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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 SASL Howto</title>
<meta http-equiv="Content-Type" content="text/html; charset=us-ascii">
</head>
<body>
<h1><img src="postfix-logo.jpg" width="203" height="98" ALT="">Postfix SASL Howto</h1>
<hr>
<h2>WARNING</h2>
<p> 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 is 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>
<h2><a name="intro">How Postfix uses SASL authentication information</a></h2>
<p> Postfix SASL support (RFC 2554) can be used to authenticate
remote SMTP clients to the Postfix SMTP server, and to authenticate
the Postfix SMTP client to a remote SMTP server. </p>
<p> When receiving mail, Postfix logs the client-provided username,
authentication method, and sender address to the maillog file, and
optionally grants mail access via the permit_sasl_authenticated
UCE restriction. </p>
<p> When sending mail, Postfix can look up the server hostname or
destination domain (the address right-hand part) in a Postfix SASL password
table, and if a username/password is found, it will use that username
and password to authenticate to the server. And as of version 2.3,
Postfix can be configured to search its SASL password table by the
sender email address. </p>
<p>This document covers the following topics: </p>
<ul>
<li><a href="#versions">What SASL implementations are supported</a>
<li><a href="#build_dovecot">Building Postfix with Dovecot SASL
support</a></li>
<li><a href="#build_sasl">Building the Cyrus SASL library</a>
<li><a href="#build_postfix">Building Postfix with Cyrus SASL
support</a></li>
<li><a href="#server_sasl">Enabling SASL authentication in the
Postfix SMTP server</a></li>
<li><a href="#server_dovecot">Dovecot SASL configuration for the Postfix
SMTP server</a></li>
<li><a href="#server_cyrus">Cyrus SASL configuration for the Postfix
SMTP server</a></li>
<li><a href="#server_test">Testing SASL authentication in the
Postfix SMTP server</a></li>
<li><a href="#debugging">Trouble shooting the SASL internals</a>
<li><a href="#client_sasl">Enabling SASL authentication in the
Postfix SMTP client</a></li>
<li><a href="#credits">Credits</a>
</ul>
<h2><a name="versions">What SASL implementations are supported</a></h2>
<p> This document describes Postfix with the following SASL
implementations: </p>
<ul>
<li> <p> Cyrus SASL version 1 (client and server). </p>
<li> <p> Cyrus SASL version 2 (client and server). </p>
<li> <p> Dovecot protocol version 1 (server only, Postfix version
2.3 and later) </p>
</ul>
<p> Postfix version 2.3 introduces a plug-in mechanism that provides
support for multiple SASL implementations. To find out what
implementations are built into Postfix, use the following commands:
</p>
<blockquote>
<pre>
% postconf -a (SASL support in the SMTP server)
% postconf -A (SASL support in the SMTP+LMTP client)
</pre>
</blockquote>
<p> Needless to say, these commands are not available in earlier
Postfix versions. </p>
<h2><a name="build_dovecot">Building Postfix with Dovecot SASL
support</a></h2>
<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 to your network provider's server. 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>
% make makefiles CCARGS='-DUSE_SASL_AUTH -DDEF_SERVER_SASL_TYPE=\"dovecot\"'
</pre>
</blockquote>
<p> After this, proceed with "<tt>make</tt>" as described in the
INSTALL document. </p>
<p> Notes: </p>
<ul>
<li> <p> The "-DDEF_SERVER_SASL_TYPE" stuff 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. </p>
<li> <p> If you also want support for LDAP or TLS, you will have to merge
their CCARGS and AUXLIBS into the above command line. </p>
</ul>
<h2><a name="build_sasl">Building the Cyrus SASL library</a></h2>
<p> Postfix appears to work with cyrus-sasl-1.5.5 or cyrus-sasl-2.1.1,
which are available from: </p>
<blockquote>
<pre>
ftp://ftp.andrew.cmu.edu/pub/cyrus-mail/
</pre>
</blockquote>
<p> IMPORTANT: if you install the Cyrus SASL libraries as per the
default, you will have to symlink /usr/lib/sasl -> /usr/local/lib/sasl
for version 1.5.5 or /usr/lib/sasl2 -> /usr/local/lib/sasl2 for
version 2.1.1. </p>
<p> Reportedly, Microsoft Internet Explorer version 5 requires the
non-standard SASL LOGIN authentication method. To enable this
authentication method, specify ``./configure --enable-login''. </p>
<h2><a name="build_postfix">Building Postfix with Cyrus SASL support</a></h2>
<p> The following
assumes that the Cyrus SASL include files are in /usr/local/include,
and that the Cyrus SASL libraries are in /usr/local/lib. </p>
<p> On some systems this generates the necessary Makefile definitions:
</p>
<dl>
<dt> (for Cyrus SASL version 1.5.5):
<dd>
<pre>
% make tidy # if you have left-over files from a previous build
% make makefiles CCARGS="-DUSE_SASL_AUTH -DUSE_CYRUS_SASL \
-I/usr/local/include" AUXLIBS="-L/usr/local/lib -lsasl"
</pre>
<dt> (for Cyrus SASL version 2.1.1):
<dd>
<pre>
% make tidy # if you have left-over files from a previous build
% make makefiles CCARGS="-DUSE_SASL_AUTH -DUSE_CYRUS_SASL \
-I/usr/local/include/sasl" AUXLIBS="-L/usr/local/lib -lsasl2"
</pre>
</dl>
<p> On Solaris 2.x you need to specify run-time link information,
otherwise ld.so will not find the SASL shared library: </p>
<dl>
<dt> (for Cyrus SASL version 1.5.5):
<dd>
<pre>
% make tidy # if you have left-over files from a previous build
% make makefiles CCARGS="-DUSE_SASL_AUTH -DUSE_CYRUS_SASL \
-I/usr/local/include" AUXLIBS="-L/usr/local/lib \
-R/usr/local/lib -lsasl"
</pre>
<dt> (for Cyrus SASL version 2.1.1):
<dd>
<pre>
% make tidy # if you have left-over files from a previous build
% make makefiles CCARGS="-DUSE_SASL_AUTH -DUSE_CYRUS_SASL \
-I/usr/local/include/sasl" AUXLIBS="-L/usr/local/lib \
-R/usr/local/lib -lsasl2"
</pre>
</dl>
<h2><a name="server_sasl">Enabling SASL authentication in the Postfix
SMTP server</a></h2>
<p> In order to enable SASL support in the SMTP server: </p>
<blockquote>
<pre>
/etc/postfix/main.cf:
smtpd_sasl_auth_enable = yes
</pre>
</blockquote>
<p> In order to allow mail relaying by authenticated clients: </p>
<blockquote>
<pre>
/etc/postfix/main.cf:
smtpd_recipient_restrictions =
permit_mynetworks permit_sasl_authenticated ...
</pre>
</blockquote>
<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>
<p> Note: the SASL login names will be shared with the entire world.
</p>
<p> Older Microsoft SMTP client software implements a non-standard
version of the AUTH protocol syntax, and expects that the SMTP
server replies to EHLO with "250 AUTH=stuff" instead of "250 AUTH
stuff". To accommodate such clients (in addition to conformant
clients) use the following: </p>
<blockquote>
<pre>
/etc/postfix/main.cf:
broken_sasl_auth_clients = yes
</pre>
</blockquote>
<h2><a name="server_dovecot">Dovecot SASL configuration for the
Postfix SMTP server</a></h2>
<p> Dovecot SASL support is available in Postfix 2.3 and later. On
the Postfix side you need to specify the location of the
Dovecot authentication daemon socket. We use a pathname relative
to the Postfix queue directory, so that it will work whether or not
Postfix runs chrooted: </p>
<blockquote>
<pre>
/etc/postfix/main.cf:
smtpd_sasl_type = dovecot
smtpd_sasl_path = private/auth
</pre>
</blockquote>
<p> On the Dovecot side you also need to specify the Dovecot
authentication daemon socket. In this case we specify an
absolute pathname. In the example we assume that the
Postfix queue is under /var/spool/postfix/. </p>
<blockquote>
<pre>
/some/where/dovecot.conf:
auth default {
mechanisms = plain login
passdb pam {
}
userdb passwd {
}
socket listen {
client {
path = /var/spool/postfix/private/auth
mode = 0660
user = postfix
group = postfix
}
}
}
</pre>
</blockquote>
<p> See the Dovecot documentation for how to configure and operate
the Dovecot authentication server. </p>
<h2><a name="server_cyrus">Cyrus SASL configuration for the Postfix
SMTP server</a></h2>
<p> In /usr/local/lib/sasl/smtpd.conf (Cyrus SASL version 1.5.5) or
/usr/local/lib/sasl2/smtpd.conf (Cyrus SASL version 2.1.1) you need to
specify how the server should validate client passwords. </p>
<p> Note: some Postfix distributions are modified and look for
the smtpd.conf file in /etc/postfix. </p>
<p> Note: some Cyrus SASL distributions look for the smtpd.conf
file in /etc/sasl2. </p>
<ul>
<li> <p> To authenticate against the UNIX password database, try: </p>
<dl>
<dt> (Cyrus SASL version 1.5.5)
<dd>
<pre>
/usr/local/lib/sasl/smtpd.conf:
pwcheck_method: pwcheck
</pre>
<dt> (Cyrus SASL version 2.1.1)
<dd>
<pre>
/usr/local/lib/sasl2/smtpd.conf:
pwcheck_method: pwcheck
</pre>
</dl>
<p> The name of the file in /usr/local/lib/sasl (Cyrus SASL version
1.5.5) or /usr/local/lib/sasl2 (Cyrus SASL version 2.1.1) used by
the SASL
library for configuration can be set with: </p>
<blockquote>
<pre>
/etc/postfix/main.cf:
smtpd_sasl_application_name = smtpd (Postfix < 2.3)
smtpd_sasl_path = smtpd (Postfix 2.3 and later)
</pre>
</blockquote>
<p> The pwcheck daemon is contained in the cyrus-sasl source tarball. </p>
<p> IMPORTANT: postfix processes need to have group read+execute
permission for the /var/pwcheck directory, otherwise authentication
attempts will fail. </p>
<li> <p> Alternately, in Cyrus SASL 1.5.26 and later (including
2.1.1), try: </p>
<dl>
<dt> (Cyrus SASL version 1.5.26)
<dd>
<pre>
/usr/local/lib/sasl/smtpd.conf:
pwcheck_method: saslauthd
</pre>
<dt> (Cyrus SASL version 2.1.1)
<dd>
<pre>
/usr/local/lib/sasl2/smtpd.conf:
pwcheck_method: saslauthd
</pre>
</dl>
<p> The saslauthd daemon is also contained in the cyrus-sasl source
tarball. It is more flexible than the pwcheck daemon, in that it
can authenticate against PAM and various other sources. To use PAM,
start saslauthd with "-a pam". </p>
<li> <p> To authenticate against Cyrus SASL's own password database: </p>
<dl>
<dt> (Cyrus SASL version 1.5.5)
<dd>
<pre>
/usr/local/lib/sasl/smtpd.conf:
pwcheck_method: sasldb
</pre>
<dt> (Cyrus SASL version 2.1.1)
<dd>
<pre>
/usr/local/lib/sasl2/smtpd.conf:
pwcheck_method: auxprop
</pre>
</dl>
<p> This will use the Cyrus SASL password file (default: /etc/sasldb in
version 1.5.5, or /etc/sasldb2 in version 2.1.1), which is maintained
with the saslpasswd or saslpasswd2 command (part of the Cyrus SASL
software). On some poorly-supported systems the saslpasswd command needs
to be run multiple times before it stops complaining. The Postfix SMTP
server needs read access to the sasldb file - you may have to play games
with group access permissions. With the OTP authentication mechanism,
the SMTP server also needs WRITE access to /etc/sasldb2 or /etc/sasldb
(or the back end SQL database, if used). </p>
<p> IMPORTANT: To get sasldb running, make sure that you set the SASL
domain (realm) to a fully qualified domain name. </p>
<p> EXAMPLE: </p>
<dl>
<dt> (Cyrus SASL version 1.5.5)
<dd>
<pre>
% saslpasswd -c -u `postconf -h myhostname` exampleuser
</pre>
<dt> (Cyrus SASL version 2.1.1)
<dd>
<pre>
% saslpasswd2 -c -u `postconf -h myhostname` exampleuser
</pre>
</dl>
<p> You can find out SASL's idea about the realms of the users
in sasldb with <i>sasldblistusers</i> (Cyrus SASL version 1.5.5) or
<i>sasldblistusers2</i> (Cyrus SASL version 2.1.1). </p>
<p> On the Postfix side, you can have only one realm per smtpd
instance, and only the users belonging to that realm would be able to
authenticate. The Postfix variable smtpd_sasl_local_domain controls the
realm used by smtpd: </p>
<blockquote>
<pre>
/etc/postfix/main.cf:
smtpd_sasl_local_domain = $myhostname
</pre>
</blockquote>
</ul>
<p> IMPORTANT: all users must be able to authenticate using ALL
authentication mechanisms advertised by Postfix, otherwise the
negotiation might end up with an unsupported mechanism, and
authentication would fail. For example if you configure SASL to
use <i>saslauthd</i> for authentication against PAM (pluggable
authentication modules), only the PLAIN and LOGIN mechanisms are
supported and stand a chance to succeed, yet the SASL library would also
advertise other mechanisms, such as DIGEST-MD5. This happens because
those mechanisms are made available by other plugins, and the SASL
library have no way to know that your only valid authentication source
is PAM. Thus you might need to limit the list of mechanisms advertised
by Postfix. </p>
<ul>
<li> <p> With older Cyrus SASL versions you remove the corresponding
library files from the SASL plug-in directory (and again whenever
the system is updated). </p>
<li> <p> With Cyrus SASL version 2.1.1 or later: </p>
<blockquote>
<pre>
/usr/local/lib/sasl2/smtpd.conf:
mech_list: plain login
</pre>
</blockquote>
</ul>
<p> For the same reasons you might want to limit the list of plugins
used for authentication. </p>
<ul>
<li> <p> With Cyrus SASL version 1.5.5 your only choice is to
delete the corresponding library files from the SASL plug-in
directory. </p>
<li> <p> With SASL version 2.1.1: </p>
<blockquote>
<pre>
/usr/local/lib/sasl2/smtpd.conf:
pwcheck_method: auxprop
auxprop_plugin: sql
</pre>
</blockquote>
</ul>
<p> To run software chrooted with SASL support is an interesting
exercise. It probably is not worth the trouble. </p>
<h2><a name="server_test">Testing SASL authentication in the Postfix
SMTP server</a></h2>
<p> To test the server side, connect to the SMTP server, and you should
be able to have a conversation as shown below. Information sent by the
client is shown in bold font. </p>
<blockquote>
<pre>
220 server.example.com ESMTP Postfix
<b>EHLO client.example.com</b>
250-server.example.com
250-PIPELINING
250-SIZE 10240000
250-ETRN
250-AUTH DIGEST-MD5 PLAIN CRAM-MD5
250 8BITMIME
<b>AUTH PLAIN dGVzdAB0ZXN0AHRlc3RwYXNz</b>
235 Authentication successful
</pre>
</blockquote>
<p> Instead of dGVzdAB0ZXN0AHRlc3RwYXNz, specify the base64 encoded
form of username\0username\0password (the \0 is a null byte). The
example above is for a user named `test' with password `testpass'.
</p>
<p> In order to generate base64 encoded authentication information
you can use one of the following commands: </p>
<blockquote>
<pre>
% printf 'username\0username\0password' | mmencode
</pre>
</blockquote>
<blockquote>
<pre>
% perl -MMIME::Base64 -e \
'print encode_base64("username\0username\0password");'
</pre>
</blockquote>
<p> The mmencode command is part of the metamail software.
MIME::Base64 is available from http://www.cpan.org/. </p>
<p> Caution: 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>
<h2><a name="debugging">Trouble shooting the SASL internals</a></h2>
<p> In the Cyrus SASL sources you'll find a subdirectory named
"sample". Run make there, "su" to the user <i>postfix</i> (or
whatever your <i>mail_owner</i> directive is set to):
<blockquote>
<pre>
% su postfix
</pre>
</blockquote>
<p> then run the resulting sample server and client in separate
terminals. Strace / ktrace / truss the server to see what makes
it unhappy, and fix the problem. Repeat the previous step until
you can successfully authenticate with the sample client. Only
then get back to Postfix. </p>
<h2><a name="client_sasl">Enabling SASL authentication in the
Postfix SMTP client</a></h2>
<p> Turn on client-side SASL authentication, and specify a table
with per-host or per-destination username and password information.
Postfix first searches the table for an entry with the server
hostname; if no entry is found, then Postfix searches the table for
an entry with the next-hop destination. Usually, that is the
right-hand part of an email address, but it can also be the information
that is specified with the relayhost parameter or with a transport(5)
table. </p>
<blockquote>
<pre>
/etc/postfix/main.cf:
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
smtp_sasl_type = cyrus
/etc/postfix/sasl_passwd:
foo.com username:password
bar.com username
[mail.myisp.net] username:password
[mail.myisp.net]:submission username:password
</pre>
</blockquote>
<p> Postfix version 2.3 supports-per-sender SASL password
information. To search the Postfix SASL password by sender
before it searches by destination, specify: </p>
<blockquote>
<pre>
/etc/postfix/main.cf:
smtp_sender_dependent_authentication = yes
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
/etc/postfix/sasl_passwd:
user@example.com username:password
bar.com username
[mail.myisp.net] username:password
[mail.myisp.net]:submission username:password
</pre>
</blockquote>
<p> Note: some SMTP servers support PLAIN or LOGIN authentication only.
By default, the Postfix SMTP client does not use authentication
methods that send plaintext passwords, and defers delivery with
the following error message: "Authentication failed: cannot SASL
authenticate to server". To enable plaintext authentication specify,
for example: </p>
<blockquote>
<pre>
/etc/postfix/main.cf:
smtp_sasl_security_options = noanonymous
</pre>
</blockquote>
<p> The Postfix SASL client password file is opened before the SMTP server
enters the optional chroot jail, so you can keep the file in
/etc/postfix. </p>
<p> Note: Some SMTP servers support authentication mechanisms that,
although available on the client system, may not in practice work or
possess the appropriate credentials to authenticate to the server. It
is possible via the smtp_sasl_mechanism_filter parameter to further
restrict the list of server mechanisms that the smtp(8) client will take
into consideration: </p>
<blockquote>
<pre>
/etc/postfix/main.cf:
smtp_sasl_mechanism_filter = !gssapi, !external, static:all
</pre>
</blockquote>
<p> In the above example, Postfix will decline to use mechanisms
that require special infrastructure such as Kerberos. </p>
<p> The Postfix SMTP client is backwards compatible with SMTP
servers that use the non-standard "AUTH=method..." syntax in response
to the EHLO command; there is no Postfix client configuration needed
to work around it. </p>
<h2><a name="credits">Credits</a></h2>
<ul>
<li> Postfix SASL support was originally implemented by Till Franke
of SuSE Rhein/Main AG.
<li> Wietse trimmed down the code to only the bare necessities.
<li> Support for Cyrus SASL version 2 was contributed by Jason Hoos.
<li> Liviu Daia added smtpd_sasl_application_name, split
reject_sender_login_mismatch into
reject_authenticated_sender_login_mismatch and
reject_unauthenticated_sender_login_mismatch, and revised the docs.
<li> Wietse made another iteration through the code to add plug-in
support for multiple SASL implementations, and changed
smtpd_sasl_application_name into smtpd_sasl_path.
<li> The Dovecot SMTP server-only plug-in was originally implemented by
Timo Sirainen of Procontrol, Finland.
</ul>
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