ClamAV

ClamAV is a cross-platform antivirus. While it is generic and can scan files in many situations, in this guide we will only configure it to scan emails. Using an antivirus on a mail server serve two goals:

  • You wouldn't like you or someone else getting a virus that transited trough your mail server
  • The huge majority of emails containing viruses is actually spam. This makes a great complement to Spamassassin.

Prerequisite

This article is part of the emails series. It is assumed that you already covered Exim.

Install

$ sudo apt install clamav clamdscan clamav-daemon

Configure

clamav-unofficial-sigs

The default signatures of ClamAV are quite useless against the rapidly changing zip attachments you find in spam.

Fortunately Sanesecurity provide for free some hourly updated databases that will detect them. Getting them on your system is as simple as

$ sudo apt install clamav-unofficial-sigs

As the version in Debian is quite outdated, it tries by default to download some files that don't exist anymore. This is fixed by creating file /etc/clamav-unofficial-sigs.conf.d/local.conf

# https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=783228
si_dbs=""

# https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=831607
mbl_dbs=""

Integrate with Exim

Permissions

The ClamAV daemon is running under user clamav. We need to get this user access to the mail spool for the scan to be possible.

$ sudo usermod -a -G Debian-exim clamav
$ sudo systemctl restart clamav-daemon.service

Exim setup

Create file /etc/exim4/local-acl with the following content

deny
  malware = *
  message = This message was detected as possible malware ($malware_name).

and append the following lines to /etc/exim4/conf.d/main/00_local_settings

CHECK_DATA_LOCAL_ACL_FILE = /etc/exim4/local-acl
av_scanner = clamd:/var/run/clamav/clamd.ctl

Finally restart Exim

$ sudo systemctl restart exim4.service

Test

Testing anti-virus software is commonly done using EICAR anti malware testfile. Although this file is not a virus, it will be recognised by all major anti-virus programs. This allows you to test them without risking infecting yourself.

To get the file, simply create a file with the following content

X5O!P%@AP[4\PZX54(P^)7CC)7}$EICAR-STANDARD-ANTIVIRUS-TEST-FILE!$H+H*

You can then email this file to yourself. It must be blocked by Exim.

Check the content of /var/log/exim4/rejectlog and /var/log/exim4/paniclog for any issue.