Intrusion Detection with the Snort IDS

Prevention or Detection

Snort provides several options for preventing (and detecting) intrusion. The three main modes for preventing intrusion are inline filtering, cooperation with an existing iptables-based firewall, and TCP-RST mode.

When Snort is working as an inline filter, all traffic must pass through the Snort system before it passes to the internal network. If the traffic triggers a rule in the Snort system, the packets are dropped. The inline solution offers advanced firewall-style security with a regularly updated rule set. However, the IPS can also prevent access to systems through false positives and will slow down your network if you have more traffic than the Snort sensor to handle. For inline mode, you'll need to add – enable-inline to your ./configure command.

If you already have an existing iptables-based firewall, you can configure Snort to provide dynamic rule changes. The iptables option reduces some of the lag on inbound traffic, but as a trade-off, your system will be slower to respond to attacks. Once the malicious traffic triggers an alert, Snort sends a command to the iptables system to block the attacker. This style of IPS, if not correctly configured, can be manipulated by a creative attacker to force a denial of service on your own systems.

If an attacker spoofs malicious traffic from your ISP's router or DNS server, you could end up blacklisting services you need to maintain a reliable network presence. To combat this, use a whitelist of addresses you never ban. However, an attacker who discovers the address of your whitelist can spoof attacks from this address without fear of being blocked.

The final option is to allow Snort to disconnect unwanted connections through the use of TCP-RST packets (through the use of the flexresp2 patch). This option can terminate an unwanted connection from both ends. However, this solution causes a race condition between your IPS and the malicious traffic. The IPS attempts to close the connection before the attacker can complete the attack. The attacker will already have an advantage in this case, because the malicious traffic is already inside your network before Snort can act. This mode of operation helps prevent certain attacks, but it might be less reliable than the other techniques.

How you configure your IDS/IPS is dependent on your security requirements. If you intend to set up Snort as an IPS, test the server in IDS mode until you've correctly tuned the configuration and reduced false positives.

Once you're happy with the configuration, move Snort to its new role as a prevention system.


Snort has many other features to discover. For example, I never got to mention the retro ASCII art pig (Figure 5).

Figure 5: Old school ASCII art: See the Snort pig mascot on the left.

Numerous books and online resources will help you get started with the Snort intrusion detection system. The Snort project website offers a great number documents that can help solve problems. Snort's website also offer a community forum that provides user assistance and news.


  1. Snort homepage:
  2. Sourcefire:
  3. BASE: Basic Analysis and Security Engine:
  4. ADOdb database abstraction library for PHP:
  5. BPF (Berkeley Packet Filter):

The Author

Chris Riley is an IT Security Analyst living and working in Austria. After 12 years working as a server administrator in the UK and Germany, he is now spending a majority of his time performing penetration tests in and around Vienna.

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