Integrating amavisd-new Into Postfix For Spam- And Virus-Scanning

Version 1.0
Author: Falko Timme

This article shows how to integrate amavisd-new into a Postfix mail server for spam- and virus-scanning. amavisd-new is a high-performance interface between MTAs such as Postfix and content checkers: virus scanners, and/or SpamAssassin. We will use ClamAV for virus scanning and SpamAssassin for spam scanning in this tutorial.

I want to say first that this is not the only way of setting up such a system. There are many ways of achieving this goal but this is the way I take. I do not issue any guarantee that this will work for you!

1 Preliminary Note

In this tutorial I will describe how to install/configure amavisd-new on Debian Etch/Ubuntu 6.10 Edgy Eft (the steps are identical for both distributions) and on Debian Sarge. The amavisd-new configuration is suitable for systems where system users are used for email accounts; if you use virtual users, a few modifications to the amavisd-new configuration might or might not be required (but that depends on the actual setup). If you use virtual email users, take a look at these two tutorials:

• Virtual Users And Domains With Postfix, Courier And MySQL (Fedora Core 5)
• Virtual Users And Domains With Postfix, Courier And MySQL (Debian Sarge)

amavisd-new works like this: Postfix receives an email on port 25, passes it to amavisd-new on port 10024 which then invokes ClamAV and SpamAssassin, and afterwards amavisd-new re-injects the mail into Postfix on port 10025 which then finally delivers the mail (if it's clean). Here's a small layout of the process:

                                      [SpamAssassin]
                                             ^
                                             |
 Email --> [(Port 25) Postfix] --> [(10024) amavisd-new] --> [(10025) Postfix] --> Mailbox
                                             |
                                             v
                                          [ClamAV]

I assume that Postfix and your email accounts are already set up and working as I don't cover Postfix installation/configuration here.

All steps in this tutorial are done as root, so make sure you're logged in as root.

2 Debian Etch/Ubuntu 6.10 Edgy Eft

First we install amavisd-new, SpamAssassin, and ClamAV together with a few other programs (mainly programs that amavisd-new needs to unpack archives, because emails can contain archives as attachments):

apt-get install amavisd-new spamassassin clamav clamav-daemon zoo unzip bzip2 unzoo libnet-ph-perl libnet-snpp-perl libnet-telnet-perl nomarch lzop

Afterwards we must configure amavisd-new. The configuration is split up in various files which reside in the /etc/amavis/conf.d directory. Take a look at each of them to become familiar with the configuration. Most settings are fine, however we must modify two files:

First we must enable ClamAV and SpamAssassin in /etc/amavis/conf.d/15-content_filter_mode by uncommenting the @bypass_virus_checks_maps and the @bypass_spam_checks_maps lines:

vi /etc/amavis/conf.d/15-content_filter_mode

The file should look like this:

use strict;

# You can modify this file to re-enable SPAM checking through spamassassin
# and to re-enable antivirus checking.

#
# Default antivirus checking mode
# Uncomment the two lines below to enable it back
#

@bypass_virus_checks_maps = (
   \%bypass_virus_checks, \@bypass_virus_checks_acl, \$bypass_virus_checks_re);


#
# Default SPAM checking mode
# Uncomment the two lines below to enable it back
#

@bypass_spam_checks_maps = (
   \%bypass_spam_checks, \@bypass_spam_checks_acl, \$bypass_spam_checks_re);

1;  # insure a defined return

And then you should take a look at the spam settings and the actions for spam-/virus-mails in /etc/amavis/conf.d/20-debian_defaults. There's no need to change anything if the default settings are ok for you. The file contains many explanations so there's no need to explain the settings here:

vi /etc/amavis/conf.d/20-debian_defaults

$QUARANTINEDIR = "$MYHOME/virusmails";

$log_recip_templ = undef;    # disable by-recipient level-0 log entries
$DO_SYSLOG = 1;              # log via syslogd (preferred)
$syslog_ident = 'amavis';    # syslog ident tag, prepended to all messages
$syslog_facility = 'mail';
$syslog_priority = 'debug';  # switch to info to drop debug output, etc

$enable_db = 1;              # enable use of BerkeleyDB/libdb (SNMP and nanny)
$enable_global_cache = 1;    # enable use of libdb-based cache if $enable_db=1

$inet_socket_port = 10024;   # default listenting socket

$sa_spam_subject_tag = '***SPAM*** ';
$sa_tag_level_deflt  = 2.0;  # add spam info headers if at, or above that level
$sa_tag2_level_deflt = 6.31; # add 'spam detected' headers at that level
$sa_kill_level_deflt = 6.31; # triggers spam evasive actions
$sa_dsn_cutoff_level = 10;   # spam level beyond which a DSN is not sent

$sa_mail_body_size_limit = 200*1024; # don't waste time on SA if mail is larger
$sa_local_tests_only = 0;    # only tests which do not require internet access?

[...]
$final_virus_destiny      = D_DISCARD;  # (data not lost, see virus quarantine)
$final_banned_destiny     = D_BOUNCE;   # D_REJECT when front-end MTA
$final_spam_destiny       = D_BOUNCE;
$final_bad_header_destiny = D_PASS;     # False-positive prone (for spam)
[...]

Afterwards, run these commands to add the clamav user to the amavis group and to restart amavisd-new and ClamAV:

adduser clamav amavis
 /etc/init.d/amavis restart
 /etc/init.d/clamav-daemon restart

Next we must edit the configuration file of the Freshclam daemon (that's the daemon that regularly and automatically fetches the newest virus signatures from a ClamAV mirror) because it contains a small bug. Open /etc/clamav/freshclam.conf and modify the NotifyClamd line as shown below:

vi /etc/clamav/freshclam.conf

[...]
NotifyClamd /etc/clamav/clamd.conf
[...]

Then restart Freshclam (make sure no other Freshclam process (maybe of another ClamAV installation) is running because then our Freshclam will fail to start):

/etc/init.d/clamav-freshclam restart

Now we have to configure Postfix to pipe incoming email through amavisd-new:

postconf -e 'content_filter = amavis:[127.0.0.1]:10024'
 postconf -e 'receive_override_options = no_address_mappings'

Afterwards append the following lines to /etc/postfix/master.cf:

vi /etc/postfix/master.cf

[...]
amavis unix - - - - 2 smtp
        -o smtp_data_done_timeout=1200
        -o smtp_send_xforward_command=yes

127.0.0.1:10025 inet n - - - - smtpd
        -o content_filter=
        -o local_recipient_maps=
        -o relay_recipient_maps=
        -o smtpd_restriction_classes=
        -o smtpd_client_restrictions=
        -o smtpd_helo_restrictions=
        -o smtpd_sender_restrictions=
        -o smtpd_recipient_restrictions=permit_mynetworks,reject
        -o mynetworks=127.0.0.0/8
        -o strict_rfc821_envelopes=yes
        -o receive_override_options=no_unknown_recipient_checks,no_header_body_checks
        -o smtpd_bind_address=127.0.0.1

Then restart Postfix:

/etc/init.d/postfix restart

Now run

netstat -tap

and you should see Postfix (master) listening on port 25 (smtp) and 10025, and amavisd-new on port 10024:

server1:~# netstat -tap
 Active Internet connections (servers and established)
 Proto Recv-Q Send-Q Local Address           Foreign Address         State       PID/Program name
 tcp        0      0 *:4069                  *:*                     LISTEN     3457/rpc.statd
 tcp        0      0 localhost.localdo:10024 *:*                     LISTEN     6886/amavisd (maste
 tcp        0      0 localhost.localdo:10025 *:*                     LISTEN     7268/master
 tcp        0      0 *:sunrpc                *:*                     LISTEN     3083/portmap
 tcp        0      0 *:auth                  *:*                     LISTEN     3417/inetd
 tcp        0      0 *:smtp                  *:*                     LISTEN     7268/master
 tcp6       0      0 *:imaps                 *:*                     LISTEN     4952/couriertcpd
 tcp6       0      0 *:pop3s                 *:*                     LISTEN     4872/couriertcpd
 tcp6       0      0 *:pop3                  *:*                     LISTEN     4815/couriertcpd
 tcp6       0      0 *:imap2                 *:*                     LISTEN     4905/couriertcpd
 tcp6       0      0 *:ssh                   *:*                     LISTEN     3438/sshd
 tcp6       0      0 *:smtp                  *:*                     LISTEN     7268/master
 tcp6       0    148 server1.example.com:ssh localhost:3117          ESTABLISHED3519/0

If you like you can now add Razor, Pyzor, and DCC to SpamAssassin to improve its filtering performance. Razor, Pyzor and DCC are spamfilters that use a collaborative filtering network. To install them, run

apt-get install razor pyzor dcc-client

Now we have to tell SpamAssassin to use these three programs. Edit /etc/spamassassin/local.cf and add the following lines to it:

vi /etc/spamassassin/local.cf

[...]

# dcc
use_dcc 1
dcc_path /usr/bin/dccproc
dcc_add_header 1
dcc_dccifd_path /usr/sbin/dccifd

#pyzor
use_pyzor 1
pyzor_path /usr/bin/pyzor
pyzor_add_header 1

#razor
use_razor2 1
razor_config /etc/razor/razor-agent.conf

#bayes
use_bayes 1
use_bayes_rules 1
bayes_auto_learn 1

Restart amavisd-new afterwards:

/etc/init.d/amavis restart

That's it already. Now watch your mail log (/var/log/mail.log) to see if amavisd-new is working properly. amavisd-new will log whenever it finds a spam or virus email. When you (re)start amavisd-new it should also log that it loads its spam and virus scanning code (if not, you probably did something wrong).

For taking a live look at your mail log, you can use this command:

tail -f /var/log/mail.log

(Press CTRL + c to leave the log.)