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Postfix SASL Howto


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.

This document covers the following topics:

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 ...

To report SASL login names in Received: message headers (Postfix version 2.3 and later):

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

Note: the SASL login names will be shared with the entire world.

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 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.

/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
    [mail.myisp.net]            username:password
    [mail.myisp.net]:submission username:password

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