Postfix SASL Howto

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WARNING

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 SASL authentication enabled in the Postfix SMTP client and
SMTP server, Postfix becomes as secure as other mail systems that use the Cyrus
SASL library.

How Postfix uses SASL authentication information

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.

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.

Postfix does not record the client's SASL authentication information in message
headers, and does not pass it on via SMTP commands when forwarding mail,
because it is no-one else's business to know the client username and
authentication method. People who need to know can find the information in the
local Postfix maillog file. Some day, Postfix message headers will be
configurable and then one can record the SASL username without having to edit C
code.

This document covers the following topics:

  * What SASL versions are supported
  * Building the SASL library
  * Building Postfix with SASL authentication support
  * Enabling SASL authentication in the Postfix SMTP server
  * Testing SASL authentication in the Postfix SMTP server
  * Trouble shooting the SASL internals
  * Enabling SASL authentication in the Postfix SMTP client
  * Credits

When sending mail, Postfix can look up the server hostname or destination
domain (the address right-hand part) in a table, and if a username/password is
found, it will use that username and password to authenticate to the server.

What SASL versions are supported

Postfix+SASL 1.5.5 was seen working on RedHat 6.1 (pwcheck_method set to shadow
or sasldb), Solaris 2.7 (pwcheck_method set to shadow or sasldb), and FreeBSD
3.4 (pwcheck_method set to sasldb). On RedHat 6.1, SASL 1.5.5 insisted on write
access to /etc/sasldb. Note that this seems to be related to the
auto_transition switch in SASL. Note also that the Cyrus SASL documentation
says that it is pointless to enable that if you use "sasldb" for
"pwcheck_method". Later versions of the SASL 1.5.x series should also work.

Postfix+SASL 2.1.1 appears to work on Mandrake Linux 8.1 (pwcheck_method set to
saslauthd or auxprop). Note that the 'auxprop' pwcheck_method replaces the
'sasldb' method from SASL 1.5.x. Postfix may need write access to /etc/sasldb2
if you use the auto_transition feature, or if you use an authentication
mechanism such as OTP (one-time passwords) that needs to update secrets in the
database.

Building the SASL library

Postfix appears to work with cyrus-sasl-1.5.5 or cyrus-sasl-2.1.1, which are
available from:

    ftp://ftp.andrew.cmu.edu/pub/cyrus-mail/.

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.

Reportedly, Microsoft Internet Explorer version 5 requires the non-standard
SASL LOGIN authentication method. To enable this authentication method, specify
``./configure --enable-login''.

Building Postfix with SASL authentication support

To build Postfix with SASL authentication support, 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.

On some systems this generates the necessary Makefile definitions:

(for SASL version 1.5.5):

    % make tidy # if you have left-over files from a previous build
    % make makefiles CCARGS="-DUSE_SASL_AUTH -I/usr/local/include" \
        AUXLIBS="-L/usr/local/lib -lsasl"

(for SASL version 2.1.1):

    % make tidy # if you have left-over files from a previous build
    % make makefiles CCARGS="-DUSE_SASL_AUTH -I/usr/local/include/sasl" \
        AUXLIBS="-L/usr/local/lib -lsasl2"

On Solaris 2.x you need to specify run-time link information, otherwise ld.so
will not find the SASL shared library:

(for SASL version 1.5.5):

    % make tidy # if you have left-over files from a previous build
    % make makefiles CCARGS="-DUSE_SASL_AUTH -I/usr/local/include" \
        AUXLIBS="-L/usr/local/lib -R/usr/local/lib -lsasl"

(for SASL version 2.1.1):

    % make tidy # if you have left-over files from a previous build
    % make makefiles CCARGS="-DUSE_SASL_AUTH -I/usr/local/include/sasl" \
        AUXLIBS="-L/usr/local/lib -R/usr/local/lib -lsasl2"

Enabling SASL authentication in the Postfix SMTP server

In order to enable SASL support in the SMTP server:

    /etc/postfix/main.cf:
        smtpd_sasl_auth_enable = yes

In order to allow mail relaying by authenticated clients:

    /etc/postfix/main.cf:
        smtpd_recipient_restrictions =
            permit_mynetworks permit_sasl_authenticated ...

In /usr/local/lib/sasl/smtpd.conf (SASL version 1.5.5) or /usr/local/lib/sasl2/
smtpd.conf (SASL version 2.1.1) you need to specify how the server should
validate client passwords.

In order to authenticate against the UNIX password database, try:

(SASL version 1.5.5)

    /usr/local/lib/sasl/smtpd.conf:
        pwcheck_method: pwcheck

(SASL version 2.1.1)

    /usr/local/lib/sasl2/smtpd.conf:
        pwcheck_method: pwcheck

The name of the file in /usr/local/lib/sasl (SASL version 1.5.5) or /usr/local/
lib/sasl2 (SASL version 2.1.1) used by the SASL library for configuration can
be set with:

    /etc/postfix/main.cf:
        smtpd_sasl_application_name = smtpd

The pwcheck daemon is contained in the cyrus-sasl source tarball.

IMPORTANT: postfix processes need to have group read+execute permission for the
/var/pwcheck directory, otherwise authentication attempts will fail.

Alternately, in SASL 1.5.26 and later (including 2.1.1), try:

(SASL version 1.5.26)

    /usr/local/lib/sasl/smtpd.conf:
        pwcheck_method: saslauthd

(SASL version 2.1.1)

    /usr/local/lib/sasl2/smtpd.conf:
        pwcheck_method: saslauthd

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".

In order to authenticate against SASL's own password database:

(SASL version 1.5.5)

    /usr/local/lib/sasl/smtpd.conf:
        pwcheck_method:  sasldb

(SASL version 2.1.1)

    /usr/local/lib/sasl2/smtpd.conf:
        pwcheck_method:  auxprop

This will use the 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).

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 saslauthd 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. This is only possible with SASL version
2.1.1 or later:

    /usr/local/lib/sasl2/smtpd.conf:
        mech_list: plain login

For the same reasons you might want to limit the list of plugins used for
authentication. With SASL version 1.5.5 your only choice is to delete the
corresponding libraries from /usr/local/lib/sasl. With SASL version 2.1.1:

    /usr/local/lib/sasl2/smtpd.conf:
        pwcheck_method:  auxprop
        auxprop_plugin:  sql

IMPORTANT: To get sasldb running, make sure that you set the SASL domain
(realm) to a fully qualified domain name.

EXAMPLE:

(SASL version 1.5.5)

    % saslpasswd -c -u `postconf -h myhostname` exampleuser

(SASL version 2.1.1)

    % saslpasswd2 -c -u `postconf -h myhostname` exampleuser

You can find out SASL's idea about the realms of the users in sasldb with
sasldblistusers (SASL version 1.5.5) or sasldblistusers2 (SASL version 2.1.1).

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:

    /etc/postfix/main.cf:
        smtpd_sasl_local_domain = $myhostname

To run software chrooted with SASL support is an interesting exercise. It
probably is not worth the trouble.

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, set "broken_sasl_auth_clients = yes" in the
main.cf file.

Testing SASL authentication in the Postfix SMTP server

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.

    220 server.host.tld ESMTP Postfix
    EHLO client.host.tld
    250-server.host.tld
    250-PIPELINING
    250-SIZE 10240000
    250-ETRN
    250-AUTH DIGEST-MD5 PLAIN CRAM-MD5
    250 8BITMIME
    AUTH PLAIN dGVzdAB0ZXN0AHRlc3RwYXNz
    235 Authentication successful

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'.

In order to generate base64 encoded authentication information you can use one
of the following commands:

    % printf 'username\0username\0password' | mmencode

    % perl -MMIME::Base64 -e \
        'print encode_base64("username\0username\0password");'

The mmencode command is part of the metamail software. MIME::Base64 is
available from http://www.cpan.org/.

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.

Trouble shooting the SASL internals

In the Cyrus SASL sources you'll find a subdirectory named "sample". Run make
there, "su" to the user postfix (or whatever your mail_owner directive is set
to):

    % su postfix

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.

Enabling SASL authentication in the Postfix SMTP client

Turn on client-side SASL authentication, and specify a table with per-host or
per-destination username and password information. Postfix first looks up the
server hostname; if no entry is found, then Postfix looks up the destination
domain name (usually, the right-hand part of an email address).

    /etc/postfix/main.cf:
        smtp_sasl_auth_enable = yes
        smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd

    /etc/postfix/sasl_passwd:
        foo.com             username:password
        bar.com             username

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:

    /etc/postfix/main.cf:
        smtp_sasl_security_options =

The SASL client password file is opened before the SMTP server enters the
optional chroot jail, so you can keep the file in /etc/postfix.

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.

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.

Credits

  * Postfix SASL support was originally implemented by Till Franke of SuSE
    Rhein/Main AG.
  * Wietse trimmed down the code to only the bare necessities.
  * Support for SASL version 2 was contributed by Jason Hoos.
  * 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.

