Scanning

Table Of Contents

• Scanning
  • Daemon
    • clamd
    • clamdscan
    • clamdtop
    • On-Access Scanning
  • One-Time Scanning
    • clamscan

Daemon

clamd

clamd is a multi-threaded daemon that uses libclamav to scan files for viruses. Scanning behaviour can be fully configured to fit most needs by modifying clamd.conf.

As clamd requires a virus signature database to run, we recommend setting up ClamAV’s official signatures before running clamd using freshclam.

The daemon works by listening for commands on the sockets specified in clamd.conf. Listening is supported over both unix local sockets and TCP sockets.

IMPORTANT: clamd does not currently protect or authenticate traffic coming over the TCP socket, meaning it will accept any and all of the following commands listed from any source. Thus, we strongly recommend following best networking practices when setting up your clamd instance. I.e. don’t expose your TCP socket to the Internet.

Here is a quick list of the commands accepted by clamd over the socket.

• PING
• VERSION
• RELOAD
• SHUTDOWN
• SCAN file/directory
• RAWSCAN file/directory
• CONTSCAN file/directory
• MULTISCAN file/directory
• ALLMATCHSCAN file/directory
• INSTREAM
• FILDES
• STATS
• IDSESSION, END

As with most ClamAV tools, you can find out more about these by invoking the command:

$ man clamd

The daemon also handles the following signals as so:

• SIGTERM - perform a clean exit
• SIGHUP - reopen the log file
• SIGUSR2 - reload the database

It should be noted that clamd should not be started using the shell operator & or other external tools which would start it as a background process. Instead, you should run clamd which will load the database and then daemonize itself (unless you have specified otherwise in clamd.conf). After that, clamd is ready to accept connections and perform file scanning.

Once you have set up your configuration to your liking, and understand how you will be sending commands to the daemon, running clamd itself is simple. Simply execute the command:

$ clamd

clamdscan

clamdscan is a clamd client, which greatly simplifies the task of scanning files with clamd. It sends commands to the clamd daemon across the socket specified in clamd.conf and generates a scan report after all requested scanning has been completed by the daemon.

Thus, to run clamdscan, you must have an instance of clamd already running as well.

Please keep in mind, that as a simple scanning client, clamdscan cannot change scanning and engine configurations. These are tied to the clamd instance and the configuration you set up in clamd.conf. Therefore, while clamdscan will accept many of the same commands as its sister tool clamscan, it will simply ignore most of them as (by design) no mechanism exists to make ClamAV engine configuration changes over the clamd socket.

Again, running clamdscan, once you have a working clamd instance, is simple:

$ clamdscan [*options*] [*file/directory/-*]

clamdtop

clamdtop is a tool to monitor one or multiple instances of clamd. It has a colorized ncurses interface, which shows each job queued, memory usage, and information about the loaded signature database for the connected clamd instance(s). By default it will attempt to connect to the local clamd as defined in clamd.conf. However, you can specify other clamd instances at the command line.

To learn more, use the commands

$ man clamdtop

or

$ clamdtop --help

On-Access Scanning

For versions >= 0.102.0

ClamAV’s On-Access Scanning (clamonacc) is a client that runs in its own application alongside, but separately from the clamd instance. The On-Access Scanner is capable of blocking access to/from any malicious files it discovers–based on the verdict it receives from clamd–but by default it is configured to run in notify-only mode, which means it will simply alert the user if a malicious file is detected, then take any additional actions that the user may have specified at the command line, but it will not actively prevent processes from reading or writing to that file.

On-Access Scanning is primarily set up through clamd.conf. However, you can learn more about all the configuration and command line options available to you by reading the On-Access Scanning User Guide.

Once you have set up the On-Access Scanner (and clamd) to your liking, you will first need to run clamd before you can start it. If your clamd instance is local, it is required you run clamd as a user that is excluded (via OnAccessExcludeUname or OnAccessExcludeUID) from On-Access scanning events (e.g.) to prevent clamonacc from triggering events endlessly as it sends scan requests to clamd:

$ su - clamuser -c "/usr/local/bin/clamd

After the daemon is running, you can start the On-Access Scanner. clamonacc must be run as root in order to utilize its kernel event detection and intervention features:

$ sudo clamonacc

It will run a number of startup checks to test for a sane configuration, and ensure it can connect to clamd, and if everything checks out clamonacc will automatically fork to the background and begin monitoring your system for events.

For versions <= 0.101.x

In older versions, ClamAV’s On-Access Scanner is a thread that runs within a clamd instance. The On-Access Scanner is capable of blocking access to/from any malicious files it discovers–based on the verdict it finds using the engine it shares with clamd–but by default it is configured to run in notify-only mode, which means it will simply alert the user if a malicious file is detected, but it will not actively prevent processes from reading or writing to that file.

On-Access Scanning is primarily set up through clamd.conf. However, you can learn more about all the configuration and command line options available to you by reading the On-Access Scanning User Guide.

Once you have set up the On-Access Scanner to your liking, you will need to run clamd will elevated permissions to start it.

$ sudo clamd

One-Time Scanning

clamscan

clamscan is a command line tool which uses libclamav to scan files and/or directories for viruses. Unlike clamdscan, clamscan does not require a running clamd instance to function. Instead, clamscan will create a new engine and load in the virus database each time it is run. It will then scan the files and/or directories specified at the command line, create a scan report, and exit.

By default, when loading databases, clamscan will check the location to which freshclam installed the virus database signatures. This behaviour, along with a myriad of other scanning and engine controls, can be modified by providing flags and other options at the command line.

There are too many options to list all of them here. So we’ll only cover a few common and more interesting ones:

• --log=FILE - save scan report to FILE
• --database=FILE/DIR - load virus database from FILE or load all supported db files from DIR
• --official-db-only[=yes/no(*)] - only load official signatures
• --max-filesize=#n - files larger than this will be skipped and assumed clean
• --max-scansize=#n - the maximum amount of data to scan for each container file
• --leave-temps[=yes/no(*)]- do not remove temporary files
• --file-list=FILE - scan files from FILE
• --quiet - only output error messages
• --bell - sound bell on virus detection
• --cross-fs[=yes(*)/no] - scan files and directories on other filesystems
• --move=DIRECTORY - move infected files into DIRECTORY
• --copy=DIRECTORY - copy infected files into DIRECTORY
• --bytecode-timeout=N - set bytecode timeout (in milliseconds)
• --heuristic-alerts[=yes(*)/no] - toggles heuristic alerts
• --alert-encrypted[=yes/no(*)] - alert on encrypted archives and documents
• --nocerts - disable authenticode certificate chain verification in PE files
• --disable-cache - disable caching and cache checks for hash sums of scanned files

To learn more about the options available when using clamscan please reference:

$ man clamscan

and

$ clamscan --help

Otherwise, the general usage of clamscan is:

clamscan [options] [file/directory/-]