ClamAV

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Clam AntiVirus is an open source (GPL) anti-virus toolkit for UNIX. It provides a number of utilities including a flexible and scalable multi-threaded daemon, a command line scanner and advanced tool for automatic database updates. Because ClamAV's main use is on file/mail servers for Windows desktops, it primarily detects Windows viruses and malware with its built-in signatures.

Installation

Install the clamav package.

Updating database

Update the virus definitions with:

# freshclam

If you are behind a proxy, edit /etc/clamav/freshclam.conf and update HTTPProxyServer, HTTPProxyPort, HTTPProxyUsername and HTTPProxyPassword.

The database files are saved in:

/var/lib/clamav/daily.cvd
/var/lib/clamav/main.cvd
/var/lib/clamav/bytecode.cvd

Start/enable clamav-freshclam.service so that the virus definitions are kept recent.

Starting the daemon

Note:
  • You will need to run freshclam before starting the service for the first time or you will run into trouble/errors which will prevent ClamAV from starting correctly.
  • The daemon is not needed if you only want to perform stand-alone scans. See Scan for viruses below.

The service is called clamav-daemon.service. Start it and enable it to start at boot.

Testing the software

In order to make sure ClamAV and the definitions are installed correctly, scan the EICAR test file (a harmless signature with no virus code) with clamscan.

$ curl https://secure.eicar.org/eicar.com.txt | clamscan -

The output must include:

stdin: Win.Test.EICAR_HDB-1 FOUND

Otherwise; read the Troubleshooting part or ask for help in the Arch Forums.

Adding more databases/signatures repositories

ClamAV can use databases/signature from other repositories or security vendors.

To add the most important ones in a single step, install either clamav-unofficial-sigsAUR (see GitHub description) or python-fangfrischAUR (see online documentation). Both will add signatures/databases from popular providers, e.g. MalwarePatrol, SecuriteInfo, Yara, Linux Malware Detect, etc.

Note: You still must have the clamav-freshclam.service started in order to have official signature updates from ClamAV mirrors.

Option #1: Set up Fangfrisch

Fangfrisch was designed as a more secure, flexible and convenient replacement for clamav-unofficial-sigs, and requires very little configuration.

Most importantly, Fangfrisch never needs to be run with root permissions, unlike clamav-unofficial-sigs.

Create database structure by running:

# sudo -u clamav /usr/bin/fangfrisch --conf /etc/fangfrisch/fangfrisch.conf initdb

Enable the fangfrisch.timer.

Option #2: Set up clamav-unofficial-sigs

Enable the clamav-unofficial-sigs.timer.

This will regularly update the unofficial signatures based on the configuration files in the directory /etc/clamav-unofficial-sigs.

To update signatures manually, run the following:

# clamav-unofficial-sigs.sh

To change any default settings, refer and modify /etc/clamav-unofficial-sigs/user.conf.

MalwarePatrol database

If you would like to use the MalwarePatrol database, sign up for an account at https://www.malwarepatrol.net/free-guard-upgrade-option.

In /etc/clamav-unofficial-sigs/user.conf, change the following to enable this functionality:

malwarepatrol_receipt_code="YOUR-RECEIPT-NUMBER" # enter your receipt number here
malwarepatrol_product_code="8" # Use 8 if you have a Free account or 15 if you are a Premium customer.
malwarepatrol_list="clamav_basic" # clamav_basic or clamav_ext
malwarepatrol_free="yes" # Set to yes if you have a Free account or no if you are a Premium customer.

Source: https://www.malwarepatrol.net/clamav-configuration-guide/

Scan for viruses

There are two options for on-demand scanning:

using the stand-alone scanner

clamscan can be used to scan certain files, home directories, or an entire system:

$ clamscan myfile
$ clamscan --recursive --infected /home
# clamscan --recursive --infected --exclude-dir='^/sys|^/dev' /

If you would like clamscan to remove the infected file add to the command the --remove option, or you can use --move=/dir to quarantine them.

You may also want clamscan to scan larger files. In this case, append the options --max-filesize=4000M and --max-scansize=4000M to the command. '4000M' is the largest possible value, and may be lowered as necessary.

Using the -l /path/to/file option will print the clamscan logs to a text file for locating reported infections.

using the daemon

clamdscan is similar to the above but utilizes the daemon, which must be running for the command to work. Most options are ignored since the daemon reads the the settings specified in /etc/clamav/clamd.conf.

Using the milter

Milter will scan your sendmail server for email containing virus. Adjust /etc/clamav/clamav-milter.conf to your needs. For example:

/etc/clamav/clamav-milter.conf
MilterSocket /tmp/clamav-milter.socket
MilterSocketMode 660
FixStaleSocket yes
User clamav
MilterSocketGroup clamav
PidFile /run/clamav/clamav-milter.pid
TemporaryDirectory /tmp
ClamdSocket unix:/run/clamav/clamd.ctl
LogSyslog yes
LogInfected Basic

Create /etc/systemd/system/clamav-milter.service:

/etc/systemd/system/clamav-milter.service
[Unit]
Description='ClamAV Milter'
After=clamav-daemon.service

[Service]
Type=forking
ExecStart=/usr/bin/clamav-milter --config-file /etc/clamav/clamav-milter.conf

[Install]
WantedBy=multi-user.target

Enable and start clamav-milter.service.

For Postfix add the following lines to /etc/postfix/main.cf:

etc/postfix/main.cf
smtpd_milters = unix:/tmp/clamav-milter.socket
milter_default_action = tempfail

Check journalctl if the permission to access clamav-milter.socket for postfix is set accordingly, if not, add user postfix to group clamav.

OnAccessScan

(Original source: https://blog.clamav.net/2016/03/configuring-on-access-scanning-in-clamav.html)

On-access scanning will scan the file while reading, writing or executing it.

Warning: At this point (November 2021), on-access scanning in ClamAV is still immature, you might see unexplained clamd crashes, and containerized processes do not get the protection. Therefore, it is debatable whether it is a security improvement or not. Do not enable it unless regulations, contracts, or your own curiosity force you to do so.

On-access scanning requires the kernel to be compiled with the fanotify kernel module (kernel >= 3.8), which is true for official Arch Linux kernel packages. Check if fanotify has been enabled before enabling on-access scanning.

$ zgrep FANOTIFY /proc/config.gz

First, edit the /etc/clamav/clamd.conf configuration file by adding the following to the end of the file (you can also change the individual options).

Warning: Please note the following warning about clamd port security if changing other parts of the file. Generally use an local unix socket but note that this port is not authenticated or protected. See https://docs.clamav.net/manual/Usage/Scanning.html#clamd-v0101 for more details in the Daemon and then ClamD section.
/etc/clamav/clamd.conf
# Set the mount point where to recursively perform the scan,
# this could be every path or multiple path (one line for path)
OnAccessMountPath /
OnAccessExcludePath /proc
OnAccessExcludePath /run
OnAccessExcludePath /sys

# Alternatively, add some directories instead of mount points
# OnAccessIncludePath /home

# Prevention doesn't work with OnAccessMountPath.
# It works with OnAccessIncludePath, as long as /usr and /etc are not included.
# Including /var while activating prevention is also not recommended, because
# this would slow down package installation by a factor of 1000.
OnAccessPrevention false

# Perform scans on newly created, moved, or renamed files
OnAccessExtraScanning true

# Exclude the UID of the scanner itself from checking, to prevent loops
OnAccessExcludeUname clamav

# Optionally, also exclude root-owned processes
# OnAccessExcludeRootUID true

# Specify an action to perform when clamav detects a malicious file
# it is possible to specify an inline command too
VirusEvent /etc/clamav/virus-event.bash

# WARNING: for security reasons, clamd should NEVER run as root.
# Previous instructions in this wiki included this line, remove it:
# User root    # REMOVE THIS
# Add this instead:
User clamav

Next, allow the clamav user to run notify-send as any user with custom environment variables via sudo:

/etc/sudoers.d/clamav
clamav ALL = (ALL) NOPASSWD: SETENV: /usr/bin/notify-send

Next, create the file /etc/clamav/virus-event.bash and add the following. This allows you to change/specify the message when a virus has been detected by clamd's on-access scanning service:

/etc/clamav/virus-event.bash
#!/bin/bash
PATH=/usr/bin
ALERT="Signature detected by clamav: $CLAM_VIRUSEVENT_VIRUSNAME in $CLAM_VIRUSEVENT_FILENAME"

# Send an alert to all graphical users.
for ADDRESS in /run/user/* ; do
    USERID=${ADDRESS#/run/user/}
    /usr/bin/sudo -u "#$USERID" DBUS_SESSION_BUS_ADDRESS="unix:path=$ADDRESS/bus" PATH=${PATH} \
        /usr/bin/notify-send -i dialog-warning "Virus found!" "$ALERT"
done

Restart the clamav-daemon.service.

By default, clamonacc passes clamav the names of just-accessed files for scanning. This is a problem, because files inaccessible to the clamav user cannot be scanned this way. Instead, it is possible to instruct clamonacc (which always runs as root) to use file descriptor passing. Edit clamav-clamonacc.service with the following:

[Service]
ExecStart=
ExecStart=/usr/sbin/clamonacc -F --fdpass --log=/var/log/clamav/clamonacc.log

Lastly, you will need to start/enable the clamav-clamonacc.service.

If you get AppArmor denials about clamd, set the profile to a complain-only mode:

# aa-complain clamd

Troubleshooting

Note: Make sure the user from which you are running clamscan has read permissions for the database files (/var/lib/clamav/*.cvd)

Error: Clamd was NOT notified

If you get the following messages after running freshclam:

WARNING: Clamd was NOT notified: Cannot connect to clamd through 
/var/lib/clamav/clamd.sock connect(): No such file or directory

Add a sock file for ClamAV

Warning: See the warning in #OnAccessScan about clamd port security:
# touch /run/clamav/clamd.ctl
# chown clamav:clamav /run/clamav/clamd.ctl

Then, edit /etc/clamav/clamd.conf - uncomment this line:

LocalSocket /run/clamav/clamd.ctl

Save the file and restart clamav-daemon.service.

Error: No supported database files found

If you get the next error when starting the daemon:

LibClamAV Error: cli_loaddb(): No supported database files found
in /var/lib/clamav ERROR: Not supported data format

This happens because of mismatch between /etc/clamav/freshclam.conf setting DatabaseDirectory and /etc/clamav/clamd.conf setting DatabaseDirectory. /etc/clamav/freshclam.conf pointing to /var/lib/clamav, but /etc/clamav/clamd.conf (default directory) pointing to /usr/share/clamav, or other directory. Edit in /etc/clamav/clamd.conf and replace with the same DatabaseDirectory as in /etc/clamav/freshclam.conf. After that clamav will start up successfully.

Error: Can't create temporary directory

If you get the following error, along with a 'HINT' containing a UID and a GID number:

# can't create temporary directory

Correct permissions:

# chown UID:GID /var/lib/clamav && chmod 755 /var/lib/clamav

Tips and tricks

Run in multiple threads

Using clamscan

When scanning a file or directory from command line using clamscan only single CPU thread is used. This may be ok in cases when timing is not critical or you do not want computer to become sluggish. If there is a need to scan large directory or USB drive quickly you may want to use all available CPUs to speed up the process.

clamscan is designed to be single-threaded, so xargs can be used to run the scan in parallel:

$ find /home/archie -type f -print0 | xargs -0 -P $(nproc) clamscan

In this example the -P parameter for xargs runs clamscan in as many processes as there are CPUs (reported by nproc) at the same time. --max-lines and --max-args options will allow even finer control of batching the workload across the threads.

Using clamdscan

If you already have clamd daemon running clamdscan can be used instead (see #Starting the daemon):

$ clamdscan --multiscan --fdpass /home/archie

Here the --multiscan parameter enables clamd to scan the contents of the directory in parallel using available threads. --fdpass parameter is required to pass the file descriptor permissions to clamd as the daemon is running under clamav user and group.

The number of available threads for clamdscan is determined in /etc/clamav/clamd.conf via MaxThreads parameter clamd.conf(5). Even though you may see that the number of MaxThreads specified is more than one (current default is 10), when you start the scan using clamdscan from command line and do not specify --multiscan option, only one effective CPU thread will be used for scanning.

See also