ClamSMTP: An SMTP Virus Filter
Clamsmtp is a very simple virus filtering tool for any SMTP server. It is very usable with the Postfix MTA, so the following article applies to this and gives you an example of a simple configuration.
The basic requirements are a working Postfix installation with a bunch of users and a working Clamav daemon, so be sure you have installed and configured them properly.
Before you install Clamsmtp, install and configure Postfix, create users for your SMTP server, and test if it is working. Install Clamav, and test it too.
If both of your tools work well, you can install clamsmtp from AUR, by makepkg or by yaourt:
# yaourt -S clamsmtpd
change the line:
Now, we will configure the daemon, by editing
/etc/clamav/clamsmtpd.conf. You can erase the original file or simply make a backup of it. Create a new file with this contents:
# Simple clamsmtpd config file OutAddress: 10025 Listen: 127.0.0.1:10026 TempDirectory: /var/spool/clamsmtp User: clamav
Clamsmtp works as a daemon. The workflow is simple, it listens on a port specified in its configuration file, catches the mails, scans them via Clamav, and then it pushes them back to Postfix via another port. Here, the daemon will listen on port 10026, then scan the mails as user clamav, and will send them back to Postfix on port 10025.
Next we create the cache for clamsmtp by:
mkdir /var/spool/clamsmtp chown clamav:clamav /var/spool/clamsmtp
(for whatever reason, the default TempDirectory:
/tmp returns permission errors )
/etc/clamav/clamd.conf, and uncomment the line ( normally, it is already done ):
Now, we have to configure Postfix to work together with Clamsmtp. Edit
/etc/postfix/main.cf, and add this two lines to the end of the file:
content_filter = scan:127.0.0.1:10026 receive_override_options = no_address_mappings
Postfix will send mails to localhost on port 10026.
scan unix - - n - 16 smtp -o smtp_send_xforward_command=yes # For injecting mail back into postfix from the filter 127.0.0.1:10025 inet n - n - 16 smtpd -o content_filter= -o receive_override_options=no_unknown_recipient_checks,no_header_body_checks -o smtpd_helo_restrictions= -o smtpd_client_restrictions= -o smtpd_sender_restrictions= -o smtpd_recipient_restrictions=permit_mynetworks,reject -o mynetworks_style=host -o smtpd_authorized_xforward_hosts=127.0.0.0/8
The first two lines create the service „scan”, the others take charge of accepting the already scanned mail from Clamsmtp from port 10025 and delivering them to the recipients.
Now, test your server:
/etc/rc.d/clamav restart /etc/rc.d/postfix restart /etc/rc.d/clamsmtp restart
Send yourself a mail, without any viruses
If you do not have any arriving mails, check
/var/log/mail.log for errors
Then, download a test-virus
and send it as an attachment.
Check your server's logfile again, you should get something similar to this:
May 23 00:04:08 servername postfix/smtp: A9B941F911: to=<firstname.lastname@example.org>, relay=127.0.0.1[127.0.0.1]:10026, delay=0.13, delays=0.08/0/0.04/0, dsn=2.0.0, status=sent (250 Virus Detected; Discarded Email)