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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 legacy TLS Support </title>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<link rel='stylesheet' type='text/css' href='postfix-doc.css'>
</head>
<body>
<h1><img src="postfix-logo.jpg" width="203" height="98" ALT="">Postfix legacy TLS Support
</h1>
<hr>
<h2> NOTE </h2>
<p> This document describes an old TLS user interface that is based
on a third-party TLS patch by Lutz Jänicke. As of Postfix
version 2.3, the old user interface still exists to allow migration
from earlier Postfix releases, but its functionality is frozen. </p>
<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> Postfix version 2.2 introduces support for TLS as described in
<a href="https://tools.ietf.org/html/rfc3207">RFC 3207</a>. TLS Support for older Postfix versions was available as
an add-on patch. The section "<a href="#compat">Compatibility with
Postfix < 2.2 TLS support</a>" below discusses the differences
between these implementations. </p>
<p> Topics covered in this document: </p>
<ul>
<li><a href="#how">How Postfix TLS support works</a>
<li><a href="#build_tls">Building Postfix with TLS support</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="#problems"> Reporting problems </a>
<li><a href="#compat">Compatibility with Postfix < 2.2 TLS support</a>
<li><a href="#credits"> Credits </a>
</ul>
<p> And last but not least, for the impatient: </p>
<ul>
<li><a href="#quick-start">Getting started, quick and dirty</a>
</ul>
<h2><a name="how">How Postfix TLS support works</a></h2>
<p> The diagram below shows the main elements of the Postfix TLS
architecture and their relationships. Colored boxes with numbered
names represent Postfix daemon programs. Other colored boxes
represent storage elements. </p>
<ul>
<li> <p> The <a href="smtpd.8.html">smtpd(8)</a> server implements the SMTP over TLS server
side. </p>
<li> <p> The <a href="smtp.8.html">smtp(8)</a> client implements the SMTP over TLS client
side. </p>
<li> <p> The <a href="tlsmgr.8.html">tlsmgr(8)</a> server maintains the pseudo-random number
generator (PRNG) that seeds the TLS engines in the <a href="smtpd.8.html">smtpd(8)</a> server
and <a href="smtp.8.html">smtp(8)</a> client processes, and maintains the TLS session key
cache files. </p>
</ul>
<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><-session-> </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><-session->
</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="build_tls">Building Postfix with TLS support</a></h2>
<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> 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, PosgreSQL, LDAP or SASL), see the respective
Postfix README documents, and combine their "<tt>make makefiles</tt>"
instructions with the instructions above: </p>
<blockquote>
<pre>
% <b>make tidy</b> # if you have left-over files from a previous build
% <b>make makefiles CCARGS="-DUSE_TLS \
<i>(other -D or -I options)</i>" \
AUXLIBS="-lssl -lcrypto \
<i>(other -l options for libraries in /usr/lib)</i> \
<i>(-L/path/name + -l options for other libraries)</i>"</b>
</pre>
</blockquote>
<p> To complete the build process, see the Postfix <a href="INSTALL.html">INSTALL</a>
instructions. Postfix has TLS support turned off by default, so
you can start using Postfix as soon as it is installed. </p>
<h2><a name="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_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 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. Both certificate and private key may be in the same
file. </p>
<p> Both RSA and DSA certificates are supported. Typically you will
only have RSA certificates issued by a commercial CA. In addition,
the tools supplied with OpenSSL will by default issue RSA certificates.
You can have both at the same time, in which case the cipher used
determines which certificate is presented. For Netscape and OpenSSL
clients without special cipher choices, the RSA certificate is
preferred. </p>
<p> In order for remote SMTP clients to check the Postfix SMTP
server certificates, the CA certificate (in case of a certificate
chain, all CA certificates) must be available. You should add
these certificates to the server certificate, the server certificate
first, then the issuing CA(s). </p>
<p> Example: the certificate for "server.dom.ain" was issued by
"intermediate CA" which itself has a certificate issued by "root
CA". Create the server.pem file with: </p>
<blockquote>
<pre>
% <b>cat server_cert.pem intermediate_CA.pem > server.pem</b>
</pre>
</blockquote>
<p> A Postfix SMTP server certificate supplied here must be usable
as an SSL server certificate and hence pass the "openssl verify -purpose
sslserver ..." test. </p>
<p> A client 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 "server.pem" file reduces
the overhead of the TLS exchange. </p>
<p> If you want the Postfix SMTP server to accept remote SMTP client
certificates issued by these CAs, append the root certificate to
$<a href="postconf.5.html#smtpd_tls_CAfile">smtpd_tls_CAfile</a> or install it in the $<a href="postconf.5.html#smtpd_tls_CApath">smtpd_tls_CApath</a> directory. When
you configure trust in a root CA, it is not necessary to explicitly trust
intermediary CAs signed by the root CA, unless $<a href="postconf.5.html#smtpd_tls_ccert_verifydepth">smtpd_tls_ccert_verifydepth</a>
is less than the number of CAs in the certificate chain for the clients
of interest. With a verify depth of 1 you can only verify certificates
directly signed by a trusted CA, and all trusted intermediary CAs need to
be configured explicitly. With a verify depth of 2 you can verify clients
signed by a root CA or a direct intermediary CA (so long as the client
is correctly configured to supply its intermediate CA certificate). </p>
<p> RSA key and certificate examples: </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#smtpd_tls_cert_file">smtpd_tls_cert_file</a> = /etc/postfix/server.pem
<a href="postconf.5.html#smtpd_tls_key_file">smtpd_tls_key_file</a> = $<a href="postconf.5.html#smtpd_tls_cert_file">smtpd_tls_cert_file</a>
</pre>
</blockquote>
<p> Their DSA counterparts: </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#smtpd_tls_dcert_file">smtpd_tls_dcert_file</a> = /etc/postfix/server-dsa.pem
<a href="postconf.5.html#smtpd_tls_dkey_file">smtpd_tls_dkey_file</a> = $<a href="postconf.5.html#smtpd_tls_dcert_file">smtpd_tls_dcert_file</a>
</pre>
</blockquote>
<p> To verify a remote SMTP client certificate, the Postfix SMTP
server needs to trust the certificates of the issuing Certification
Authorities. These certificates in "pem" format can be stored in a
single $<a href="postconf.5.html#smtpd_tls_CAfile">smtpd_tls_CAfile</a> or in multiple files, one CA per file in
the $<a href="postconf.5.html#smtpd_tls_CApath">smtpd_tls_CApath</a> directory. If you use a directory, don't forget
to create the necessary "hash" links with: </p>
<blockquote>
<pre>
# <b>$OPENSSL_HOME/bin/c_rehash <i>/path/to/directory</i> </b>
</pre>
</blockquote>
<p> The $<a href="postconf.5.html#smtpd_tls_CAfile">smtpd_tls_CAfile</a> contains the CA certificates of one or
more trusted CAs. The file is opened (with root privileges) before
Postfix enters the optional chroot jail and so need not be accessible
from inside the chroot jail. </p>
<p> Additional trusted CAs can be specified via the $<a href="postconf.5.html#smtpd_tls_CApath">smtpd_tls_CApath</a>
directory, in which case the certificates are read (with $<a href="postconf.5.html#mail_owner">mail_owner</a>
privileges) from the files in the directory when the information
is needed. Thus, the $<a href="postconf.5.html#smtpd_tls_CApath">smtpd_tls_CApath</a> directory needs to be
accessible inside the optional chroot jail. </p>
<p> When you configure Postfix to request client certificates (by
setting $<a href="postconf.5.html#smtpd_tls_ask_ccert">smtpd_tls_ask_ccert</a> = yes), any certificates in
$<a href="postconf.5.html#smtpd_tls_CAfile">smtpd_tls_CAfile</a> are sent to the client, in order to allow it to
choose an identity signed by a CA you trust. If no $<a href="postconf.5.html#smtpd_tls_CAfile">smtpd_tls_CAfile</a>
is specified, no preferred CA list is sent, and the client is free
to choose an identity signed by any CA. Many clients use a fixed
identity regardless of the preferred CA list and you may be able
to reduce TLS negotiation overhead by installing client CA certificates
mostly or only in $<a href="postconf.5.html#smtpd_tls_CApath">smtpd_tls_CApath</a>. In the latter case you need
not specify a $<a href="postconf.5.html#smtpd_tls_CAfile">smtpd_tls_CAfile</a>. </p>
<p> Note, that unless client certificates are used to allow greater
access to TLS authenticated clients, it is best to not ask for
client certificates at all, as in addition to increased overhead
some clients (notably in some cases qmail) are unable to complete
the TLS handshake when client certificates are requested. </p>
<p> Example: </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#smtpd_tls_CAfile">smtpd_tls_CAfile</a> = /etc/postfix/CAcert.pem
<a href="postconf.5.html#smtpd_tls_CApath">smtpd_tls_CApath</a> = /etc/postfix/certs
</pre>
</blockquote>
<h3><a name="server_logging"> Server-side TLS activity logging </a> </h3>
<p> To get additional information about Postfix SMTP server 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>
<tr> <td> 0 </td> <td> Disable logging of TLS activity.</td> </tr>
<tr> <td> 1 </td> <td> Log TLS handshake and certificate information.
</td> </tr>
<tr> <td> 2 </td> <td> Log levels during TLS negotiation. </td>
</tr>
<tr> <td> 3 </td> <td> Log hexadecimal and ASCII dump of TLS
negotiation process </td> </tr>
<tr> <td> 4 </td> <td> Log hexadecimal and ASCII dump of complete
transmission after STARTTLS </td> </tr>
</table>
</blockquote>
<p> Use loglevel 3 only in case of problems. Use of loglevel 4 is
strongly discouraged. </p>
<p> Example: </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#smtpd_tls_loglevel">smtpd_tls_loglevel</a> = 0
</pre>
</blockquote>
<p> To include information about the protocol and cipher used as
well as the client and issuer CommonName into the "Received:"
message header, set the <a href="postconf.5.html#smtpd_tls_received_header">smtpd_tls_received_header</a> variable to true.
The default is no, as the information is not necessarily authentic.
Only information recorded at the final destination is reliable,
since the headers may be changed by intermediate servers. </p>
<p> Example: </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#smtpd_tls_received_header">smtpd_tls_received_header</a> = yes
</pre>
</blockquote>
<h3><a name="server_enable">Enabling TLS in the Postfix SMTP server </a> </h3>
<p> By default, TLS is disabled in the Postfix SMTP server, so no
difference to plain Postfix is visible. Explicitly switch it on
using "<a href="postconf.5.html#smtpd_use_tls">smtpd_use_tls</a> = yes". </p>
<p> Example: </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#smtpd_use_tls">smtpd_use_tls</a> = yes
</pre>
</blockquote>
<p> With this, Postfix SMTP server announces STARTTLS support to
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 server
private key. This is intended behavior. </p>
<p> You can ENFORCE the use of TLS, so that the Postfix SMTP server
announces STARTTLS and accepts no mail without TLS encryption, by
setting "<a href="postconf.5.html#smtpd_enforce_tls">smtpd_enforce_tls</a> = yes". According to <a href="https://tools.ietf.org/html/rfc2487">RFC 2487</a> this MUST
NOT be applied in case of a publicly-referenced Postfix SMTP server.
This option is off by default and should only seldom be used. </p>
<p> Example: </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#smtpd_enforce_tls">smtpd_enforce_tls</a> = yes
</pre>
</blockquote>
<p> TLS is sometimes used in the non-standard "wrapper" mode where
a server always uses TLS, instead of announcing STARTTLS support
and waiting for clients to request TLS service. Some clients, namely
Outlook [Express] prefer the "wrapper" mode. This is true for OE
(Win32 < 5.0 and Win32 >=5.0 when run on a port<>25
and OE (5.01 Mac on all ports). </p>
<p> It is strictly discouraged to use this mode from <a href="postconf.5.html">main.cf</a>. If
you want to support this service, enable a special port in <a href="master.5.html">master.cf</a>
and specify "-o <a href="postconf.5.html#smtpd_tls_wrappermode">smtpd_tls_wrappermode</a> = yes" as an <a href="smtpd.8.html">smtpd(8)</a> command
line option. Port 465 (smtps) was once chosen for this feature.
</p>
<p> Example: </p>
<blockquote>
<pre>
/etc/postfix/<a href="master.5.html">master.cf</a>:
smtps inet n - n - - smtpd
-o <a href="postconf.5.html#smtpd_tls_wrappermode">smtpd_tls_wrappermode</a>=yes -o <a href="postconf.5.html#smtpd_sasl_auth_enable">smtpd_sasl_auth_enable</a>=yes
</pre>
</blockquote>
<h3><a name="server_vrfy_client">Client certificate verification</a> </h3>
<p> To receive a remote SMTP client certificate, the Postfix SMTP
server must explicitly ask for one (any contents of $<a href="postconf.5.html#smtpd_tls_CAfile">smtpd_tls_CAfile</a>
are also sent to the client as a hint for choosing a certificate
from a suitable CA). Unfortunately, Netscape clients will either
complain if no matching client certificate is available or will
offer the user client a list of certificates to choose from.
Additionally some MTAs (notably some versions of qmail) are unable
to complete TLS negotiation when client certificates are requested,
and abort the SMTP session. So this option is "off" by default.
You will however need the certificate if you want to use certificate
based relaying with, for example, the <a href="postconf.5.html#permit_tls_clientcerts">permit_tls_clientcerts</a>
feature. </p>
<p> Example: </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#smtpd_tls_ask_ccert">smtpd_tls_ask_ccert</a> = no
</pre>
</blockquote>
<p> You may also decide to REQUIRE a remote SMTP client certificate
before allowing TLS connections. This feature is included for
completeness, and implies "<a href="postconf.5.html#smtpd_tls_ask_ccert">smtpd_tls_ask_ccert</a> = yes". </p>
<p> Please be aware, that this will inhibit TLS connections without
a proper client certificate and that it makes sense only when
non-TLS submission is disabled (<a href="postconf.5.html#smtpd_enforce_tls">smtpd_enforce_tls</a> = yes). Otherwise,
clients could bypass the restriction by simply not using STARTTLS
at all. </p>
<p> When TLS is not enforced, the connection will be handled as
if only "<a href="postconf.5.html#smtpd_tls_ask_ccert">smtpd_tls_ask_ccert</a> = yes" is specified, and a warning is
logged. </p>
<p> Example: </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#smtpd_tls_req_ccert">smtpd_tls_req_ccert</a> = no
</pre>
</blockquote>
<p> A client certificate verification depth of 1 is sufficient if
the certificate is directly issued by a CA listed in the CA file.
The default value (5) should also suffice for longer chains (root
CA issues special CA which then issues the actual certificate...)
</p>
<p> Example: </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#smtpd_tls_ccert_verifydepth">smtpd_tls_ccert_verifydepth</a> = 5
</pre>
</blockquote>
<h3><a name="server_tls_auth">Supporting AUTH over TLS only</a></h3>
<p> Sending AUTH data over an unencrypted channel poses a security
risk. When TLS layer encryption is required (<a href="postconf.5.html#smtpd_enforce_tls">smtpd_enforce_tls</a> =
yes), the Postfix SMTP server will announce and accept AUTH only
after the TLS layer has been activated with STARTTLS. When TLS
layer encryption is optional (<a href="postconf.5.html#smtpd_enforce_tls">smtpd_enforce_tls</a> = no), it may
however still be useful to only offer AUTH when TLS is active. To
maintain compatibility with non-TLS clients, the default is to
accept AUTH without encryption. In order to change this behavior,
set "<a href="postconf.5.html#smtpd_tls_auth_only">smtpd_tls_auth_only</a> = yes". </p>
<p> Example: </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#smtpd_tls_auth_only">smtpd_tls_auth_only</a> = no
</pre>
</blockquote>
<h3><a name="server_tls_cache">Server-side TLS session cache</a> </h3>
<p> The Postfix SMTP server and the remote SMTP client negotiate
a session, which takes some computer time and network bandwidth.
By default, this session information is cached only in the <a href="smtpd.8.html">smtpd(8)</a>
process actually using this session and is lost when the process
terminates. To share the session information between multiple
<a href="smtpd.8.html">smtpd(8)</a> processes, a persistent session cache can be used. You
can specify any database type that can store objects of several
kbytes and that supports the sequence operator. DBM databases are
not suitable because they can only store small objects. The cache
is maintained by the <a href="tlsmgr.8.html">tlsmgr(8)</a> process, so there is no problem with
concurrent access. Session caching is highly recommended, because
the cost of repeatedly negotiating TLS session keys is high.</p>
<p> Example: </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#smtpd_tls_session_cache_database">smtpd_tls_session_cache_database</a> = <a href="DATABASE_README.html#types">btree</a>:/etc/postfix/smtpd_scache
</pre>
</blockquote>
<p> As of version 2.5, Postfix will no longer maintain this file
in a directory with non-Postfix ownership. As a migration aid,
attempts to open such files are redirected to the Postfix-owned
$<a href="postconf.5.html#data_directory">data_directory</a>, and a warning is logged. </p>
<p> Cached Postfix SMTP server session information expires after
a certain amount of time. Postfix/TLS does not use the OpenSSL
default of 300s, but a longer time of 3600sec (=1 hour). <a href="https://tools.ietf.org/html/rfc2246">RFC 2246</a>
recommends a maximum of 24 hours. </p>
<p> Example: </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#smtpd_tls_session_cache_timeout">smtpd_tls_session_cache_timeout</a> = 3600s
</pre>
</blockquote>
<h3><a name="server_access">Server access control</a> </h3>
<p> Postfix TLS support introduces three additional features for
Postfix SMTP server access control: </p>
<blockquote>
<dl>
<dt> <a href="postconf.5.html#permit_tls_clientcerts">permit_tls_clientcerts</a> </dt> <dd> <p> Allow the remote SMTP
client SMTP request if the client certificate passes verification,
and if its fingerprint is listed in the list of client certificates
(see <a href="postconf.5.html#relay_clientcerts">relay_clientcerts</a> discussion below). </p> </dd>
<dt> <a href="postconf.5.html#permit_tls_all_clientcerts">permit_tls_all_clientcerts</a> </dt> <dd> <p> Allow the remote
client SMTP request if the client certificate passes verification.
</p> </dd>
<dt> <a href="postconf.5.html#check_ccert_access">check_ccert_access</a> <a href="DATABASE_README.html">type:table</a></dt> <dd>
<p> If the client certificate passes verification, use its fingerprint
as a key for the specified <a href="access.5.html">access(5)</a> table. </p> </dd>
</dl>
</blockquote>
<p> The <a href="postconf.5.html#permit_tls_all_clientcerts">permit_tls_all_clientcerts</a> feature must be used with caution,
because it can result in too many access permissions. Use this
feature only if a special CA issues the client certificates, and
only if this CA is listed as a trusted CA. If other CAs are trusted,
any owner of a valid client certificate would be authorized.
The <a href="postconf.5.html#permit_tls_all_clientcerts">permit_tls_all_clientcerts</a> feature can be practical for a
specially created email relay server. </p>
<p> It is however recommended to stay with the <a href="postconf.5.html#permit_tls_clientcerts">permit_tls_clientcerts</a>
feature and list all certificates via $<a href="postconf.5.html#relay_clientcerts">relay_clientcerts</a>, as
<a href="postconf.5.html#permit_tls_all_clientcerts">permit_tls_all_clientcerts</a> does not permit any control when a
certificate must no longer be used (e.g. an employee leaving). </p>
<p> Example: </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#smtpd_recipient_restrictions">smtpd_recipient_restrictions</a> =
...
<a href="postconf.5.html#permit_tls_clientcerts">permit_tls_clientcerts</a>
<a href="postconf.5.html#reject_unauth_destination">reject_unauth_destination</a>
...
</pre>
</blockquote>
<p> The Postfix list manipulation routines give special treatment
to whitespace and some other characters, making the use of certificate
names impractical. Instead we use the certificate fingerprints as
they are difficult to fake but easy to use for lookup. 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>
<p> Example: </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#relay_clientcerts">relay_clientcerts</a> = <a href="DATABASE_README.html#types">hash</a>:/etc/postfix/relay_clientcerts
/etc/postfix/relay_clientcerts:
D7:04:2F:A7:0B:8C:A5:21:FA:31:77:E1:41:8A:EE:80 lutzpc.at.home
</pre>
</blockquote>
<h3><a name="server_cipher">Server-side cipher controls</a> </h3>
<p> To influence the Postfix SMTP server cipher selection scheme,
you can give cipherlist string. A detailed description would go
too far here; please refer to the OpenSSL documentation. If you
don't know what to do with it, simply don't touch it and leave the
(openssl-)compiled in default! </p>
<p> DO NOT USE " to enclose the string, specify just the string!!! </p>
<p> Example: </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#smtpd_tls_cipherlist">smtpd_tls_cipherlist</a> = DEFAULT
</pre>
</blockquote>
<p> If you want to take advantage of ciphers with EDH, DH parameters
are needed. Instead of using the built-in DH parameters for both
1024bit and 512bit, it is better to generate "own" parameters,
since otherwise it would "pay" for a possible attacker to start a
brute force attack against parameters that are used by everybody.
For this reason, the parameters chosen are already different from
those distributed with other TLS packages. </p>
<p> To generate your own set of DH parameters, use: </p>
<blockquote>
<pre>
% <b>openssl gendh -out /etc/postfix/dh_1024.pem -2 -rand /var/run/egd-pool 1024</b>
% <b>openssl gendh -out /etc/postfix/dh_512.pem -2 -rand /var/run/egd-pool 512</b>
</pre>
</blockquote>
<p> Examples: </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#smtpd_tls_dh1024_param_file">smtpd_tls_dh1024_param_file</a> = /etc/postfix/dh_1024.pem
<a href="postconf.5.html#smtpd_tls_dh512_param_file">smtpd_tls_dh512_param_file</a> = /etc/postfix/dh_512.pem
</pre>
</blockquote>
<h3><a name="server_misc"> Miscellaneous server controls</a> </h3>
<p> The <a href="postconf.5.html#smtpd_starttls_timeout">smtpd_starttls_timeout</a> parameter limits the time of Postfix
SMTP server write and read operations during TLS startup and shutdown
handshake procedures. </p>
<p> Example: </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#smtpd_starttls_timeout">smtpd_starttls_timeout</a> = 300s
</pre>
</blockquote>
<h2> <a name="client_tls">SMTP Client specific settings</a> </h2>
<p> Topics covered in this section: </p>
<ul>
<li><a href="#client_cert_key">Client-side certificate and private
key configuration </a>
<li><a href="#client_logging"> Client-side TLS activity logging
</a>
<li><a href="#client_tls_cache">Client-side TLS session cache</a>
<li><a href="#client_tls_enable"> Enabling TLS in the Postfix SMTP client </a>
<li><a href="#client_tls_require"> Requiring TLS encryption </a>
<li><a href="#client_tls_nopeer"> Disabling server certificate verification </a>
<li><a href="#client_tls_per_site"> Per-site TLS policies </a>
<!--
<li><a href="#client_tls_obs"> Obsolete per-site TLS policy support </a>
-->
<li><a href="#client_tls_harden"> Closing a DNS loophole with <!-- legacy --> per-site TLS policies </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_misc"> Miscellaneous client controls </a>
</ul>
<h3><a name="client_cert_key">Client-side certificate and private
key configuration </a> </h3>
<p> During TLS startup negotiation the Postfix SMTP client may present
a certificate to the remote SMTP server. The Netscape client is
rather clever here and lets the user select between only those
certificates that match CA certificates offered by the remote SMTP
server. As the Postfix SMTP client uses the "SSL_connect()" function
from the OpenSSL package, this is not possible and we have to choose
just one certificate. So for now the default is to use _no_
certificate and key unless one is explicitly specified here. </p>
<p> Both RSA and DSA certificates are supported. You can have both
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> In order for remote SMTP servers to verify the Postfix SMTP
client certificates, the CA certificate (in case of a certificate
chain, all CA certificates) must be available. You should add
these certificates to the client certificate, the client certificate
first, then the issuing CA(s). </p>
<p> Example: the certificate for "client.example.com" was issued by
"intermediate CA" which itself has a certificate of "root CA".
Create the client.pem file with: </p>
<blockquote>
<pre>
% <b>cat client_cert.pem intermediate_CA.pem > client.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 "client.pem" file reduces
the overhead of the TLS exchange. </p>
<p> If you want the Postfix SMTP client to accept remote SMTP server
certificates issued by these CAs, append the root certificate to
$<a href="postconf.5.html#smtp_tls_CAfile">smtp_tls_CAfile</a> or install it in the $<a href="postconf.5.html#smtp_tls_CApath">smtp_tls_CApath</a> directory. When
you configure trust in a root CA, it is not necessary to explicitly trust
intermediary CAs signed by the root CA, unless $<a href="postconf.5.html#smtp_tls_scert_verifydepth">smtp_tls_scert_verifydepth</a>
is less than the number of CAs in the certificate chain for the servers
of interest. With a verify depth of 1 you can only verify certificates
directly signed by a trusted CA, and all trusted intermediary CAs need to
be configured explicitly. With a verify depth of 2 you can verify servers
signed by a root CA or a direct intermediary CA (so long as the server
is correctly configured to supply its intermediate CA certificate). </p>
<p> RSA key and certificate examples: </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#smtp_tls_cert_file">smtp_tls_cert_file</a> = /etc/postfix/client.pem
<a href="postconf.5.html#smtp_tls_key_file">smtp_tls_key_file</a> = $<a href="postconf.5.html#smtp_tls_cert_file">smtp_tls_cert_file</a>
</pre>
</blockquote>
<p> Their DSA counterparts: </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#smtp_tls_dcert_file">smtp_tls_dcert_file</a> = /etc/postfix/client-dsa.pem
<a href="postconf.5.html#smtp_tls_dkey_file">smtp_tls_dkey_file</a> = $<a href="postconf.5.html#smtp_tls_dcert_file">smtp_tls_dcert_file</a>
</pre>
</blockquote>
<p> To verify a remote SMTP server certificate, the Postfix SMTP
client needs to trust the certificates of the issuing Certification
Authorities. These certificates in "pem" format can be stored in a
single $<a href="postconf.5.html#smtp_tls_CAfile">smtp_tls_CAfile</a> or in multiple files, one CA per file in
the $<a href="postconf.5.html#smtp_tls_CApath">smtp_tls_CApath</a> directory. If you use a directory, don't forget
to create the necessary "hash" links with: </p>
<blockquote>
<pre>
# <b>$OPENSSL_HOME/bin/c_rehash <i>/path/to/directory</i> </b>
</pre>
</blockquote>
<p> The $<a href="postconf.5.html#smtp_tls_CAfile">smtp_tls_CAfile</a> contains the CA certificates of one or more
trusted CAs. The file is opened (with root privileges) before Postfix
enters the optional chroot jail and so need not be accessible from inside the
chroot jail. </p>
<p> Additional trusted CAs can be specified via the $<a href="postconf.5.html#smtp_tls_CApath">smtp_tls_CApath</a>
directory, in which case the certificates are read (with $<a href="postconf.5.html#mail_owner">mail_owner</a>
privileges) from the files in the directory when the information
is needed. Thus, the $<a href="postconf.5.html#smtp_tls_CApath">smtp_tls_CApath</a> directory needs to be accessible
inside the optional chroot jail. </p>
<p> The choice between $<a href="postconf.5.html#smtp_tls_CAfile">smtp_tls_CAfile</a> and $<a href="postconf.5.html#smtp_tls_CApath">smtp_tls_CApath</a> is
a space/time tradeoff. If there are many trusted CAs, the cost of
preloading them all into memory may not pay off in reduced access time
when the certificate is needed. </p>
<p> Example: </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#smtp_tls_CAfile">smtp_tls_CAfile</a> = /etc/postfix/CAcert.pem
<a href="postconf.5.html#smtp_tls_CApath">smtp_tls_CApath</a> = /etc/postfix/certs
</pre>
</blockquote>
<h3><a name="client_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>
<tr> <td> 0 </td> <td> Disable logging of TLS activity.</td> </tr>
<tr> <td> 1 </td> <td> Log TLS handshake and certificate information.
</td> </tr>
<tr> <td> 2 </td> <td> Log levels during TLS negotiation. </td>
</tr>
<tr> <td> 3 </td> <td> Log hexadecimal and ASCII dump of TLS
negotiation process </td> </tr>
<tr> <td> 4 </td> <td> Log hexadecimal and ASCII dump of complete
transmission after STARTTLS </td> </tr>
</table>
</blockquote>
<p> Example: </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#smtp_tls_loglevel">smtp_tls_loglevel</a> = 0
</pre>
</blockquote>
<h3><a name="client_tls_cache">Client-side TLS session cache</a> </h3>
<p> The remote SMTP server and the Postfix SMTP client negotiate a
session, which takes some computer time and network bandwidth. By
default, this session information is cached only in the <a href="smtp.8.html">smtp(8)</a>
process actually using this session and is lost when the process
terminates. To share the session information between multiple
<a href="smtp.8.html">smtp(8)</a> processes, a persistent session cache can be used. You
can specify any database type that can store objects of several
kbytes and that supports the sequence operator. DBM databases are
not suitable because they can only store small objects. The cache
is maintained by the <a href="tlsmgr.8.html">tlsmgr(8)</a> process, so there is no problem with
concurrent access. Session caching is highly recommended, because
the cost of repeatedly negotiating TLS session keys is high. Future
Postfix SMTP servers may limit the number of sessions that a client
is allowed to negotiate per unit time.</p>
<p> Example: </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#smtp_tls_session_cache_database">smtp_tls_session_cache_database</a> = <a href="DATABASE_README.html#types">btree</a>:/etc/postfix/smtp_scache
</pre>
</blockquote>
<p> As of version 2.5, Postfix will no longer maintain this file
in a directory with non-Postfix ownership. As a migration aid,
attempts to open such files are redirected to the Postfix-owned
$<a href="postconf.5.html#data_directory">data_directory</a>, and a warning is logged. </p>
<p> Cached Postfix SMTP client session information expires after
a certain amount of time. Postfix/TLS does not use the OpenSSL
default of 300s, but a longer time of 3600s (=1 hour). <a href="https://tools.ietf.org/html/rfc2246">RFC 2246</a>
recommends a maximum of 24 hours. </p>
<p> Example: </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#smtp_tls_session_cache_timeout">smtp_tls_session_cache_timeout</a> = 3600s
</pre>
</blockquote>
<h3><a name="client_tls_enable"> Enabling TLS in the Postfix SMTP
client </a> </h3>
<p> By default, TLS is disabled in the Postfix SMTP client, so no
difference to plain Postfix is visible. If you enable TLS, the
Postfix SMTP client will send STARTTLS when TLS support is announced
by the remote SMTP server. </p>
<p> When the server accepts the STARTTLS command, but the subsequent
TLS handshake fails, and no other server is available, the Postfix SMTP
client defers the delivery attempt, and the mail stays in the queue. After
a handshake failure, the communications channel is in an indeterminate
state and cannot be used for non-TLS deliveries. </p>
<p> Example: </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#smtp_use_tls">smtp_use_tls</a> = yes
</pre>
</blockquote>
<h3><a name="client_tls_require"> Requiring TLS encryption </a>
</h3>
<p> You can ENFORCE the use of TLS, so that the Postfix SMTP client
will not deliver mail over unencrypted connections. In this mode,
the remote SMTP server hostname must match the information in the
remote server certificate, and the server certificate must be issued
by a CA that is trusted by the Postfix SMTP client. If the remote
server certificate doesn't verify or the remote SMTP server hostname
doesn't match, and no other server is available, the delivery
attempt is deferred and the mail stays in the queue. </p>
<p> The remote SMTP server hostname is verified against all names
provided as dNSNames
in the SubjectAlternativeName. If no dNSNames are specified, the
CommonName is checked. Verification may be turned off with the
<a href="postconf.5.html#smtp_tls_enforce_peername">smtp_tls_enforce_peername</a> option which is discussed below. </p>
<p> Enforcing the use of TLS is useful if you know that you will
only
connect to servers that support <a href="https://tools.ietf.org/html/rfc2487">RFC 2487</a> _and_ that present server
certificates that meet the above requirements. An example would
be a client only sends email to one specific mailhub that offers
the necessary STARTTLS support. </p>
<p> Example: </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#smtp_enforce_tls">smtp_enforce_tls</a> = yes
</pre>
</blockquote>
<h3> <a name="client_tls_nopeer"> Disabling server certificate
verification </a> </h3>
<p> As of <a href="https://tools.ietf.org/html/rfc2487">RFC 2487</a> the requirements for hostname checking for MTA
clients are not set. When TLS is required (<a href="postconf.5.html#smtp_enforce_tls">smtp_enforce_tls</a> = yes),
the option <a href="postconf.5.html#smtp_tls_enforce_peername">smtp_tls_enforce_peername</a> can be set to "no" to disable
strict remote SMTP server hostname checking. In this case, the mail
delivery will proceed regardless of the CommonName etc. listed in
the certificate. </p>
<p> Despite the potential for eliminating "man-in-the-middle" and
other attacks, mandatory certificate/peername verification is not
viable as a default Internet mail delivery policy at this time. A
significant fraction of TLS enabled MTAs uses self-signed certificates,
or certificates that are signed by a private Certification Authority.
On a machine that delivers mail to the Internet, if you set
<a href="postconf.5.html#smtp_enforce_tls">smtp_enforce_tls</a> = yes, you should probably also set
<a href="postconf.5.html#smtp_tls_enforce_peername">smtp_tls_enforce_peername</a> = no. You can use the per-site TLS
policies (see below) to enable full peer verification for specific
destinations that are known to have verifiable TLS server certificates.
</p>
<p> Example: </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#smtp_enforce_tls">smtp_enforce_tls</a> = yes
<a href="postconf.5.html#smtp_tls_enforce_peername">smtp_tls_enforce_peername</a> = no
</pre>
</blockquote>
<h3> <a name="client_tls_per_site"> Per-site TLS policies </a> </h3>
<p> A small fraction of servers offer STARTTLS but the negotiation
consistently fails, leading to mail aging out of the queue and
bouncing back to the sender. In such cases, you can use the per-site
policies to disable TLS for the problem sites. Alternatively, you
can enable TLS for just a few specific sites and not enable it for
all sites. </p>
<!-- insert new-style TLS policy mechanism here
<h3> <a name="client_tls_obs"> Obsolete per-site TLS policy support
</a> </h3>
<p> This section describes an obsolete per-site TLS policy mechanism.
Unlike the newer mechanism it supports TLS policy lookup by server
hostname, and lacks control over what names can appear in server
certificates. Because of this, the obsolete mechanism is vulnerable
to false DNS hostname information in MX or CNAME records. These
attacks can be eliminated only with great difficulty. </p>
-->
<p> The <a href="postconf.5.html#smtp_tls_per_site">smtp_tls_per_site</a> table is searched for a policy that matches
the following information: </p>
<blockquote>
<dl>
<dt> remote SMTP server hostname </dt> <dd> This is simply the DNS
name of the server that the Postfix SMTP client connects to; this
name may be obtained from other DNS lookups, such as MX lookups or
CNAME lookups. </dd>
<dt> next-hop destination </dt> <dd> This is normally the domain
portion of the recipient address, but it may be overruled by
information from the <a href="transport.5.html">transport(5)</a> table, from the <a href="postconf.5.html#relayhost">relayhost</a> parameter
setting, or from the <a href="postconf.5.html#relay_transport">relay_transport</a> setting. When it's not the
recipient domain, the next-hop destination can have the Postfix-specific
form "<tt>[name]</tt>", <tt>[name]:port</tt>", "<tt>name</tt>" or
"<tt>name:port</tt>". </dd>
</dl>
</blockquote>
<p> When both the hostname lookup and the next-hop lookup succeed,
the host policy does not automatically override the next-hop policy.
Instead, precedence is given to either the more specific or the
more secure per-site policy as described below. </p>
<p> The <a href="postconf.5.html#smtp_tls_per_site">smtp_tls_per_site</a> table uses a simple "<i>name whitespace
value</i>" format. Specify host names or next-hop destinations on
the left-hand side; no wildcards are allowed. On the right hand
side specify one of the following keywords: </p>
<blockquote>
<dl>
<dt> NONE </dt> <dd> Don't use TLS at all. This overrides a less
specific <b>MAY</b> lookup result from the alternate host or next-hop
lookup key, and overrides the global <a href="postconf.5.html#smtp_use_tls">smtp_use_tls</a>, <a href="postconf.5.html#smtp_enforce_tls">smtp_enforce_tls</a>,
and <a href="postconf.5.html#smtp_tls_enforce_peername">smtp_tls_enforce_peername</a> settings. </dd>
<dt> MAY </dt> <dd> Try to use TLS if the server announces support,
otherwise use the unencrypted connection. This has less precedence
than a more specific result (including <b>NONE</b>) from the alternate
host or next-hop lookup key, and has less precedence than the more
specific global "<a href="postconf.5.html#smtp_enforce_tls">smtp_enforce_tls</a> = yes" or "<a href="postconf.5.html#smtp_tls_enforce_peername">smtp_tls_enforce_peername</a>
= yes". </dd>
<dt> MUST_NOPEERMATCH </dt> <dd> Require TLS encryption, but do not
require that the remote SMTP server hostname matches the information
in the remote SMTP server certificate, or that the server certificate
was issued by a trusted CA. This overrides a less secure <b>NONE</b>
or a less specific <b>MAY</b> lookup result from the alternate host
or next-hop lookup key, and overrides the global <a href="postconf.5.html#smtp_use_tls">smtp_use_tls</a>,
<a href="postconf.5.html#smtp_enforce_tls">smtp_enforce_tls</a> and <a href="postconf.5.html#smtp_tls_enforce_peername">smtp_tls_enforce_peername</a> settings. </dd>
<dt> MUST </dt> <dd> Require TLS encryption, require that the remote
SMTP server hostname matches the information in the remote SMTP
server certificate, and require that the remote SMTP server certificate
was issued by a trusted CA. This overrides a less secure <b>NONE</b>
and <b>MUST_NOPEERMATCH</b> or a less specific <b>MAY</b> lookup
result from the alternate host or next-hop lookup key, and overrides
the global <a href="postconf.5.html#smtp_use_tls">smtp_use_tls</a>, <a href="postconf.5.html#smtp_enforce_tls">smtp_enforce_tls</a> and <a href="postconf.5.html#smtp_tls_enforce_peername">smtp_tls_enforce_peername</a>
settings. </dd>
</dl>
</blockquote>
<p> The precedences between global (<a href="postconf.5.html">main.cf</a>) and per-site TLS
policies can be summarized as follows: </p>
<ul>
<li> <p> When neither the remote SMTP server hostname nor the
next-hop destination are found in the <a href="postconf.5.html#smtp_tls_per_site">smtp_tls_per_site</a> table, the
policy is based on <a href="postconf.5.html#smtp_use_tls">smtp_use_tls</a>, <a href="postconf.5.html#smtp_enforce_tls">smtp_enforce_tls</a> and
<a href="postconf.5.html#smtp_tls_enforce_peername">smtp_tls_enforce_peername</a>. Note: "<a href="postconf.5.html#smtp_enforce_tls">smtp_enforce_tls</a> = yes" and
"<a href="postconf.5.html#smtp_tls_enforce_peername">smtp_tls_enforce_peername</a> = yes" imply "<a href="postconf.5.html#smtp_use_tls">smtp_use_tls</a> = yes". </p>
<li> <p> When both hostname and next-hop destination lookups produce
a result, the more specific per-site policy (NONE, MUST, etc.)
overrides the less specific one (MAY), and the more secure per-site
policy (MUST, etc.) overrides the less secure one (NONE). </p>
<li> <p> After the per-site policy lookups are combined, the result
generally overrides the global policy. The exception is the less
specific <b>MAY</b> per-site policy, which is overruled by the more
specific global "<a href="postconf.5.html#smtp_enforce_tls">smtp_enforce_tls</a> = yes" with server certificate
verification as specified with the <a href="postconf.5.html#smtp_tls_enforce_peername">smtp_tls_enforce_peername</a>
parameter. </p>
</ul>
<h3> <a name="client_tls_harden"> Closing a DNS loophole with
<!-- legacy --> per-site TLS policies </a> </h3>
<p> As long as no secure DNS lookup mechanism is available, false
hostnames in MX or CNAME responses can change the server hostname
that Postfix uses for TLS policy lookup and server certificate
verification. Even with a perfect match between the server hostname
and the server certificate, there is no guarantee that Postfix is
connected to the right server. To avoid this loophole take the
following steps: </p>
<ul>
<li> <p> Eliminate MX lookups. Specify local <a href="transport.5.html">transport(5)</a> table
entries for sensitive domains with explicit <a href="smtp.8.html">smtp</a>:[<i>mailhost</i>]
or <a href="smtp.8.html">smtp</a>:[<i>mailhost</i>]:<i>port</i> destinations (you can assure
security of this table unlike DNS); in the <a href="postconf.5.html#smtp_tls_per_site">smtp_tls_per_site</a> table
specify the value <b>MUST</b> for the key [<i>mailhost</i>] or
<a href="smtp.8.html">smtp</a>:[<i>mailhost</i>]:<i>port</i>. This prevents false hostname
information in DNS MX records from changing the server hostname
that Postfix uses for TLS policy lookup and server certificate
verification. </p>
<li> <p> Disallow CNAME hostname overrides. In <a href="postconf.5.html">main.cf</a> specify
"<a href="postconf.5.html#smtp_cname_overrides_servername">smtp_cname_overrides_servername</a> = no". This prevents false hostname
information in DNS CNAME records from changing the server hostname
that Postfix uses for TLS policy lookup and server certificate
verification. This feature requires Postfix 2.2.9 or later. </p>
</ul>
<p> Example: </p>
<blockquote> <pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#smtp_tls_per_site">smtp_tls_per_site</a> = <a href="DATABASE_README.html#types">hash</a>:/etc/postfix/tls_per_site
<a href="postconf.5.html#relayhost">relayhost</a> = [msa.example.net]:587
/etc/postfix/tls_per_site:
# <a href="postconf.5.html#relayhost">relayhost</a> exact nexthop match
[msa.example.net]:587 MUST
# TLS should not be used with the <i>example.org</i> MX hosts.
example.org NONE
# TLS should not be used with the host <i>smtp.example.com</i>.
[smtp.example.com] NONE
</pre>
</blockquote>
<h3> <a name="client_tls_discover"> Discovering servers that support
TLS </a> </h3>
<p> As we decide on a "per site" basis whether or not to use TLS,
it would be good to have a list of sites that offered "STARTTLS".
We can collect it ourselves with this option. </p>
<p> If the <a href="postconf.5.html#smtp_tls_note_starttls_offer">smtp_tls_note_starttls_offer</a> feature is enabled and a
server offers STARTTLS while TLS is not already enabled for that
server, the Postfix SMTP client logs a line as follows: </p>
<blockquote>
<pre>
postfix/smtp[pid]: Host offered STARTTLS: [hostname.example.com]
</pre>
</blockquote>
<p> Example: </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#smtp_tls_note_starttls_offer">smtp_tls_note_starttls_offer</a> = yes
</pre>
</blockquote>
<h3><a name="client_vrfy_server">Server certificate verification depth</a> </h3>
<p> When verifying a remote SMTP server certificate, a verification
depth of 1 is sufficient if the certificate is directly issued by
a CA specified with <a href="postconf.5.html#smtp_tls_CAfile">smtp_tls_CAfile</a> or <a href="postconf.5.html#smtp_tls_CApath">smtp_tls_CApath</a>. The default
value of 5 should also suffice for longer chains (root CA issues
special CA which then issues the actual certificate...) </p>
<p> Example: </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#smtp_tls_scert_verifydepth">smtp_tls_scert_verifydepth</a> = 5
</pre>
</blockquote>
<h3> <a name="client_cipher">Client-side cipher controls </a> </h3>
<p> To influence the Postfix SMTP client cipher selection scheme,
you can give cipherlist string. A detailed description would go
too far here; please refer to the OpenSSL documentation. If you
don't know what to do with it, simply don't touch it and leave the
(openssl-)compiled in default! </p>
<p> DO NOT USE " to enclose the string, specify just the string!!! </p>
<p> Example: </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#smtp_tls_cipherlist">smtp_tls_cipherlist</a> = DEFAULT
</pre>
</blockquote>
<h3> <a name="client_misc"> Miscellaneous client controls </a> </h3>
<p> The <a href="postconf.5.html#smtp_starttls_timeout">smtp_starttls_timeout</a> parameter limits the time of Postfix
SMTP client write and read operations during TLS startup and shutdown
handshake procedures. In case of problems the Postfix SMTP client
tries the next network address on the mail exchanger list, and
defers delivery if no alternative server is available. </p>
<p> Example: </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#smtp_starttls_timeout">smtp_starttls_timeout</a> = 300s
</pre>
</blockquote>
<h2><a name="tlsmgr_controls"> TLS manager specific settings </a> </h2>
<p> The security of cryptographic software such as TLS depends
critically on the ability to generate unpredictable numbers for
keys and other information. To this end, the <a href="tlsmgr.8.html">tlsmgr(8)</a> process
maintains a Pseudo Random Number Generator (PRNG) pool. This is
queried by the <a href="smtp.8.html">smtp(8)</a> and <a href="smtpd.8.html">smtpd(8)</a> processes when they initialize.
By default, these daemons request 32 bytes, the equivalent to 256
bits. This is more than sufficient to generate a 128bit (or 168bit)
session key. </p>
<p> Example: </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#tls_daemon_random_bytes">tls_daemon_random_bytes</a> = 32
</pre>
</blockquote>
<p> In order to feed its in-memory PRNG pool, the <a href="tlsmgr.8.html">tlsmgr(8)</a> reads
entropy from an external source, both at startup and during run-time.
Specify a good entropy source, like EGD or /dev/urandom; be sure
to only use non-blocking sources (on OpenBSD, use /dev/arandom
when <a href="tlsmgr.8.html">tlsmgr(8)</a> complains about /dev/urandom timeout errors).
If the entropy source is not a
regular file, you must prepend the source type to the source name:
"dev:" for a device special file, or "egd:" for a source with EGD
compatible socket interface. </p>
<p> Examples (specify only one in <a href="postconf.5.html">main.cf</a>): </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#tls_random_source">tls_random_source</a> = dev:/dev/urandom
<a href="postconf.5.html#tls_random_source">tls_random_source</a> = egd:/var/run/egd-pool
</pre>
</blockquote>
<p> By default, <a href="tlsmgr.8.html">tlsmgr(8)</a> reads 32 bytes from the external entropy
source at each seeding event. This amount (256bits) is more than
sufficient for generating a 128bit symmetric key. With EGD and
device entropy sources, the <a href="tlsmgr.8.html">tlsmgr(8)</a> limits the amount of data
read at each step to 255 bytes. If you specify a regular file as
entropy source, a larger amount of data can be read. </p>
<p> Example: </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#tls_random_bytes">tls_random_bytes</a> = 32
</pre>
</blockquote>
<p> In order to update its in-memory PRNG pool, the <a href="tlsmgr.8.html">tlsmgr(8)</a>
queries the external entropy source again after a pseudo-random
amount of time. The time is calculated using the PRNG, and is
between 0 and the maximal time specified with <a href="postconf.5.html#tls_random_reseed_period">tls_random_reseed_period</a>.
The default maximal time interval is 1 hour. </p>
<p> Example: </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#tls_random_reseed_period">tls_random_reseed_period</a> = 3600s
</pre>
</blockquote>
<p> The <a href="tlsmgr.8.html">tlsmgr(8)</a> process saves the PRNG state to a persistent
exchange file at regular times and when the process terminates, so
that it can recover the PRNG state the next time it starts up.
This file is created when it does not exist. Its default location
is under the Postfix configuration directory, which is not the
proper place for information that is modified by Postfix. Instead,
the file location should probably be on the /var partition (but
<b>not</b> inside the chroot jail). </p>
<p> Examples: </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#tls_random_exchange_name">tls_random_exchange_name</a> = /etc/postfix/prng_exch
<a href="postconf.5.html#tls_random_prng_update_period">tls_random_prng_update_period</a> = 3600s
</pre>
</blockquote>
<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, your Postfix public key certificate
needs to be signed by a recognized Certification Authority, and
Postfix needs to be configured with a list of public key certificates
of Certification Authorities, so that Postfix can verify the public key
certificates of remote hosts. </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> Become your own Certification Authority, so that you can
sign your own public keys. This example uses the CA.pl script that
ships with OpenSSL. By default, OpenSSL installs this as
<tt>/usr/local/ssl/misc/CA.pl</tt>, but your mileage may vary.
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 and create
an unsigned public key certificate. </p>
<blockquote>
<pre>
% <b>openssl req -new -nodes -keyout FOO-key.pem -out FOO-req.pem -days 365</b>
Using configuration from /etc/ssl/openssl.cnf
Generating a 1024 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</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 with the
Certification Authority private key that we created a few
steps ago. </p>
<blockquote>
<pre>
% <b>openssl ca -out FOO-cert.pem -infiles FOO-req.pem</b>
Uing 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'
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>
<blockquote>
<pre>
# <b>cp demoCA/cacert.pem FOO-key.pem FOO-cert.pem /etc/postfix</b>
# <b>chmod 644 /etc/postfix/FOO-cert.pem /etc/postfix/cacert.pem</b>
# <b>chmod 400 /etc/postfix/FOO-key.pem</b>
</pre>
</blockquote>
<li> <p> Configure Postfix, by adding the following to
<tt>/etc/postfix/<a href="postconf.5.html">main.cf</a> </tt>. </p>
<blockquote>
<pre>
<a href="postconf.5.html#smtp_tls_CAfile">smtp_tls_CAfile</a> = /etc/postfix/cacert.pem
<a href="postconf.5.html#smtp_tls_cert_file">smtp_tls_cert_file</a> = /etc/postfix/FOO-cert.pem
<a href="postconf.5.html#smtp_tls_key_file">smtp_tls_key_file</a> = /etc/postfix/FOO-key.pem
<a href="postconf.5.html#smtp_tls_session_cache_database">smtp_tls_session_cache_database</a> = <a href="DATABASE_README.html#types">btree</a>:/var/run/smtp_tls_session_cache
<a href="postconf.5.html#smtp_use_tls">smtp_use_tls</a> = yes
<a href="postconf.5.html#smtpd_tls_CAfile">smtpd_tls_CAfile</a> = /etc/postfix/cacert.pem
<a href="postconf.5.html#smtpd_tls_cert_file">smtpd_tls_cert_file</a> = /etc/postfix/FOO-cert.pem
<a href="postconf.5.html#smtpd_tls_key_file">smtpd_tls_key_file</a> = /etc/postfix/FOO-key.pem
<a href="postconf.5.html#smtpd_tls_received_header">smtpd_tls_received_header</a> = yes
<a href="postconf.5.html#smtpd_tls_session_cache_database">smtpd_tls_session_cache_database</a> = <a href="DATABASE_README.html#types">btree</a>:/var/run/smtpd_tls_session_cache
<a href="postconf.5.html#smtpd_use_tls">smtpd_use_tls</a> = yes
<a href="postconf.5.html#tls_random_source">tls_random_source</a> = dev:/dev/urandom
</pre>
</blockquote>
</ul>
<h2> <a name="problems"> Reporting problems </a> </h2>
<p> When reporting a problem, please be thorough in the report.
Patches, when possible, are greatly appreciated too. </p>
<p> Please differentiate when possible between: </p>
<ul>
<li> Problems in the TLS code: <postfix_tls@aet.tu-cottbus.de>
<li> Problems in vanilla Postfix: <postfix-users@postfix.org>
</ul>
<h2><a name="compat">Compatibility with Postfix < 2.2 TLS support</a></h2>
<p> Postfix version 2.2 TLS support is based on the Postfix/TLS
patch by Lutz Jänicke, but differs in a few minor ways. </p>
<ul>
<li> <p> <a href="postconf.5.html">main.cf</a>: Specify "btree" instead of "sdbm" for TLS
session cache databases. </p>
<p> TLS session cache databases are now accessed only by the
<a href="tlsmgr.8.html">tlsmgr(8)</a> process, so there are no more concurrency issues. Although
Postfix has an sdbm client, the sdbm library (1000
lines of code) is not included with Postfix. </p>
<p> TLS session caches can use any database that can store objects
of several kbytes or more, and that implements the sequence operation.
In most cases, btree databases should be adequate. </p>
<p> NOTE: You cannot use dbm databases. TLS session objects
are too large. </p>
<li> <p> <a href="master.5.html">master.cf</a>: Specify "unix" instead of "fifo" as
the tlsmgr service type. </p>
<p> The <a href="smtp.8.html">smtp(8)</a> and <a href="smtpd.8.html">smtpd(8)</a> processes now use a client-server
protocol in order to access the <a href="tlsmgr.8.html">tlsmgr(8)</a> pseudo-random number
generation (PRNG) pool, and in order to access the TLS session
cache databases. Such a protocol cannot be run across fifos. </p>
<li> <p> <a href="postconf.5.html#smtp_tls_per_site">smtp_tls_per_site</a>: the MUST_NOPEERMATCH per-site policy
cannot override the global "<a href="postconf.5.html#smtp_tls_enforce_peername">smtp_tls_enforce_peername</a> = yes" setting.
</p>
<li> <p> <a href="postconf.5.html#smtp_tls_per_site">smtp_tls_per_site</a>: a combined (NONE + MAY) lookup result
for (hostname and next-hop destination) produces counter-intuitive
results for different <a href="postconf.5.html">main.cf</a> settings. TLS is enabled with
"<a href="postconf.5.html#smtp_tls_enforce_peername">smtp_tls_enforce_peername</a> = no", but it is disabled when both
"<a href="postconf.5.html#smtp_enforce_tls">smtp_enforce_tls</a> = yes" and "<a href="postconf.5.html#smtp_tls_enforce_peername">smtp_tls_enforce_peername</a> = yes".
</p>
</ul>
<p> The <a href="postconf.5.html#smtp_tls_per_site">smtp_tls_per_site</a> limitations were removed by the end of
the Postfix 2.2 support cycle. </p>
<h2><a name="credits">Credits </a> </h2>
<ul>
<li> TLS support for Postfix was originally developed by Lutz
Jänicke at Cottbus Technical University.
<li> Wietse Venema adopted the code, did some restructuring, and
compiled this part of the documentation from Lutz's documents.
<li> Victor Duchovni was instrumental with the re-implementation
of the <a href="postconf.5.html#smtp_tls_per_site">smtp_tls_per_site</a> code in terms of enforcement levels, which
simplified the implementation greatly.
</ul>
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