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.
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:
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.
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.
Postfix appears to work with cyrus-sasl-1.5.5 or cyrus-sasl-2.1.1, which are available from:
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''.
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:
% 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"
% 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:
% 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"
% 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"
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:
/usr/local/lib/sasl/smtpd.conf: pwcheck_method: pwcheck
/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:
/usr/local/lib/sasl/smtpd.conf: pwcheck_method: saslauthd
/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:
/usr/local/lib/sasl/smtpd.conf: pwcheck_method: sasldb
/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.
% saslpasswd -c -u `postconf -h myhostname` exampleuser
% 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.
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.
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.
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.
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.