ClamAV
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
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
- You will need to run
freshclambefore 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.
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
On-access scanning will scan the file while reading, writing or executing it.
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):
/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.
# systemctl edit clamav-clamonacc.service
[Service] ExecStart= ExecStart=/usr/sbin/clamonacc -F --fdpass --log=/var/log/clamav/clamonacc.log
Lastly, you will need to enable and start the clamav-clamonacc.service.
If you get AppArmor denials about clamd, set the profile to a complain-only mode:
# aa-complain clamd
Troubleshooting
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:
# 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 like 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 folder 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.