Updating Signature Databases - ClamAV Documentation
Description: An open source malware detection toolkit and antivirus engine.

# Signature Testing and Management

Table Of Contents

- Signature Testing and Management
  - FreshClam
  - SigTool
  - ClamBC
  - Next Steps
  - Create your own signatures

> Tip: The commands on Windows are generally the same, but you may need to add the `.exe` extension to run the ClamAV applications.

## FreshClam

Before you can start the ClamAV scanning engine (using either `clamd` or `clamscan`), you must first have ClamAV Virus Database (.cvd) file(s) installed in the appropriate location on your system.

The tool `freshclam` is used to download and update ClamAV’s official virus signature databases. While easy to use in its base configuration, `freshclam` does require a working `freshclam.conf` configuration file to run (the location of which can be passed in via command line if the default search location does not fit your needs).

Once you have a valid configuration file, you can invoke FreshClam with the following command:

    freshclam

By default, `freshclam` will then attempt to connect to ClamAV's virus signature database distribution network. If no databases exist in the directory specified, `freshclam` will do a fresh download of the requested databases. Otherwise, `freshclam` will attempt to update existing databases, pairing them against downloaded cdiffs. If a database is found to be corrupted, it is not updated and instead replaced with a fresh download.

Of course, all this behavior--and more--can be changed to suit your needs by modifying `freshclam.conf` and/or using various command line options.

You can find more information about FreshClam with the commands:

Unix/Linux:

    freshclam --help

Or (Unix/Linux only):

    man freshclam

> Tip: Newer versions of FreshClam will create your database directory if it doesn't already exist. Older versions won't, and may fail unless you create it first.

> Important: It is common on Ubuntu after a fresh install to see the following error the first time you use ClamAV:

>     freshclam

>     freshclam: error while loading shared libraries: libclamav.so.7: cannot open shared object   file: No such file or directory

> You can fix this error by using `ldconfig` to rebuild the library search path.

>     sudo ldconfig

If you are having issues updating the signature databases with `freshclam`, please review the `freshclam` FAQ.

## SigTool

ClamAV provides `sigtool` as a command-line testing tool for assisting users in their efforts creating and working with virus signatures. While sigtool has many uses--including crafting signatures--of particular note, is sigtool's ability to help users and analysts in determining if a file detected by libclamav's virus signatures is a false positive.

This can be accomplished by using the command:

    sigtool --unpack=FILE

Where FILE points to your virus signature databases. Then, once `sigtool` has finished unpacking the database into the directory from which you ran the command, you can search for the offending signature name (provided either by `clamscan` scan reports or `clamd` logs). As an example:

    grep "Win.Test.EICAR" ./*

Or, do all that in one step with:

    sigtool --find="Win.Test.EICAR"

This should give you the offending signature(s) in question, which can then be included as part of your false positive report.

To learn more in depth information on how `sigtool` can be used to help create virus signatures and work with malicious (and non-malicious) files please reference the many online tutorials on the topic.

Otherwise, information on available sigtool functions can be easily referenced with:

    sigtool --help

Or (Unix/Linux only):

    man sigtool

## ClamBC

`clambc` is Clam Anti-Virus’ bytecode signature testing tool. It can be used to test newly crafted bytecode signatures or to help verify existing bytecode is executing against a sample as expected.

For more detailed help, please use:

    clambc --help

Or (Unix/Linux only):

    man clambc

## Next Steps

Now that you know more about FreshClam and tools to work with the signature databases, it's time to run your first scan.

## Create your own signatures

There is a whole community of malware researchers and signature writers. If you'd like to learn how to craft your own signatures, you can!