Postfix SASL HowtoPostfix SASL HowtoPeople 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.
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.
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.
This document covers the following topics:
This document describes Postfix with the following SASL implementations:
Cyrus SASL version 1 (client and server).
Cyrus SASL version 2 (client and server).
Dovecot protocol version 1 (server only, Postfix version 2.3 and later)
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:
% postconf -a (SASL support in the SMTP server) % postconf -A (SASL support in the SMTP+LMTP client)
Needless to say, these commands are not available in earlier Postfix versions.
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.
To generate the necessary Makefiles, execute the following in the Postfix top-level directory:
% make makefiles CCARGS='-DUSE_SASL_AUTH -DDEF_SERVER_SASL_TYPE=\"dovecot\"'
After this, proceed with "make" as described in the INSTALL document.
Notes:
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.
If you also want support for LDAP or TLS, you will have to merge their CCARGS and AUXLIBS into the above command line.
Postfix appears to work with cyrus-sasl-1.5.x or cyrus-sasl-2.1.x, 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.x or /usr/lib/sasl2 -> /usr/local/lib/sasl2 for version 2.1.x.
Reportedly, Microsoft Outlook (Express) requires the non-standard LOGIN authentication method. To enable this authentication method, specify ``./configure --enable-login''.
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 -DUSE_CYRUS_SASL \
-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 -DUSE_CYRUS_SASL \
-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 -DUSE_CYRUS_SASL \
-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 -DUSE_CYRUS_SASL \
-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 ...
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.
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=mechanism-list" instead of "250 AUTH mechanism-list". To accommodate such clients (in addition to conformant clients) use the following:
/etc/postfix/main.cf:
broken_sasl_auth_clients = yes
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:
/etc/postfix/main.cf:
smtpd_sasl_type = dovecot
smtpd_sasl_path = private/auth
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/.
/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
}
}
}
See the Dovecot documentation for how to configure and operate the Dovecot authentication server.
You need to configure how the Cyrus SASL library should authenticate a client's username and password. These settings must be stored in a separate configuration file.
The name of the configuration file (default: smtpd.conf) will be constructed from a value sent by Postfix to the Cyrus SASL library, which adds the suffix .conf. The value is configured using one of the following variables:
/etc/postfix/main.cf:
# Postfix 2.3 and later
smtpd_sasl_path = smtpd
# Postfix < 2.3
smtpd_sasl_application_name = smtpd
Cyrus SASL searches for the configuration file in /usr/local/lib/sasl/ (Cyrus SASL version 1.5.5) or /usr/local/lib/sasl2/ (Cyrus SASL version 2.1.x).
Note: some Postfix distributions are modified and look for the smtpd.conf file in /etc/postfix/sasl.
Note: some Cyrus SASL distributions look for the smtpd.conf file in /etc/sasl2.
To authenticate against the UNIX password database, use:
/usr/local/lib/sasl/smtpd.conf:
pwcheck_method: pwcheck
IMPORTANT: pwcheck establishes a UNIX domain socket in /var/pwcheck and waits for authentication requests. Postfix processes must have read+execute permission to this directory or authentication attempts will fail.
The pwcheck daemon is contained in the cyrus-sasl source tarball.
/usr/local/lib/sasl/smtpd.conf:
pwcheck_method: saslauthd
/usr/local/lib/sasl2/smtpd.conf:
pwcheck_method: saslauthd
mech_list: PLAIN LOGIN
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".
IMPORTANT: saslauthd usually establishes a UNIX domain socket in /var/run/saslauthd and waits for authentication requests. Postfix processes must have read+execute permission to this directory or authentication attempts will fail.
Note: The directory where saslauthd puts the socket is configurable. See the command-line option "-m /path/to/socket" in the saslauthd --help listing.
To authenticate against Cyrus SASL's own password database:
/usr/local/lib/sasl/smtpd.conf:
pwcheck_method: sasldb
/usr/local/lib/sasl2/smtpd.conf:
pwcheck_method: auxprop
auxprop_plugin: sasldb
mech_list: PLAIN LOGIN CRAM-MD5 DIGEST-MD5
This will use the Cyrus SASL password file (default: /etc/sasldb in version 1.5.x, or /etc/sasldb2 in version 2.1.x), 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: To get sasldb running, make sure that you set the SASL domain (realm) to a fully qualified domain name.
EXAMPLE:
% 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 (Cyrus SASL version 1.5.x) or sasldblistusers2 (Cyrus SASL version 2.1.x).
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
IMPORTANT: The Cyrus SASL password verification services pwcheck and saslauthd can only support the plaintext mechanisms PLAIN or LOGIN. However, the Cyrus SASL library doesn't know this, and will happily advertise other authentication mechanisms that the SASL library implements, such as DIGEST-MD5. As a result, if an SMTP client chooses any mechanism other than PLAIN or LOGIN while pwcheck or saslauthd are used, authentication will fail. Thus you may need to limit the list of mechanisms advertised by Postfix.
With older Cyrus SASL versions you remove the corresponding library files from the SASL plug-in directory (and again whenever the system is updated).
With Cyrus SASL version 2.1.x or later the mech_list variable can specify a list of authentication mechanisms that Cyrus SASL may offer:
/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 Cyrus SASL version 1.5.x your only choice is to delete the corresponding library files from the SASL plug-in directory.
With SASL version 2.1.x:
/usr/local/lib/sasl2/smtpd.conf:
pwcheck_method: auxprop
auxprop_plugin: sql
To run software chrooted with SASL support is an interesting exercise. It probably is not worth the trouble.
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.example.com ESMTP Postfix EHLO client.example.com 250-server.example.com 250-PIPELINING 250-SIZE 10240000 250-ETRN 250-AUTH DIGEST-MD5 PLAIN CRAM-MD5 250 8BITMIME AUTH PLAIN AHRlc3QAdGVzdHBhc3M= 235 Authentication successful
Instead of AHRlc3QAdGVzdHBhc3M=, specify the base64 encoded form of \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 '\0username\0password' | mmencode
% perl -MMIME::Base64 -e \
'print encode_base64("\0username\0password");'
The mmencode command is part of the metamail software. MIME::Base64 is available from http://www.cpan.org/.
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.
In the Cyrus SASL sources you'll find a subdirectory named "sample". Run make there, then create a symbolic link from sample.conf to smtpd.conf in your Cyrus SASL library directory /usr/local/lib/sasl2. "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. The sample applications send log messages to the syslog facility auth. Check the log to fix the problem or run strace / ktrace / truss on the server to see what makes it unhappy. 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 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
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
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 and set permissions read / write only for root to keep the username:password combinations away from other system users.
Postfix version 2.3 supports-per-sender SASL password information. To search the Postfix SASL password by sender before it searches by destination, specify:
/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
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 = noanonymous
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:
/etc/postfix/main.cf:
smtp_sasl_mechanism_filter = !gssapi, !external, static:all
In the above example, Postfix will decline to use mechanisms that require special infrastructure such as Kerberos or TLS.
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.