The Internet has become more dangerous over the years, with constantly changing threats and ever more sophisticated attackers. As have most security professionals, we have seen all manner of clever schemes designed to part businesses from their money and data. To help discover and manage threats, we rely heavily on free software tools. One of the most popular security tools is the Nessus vulnerability scanner by Tenable Network Security [1], which many have used to look for vulnerabilities on customer networks.

The decision by Tenable to change the licensing for commercial use was viewed with frustration by a number of security professionals. Prompted by the final announcement on Slashdot in October 2005 that Nessus would be closed source, Tim Brown made the decision to announce a fork. The result of that effort is OpenVAS [2], a free software fork of the Nessus vulnerability scanner predominantly licensed under the GPL.

At this time, OpenVAS has a thriving community, with contributions from both individuals and corporations from all over the world. The OpenVAS project is a member of Software in the Public Interest, a non-profit organization dedicated to helping organizations develop free software. A recent successful developers conference had 16 participants from four continents, and we have more than 13,000 vulnerability checks committed to our public SVN repository.

In this article, we describe the OpenVAS project and show how to use it to assess your network security systems.

OpenVAS Architecture

As with Nessus, OpenVAS follows a client-server model. A server component is responsible for scheduling and running scans, and a client component is used to configure scans and access the results. The server is normally installed on a Linux or Unix server, and the client is typically run from the administrator's own workstation. Currently, the client must be connected to the server for the entire duration of the scan; however, some of the developers are currently working on a new component that will support much more RESTful interfaces.

The Internet Assigned Numbers Authority officially assigned OpenVAS TCP port 9390. OpenVAS is a security-conscious project, and the connection to this port from an OpenVAS Client is always tunneled over SSL with strong ciphers to ensure that only the intended user can access the data generated by OpenVAS.

Installation

OpenVAS is available in binary package form for most major Linux distributions, and you can get it from third-party repositories or obtain it directly from the OpenVAS website.

Some Linux distributions ship incompatible versions of the server and client. Version 1 clients are not compatible with version 2 servers. Keep this in mind when installing from your standard Linux distribution repository.

If you would prefer to compile the code yourself, you will find the source code at the OpenVAS website [2] – either in standard tarball form or via anonymous SVN. Compiling is not difficult, but you must compile the openvas-libraries module first. Be sure to meet the following dependencies when compiling: gpgme-devel, gcrypt-devel, gnutls-devel, glib2, glib2-devel, glib2-data, and Gtk+ 2.x (for the Gtk client).

The next step is to add the first OpenVAS user. The OpenVAS server implements user accounts independently of the operating system. (LDAP support is planned for a future version.) To add a user, simply run the openvas-useradd script

# openvas-adduser johndoe

and you will be prompted for rules to supply with the user information. These rules let you specify which nodes an OpenVAS user can and cannot scan. This feature protects some sensitive systems from being scanned without proper supervision.

The rules syntax is

accept|deny ip/mask
default accept/deny

as in:

accept 192.168.1.0/24
default deny

The preceding rules allow the user to scan all systems in the 192.168.1.0 network but deny access to any other network. User settings are stored in the /var/lib/openvas/users/ directory.

Getting Started

As you will learn later in this article, OpenVAS uses plugins to define scanning scenarios. To update the plugins, simply run the following command:

# openvas-nvt-sync

If you run this command regularly, your plugins remain up to date. Without up-to-date plugins, the scanner will miss new vulnerabilities. After you run openvas-nvt-sync, restart the server.

If you installed from a binary package, use the service command to start the server. If you installed from source, start the server by typing:

PATH_TO_SERVER/openvasd -D