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<h1><img src="postfix-logo.jpg" width="203" height="98" ALT="">Postfix SASL Howto</h1>
<hr>
<h2><a name="intro">How Postfix uses SASL authentication</a></h2>
<p> SMTP servers need to decide whether an SMTP client is authorized
to send mail to remote destinations, or only to destinations that
the server itself is responsible for. Usually, SMTP servers accept
mail to remote destinations when the client's IP address is in the
"same network" as the server's IP address. </p>
<p> SMTP clients outside the SMTP server's network need a different
way to get "same network" privileges. To address this need, Postfix
supports SASL authentication (RFC 4954, formerly RFC 2554). With
this a remote SMTP client can authenticate to the Postfix SMTP
server, and the Postfix SMTP client can authenticate to a remote
SMTP server. Once a client is authenticated, a server can give it
"same network" privileges. </p>
<p> Postfix does not implement SASL itself, but instead uses existing
implementations as building blocks. This means that some SASL-related
configuration files will belong to Postfix, while other
configuration files belong to the specific SASL
implementation that Postfix will use. This document covers both the
Postfix and non-Postfix configuration. </p>
<p> NOTE: People who go to the trouble of installing Postfix may
have the expectation that Postfix is more secure than some other
mailers. The Cyrus SASL library contains a lot of code. With this,
Postfix becomes as secure as other mail systems that use the Cyrus
SASL library. Dovecot provides an alternative that may be worth
considering. </p>
<p> You can read more about the following topics: </p>
<ul>
<li><a href="#server_sasl">Configuring SASL authentication in the
Postfix SMTP server</a></li>
<li><a href="#client_sasl">Configuring SASL authentication in the Postfix SMTP/LMTP client</a></li>
<li><a href="#postfix_build">Building Postfix with SASL support</a></li>
<li><a href="#cyrus_legacy">Using Cyrus SASL version 1.5.x</a></li>
<li><a href="#credits">Credits</a></li>
</ul>
<h2><a name="server_sasl">Configuring SASL authentication in the
Postfix SMTP server</a></h2>
<p> As mentioned earlier, SASL is implemented separately from
Postfix. For this reason, configuring SASL authentication in the
Postfix SMTP server involves two different steps: </p>
<ul>
<li> <p> Configuring the SASL implementation to offer a list of
mechanisms that are suitable for SASL authentication and, depending
on the SASL implementation used, configuring authentication backends
that verify the remote SMTP client's authentication data against
the system password file or some other database. </p> </li>
<li> <p> Configuring the Postfix SMTP server to enable SASL
authentication, and to authorize clients to relay mail or to control
what envelope sender addresses the client may use. </p> </li>
</ul>
<p> Successful authentication in the Postfix SMTP server requires
a functional SASL framework. Configuring SASL should therefore
always be the first step, before configuring Postfix. </p>
<p> You can read more about the following topics: </p>
<ul>
<li><a href="#server_which">Which SASL Implementations are
supported?</a></li>
<li><a href="#server_dovecot">Configuring Dovecot SASL</a>
<ul>
<li><a href="#server_dovecot_comm">Postfix to Dovecot SASL
communication</a></li>
</ul> </li>
<li><a href="#server_cyrus">Configuring Cyrus SASL</a>
<ul>
<li><a href="#server_cyrus_name">Cyrus SASL configuration file
name</a></li>
<li><a href="#server_cyrus_location">Cyrus SASL configuration
file location</a></li>
<li><a href="#server_cyrus_comm">Postfix to Cyrus SASL
communication</a></li>
</ul> </li>
<li><a href="#server_sasl_enable">Enabling SASL authentication and
authorization in the Postfix SMTP server</a>
<ul>
<li><a href="#server_sasl_authc">Enabling SASL authentication in
the Postfix SMTP server</a></li>
<li><a href="#smtpd_sasl_security_options">Postfix SMTP Server
policy - SASL mechanism properties</a></li>
<li><a href="#server_sasl_authz">Enabling SASL authorization in the
Postfix SMTP server</a></li>
<li><a href="#server_sasl_other">Additional SMTP Server SASL
options</a></li>
</ul></li>
<li><a href="#server_test">Testing SASL authentication in the Postfix
SMTP server</a></li>
</ul>
<h3><a name="server_which">Which SASL Implementations are
supported?</a></h3>
<p> Currently the Postfix SMTP server supports the Cyrus SASL and
Dovecot SASL implementations. </p>
<blockquote>
<strong>Note</strong>
<p> Current Postfix versions have a plug-in architecture that can
support multiple SASL implementations. Before Postfix version 2.3,
Postfix had support only for Cyrus SASL. </p>
</blockquote>
<p> To find out what SASL implementations are compiled into Postfix,
use the following commands: </p>
<blockquote>
<pre>
% <strong><code>postconf -a</code></strong> (SASL support in the SMTP server)
% <strong><code>postconf -A</code></strong> (SASL support in the SMTP+LMTP client)
</pre>
</blockquote>
<p> These commands are available only with Postfix version 2.3 and
later. </p>
<h3><a name="server_dovecot">Configuring Dovecot SASL</a></h3>
<p> Dovecot is a POP/IMAP server that has its own configuration to
authenticate POP/IMAP clients. When the Postfix SMTP server uses
Dovecot SASL, it reuses parts of this configuration. Consult the
<a href="http://wiki.dovecot.org">Dovecot documentation</a> for how
to configure and operate the Dovecot authentication server. </p>
<h4><a name="server_dovecot_comm">Postfix to Dovecot SASL communication</a></h4>
<p> Communication between the Postfix SMTP server and Dovecot SASL
happens over a UNIX-domain socket or over a TCP socket. We will
be using a UNIX-domain socket for better privacy. </p>
<p> The following fragment for Dovecot version 2 assumes that the
Postfix queue is under <code>/var/spool/postfix/</code>. </p>
<blockquote>
<pre>
1 conf.d/10-master.conf:
2 service auth {
3 ...
4 unix_listener /var/spool/postfix/private/auth {
5 mode = 0660
6 # Assuming the default Postfix user and group
7 user = postfix
8 group = postfix
9 }
10 ...
11 }
12
13 conf.d/10-auth.conf
14 auth_mechanisms = plain login
</pre>
</blockquote>
<p> Line 4 places the Dovecot SASL socket in
<code>/var/spool/postfix/private/auth</code>, lines 5-8 limit
read+write permissions to user and group <code>postfix</code> only,
and line 14 provides <code>plain</code> and <code>login</code> as
mechanisms for the Postfix SMTP server. </p>
<p> Proceed with the section "<a href="#server_sasl_enable">Enabling
SASL authentication and authorization in the Postfix SMTP server</a>"
to turn on and use SASL in the Postfix SMTP server. </p>
<h3><a name="server_cyrus">Configuring Cyrus SASL</a></h3>
<p> The Cyrus SASL framework supports a wide variety of applications
(POP, IMAP, SMTP, etc.). Different applications may require different
configurations. As a consequence each application may have its own
configuration file. </p>
<p> The first step configuring Cyrus SASL is to determine name and
location of a configuration file that describes how the Postfix
SMTP server will use the SASL framework. </p>
<h4><a name="server_cyrus_name">Cyrus SASL configuration file name</a></h4>
<p> The name of the configuration file (default: <code>smtpd.conf</code>)
is configurable. It is a concatenation from a value that the Postfix
SMTP server sends to the Cyrus SASL library, and the suffix
<code>.conf</code>, added by Cyrus SASL. </p>
<p> The value sent by Postfix is the name of the server component
that will use Cyrus SASL. It defaults to <code>smtpd</code> and
is configured with one of the following variables: </p>
<blockquote>
<pre>
/etc/postfix/main.cf:
# Postfix 2.3 and later
smtpd_sasl_path = smtpd
# Postfix < 2.3
smtpd_sasl_application_name = smtpd
</pre>
</blockquote>
<h4><a name="server_cyrus_location">Cyrus SASL configuration file
location</a></h4>
<p> The location where Cyrus SASL searches for the named file depends
on the Cyrus SASL version and the OS/distribution used. </p>
<p> You can read more about the following topics: </p>
<ul>
<li> <p> Cyrus SASL version 2.x searches for the configuration file
in <code>/usr/lib/sasl2/</code>. </p> </li>
<li> <p> Cyrus SASL version 2.1.22 and newer additionally search
in <code>/etc/sasl2/</code>. </p> </li>
<li> <p> Some Postfix distributions are modified and look for the
Cyrus SASL configuration file in <code>/etc/postfix/sasl/</code>,
<code>/var/lib/sasl2/</code> etc. See the distribution-specific
documentation to determine the expected location. </p> </li>
</ul>
<blockquote>
<strong>Note</strong>
<p> Cyrus SASL searches <code>/usr/lib/sasl2/</code> first. If it
finds the specified configuration file there, it will not examine
other locations. </p>
</blockquote>
<h4><a name="server_cyrus_comm">Postfix to Cyrus SASL communication</a></h4>
<p> As the Postfix SMTP server is linked with the Cyrus SASL library
<code>libsasl</code>, communication between Postfix and Cyrus SASL
takes place by calling functions in the SASL library. </p>
<p> The SASL library may use an external password verification
service, or an internal plugin to connect to authentication backends
and verify the SMTP client's authentication data against the system
password file or other databases. </p>
<p> The following table shows typical combinations discussed in
this document: </p>
<blockquote>
<table border="1">
<tr>
<th align="center">authentication backend</th>
<th align="center">password verification service / plugin</th>
</tr>
<tr>
<td>/etc/shadow</td>
<td><a href="#saslauthd">saslauthd</a></td>
</tr>
<tr>
<td>PAM</td>
<td><a href="#saslauthd">saslauthd</a></td>
</tr>
<tr>
<td>IMAP server</td>
<td><a href="#saslauthd">saslauthd</a></td>
</tr>
<tr>
<td>sasldb</td>
<td><a href="#auxprop_sasldb">sasldb</a></td>
</tr>
<tr>
<td>MySQL, PostgreSQL, SQLite</td>
<td><a href="#auxprop_sql">sql</a></td>
</tr>
<tr>
<td>LDAP</td>
<td><a href="#auxprop_ldapdb">ldapdb</a></td>
</tr>
</table>
</blockquote>
<blockquote>
<strong>Note</strong>
<p> Read the Cyrus SASL documentation for other backends it can
use. </p>
</blockquote>
<h4><a name="saslauthd">saslauthd - Cyrus SASL password verification service</a></h4>
<p> Communication between the Postfix SMTP server (read: Cyrus SASL's
<code>libsasl</code>) and the <code>saslauthd</code> server takes
place over a UNIX-domain socket. </p>
<p> <code>saslauthd</code> usually establishes the UNIX domain
socket in <code>/var/run/saslauthd/</code> and waits for authentication
requests. The Postfix SMTP server must have read+execute permission
to this directory or authentication attempts will fail. </p>
<blockquote>
<strong>Important</strong>
<p> Some distributions require the user <code>postfix</code> to be
member of a special group e.g. <code>sasl</code>, otherwise it
will not be able to access the <code>saslauthd</code> socket
directory. </p>
</blockquote>
<p> The following example configures the Cyrus SASL library to
contact <code>saslauthd</code> as its password verification service:
</p>
<blockquote>
<pre>
/etc/sasl2/smtpd.conf:
pwcheck_method: saslauthd
mech_list: PLAIN LOGIN
</pre>
</blockquote>
<blockquote>
<strong>Important</strong>
<p> Do not specify any other mechanisms in <code>mech_list</code>
than <code>PLAIN</code> or <code>LOGIN</code> when using
<code>saslauthd</code>! It can only handle these two mechanisms,
and authentication will fail if clients are allowed to choose other
mechanisms. </p>
</blockquote>
<blockquote>
<strong>Important</strong>
<p> Plaintext mechanisms (<code>PLAIN</code>, <code>LOGIN</code>)
send credentials unencrypted. This information should be protected
by an additional security layer such as a TLS-encrypted SMTP session
(see: TLS_README). </p>
</blockquote>
<p> Additionally the <code>saslauthd</code> server itself must be
configured. It must be told which authentication backend to turn
to for password verification. The backend is selected with a
<code>saslauthd</code> command-line option and will be shown in the
following examples. </p>
<blockquote>
<strong>Note</strong>
<p> Some distributions use a configuration file to provide saslauthd
command line options to set e.g. the authentication backend. Typical
locations are <code>/etc/sysconfig/saslauthd</code> or
<code>/etc/default/saslauthd</code>. </p>
</blockquote>
<h4><a name="saslauthd_shadow">Using saslauthd with /etc/shadow</a></h4>
<p> Access to the <code>/etc/shadow</code> system password file
requires <code>root</code> privileges. The Postfix SMTP server
(and in consequence <code>libsasl</code> linked to the server) runs
with the least privilege possible. Direct access to
<code>/etc/shadow</code> would not be possible without breaking the
Postfix security architecture. </p>
<p> The <code>saslauthd</code> socket builds a safe bridge. Postfix,
running as limited user <code>postfix</code>, can access the
UNIX-domain socket that <code>saslauthd</code> receives commands
on; <code>saslauthd</code>, running as privileged user <code>root</code>,
has the privileges required to access the shadow file. </p>
<p> The <code>saslauthd</code> server verifies passwords against the
authentication backend <code>/etc/shadow</code> if started like this: </p>
<blockquote>
<pre>
% <strong><code>saslauthd -a shadow</code></strong>
</pre>
</blockquote>
<p> See section "<a href="#testing_saslauthd">Testing saslauthd
authentication</a>" for test instructions. </p>
<h4><a name="saslauthd_pam">Using saslauthd with PAM</a></h4>
<p> Cyrus SASL can use the PAM framework to authenticate credentials.
<code>saslauthd</code> uses the PAM framework when started like
this: </p>
<blockquote>
<pre>
% <strong><code>saslauthd -a pam</code></strong>
</pre>
</blockquote>
<blockquote>
<strong>Note</strong>
<p> PAM configuration for the Postfix SMTP server is usually given
in <code>/etc/pam.d/smtp</code> and is beyond the scope of this
document. </p>
</blockquote>
<p> See section "<a href="#testing_saslauthd">Testing saslauthd
authentication</a>" for test instructions. </p>
<h4><a name="saslauthd_imap">Using saslauthd with an IMAP server</a></h4>
<p> <code>saslauthd</code> can verify the SMTP client credentials
by using them to log into an IMAP server. If the login succeeds,
SASL authentication also succeeds. <code>saslauthd</code> contacts
an IMAP server when started like this: </p>
<blockquote>
<pre>
% <strong><code>saslauthd -a rimap -O imap.example.com</code></strong>
</pre>
</blockquote>
<blockquote>
<strong>Note</strong>
<p> The option "<code>-O imap.example.com</code>" specifies the
IMAP server <code>saslauthd</code> should contact when it verifies
credentials. </p>
</blockquote>
<blockquote>
<strong>Important</strong>
<p> <code>saslauthd</code> sends IMAP login information unencrypted.
Any IMAP session leaving the local host should be protected by an
additional security layer such as an SSL tunnel. </p>
</blockquote>
<p> See section "<a href="#testing_saslauthd">Testing saslauthd
authentication</a>" for test instructions. </p>
<h4><a name="testing_saslauthd">Testing saslauthd authentication</a></h4>
<p> Cyrus SASL provides the <code>testsaslauthd</code> utility to
test <code>saslauthd</code> authentication. The username and password
are given as command line arguments. The example shows the response
when authentication is successful: </p>
<blockquote>
<pre>
% <strong><code>testsaslauthd -u <em>username</em> -p <em>password</em></code></strong>
0: OK "Success."
</pre>
</blockquote>
<blockquote>
<strong>Note</strong>
<p> Sometimes the <code>testsaslauthd</code> program is not distributed
with a the Cyrus SASL main package. In that case, it may be
distributed with <code>-devel</code>, <code>-dev</code> or
<code>-debug</code> packages. </p>
</blockquote>
<p> Specify an additional "<code>-s smtp</code>" if <code>saslauthd</code>
was configured to contact the PAM authentication framework, and
specify an additional "<code>-f <em>/path/to/socketdir/mux</em></code>"
if <code>saslauthd</code> establishes the UNIX-domain socket in a
non-default location. </p>
<p> If authentication succeeds, proceed with the section "<a
href="#server_sasl_enable">Enabling SASL authentication and authorization
in the Postfix SMTP server</a>". </p>
<h4><a name="auxprop">Cyrus SASL Plugins - auxiliary property
plugins</a></h4>
<p> Cyrus SASL uses a plugin infrastructure (called <code>auxprop</code>)
to expand <code>libsasl</code>'s capabilities. Currently Cyrus
SASL sources provide three authentication plugins. </p>
<blockquote>
<table border="1">
<tr> <th>Plugin </th> <th>Description </th> </tr>
<tr> <td><a href="#auxprop_sasldb">sasldb</a></td> <td> Accounts
are stored stored in a Cyrus SASL Berkeley DB database </td> </tr>
<tr> <td><a href="#auxprop_sql">sql</a></td> <td> Accounts are
stored in a SQL database </td> </tr>
<tr> <td><a href="#auxprop_ldapdb">ldapdb</a></td> <td> Accounts
are stored stored in an LDAP database </td> </tr>
</table>
</blockquote>
<blockquote>
<strong>Important</strong>
<p> These three plugins support shared-secret mechanisms i.e.
CRAM-MD5, DIGEST-MD5 and NTLM. These mechanisms send credentials
encrypted but their verification process requires the password to
be available in plaintext. Consequently passwords cannot (!) be
stored in encrypted form. </p>
</blockquote>
<h4><a name="auxprop_sasldb">The sasldb plugin</a></h4>
<p> The sasldb auxprop plugin authenticates SASL clients against
credentials that are stored in a Berkeley DB database. The database
schema is specific to Cyrus SASL. The database is usually located
at <code>/etc/sasldb2</code>. </p>
<blockquote>
<strong>Note</strong>
<p> The <code>sasldb2</code> file contains passwords in
plaintext, and should have read+write access only to user
<code>postfix</code> or a group that <code>postfix</code> is member
of. </p>
</blockquote>
<p> The <code>saslpasswd2</code> command-line utility creates
and maintains the database: </p>
<blockquote>
<pre>
% <strong>saslpasswd2 -c -u <em>example.com</em> <em>username</em></strong>
Password:
Again (for verification):
</pre>
</blockquote>
<p> This command creates an account
<code><em>username@example.com</em></code>. </p>
<blockquote>
<strong>Important</strong>
<p> users must specify <code><em>username@example.com</em></code>
as login name, not <code><em>username</em></code>. </p>
</blockquote>
<p> Run the following command to reuse the Postfix <code>mydomain</code>
parameter value as the login domain: </p>
<blockquote>
<pre>
% <strong>saslpasswd2 -c -u `postconf -h mydomain` <em>username</em></strong>
Password:
Again (for verification):
</pre>
</blockquote>
<blockquote>
<strong>Note</strong>
<p> Run <code>saslpasswd2</code> without any options for further
help on how to use the command. </p>
</blockquote>
<p> The <code>sasldblistusers2</code> command lists all existing
users in the sasldb database: </p>
<blockquote>
<pre>
% <strong>sasldblistusers2</strong>
username1@example.com: password1
username2@example.com: password2
</pre>
</blockquote>
<p> Configure libsasl to use sasldb with the following instructions: </p>
<blockquote>
<pre>
/etc/sasl2/smtpd.conf:
pwcheck_method: auxprop
auxprop_plugin: sasldb
mech_list: PLAIN LOGIN CRAM-MD5 DIGEST-MD5 NTLM
</pre>
</blockquote>
<blockquote>
<strong>Note</strong>
<p> In the above example adjust <code>mech_list</code> to the
mechanisms that are applicable for your environment. </p>
</blockquote>
<h4><a name="auxprop_sql">The sql plugin</a></h4>
<p> The sql auxprop plugin is a generic SQL plugin. It provides
access to credentials stored in a MySQL, PostgreSQL or SQLite
database. This plugin requires that SASL client passwords are
stored as plaintext. </p>
<blockquote>
<strong>Tip</strong>
<p> If you must store encrypted passwords, you cannot use the sql
auxprop plugin. Instead, see section "<a href="#saslauthd_pam">Using
saslauthd with PAM</a>", and configure PAM to look up the encrypted
passwords with, for example, the <code>pam_mysql</code> module.
You will not be able to use any of the methods that require access
to plaintext passwords, such as the shared-secret methods CRAM-MD5
and DIGEST-MD5. </p>
</blockquote>
<p> The following example configures libsasl to use the sql plugin
and connects it to a PostgreSQL server: </p>
<blockquote>
<pre>
/etc/sasl2/smtpd.conf:
pwcheck_method: auxprop
auxprop_plugin: sql
mech_list: PLAIN LOGIN CRAM-MD5 DIGEST-MD5 NTLM
sql_engine: pgsql
sql_hostnames: 127.0.0.1, 192.0.2.1
sql_user: username
sql_passwd: secret
sql_database: dbname
sql_select: SELECT password FROM users WHERE user = '%u@%r'
</pre>
</blockquote>
<blockquote>
<strong>Note</strong>
<p> Set appropriate permissions if <code>smtpd.conf</code> contains
a password. The file should be readable by the <code>postfix</code>
user. </p>
</blockquote>
<blockquote>
<strong>Note</strong>
<p> In the above example, adjust <code>mech_list</code> to the
mechanisms that are applicable for your environment. </p>
</blockquote>
<p> The sql plugin has the following configuration options: </p>
<blockquote>
<dl>
<dt>sql_engine</dt>
<dd>
<p> Specify <code>mysql</code> to connect to a MySQL server,
<code>pgsql</code> for a PostgreSQL server or <code>sqlite</code>
for an SQLite database </p>
</dd>
<dt>sql_hostnames</dt>
<dd>
<p> Specify one or more servers (hostname or hostname:port) separated
by commas. </p>
<blockquote>
<strong>Note</strong>
<p> With MySQL servers, specify <code>localhost</code> to connect
over a UNIX-domain socket, and specify <code>127.0.0.1</code> to
connect over a TCP socket. </p>
</blockquote>
</dd>
<dt>sql_user</dt>
<dd>
<p> The login name to gain access to the database. </p>
</dd>
<dt>sql_passwd</dt>
<dd>
<p> The password to gain access to the database. </p>
</dd>
<dt>sql_database</dt>
<dd>
<p> The name of the database to connect to. </p>
</dd>
<dt>sql_select</dt>
<dd>
<p> The SELECT statement that should retrieve the plaintext password
from a database table. </p>
<blockquote>
<strong>Important</strong>
<p> Do not enclose the statement in quotes! Use single quotes to
escape macros! </p>
</blockquote>
</dd>
</dl>
</blockquote>
<p> The sql plugin provides macros to build <code>sql_select</code>
statements. They will be replaced with arguments sent from the client. The
following macros are available: </p>
<blockquote>
<dl>
<dt>%u</dt>
<dd>
<p> The name of the user whose properties are being selected. </p>
</dd>
<dt>%p</dt>
<dd>
<p> The name of the property being selected. While this could technically be
anything, Cyrus SASL will try userPassword and cmusaslsecretMECHNAME (where
MECHNAME is the name of a SASL mechanism). </p>
</dd>
<dt>%r</dt>
<dd>
<p> The name of the realm to which the user belongs. This could be
the KERBEROS realm, the fully-qualified domain name of the computer
the SASL application is running on, or the domain after the "@" in a
username. </p>
</dd>
</dl>
</blockquote>
<h4><a name="auxprop_ldapdb">The ldapdb plugin</a></h4>
<p> The ldapdb auxprop plugin provides access to credentials stored
in an LDAP server. This plugin requires that SASL client passwords are
stored as plaintext. </p>
<blockquote>
<strong>Tip</strong>
<p> If you must store encrypted passwords, you cannot use the ldapdb
auxprop plugin. Instead, you can use "<code>saslauthd -a ldap</code>"
to query the LDAP database directly, with appropriate configuration
in <code>saslauthd.conf</code>, <a
href="http://git.cyrusimap.org/cyrus-sasl/tree/saslauthd/LDAP_SASLAUTHD">as
described here</a>. You will not be able to use any of the
methods that require access to plaintext passwords, such as the
shared-secret methods CRAM-MD5 and DIGEST-MD5. </p>
</blockquote>
<p> The ldapdb plugin implements proxy authorization. This means
that the ldapdb plugin uses its own username and password to
authenticate with the LDAP server, before it asks the LDAP server
for the remote SMTP client's password. The LDAP server then decides
if the ldapdb plugin is authorized to read the remote SMTP client's
password. </p>
<p> In a nutshell: Configuring ldapdb means authentication and
authorization must be configured twice - once in the Postfix SMTP
server to authenticate and authorize the remote SMTP client, and
once in the LDAP server to authenticate and authorize the ldapdb
plugin. </p>
<p> This example configures libsasl to use the ldapdb plugin and
the plugin to connect to an LDAP server: </p>
<blockquote>
<pre>
/etc/sasl2/smtpd.conf:
pwcheck_method: auxprop
auxprop_plugin: ldapdb
mech_list: PLAIN LOGIN NTLM CRAM-MD5 DIGEST-MD5
ldapdb_uri: ldap://localhost
ldapdb_id: proxyuser
ldapdb_pw: password
ldapdb_mech: DIGEST-MD5
</pre>
</blockquote>
<blockquote>
<strong>Important</strong>
<p> Set appropriate permissions if <code>smtpd.conf</code> contains a
password. The file should be readable by the <code>postfix</code>
user. </p>
</blockquote>
<blockquote>
<strong>Note</strong>
<p> The shared-secret mechanisms (CRAM-MD5, etc.) require that the
SASL client passwords are stored as plaintext. </p>
</blockquote>
<p> The following is a summary of applicable <code>smtpd.conf</code>
file entries: </p>
<blockquote>
<dl>
<dt>auxprop_plugin</dt>
<dd> <p> Specify <code>ldapdb</code> to enable the plugin. </p> </dd>
<dt>ldapdb_uri</dt>
<dd> <p> Specify either <code>ldapi://</code> to connect over
a UNIX-domain socket, <code>ldap://</code> for an unencrypted TCP
connection, or <code>ldaps://</code> for an encrypted TCP connection.
</p> </dd>
<dt>ldapdb_id</dt>
<dd> <p> The login name to authenticate the ldapdb plugin to the
LDAP server (proxy authorization). </p> </dd>
<dt>ldapdb_pw</dt>
<dd> <p> The password (in plaintext) to authenticate the ldapdb
plugin to the LDAP server (proxy authorization). </p> </dd>
<dt>ldapdb_mech</dt>
<dd> <p> The mechanism to authenticate the ldapdb plugin to the
LDAP server. </p>
<blockquote>
<strong>Note</strong>
<p> Specify a mechanism here that is supported by the LDAP server.
</p>
</blockquote>
</dd>
<dt>ldapdb_rc (optional)</dt>
<dd> <p> The path to a file containing individual configuration
options for the ldapdb LDAP client (libldap). This allows to specify
a TLS client certificate which in turn can be used to use the SASL
EXTERNAL mechanism. </p>
<blockquote>
<strong>Note</strong>
<p> This mechanism supports authentication over an encrypted transport
layer, which is recommended if the plugin must connect to an OpenLDAP
server on a remote machine. </p>
</blockquote>
</dd>
<dt>ldapdb_starttls (optional)</dt>
<dd> <p> The TLS policy for connecting to the LDAP server. Specify
either <code>try</code> or <code>demand</code>. If the option is
<code>try</code> the plugin will attempt to establish a TLS-encrypted
connection with the LDAP server, and will fallback to an unencrypted
connection if TLS fails. If the policy is <code>demand</code> and
a TLS-encrypted connection cannot be established, the connection
fails immediately. </p> </dd>
</dl>
</blockquote>
<p> When the ldapdb plugin connects to the OpenLDAP server and
successfully authenticates, the OpenLDAP server decides if the
plugin user is authorized to read SASL account information. </p>
<p> The following configuration gives an example of authorization configuration
in the OpenLDAP slapd server: </p>
<blockquote>
<pre>
/etc/openldap/slapd.conf:
authz-regexp
uid=(.*),cn=.*,cn=auth
ldap:///dc=example,dc=com??sub?cn=$1
authz-policy to
</pre>
</blockquote>
<p> Here, the <code>authz-regexp</code> option serves for authentication
of the ldapdb user. It maps its login name to a DN in the LDAP
directory tree where <code>slapd</code> can look up the SASL account
information. The <code>authz-policy</code> options defines the
authentication policy. In this case it grants authentication
privileges "<code>to</code>" the ldapdb plugin. </p>
<p> The last configuration step is to tell the OpenLDAP <code>slapd</code>
server where ldapdb may search for usernames matching the one given
by the mail client. The example below adds an additional attribute
ldapdb user object (here: <code>authzTo</code> because the authz-policy
is "<code>to</code>") and configures the scope where the login name
"proxyuser" may search: </p>
<blockquote>
<pre>
dn: cn=proxyuser,dc=example,dc=com
changetype: modify
add: authzTo
authzTo: dn.regex:uniqueIdentifier=(.*),ou=people,dc=example,dc=com
</pre>
</blockquote>
<p> Use the <code>ldapmodify</code> or <code>ldapadd</code> command
to add the above attribute. </p>
<blockquote>
<strong>Note</strong>
<p> Read the chapter "Using SASL" in the <a
href="http://www.openldap.org/doc/admin">OpenLDAP Admin Guide</a>
for more detailed instructions to set up SASL authentication in
OpenLDAP. </p>
</blockquote>
<h3><a name="server_sasl_enable">Enabling SASL authentication and
authorization in the Postfix SMTP server</a></h3>
<p> By default the Postfix SMTP server uses the Cyrus SASL
implementation. If the Dovecot SASL implementation should be used,
specify an <code>smtpd_sasl_type</code> value of <code>dovecot</code>
instead of <code>cyrus</code>: </p>
<blockquote>
<pre>
/etc/postfix/main.cf:
smtpd_sasl_type = dovecot
</pre>
</blockquote>
<p> Additionally specify how Postfix SMTP server can find the Dovecot
authentication server. This depends on the settings that you have
selected in the section "<a href="#server_dovecot_comm">Postfix to
Dovecot SASL communication</a>". </p>
<ul>
<li> <p> If you configured Dovecot for UNIX-domain socket communication,
configure Postfix as follows: </p>
<pre>
/etc/postfix/main.cf:
smtpd_sasl_path = private/auth
</pre>
<strong>Note</strong>
<p> This example uses a pathname relative to the Postfix queue
directory, so that it will work whether or not the Postfix SMTP
server runs chrooted. </p>
<li> <p> If you configured Dovecot for TCP socket communication,
configure Postfix as follows. If Dovecot runs on a different machine,
replace 127.0.0.1 by that machine's IP address. </p>
<pre>
/etc/postfix/main.cf:
smtpd_sasl_path = inet:127.0.0.1:12345
</pre>
<strong>Note</strong>
<p> If you specify a remote IP address, information
will be sent as plaintext over the network. </p>
</ul>
<h4><a name="server_sasl_authc">Enabling SASL authentication
in the Postfix SMTP server</a></h4>
<p> Regardless of the SASL implementation type, enabling SMTP
authentication in the Postfix SMTP server always requires setting
the <code>smtpd_sasl_auth_enable</code> option: </p>
<blockquote>
<pre>
/etc/postfix/main.cf:
smtpd_sasl_auth_enable = yes
</pre>
</blockquote>
<p> After a "postfix reload", SMTP clients will see the additional
capability AUTH in an SMTP session, followed by a list of
authentication mechanisms the server supports: </p>
<blockquote>
<pre>
% <strong>telnet server.example.com 25</strong>
...
220 server.example.com ESMTP Postfix
<strong>EHLO client.example.com</strong>
250-server.example.com
250-PIPELINING
250-SIZE 10240000
250-AUTH DIGEST-MD5 PLAIN CRAM-MD5
...
</pre>
</blockquote>
<p> However not all clients recognize the AUTH capability as defined
by the SASL authentication RFC. Some historical implementations expect the
server to send an "<code>=</code>" as separator between the AUTH
verb and the list of mechanisms that follows it. </p>
<p> The <code>broken_sasl_auth_clients</code> configuration option
lets Postfix repeat the AUTH statement in a form that these broken
clients understand: </p>
<blockquote>
<pre>
/etc/postfix/main.cf:
broken_sasl_auth_clients = yes
</pre>
</blockquote>
<blockquote>
<strong>Note</strong>
<p> Enable this option for Outlook up to and including version 2003
and Outlook Express up to version 6. This option does not hurt other
clients. </p>
</blockquote>
<p> After "postfix reload", the Postfix SMTP server will propagate
the AUTH capability twice - once for compliant and once for broken
clients: </p>
<blockquote>
<pre>
% <strong>telnet server.example.com 25</strong>
...
220 server.example.com ESMTP Postfix
<strong>EHLO client.example.com</strong>
250-server.example.com
250-PIPELINING
250-SIZE 10240000
250-AUTH DIGEST-MD5 PLAIN CRAM-MD5
250-AUTH=DIGEST-MD5 PLAIN CRAM-MD5
...
</pre>
</blockquote>
<h4><a name="smtpd_sasl_security_options">Postfix SMTP Server policy
- SASL mechanism properties</a></h4>
<p> The Postfix SMTP server supports policies that limit the SASL
mechanisms that it makes available to clients, based on the properties
of those mechanisms. The next two sections give examples of how
these policies are used. </p>
<blockquote>
<table border="1">
<tr> <th>Property</th> <th>Description</th> </tr>
<tr> <td>noanonymous</td> <td> Don't use mechanisms that permit
anonymous authentication. </td> </tr>
<tr> <td>noplaintext</td> <td> Don't use mechanisms that transmit
unencrypted username and password information. </td> </tr>
<tr> <td>nodictionary</td> <td> Don't use mechanisms that are
vulnerable to dictionary attacks. </td> </tr>
<tr> <td>forward_secrecy</td> <td> Require forward secrecy between
sessions (breaking one session does not break earlier sessions).
</td> </tr>
<tr> <td>mutual_auth</td> <td> Use only mechanisms that authenticate
both the client and the server to each other. </td> </tr>
</table>
</blockquote>
<h4><a name="id396877">Unencrypted SMTP session</a></h4>
<p> The default policy is to allow any mechanism in the Postfix SMTP server
except for those based on anonymous authentication: </p>
<blockquote>
<pre>
/etc/postfix/main.cf:
# Specify a list of properties separated by comma or whitespace
smtpd_sasl_security_options = noanonymous
</pre>
</blockquote>
<blockquote>
<strong>Important</strong>
<p> Always set at least the <code>noanonymous</code> option.
Otherwise, the Postfix SMTP server can give strangers the same
authorization as a properly-authenticated client. </p>
</blockquote>
<h4><a name="id396969">Encrypted SMTP session (TLS)</a></h4>
<p> A separate parameter controls Postfix SASL mechanism policy
during a TLS-encrypted SMTP session. The default is to copy the
settings from the unencrypted session: </p>
<blockquote>
<pre>
/etc/postfix/main.cf:
smtpd_sasl_tls_security_options = $smtpd_sasl_security_options
</pre>
</blockquote>
<p> A more sophisticated policy allows plaintext mechanisms, but
only over a TLS-encrypted connection: </p>
<blockquote>
<pre>
/etc/postfix/main.cf:
smtpd_sasl_security_options = noanonymous, noplaintext
smtpd_sasl_tls_security_options = noanonymous
</pre>
</blockquote>
<p> To offer SASL authentication only after a TLS-encrypted session has been
established specify this: </p>
<blockquote>
<pre>
/etc/postfix/main.cf:
smtpd_tls_auth_only = yes
</pre>
</blockquote>
<h4><a name="server_sasl_authz">Enabling SASL authorization in the Postfix
SMTP server</a></h4>
<p> After the client has authenticated with SASL, the Postfix SMTP
server decides what the remote SMTP client will be authorized
for. Examples of possible SMTP clients authorizations are: </p>
<ul>
<li> <p> Send a message to a remote recipient. </p> </li>
<li> <p> Use a specific envelope sender in the MAIL FROM command. </p> </li>
</ul>
<p> These permissions are not enabled by default. </p>
<h4><a name="server_sasl_authz_relay">Mail relay authorization</a></h4>
<p> With <code>permit_sasl_authenticated</code> the Postfix SMTP
server can allow
SASL-authenticated SMTP clients to send mail to remote destinations.
Examples:
</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
<strong>permit_sasl_authenticated</strong>
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
<strong>permit_sasl_authenticated</strong>
reject_unauth_destination
...other rules...
</pre>
</blockquote>
<h4><a name="server_sasl_authz_envelope">Envelope sender address
authorization</a></h4>
<p> By default an SMTP client may specify any envelope sender address
in the MAIL FROM command. That is because the Postfix SMTP server
only knows the remote SMTP client hostname and IP address, but not
the user who controls the remote SMTP client. </p>
<p> This changes the moment an SMTP client uses SASL authentication.
Now, the Postfix SMTP server knows who the sender is. Given a table
of envelope sender addresses and SASL login names, the Postfix SMTP
server can decide if the SASL authenticated client is allowed to
use a particular envelope sender address: </p>
<blockquote>
<pre>
/etc/postfix/main.cf:
<strong>smtpd_sender_login_maps = hash:/etc/postfix/controlled_envelope_senders</strong>
smtpd_recipient_restrictions =
...
<strong>reject_sender_login_mismatch</strong>
permit_sasl_authenticated
...
</pre>
</blockquote>
<p> The <code>controlled_envelope_senders</code> table specifies
the binding between a sender envelope address and the SASL login
names that own that address: </p>
<blockquote>
<pre>
/etc/postfix/controlled_envelope_senders
# envelope sender owners (SASL login names)
john@example.com john@example.com
helpdesk@example.com john@example.com, mary@example.com
postmaster admin@example.com
@example.net barney, fred, john@example.com, mary@example.com
</pre>
</blockquote>
<p> With this, the <code>reject_sender_login_mismatch</code>
restriction above will reject the sender address in the MAIL FROM
command if <code>smtpd_sender_login_maps</code> does not specify
the SMTP client's login name as an owner of that address. </p>
<p> See also <code>reject_authenticated_sender_login_mismatch</code>,
<code>reject_known_sender_login_mismatch</code>, and
<code>reject_unauthenticated_sender_login_mismatch</code> for additional
control over the SASL login name and the envelope sender. </p>
<h4><a name="server_sasl_other">Additional SMTP Server SASL options</a></h4>
<p> Postfix provides a wide range of SASL authentication configuration
options. The next section lists a few that are discussed frequently.
See postconf(5) for a complete list. </p>
<h4><a name="sasl_access">Per-account access control</a></h4>
<p> Postfix can implement policies that depend on the SASL login
name (Postfix 2.11 and later). Typically this is used to HOLD or
REJECT mail from accounts whose credentials have been compromised.
</p>
<blockquote>
<pre>
/etc/postfix/main.cf:
smtpd_recipient_restrictions =
permit_mynetworks
check_sasl_access hash:/etc/postfix/sasl_access
permit_sasl_authenticated
...
/etc/postfix/sasl_access:
# Use this when smtpd_sasl_local_domain is empty.
username HOLD
# Use this when smtpd_sasl_local_domain=example.com.
username@example.com HOLD
</pre>
</blockquote>
<h4><a name="id397172">Default authentication domain</a></h4>
<p> Postfix can append a domain name (or any other string) to a
SASL login name that does not have a domain part, e.g. "<code>john</code>"
instead of "<code>john@example.com</code>": </p>
<blockquote>
<pre>
/etc/postfix/main.cf:
smtpd_sasl_local_domain = example.com
</pre>
</blockquote>
<p> This is useful as a default setting and safety net for misconfigured
clients, or during a migration to an authentication method/backend
that requires an authentication REALM or domain name, before all
SMTP clients are configured to send such information. </p>
<h4><a name="id397205">Hiding SASL authentication from clients or
networks</a></h4>
<p> Some clients insist on using SASL authentication if it is offered, even
when they are not configured to send credentials - and therefore
they will always fail and disconnect. </p>
<p> Postfix can hide the AUTH capability from these clients/networks: </p>
<blockquote>
<pre>
/etc/postfix/main.cf:
smtpd_sasl_exceptions_networks = !192.0.2.171/32, 192.0.2.0/24
</pre>
</blockquote>
<h4><a name="id397226">Adding the SASL login name to mail headers</a></h4>
<p> To report SASL login names in Received: message headers (Postfix
version 2.3 and later): </p>
<blockquote>
<pre>
/etc/postfix/main.cf:
smtpd_sasl_authenticated_header = yes
</pre>
</blockquote>
<blockquote>
<strong>Note</strong>
<p> The SASL login names will be shared with the entire world. </p>
</blockquote>
<h3><a name="server_test">Testing SASL authentication in the Postfix SMTP Server</a></h3>
<p> To test the server side, connect (for example, with
<code>telnet</code>) to the Postfix SMTP server port and you should
be able to have a conversation as shown below. Information sent by
the client (that is, you) is shown in <strong>bold</strong> font.
</p>
<blockquote>
<pre>
% <strong>telnet server.example.com 25</strong>
...
220 server.example.com ESMTP Postfix
<strong>EHLO client.example.com</strong>
250-server.example.com
250-PIPELINING
250-SIZE 10240000
250-ETRN
250-AUTH DIGEST-MD5 PLAIN CRAM-MD5
250 8BITMIME
<strong>AUTH PLAIN AHRlc3QAdGVzdHBhc3M=</strong>
235 Authentication successful
</pre>
</blockquote>
<p> To test this over a connection that is encrypted with TLS, use
<code>openssl s_client</code> instead of <code>telnet</code>:
<blockquote>
<pre>
% <strong>openssl s_client -connect server.example.com:25 -starttls smtp</strong>
...
220 server.example.com ESMTP Postfix
<strong>EHLO client.example.com</strong>
...see above example for more...
</pre>
</blockquote>
<p> Instead of <code>AHRlc3QAdGVzdHBhc3M=</code>, specify the
base64-encoded form of <code>\0username\0password</code> (the \0
is a null byte). The example above is for a user named `<code>test</code>'
with password `<code>testpass</code>'. </p>
<blockquote>
<strong>Caution</strong>
<p> When posting logs of the SASL negotiations to public lists,
please keep in mind that username/password information is trivial
to recover from the base64-encoded form. </p>
</blockquote>
<p> You can use one of the following commands to generate base64
encoded authentication information: </p>
<ul>
<li> <p> Using a recent version of the <b>bash</b> shell: </p>
<blockquote>
<pre>
% <strong>echo -ne '\000username\000password' | openssl base64</strong>
</pre>
</blockquote>
<p> Some other shells support similar syntax. </p>
<li> <p> Using the <b>printf</b> command: </p>
<blockquote>
<pre>
% <strong>printf '\0%s\0%s' '<em>username</em>' '<em>password</em>' | openssl base64</strong>
% <strong>printf '\0%s\0%s' '<em>username</em>' '<em>password</em>' | mmencode</strong>
</pre>
</blockquote>
<p> The <strong>mmencode</strong> command is part of the metamail
software. </p>
<li> <p> Using Perl <b>MIME::Base64</b> (from http://www.cpan.org/): </p>
<blockquote>
<pre>
% <strong>perl -MMIME::Base64 -e \
'print encode_base64("\0<em>username</em>\0<em>password</em>");'</strong>
</pre>
</blockquote>
<p> If the username or password contain "@", you must specify "\@". </p>
<li> <p> Using the <b>gen-auth</b> script: </p>
<blockquote>
<pre>
% <strong>gen-auth plain</strong>
username: <strong><em>username</em></strong>
password:
</pre>
</blockquote>
<p> The <strong>gen-auth</strong> Perl script was written by John
Jetmore and can be found at http://jetmore.org/john/code/gen-auth. </p>
</ul>
<h2><a name="client_sasl">Configuring SASL authentication in the Postfix SMTP/LMTP client</a></h2>
<p> The Postfix SMTP and the LMTP client can authenticate with a
remote SMTP server via the Cyrus SASL framework. At this time, the
Dovecot SASL implementation does not provide client functionality.
</p>
<blockquote>
<strong>Note</strong>
<p> The examples in this section discuss only the SMTP client.
Replace <code>smtp_</code> with <code>lmtp_</code> to get the
corresponding LMTP client configuration. </p>
</blockquote>
<p> You can read more about the following topics: </p>
<ul>
<li><a href="#client_sasl_enable">Enabling SASL authentication in
the Postfix SMTP/LMTP client</a></li>
<li><a href="#client_sasl_sender">Configuring sender-dependent SASL
authentication</a></li>
<li><a href="#client_sasl_policy">Postfix SMTP/LMTP client policy
- SASL mechanism <em>properties</em></a></li>
<li><a href="#client_sasl_filter">Postfix SMTP/LMTP client policy
- SASL mechanism <em>names</em></a></li>
</ul>
<h3><a name="client_sasl_enable">Enabling SASL authentication in the
Postfix SMTP/LMTP client</a></h3>
<p> This section shows a typical scenario where the Postfix SMTP
client sends all messages via a mail gateway server that requires
SASL authentication. </p>
<blockquote>
<strong> Trouble solving tips: </strong>
<ul>
<li> <p> If your SASL logins fail with "SASL authentication failure:
No worthy mechs found" in the mail logfile, then see the section
"<a href="SASL_README.html#client_sasl_policy">Postfix SMTP/LMTP
client policy - SASL mechanism <em>properties</em></a>".
<li> <p> For a solution to a more obscure class of SASL authentication
failures, see "<a href="SASL_README.html#client_sasl_filter">Postfix
SMTP/LMTP client policy - SASL mechanism <em>names</em></a>".
</ul>
</blockquote>
<p> To make the example more readable we introduce it in two parts.
The first part takes care of the basic configuration, while the
second part sets up the username/password information. </p>
<blockquote>
<pre>
/etc/postfix/main.cf:
smtp_sasl_auth_enable = yes
smtp_tls_security_level = encrypt
smtp_sasl_tls_security_options = noanonymous
relayhost = [mail.isp.example]
# Alternative form:
# relayhost = [mail.isp.example]:submission
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
</pre>
</blockquote>
<ul>
<li> <p> The <code>smtp_sasl_auth_enable</code> setting enables
client-side authentication. We will configure the client's username
and password information in the second part of the example. </p>
</li>
<li> <p> The <code>smtp_tls_security_level</code> setting ensures
that the connection to the remote smtp server will be encrypted, and
<code>smtp_sasl_tls_security_options</code> removes the prohibition on
plaintext passwords. </p>
<li> <p> The <code>relayhost</code> setting forces the Postfix SMTP
to send all remote messages to the specified mail server instead
of trying to deliver them directly to their destination. </p> </li>
<li> <p> In the <code>relayhost</code> setting, the "<code>[</code>"
and "<code>]</code>" prevent the Postfix SMTP client from looking
up MX (mail exchanger) records for the enclosed name. </p> </li>
<li> <p> The <code>relayhost</code> destination may also specify a
non-default TCP port. For example, the alternative form
<code>[mail.isp.example]:submission</code> tells Postfix to connect
to TCP network port 587, which is reserved for email client
applications. </p> </li>
<li> <p> The Postfix SMTP client is compatible with SMTP servers
that use the non-standard "<code>AUTH=<em>method.</em>...</code>"
syntax in response to the EHLO command; this requires no additional
Postfix client configuration. </p> </li>
<li> <p> With the setting "smtp_tls_wrappermode = yes", the Postfix
SMTP client supports the "wrappermode" protocol, which uses TCP
port 465 on the SMTP server (Postfix 3.0 and later). </p> </li>
<li> <p> With the <code>smtp_sasl_password_maps</code> parameter,
we configure the Postfix SMTP client to send username and password
information to the mail gateway server. As discussed in the next
section, the Postfix SMTP client supports multiple ISP accounts.
For this reason the username and password are stored in a table
that contains one username/password combination for each mail gateway
server. </p>
</ul>
<blockquote>
<pre>
/etc/postfix/sasl_passwd:
# destination credentials
[mail.isp.example] username:password
# Alternative form:
# [mail.isp.example]:submission username:password
</pre>
</blockquote>
<blockquote>
<strong>Important</strong>
<p> Keep the SASL client password file in <code>/etc/postfix</code>,
and make the file read+write only for <code>root</code> to protect
the username/password combinations against other users. The Postfix
SMTP client will still be able to read the SASL client passwords.
It opens the file as user <code>root</code> before it drops privileges,
and before entering an optional chroot jail. </p>
</blockquote>
<ul>
<li> <p> Use the <code>postmap</code> command whenever you
change the <code>/etc/postfix/sasl_passwd</code> file. </p> </li>
<li> <p> If you specify the "<code>[</code>" and "<code>]</code>"
in the <code>relayhost</code> destination, then you must use the
same form in the <code>smtp_sasl_password_maps</code> file. </p>
</li>
<li> <p> If you specify a non-default TCP Port (such as
"<code>:submission</code>" or "<code>:587</code>") in the
<code>relayhost</code> destination, then you must use the same form
in the <code>smtp_sasl_password_maps</code> file. </p> </li>
</ul>
<h3><a name="client_sasl_sender">Configuring Sender-Dependent SASL
authentication</a></h3>
<p> Postfix supports different ISP accounts for different sender
addresses (version 2.3 and later). This can be useful when one
person uses the same machine for work and for personal use, or when
people with different ISP accounts share the same Postfix server.
</p>
<p> To make this possible, Postfix supports per-sender SASL passwords
and per-sender relay hosts. In the example below, the Postfix SMTP
client will search the SASL password file by sender address before
it searches that same file by destination. Likewise, the Postfix
trivial-rewrite(8) daemon will search the per-sender relayhost file,
and use the default <code>relayhost</code> setting only as a final
resort. </p>
<blockquote>
<pre>
/etc/postfix/main.cf:
smtp_sender_dependent_authentication = yes
sender_dependent_relayhost_maps = hash:/etc/postfix/sender_relay
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
relayhost = [mail.isp.example]
# Alternative form:
# relayhost = [mail.isp.example]:submission
</pre>
</blockquote>
<blockquote>
<pre>
/etc/postfix/sasl_passwd:
# Per-sender authentication; see also /etc/postfix/sender_relay.
user1@example.com username1:password1
user2@example.net username2:password2
# Login information for the default relayhost.
[mail.isp.example] username:password
# Alternative form:
# [mail.isp.example]:submission username:password
</pre>
</blockquote>
<blockquote>
<pre>
/etc/postfix/sender_relay:
# Per-sender provider; see also /etc/postfix/sasl_passwd.
user1@example.com [mail.example.com]:submission
user2@example.net [mail.example.net]
</pre>
</blockquote>
<ul>
<li> <p> If you are creative, then you can try to combine the two
tables into one single MySQL database, and configure different
Postfix queries to extract the appropriate information. </p>
<li> <p> Specify <b>dbm</b> instead of <b>hash</b> if your system uses
<b>dbm</b> files instead of <b>db</b> files. To find out what lookup
tables Postfix supports, use the command "<b>postconf -m</b>". </p>
<li> <p> Execute the command "<b>postmap /etc/postfix/sasl_passwd</b>"
whenever you change the sasl_passwd table. </p>
<li> <p> Execute the command "<b>postmap /etc/postfix/sender_relay</b>"
whenever you change the sender_relay table. </p>
</ul>
<h3><a name="client_sasl_policy">Postfix SMTP/LMTP client policy -
SASL mechanism <em>properties</em></a></h3>
<p> Just like the Postfix SMTP server, the SMTP client has a policy
that determines which SASL mechanisms are acceptable, based on their
properties. The next two sections give examples of how these policies
are used. </p>
<blockquote>
<table border="1">
<tr> <th>Property</th> <th>Description</th> </tr>
<tr> <td>noanonymous</td> <td> Don't use mechanisms that permit
anonymous authentication. </td> </tr>
<tr> <td>noplaintext</td> <td> Don't use mechanisms that transmit
unencrypted username and password information. </td> </tr>
<tr> <td>nodictionary</td> <td> Don't use mechanisms that are
vulnerable to dictionary attacks. </td> </tr>
<tr> <td>mutual_auth</td> <td> Use only mechanisms that authenticate
both the client and the server to each other. </td> </tr>
</table>
</blockquote>
<h4>Unencrypted SMTP session</h4>
<p> The default policy is stricter than that of the Postfix SMTP
server - plaintext mechanisms are not allowed (nor is any anonymous
mechanism): </p>
<blockquote>
<pre>
/etc/postfix/main.cf:
smtp_sasl_security_options = noplaintext, noanonymous
</pre>
</blockquote>
<p> This default policy, which allows no plaintext passwords, leads
to authentication failures if the remote server only offers plaintext
authentication mechanisms (the SMTP server announces "<code>AUTH
PLAIN LOGIN</code>"). In such cases the SMTP client will log the
following error message: </p>
<blockquote>
<pre>
SASL authentication failure: No worthy mechs found
</pre>
</blockquote>
<blockquote>
<strong>Note</strong>
<p> This same error message will also be logged when the
<code>libplain.so</code> or <code>liblogin.so</code> modules are
not installed in the <code>/usr/lib/sasl2</code> directory. </p>
</blockquote>
<p> The insecure approach is to lower the security standards and
permit plaintext authentication mechanisms: </p>
<blockquote>
<pre>
/etc/postfix/main.cf:
smtp_sasl_security_options = noanonymous
</pre>
</blockquote>
<p> The more secure approach is to protect the plaintext username
and password with TLS session encryption. To find out if the remote
SMTP server supports TLS, connect to the server and see if it
announces STARTTLS support as shown in the example. Information
sent by the client (that is, you) is shown in <strong>bold</strong>
font. </p>
<blockquote>
<pre>
% <strong>telnet server.example.com 25</strong>
...
220 server.example.com ESMTP Postfix
<strong>EHLO client.example.com</strong>
250-server.example.com
250-PIPELINING
250-SIZE 10240000
250-STARTTLS
...
</pre>
</blockquote>
<p> Instead of port 25 (smtp), specify port 587 (submission) where
appropriate. </p>
<h4>Encrypted SMTP session (TLS)</h4>
<p> To turn on TLS in the Postfix SMTP client, see TLS_README for
configuration details. </p>
<p> The smtp_sasl_tls_security_options parameter controls Postfix
SASL mechanism policy during a TLS-encrypted SMTP session. The
default is to copy the settings from the unencrypted session: </p>
<blockquote>
<pre>
/etc/postfix/main.cf:
smtp_sasl_tls_security_options = $smtp_sasl_security_options
</pre>
</blockquote>
<p> A more sophisticated policy allows plaintext mechanisms, but
only over a TLS-encrypted connection: </p>
<blockquote>
<pre>
/etc/postfix/main.cf:
smtp_sasl_security_options = noanonymous, noplaintext
smtp_sasl_tls_security_options = noanonymous
</pre>
</blockquote>
<h3><a name="client_sasl_filter">Postfix SMTP/LMTP client policy -
SASL mechanism <em>names</em></a></h3>
<p> Given the SASL security options of the previous section, the
Cyrus SASL library will choose the most secure authentication
mechanism that both the SMTP client and server implement. Unfortunately,
that authentication mechanism may fail because the client or server
is not configured to use that mechanism.</p>
<p> To prevent this, the Postfix SMTP client can filter the names
of the authentication mechanisms from the remote SMTP server. Used
correctly, the filter hides unwanted mechanisms from the Cyrus SASL
library, forcing the library to choose from the mechanisms the
Postfix SMTP client filter passes through. </p>
<p> The following example filters out everything but the mechanisms
<code>PLAIN</code> and <code>LOGIN</code>: </p>
<blockquote>
<pre>
/etc/postfix/main.cf:
smtp_sasl_mechanism_filter = plain, login
</pre>
</blockquote>
<blockquote>
<strong>Note</strong>
<p> If the remote server does not offer any of the mechanisms on
the filter list, authentication will fail. </p>
</blockquote>
<p> We close this section with an example that passes every mechanism
except for <code>GSSAPI</code> and <code>LOGIN</code>: </p>
<blockquote>
<pre>
/etc/postfix/main.cf:
smtp_sasl_mechanism_filter = !gssapi, !login, static:all
</pre>
</blockquote>
<h2><a name="postfix_build">Building Postfix with SASL support</a></h2>
<p> As mentioned elsewhere, Postfix supports two SASL implementations:
Cyrus SASL (SMTP client and server) and Dovecot SASL (SMTP server
only). Both implementations can be built into Postfix simultaneously.
</p>
<ul>
<li><a href="#build_dovecot">Building Dovecot SASL support</a></li>
<li><a href="#sasl_support">Building Cyrus SASL support</a></li>
</ul>
<h3><a name="build_dovecot">Building Dovecot SASL support</a></h3>
<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> Support for the Dovecot version 1 SASL protocol is available
in Postfix 2.3 and later. At the time of writing, only server-side
SASL support is available, so you can't use it to authenticate the
Postfix SMTP client to your network provider's server. </p>
<p> Dovecot uses its own daemon process for authentication. This
keeps the Postfix build process simple, because there is no need
to link extra libraries into Postfix. </p>
<p> To generate the necessary Makefiles, execute the following in
the Postfix top-level directory: </p>
<blockquote>
<pre>
% <strong>make tidy</strong> # if you have left-over files from a previous build
% <strong>make makefiles CCARGS='-DUSE_SASL_AUTH \
-DDEF_SERVER_SASL_TYPE=\"dovecot\"'</strong>
</pre>
</blockquote>
<p> After this, proceed with "<code>make</code>" as described in
the INSTALL document. </p>
<strong>Note</strong>
<ul>
<li>
<p> The <code>-DDEF_SERVER_SASL_TYPE=\"dovecot\"</code> is not
necessary; it just makes Postfix configuration a little more
convenient because you don't have to specify the SASL plug-in type
in the Postfix main.cf file (but this may cause surprises when you
switch to a later Postfix version that is built with the default
SASL type of <code>cyrus</code>). </p>
</li>
<li>
<p> If you also want support for LDAP or TLS (or for Cyrus SASL),
you need to merge their <code>CCARGS</code> and <code>AUXLIBS</code>
options into the above command line; see the LDAP_README and
TLS_README for details. </p>
<blockquote>
<pre>
% <strong>make tidy</strong> # if you have left-over files from a previous build
% <strong>make makefiles CCARGS='-DUSE_SASL_AUTH \
-DDEF_SERVER_SASL_TYPE=\"dovecot\" \
...<i>CCARGS options for LDAP or TLS etc.</i>...' \
AUXLIBS='...<i>AUXLIBS options for LDAP or TLS etc.</i>...'</strong>
</pre>
</blockquote>
</li>
</ul>
<h3><a name="sasl_support">Building Cyrus SASL support</a></h3>
<h4><a name="build_sasl">Building the Cyrus SASL library</a></h4>
<p> Postfix works with cyrus-sasl-1.5.x or cyrus-sasl-2.1.x, which are
available from https://github.com/cyrusimap/cyrus-sasl/releases. </p>
<blockquote>
<strong>Important</strong>
<p> If you install the Cyrus SASL libraries as per the default, you will have
to create a symlink <code>/usr/lib/sasl</code> ->
<code>/usr/local/lib/sasl</code> for version 1.5.x or
<code>/usr/lib/sasl2</code> -> <code>/usr/local/lib/sasl2</code>
for version 2.1.x. </p>
</blockquote>
<p> Reportedly, Microsoft Outlook (Express) requires the non-standard LOGIN
and/or NTLM authentication mechanism. To enable these authentication
mechanisms, build the Cyrus SASL libraries with: </p>
<blockquote>
<pre>
% <strong>./configure --enable-login --enable-ntlm</strong>
</pre>
</blockquote>
<h4><a name="build_postfix">Building Postfix with Cyrus SASL support</a></h4>
<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> The following assumes that the Cyrus SASL include files are in
<code>/usr/local/include</code>, and that the Cyrus SASL libraries are in
<code>/usr/local/lib</code>. </p>
<p> On some systems this generates the necessary <code>Makefile</code>
definitions: </p>
<dl>
<dt>Cyrus SASL version 2.1.x</dt>
<dd>
<pre>
% <strong>make tidy</strong> # if you have left-over files from a previous build
% <strong>make makefiles CCARGS="-DUSE_SASL_AUTH -DUSE_CYRUS_SASL \
-I/usr/local/include/sasl" AUXLIBS="-L/usr/local/lib -lsasl2"</strong>
</pre>
<p> If your Cyrus SASL shared library is in a directory that the RUN-TIME
linker does not know about, add a "-Wl,-R,/path/to/directory" option after
"-lsasl2". </p>
</dd>
<dt>Cyrus SASL version 1.5.x</dt>
<dd>
<pre>
% <strong>make tidy</strong> # if you have left-over files from a previous build
% <strong>make makefiles CCARGS="-DUSE_SASL_AUTH -DUSE_CYRUS_SASL \
-I/usr/local/include" AUXLIBS="-L/usr/local/lib -lsasl"</strong>
</pre>
</dd>
</dl>
<p> On Solaris 2.x you need to specify run-time link information,
otherwise the ld.so run-time linker will not find the SASL shared
library: </p>
<dl>
<dt>Cyrus SASL version 2.1.x</dt>
<dd>
<pre>
% <strong>make tidy</strong> # remove left-over files from a previous build
% <strong>make makefiles CCARGS="-DUSE_SASL_AUTH -DUSE_CYRUS_SASL \
-I/usr/local/include/sasl" AUXLIBS="-L/usr/local/lib \
-R/usr/local/lib -lsasl2"</strong>
</pre>
</dd>
<dt>Cyrus SASL version 1.5.x</dt>
<dd>
<pre>
% <strong>make tidy</strong> # if you have left-over files from a previous build
% <strong>make makefiles CCARGS="-DUSE_SASL_AUTH -DUSE_CYRUS_SASL \
-I/usr/local/include" AUXLIBS="-L/usr/local/lib \
-R/usr/local/lib -lsasl"</strong>
</pre>
</dd>
</dl>
<h2><a name="cyrus_legacy">Using Cyrus SASL version 1.5.x</a></h2>
<p> Postfix supports Cyrus SASL version 1.x, but you shouldn't use
it unless you are forced to. The makers of Cyrus SASL write: </p>
<blockquote> <i> This library is being deprecated and applications
should transition to using the SASLv2 library</i> (source: <a
href="http://www.cyrusimap.org/download.html">Project Cyrus:
Downloads</a>). </blockquote>
<p> If you still need to set it up, here's a quick rundown: </p>
<p> Read the regular section on SMTP server configurations for the
Cyrus SASL framework. The differences are: </p>
<ul>
<li> <p> Cyrus SASL version 1.5.x searches for configuration
(<code>smtpd.conf</code>) in <code>/usr/lib/sasl/</code> only. You
must place the configuration in that directory. Some systems may
have modified Cyrus SASL and put the files into e.g.
<code>/var/lib/sasl/</code>. </p> </li>
<li> <p> Use the <code>saslpasswd</code> command instead of
<code>saslpasswd2</code> to create users in <code>sasldb</code>.
</p> </li>
<li> <p> Use the <code>sasldblistusers</code> command instead of
<code>sasldblistusers2</code> to find users in <code>sasldb</code>.
</p> </li>
<li> <p> In the <code>smtpd.conf</code> file you can't use
<code>mech_list</code> to limit the range of mechanisms offered.
Instead, remove their libraries from <code>/usr/lib/sasl/</code>
(and remember remove those files again when a system update
re-installs new versions). </p> </li>
</ul>
<h2><a name="credits">Credits</a></h2>
<ul>
<li> Postfix SASL support was originally implemented by Till Franke
of SuSE Rhein/Main AG. </li>
<li> Wietse trimmed down the code to only the bare necessities.
</li>
<li> Support for Cyrus SASL version 2 was contributed by Jason Hoos.
</li>
<li> Liviu Daia added smtpd_sasl_application_name, separated
reject_sender_login_mismatch into
reject_authenticated_sender_login_mismatch and
reject_unauthenticated_sender_login_mismatch, and revised the docs.
</li>
<li> Wietse made another iteration through the code to add plug-in
support for multiple SASL implementations, and for reasons that
have been lost, also changed smtpd_sasl_application_name into
smtpd_sasl_path. </li>
<li> The Dovecot SMTP server-only plug-in was originally implemented
by Timo Sirainen of Procontrol, Finland. </li>
<li> Patrick Ben Koetter revised this document for Postfix 2.4 and
made much needed updates. </li>
<li> Patrick Ben Koetter revised this document again for Postfix
2.7 and made much needed updates. </li>
</ul>
</body>
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