls_CApath
directory, in which case the certificates are read (with $mail_owner
privileges) from the files in the directory when the information
is needed. Thus, the $smtp_tls_CApath directory needs to be accessible
inside the optional chroot jail. </p>
<p> The choice between $smtp_tls_CAfile and $smtp_tls_CApath 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/main.cf:
smtp_tls_CAfile = /etc/postfix/CAcert.pem
smtp_tls_CApath = /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/main.cf:
smtp_tls_connection_reuse = yes
</pre>
</blockquote>
<p> Alternatively, specify the attribute "connection_reuse=yes" in
an smtp_tls_policy_maps entry. </p>
<p> The implementation of TLS connection reuse relies on the same
scache(8) service as used for delivering plaintext SMTP mail, the
same tlsproxy(8) daemon as used by the postscreen(8) service, and
relies on the same hints from the qmgr(8) 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> smtp(8) -> remote SMTP server</pre>
<p> Reused SMTP/TLS connection, or new SMTP/TLS connection: </p>
<pre> smtp(8) -> tlsproxy(8) -> remote SMTP server </pre>
<p> Cached SMTP/TLS connection:</p>
<pre> scache(8) -> tlsproxy(8) -> 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 smtp(8)
process actually using this session and is lost when the process
terminates. To share the session information between multiple
smtp(8) 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 tlsmgr(8) 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/main.cf:
smtp_tls_session_cache_database = btree:/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 data_directory. As a migration aid, an attempt to
open the file under a non-Postfix directory is redirected to the
Postfix-owned data_directory, 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). RFC 2246
recommends a maximum of 24 hours. </p>
<p> Example: </p>
<blockquote>
<pre>
/etc/postfix/main.cf:
smtp_tls_session_cache_timeout = 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 RFC6698 (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 smtp_tls_policy_maps
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, $local_transport, $virtual_transport,
$relay_transport or $default_transport. 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 $myhostname 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 transport(5) 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 main.cf 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
"smtp_tls_ciphers", "smtp_tls_exclude_ciphers" and "smtp_tls_protocols"
configuration parameters. At this level and higher, the optional
"servername" attribute (available with Postfix ≥ 3.4) overrides the
global "smtp_tls_servername" 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 main.cf smtp_tls_mandatory_protocols
parameter, the optional "ciphers" attribute overrides the
main.cf smtp_tls_mandatory_ciphers parameter, and the optional
"exclude" attribute (Postfix ≥ 2.6) overrides the main.cf
smtp_tls_mandatory_exclude_ciphers 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 smtp_tls_mandatory_protocols), and the server certificate
must match the TLSA records. RFC 7672 (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 smtp_tls_mandatory_protocols), and the server certificate must
match the TLSA records. RFC 7672 (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 main.cf <b>smtp_tls_fingerprint_cert_match</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>smtp_tls_fingerprint_digest</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 main.cf smtp_tls_verify_cert_match
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
"smtp_tls_trust_anchor_file" 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 main.cf smtp_tls_secure_cert_match 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 "smtp_tls_trust_anchor_file" 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 transport(5) 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 main.cf, 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/main.cf:
smtp_tls_policy_maps = hash:/etc/postfix/tls_policy
# Postfix 2.5 and later
smtp_tls_fingerprint_digest = 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 smtp_tls_secure_cert_match 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 smtp_tls_note_starttls_offer 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/main.cf:
smtp_tls_note_starttls_offer = 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
main.cf smtp_tls_scert_verifydepth parameter. The default verification
depth is 9 (the OpenSSL default), for compatibility with Postfix
versions before 2.5 where smtp_tls_scert_verifydepth 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 $smtp_tls_scert_verifydepth 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/main.cf:
smtp_tls_scert_verifydepth = 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 smtp_tls_mandatory_ciphers 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 smtp_tls_policy_maps 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
"smtp_tls_mandatory_exclude_ciphers = aNULL"; and to
disable anonymous ciphers even with opportunistic TLS, set
"smtp_tls_exclude_ciphers = 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 "smtp_tls_ciphers" 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
smtp_tls_mandatory_protocols configuration parameter. The corresponding
smtp_tls_protocols parameter (Postfix ≥ 2.6) controls
the TLS protocols used with opportunistic TLS. </p>
<p> Example: </p>
<blockquote>
<pre>
/etc/postfix/main.cf:
smtp_tls_mandatory_ciphers = medium
smtp_tls_mandatory_exclude_ciphers = RC4, MD5
smtp_tls_exclude_ciphers = aNULL
smtp_tls_ciphers = medium
# Preferred form with Postfix ≥ 3.6:
smtp_tls_mandatory_protocols = >=TLSv1.2
smtp_tls_protocols = >=TLSv1
# Legacy form for Postfix < 3.6:
smtp_tls_mandatory_protocols = !SSLv2, !SSLv3, !TLSv1, !TLSv1.1
smtp_tls_protocols = !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/main.cf:
# Client-side SMTPS requires "encrypt" or stronger.
smtp_tls_security_level = encrypt
smtp_tls_wrappermode = yes
# The [] suppress MX lookups.
relayhost = [mail.example.com]:465
</pre>
</blockquote>
<p> Use "postfix reload" to make the change effective. </p>
<p> See SOHO_README 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/main.cf:
transport_maps = hash:/etc/postfix/transport
/etc/postfix/transport:
example.com relay-smtps:example.com:465
/etc/postfix/master.cf:
relay-smtps unix - - n - - smtp
# Client-side SMTPS requires "encrypt" or stronger.
-o smtp_tls_security_level=encrypt
-o smtp_tls_wrappermode=yes
</pre>
</blockquote>
<p> Use "postmap hash:/etc/postfix/transport" and "postfix reload"
to make the change effective. </p>
<p> See SOHO_README 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 relayhost feature sends all remote
mail through the local stunnel listener on port 11125: </p>
<blockquote>
<pre>
/etc/postfix/main.cf:
relayhost = [127.0.0.1]:11125
</pre>
</blockquote>
<p> Use "postfix reload" to make the change effective. </p>
<p> See SOHO_README 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/main.cf:
transport_maps = hash:/etc/postfix/transport
/etc/postfix/transport:
example.com relay:[127.0.0.1]:11125
</pre>
</blockquote>
<p> Use "postmap hash:/etc/postfix/transport" and "postfix reload"
to make the change effective. </p>
<p> See SOHO_README for additional information about SASL authentication.
</p>
<h3> <a name="client_misc"> Miscellaneous client controls </a> </h3>
<p> The smtp_starttls_timeout 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/main.cf:
smtp_starttls_timeout = 300s
</pre>
</blockquote>
<p> With Postfix 2.8 and later, the tls_disable_workarounds 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/main.cf:
tls_disable_workarounds = 0xFFFFFFFF
tls_disable_workarounds = 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 tls_ssl_options 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/main.cf:
tls_ssl_options = 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 tlsmgr(8) process
maintains a Pseudo Random Number Generator (PRNG) pool. This is
queried by the smtp(8) and smtpd(8) 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/main.cf:
tls_daemon_random_bytes = 32
</pre>
</blockquote>
<p> In order to feed its in-memory PRNG pool, the tlsmgr(8) 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 tlsmgr(8) 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 main.cf): </p>
<blockquote>
<pre>
/etc/postfix/main.cf:
tls_random_source = dev:/dev/urandom
tls_random_source = egd:/var/run/egd-pool
</pre>
</blockquote>
<p> By default, tlsmgr(8) 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 tlsmgr(8) 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/main.cf:
tls_random_bytes = 32
</pre>
</blockquote>
<p> In order to update its in-memory PRNG pool, the tlsmgr(8)
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 tls_random_reseed_period.
The default maximal time interval is 1 hour. </p>
<p> Example: </p>
<blockquote>
<pre>
/etc/postfix/main.cf:
tls_random_reseed_period = 3600s
</pre>
</blockquote>
<p> The tlsmgr(8) 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/main.cf:
tls_random_exchange_name = /var/lib/postfix/prng_exch
tls_random_prng_update_period = 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
data_directory. As a migration aid, an attempt to open the file
under a non-Postfix directory is redirected to the Postfix-owned
data_directory, and a warning is logged. If you wish to continue
using a pre-existing PRNG state file, move it to the data_directory
and change the ownership to the account specified with the mail_owner
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>
# postfix tls 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'`
# postfix tls 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>
# postfix tls 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'`
# postfix tls 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 "postfix tls"
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 postfix-tls(1) 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 main.cf with new filenames, a potential race condition
in which the key and certificate might not match is avoided).
</p>
<blockquote>
<pre>
# dir="$(postconf -h config_directory)"
# fqdn=$(postconf -h myhostname)
# 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 \
"smtpd_tls_cert_file = ${cert}" \
"smtpd_tls_key_file = ${key}" \
'smtpd_tls_security_level = may' \
'smtpd_tls_received_header = yes' \
'smtpd_tls_loglevel = 1' \
'smtp_tls_security_level = may' \
'smtp_tls_loglevel = 1' \
'smtp_tls_session_cache_database = btree:${data_directory}/smtp_scache' \
'tls_random_source = dev:/dev/urandom'
</pre>
</blockquote>
<p> Note: the last command requires both single (') and double (")
quotes. </p>
<p> The postconf(1) 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 RFC 5077 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/main.cf </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/main.cf:
smtp_tls_CAfile = /etc/postfix/cacert.pem
smtp_tls_session_cache_database =
btree:/var/lib/postfix/smtp_tls_session_cache
smtp_tls_security_level = may
smtp_tls_loglevel = 1
smtpd_tls_CAfile = /etc/postfix/cacert.pem
smtpd_tls_cert_file = /etc/postfix/foo-cert.pem
smtpd_tls_key_file = /etc/postfix/foo-key.pem
smtpd_tls_received_header = yes
smtpd_tls_session_cache_database =
btree:/var/lib/postfix/smtpd_tls_session_cache
tls_random_source = dev:/dev/urandom
smtpd_tls_security_level = may
smtpd_tls_loglevel = 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 INSTALL 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 INSTALL
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 http://www.postfix.org/lists.html 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 smtp_tls_per_site 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>
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