Guard on Duty

When most Linux users think of an open source IDS/IPS (intrusion detection|prevention system), they think of Snort. But Linux users have another option for intrusion detection and prevention: Suricata [1]. Suricata is developed by the Open Information Security Foundation (OISF), which is tasked "to build a next generation IDS/IPS engine." The funding for the OISF comes from several US government agencies and private firms, but since Suricata is licensed under the common "GPLv2 and later" license, it is true open source and also freely available.

Why Suricata?

Why choose Suricata instead of Snort? Suricata uses Snort-compatible rulesets, which is good for compatibility, but if you're already using Snort rulesets, why not just use Snort? For one thing, Suricata is multithreaded, meaning it can easily take advantage of multiple cores, so you can more easily examine large volumes of traffic without having to make sacrifices like reducing the number of rules. Suricata can also do protocol inspection, so you don't have to rely on port numbers to identify traffic  – you can easily examine HTTP traffic no matter what port it is on. Suricata also allows you to look inside of the protocol streams and extract the files, so they can then be examined. You can also block files based on their MD5 signature (more on this later). Suricata also allows you to examine TSL/SSL certificates and match them based on their fingerprints (a SHA1 hash of the certificate typically), so even if you can't intercept the TLS/SSL connection, you can still exert some control over it. However the main advantage of Suricata over Snort is that it isn't Snort. Don't get me wrong, Snort is awesome and I really like it, but Snort is also showing its age and some of the design decisions are less than optimal in the modern world (e.g., 10Gb Ethernet is cheap now)  – things that Suricata addresses. Note: The latest version of Snort addresses some of these problems – see the article on Snort elsewhere in this issue.

Suricata Install

The three primary options for installing Suricata [2] are from a package, from a source tarball, and from Git. I would only recommend using the Git version if you are a developer or need access to some bleeding-edge feature that hasn't yet made it into a release tarball. The package installation is less than optimal; at the time of this writing, Debian has a really old version of Suricata (1.2, current is 2.0.2), so unless you can get at least version 2.0.x, I would recommend installing from the release tarball. But first you're going to need some build dependencies: libpcap-devel, libnet-devel, pcre-devel, libyaml-devel, zlib-devel, libcap-ng-devel, and file-devel. If you want to use the IPS mode to block attacks, you're also going to need libnfnetlink-devel and libnetfilter_queue-devel (on RHEL and CentOS you can find these in EPEL). Then, grab the tarball and execute the usual configure, make, and make install:

wget http://www.openinfosecfoundation.org/download/suricata-2.0.2.tar.gz
tar -xvzf suricata-2.0.2.tar.gz
cd suricata-2.0.2
./configure --enable-nfqueue --prefix=/usr --sysconfdir=/etc --localstatedir=/var
make
sudo make install
sudo ldconfig

You also have some options when installing; by using make install-conf, you can install the configuration files into /etc/, the make install-rules command will get the latest emerging threat rules from Suricata and install them, and make install-full will do both. You'll then need to create the Suricata log directory:

sudo mkdir /var/log/suricata

At this point, you might also need to get the Snort rules – more on this later.

Suricata Configuration

Configuring Suricata is a lot like configuring Snort, except with even more options. The primary configuration file is /etc/suricata/suricata.yaml, with several sections you'll certainly want to configure, including outputs (logging and output of data), vars (variables), and host-os-policy (allows you to specify the IP/network of operating systems, which helps Suricata determine how best to defragment and reassemble the TCP stream). Your choice of output log will largely depend on how you want to process the data (if at all). The vars section allows you to specify various network and service configurations (such as your DNS, SMTP, and SQL servers), which are used by various signatures. The cuda section even allows you to configure a GPU to accelerate Suricata.

The final section is the app-layer configuration. Suricata understands a wide variety of protocols, and you have the option of detecting the traffic only, detecting and parsing the traffic, and not detecting or parsing it (e.g., you might want to ignore DNS traffic since you can easily log and firewall it).