Complete Virtual Mail Server/SMTP Authentication

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This article is part of the Complete Virtual Mail Server series, and may require previous parts to have been read or followed.

So far only localhost is allowed to send mail. Unfortunately postfix cannot work with courier-authlib directly. An intermediary solution exists however, dev-libs/cyrus-sasl. There are three ways cyrus-sasl can get authentication information. Either directly from the database, locally or remotely. A setup using this approach would look like this:

  courier-imap -> courier-authlib --\
                                     +--> database 
  postfix ------> cyrus-sasl -------/

Making things only slightly more complex, cyrus-sasl can be used to communicate through courier-authlib and thus letting courier-authlib do the authentication:

  courier-imap -----------\
                           +-> courier-authlib -> database
  postfix -> cyrus-sasl --/

Ideally, the last option would be the used solution, as one authentication back-end would be used, courier-authlib. The cyrus-sasl plugin to talk to courier-authlib however will only work via a unix socket, and thus if courier-authlib is not running on the same host as cyrus-sasl, this would not work. The first approach should therefore only be used if courier-authlib can not be used.

Installing cyrus-sasl

A key feature of cyrus-sasl that is required is the crypt USE flag. It needs to be enabled or crypted passwords from the database cannot be authenticated with. Cyrus-sasl with the correct USE flag should have been pulled in earlier whilst emerging postfix.

The Gentoo patch to support crypted passwords directly from the database is not available for >cyrus-sasl-2.1.23, therefore go via net-libs/courier-authlib when using such a version.

USE flags for dev-libs/cyrus-sasl The Cyrus SASL (Simple Authentication and Security Layer)

authdaemond Add Courier-IMAP authdaemond unix socket support (net-mail/courier-imap, mail-mta/courier)
berkdb Add support for sys-libs/db (Berkeley DB for MySQL)
gdbm Add support for sys-libs/gdbm (GNU database libraries)
kerberos Add kerberos support
ldapdb Enable ldapdb plugin
mysql Add mySQL Database support
openldap Add ldap support for saslauthd
pam Add support for PAM (Pluggable Authentication Modules) - DANGEROUS to arbitrarily flip
postgres Add support for the postgresql database
sample Enable sample client and server
selinux !!internal use only!! Security Enhanced Linux support, this must be set by the selinux profile or breakage will occur
sqlite Add support for sqlite - embedded sql database
srp Enable SRP authentication
ssl Add support for SSL/TLS connections (Secure Socket Layer / Transport Layer Security)
static-libs Build static versions of dynamic libraries as well
urandom Use /dev/urandom instead of /dev/random

Whatever database choice was made before, support for it will be built here as well. If for any reason, mostly security, the additional database code is not wanted, it is possible to emerge without any of the database useflags set.
The database USE flags should also be removed if going via courier-authlib rather than directly to the database, as cyrus-sasl will continually log errors when compiled with SQL support without being configured to use it.

Configuring postfix with cyrus-sasl

Postfix needs a few options to tell it to use sasl in its These are not mentioned in the default config file so they should be added:

FILE /etc/postfix/main.cfAdd sasl support to postfix.
# Postfix to SASL authentication
broken_sasl_auth_clients = no
smtpd_sasl_auth_enable = yes
smtpd_sasl_security_options = noanonymous
smtpd_sasl_local_domain =
smtpd_sasl_authenticated_header = yes
smtpd_recipient_restrictions = permit_sasl_authenticated, permit_mynetworks, reject_unauth_destination
smtpd_sasl_authenticated_header prints the username used to authenticate with cyrus when sending mail into the mail header. Especially dangerous for mails that go to publicly archived mailing lists, this header will be seen world wide. For testing purposes it is enabled here.

Configuring cyrus-sasl

With authdaemond

Postfix queries the socket created by authdaemond which is protected by the mail user and group and thus postfix needs to be granted access:

root #gpasswd -a postfix mail

Next cyrus-sasl needs to be told to authenticate with authdaemond:

FILE /etc/sasl2/smtpd.confAuthenticate with authdaemond
pwcheck_method: authdaemond
mech_list: LOGIN PLAIN
sql_select: dummy 
authdaemond_path: /var/lib/courier/authdaemon/socket
log_level: 5

With postgresql

FILE /etc/sasl2/smtpd.confDirect database authentication
sasl_pwcheck_method: auxprop
sasl_auxprop_plugin: pgsql
password_format: crypt
mech_list: LOGIN PLAIN
sql_engine: pgsql
#sql_hostnames: localhost
sql_database: postfix
sql_user: postfix
sql_passwd: $password
sql_select: SELECT password FROM mailbox WHERE local_part='%u' AND active='1'


To verify sasl support telnet can be used to check for the AUTH statement:

user $telnet 25
220 ESMTP Postfix
250-SIZE 10240000
250 DSN
221 2.0.0 Bye
Connection closed by foreign host.
If a mail-client is being used that does not work properly, broken_sasl_auth_clients = yes can be used in postfix's to get an additional entry here, 250-AUTH=LOGIN PLAIN.

The next test is to use a remote host and try to login to send a test message.

Although it is quite common to authenticate against mail servers (pop/imap/webmail/smtp) through plain text logins, it is incredibly insecure (and can be protected against, which is described later in this guide). When sending plain text login data (it is base64 encoded data so very easy to decode) over the big bad Internet, only use a testuser or at least a test password.

If perl with the base64 module is installed, it can be used to generate base64 encoded data. Otherwise base64 conversion can be done online. Again, be very careful when using production data on untrusted sites.

user $perl -MMIME::Base64 -e 'print encode_base64("testuser");'
user $telnet 25
Connected to
Escape character is '^]'.
220 ESMTP Postfix
334 VXNlcm5hbWU6 (base64 decode: 'Username:')
dGVzdHVzZXI= (base64 encoded from: 'testuser')
334 UGFzc3dvcmQ6 (base64 decode: 'Password:')
c2VjcmV0 (base64 encoded from: 'secret')
235 2.7.0 Authentication successful
250 2.1.0 Ok
rcpt to:<validuser>@<validexternaldomain>.<tld>
250 2.1.5 Ok
354 End data with <CR><LF>.<CR><LF>
Subject: Test message
Test message to ensure Postfix is only relaying with smtp authorization.
250 2.0.0 Ok: queued as 82F97606
221 2.0.0 Bye
Connection closed by foreign host.

Wrapping it up

Once everything is working as expected, debugging can be disabled (or the line can be removed entirely):

FILE /etc/sasl2/smtpd.confDisable debugging
log_level: 0

Optionally, smtpd_sasl_authenticated_header can be disabled again. It is very handy for tracking down mailing issues from users. It can however be potentially a security issue, as mentioned above, the users login name is written in the header. On the other hand, if the login name is the local_part of the e-mail address or even the e-mail address then the login name is already known anyway so no big harm there, right? Some caution is advised, but it shouldn't be a huge issue.

FILE /etc/postfix/main.cfAdd sasl support to postfix
smtpd_sasl_authenticated_header = no