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Defend your network against idle scanning

• Tags:
• Intrusion Detection,
• Idle Scanning

Michael Mullins

Published: 30 Sep 2005 16:55 BST

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If your organisation's intrusion-detection system (IDS) identifies a scan of your network, and you just block that IP address, you likely haven't addressed the real threat to your network. Black hats employ several stealth scanning techniques, and one of those threats is the idle scan.

Idle scanning is a procedure that involves scanning TCP ports. An attacker will probe a public host with SYN|ACK (synchronisation acknowledgement) and receive an RST (reset the connection) response that has the current IPID (IP identification) number. The attacker will then send a SYN (synchronisation) request packet to the port of his or her target with a source IP of the public host.

If the port is open, the target will send a SYN|ACK packet to the public host. The public host will then send an RST packet to the target and increment the IPID.

If the port is closed, the target will send an RST packet to the public host indicating that the port is closed. The attacker will then send another SYN|ACK packet to the public host and look at the IPID number to determine if sending an RST packet to the target incremented the IPID.

After several hundred of these bogus session requests, your IDS will realise that something is scanning the network, and you'll eventually block the public host. But even though you didn't identify the attacker, the intruder still managed to map your network.

This pre-emptive probe becomes an even greater problem if the public host used to scan your internal network is one of the trusted machines that sits in front of your firewall. However, you can protect your network from this type of scanning technique.

The best defence against idle scanning is to follow these simple best practices:

• Don't put a public host in front of your firewall that uses a predictable IPID sequence. Solaris and Linux are two operating systems that aren't vulnerable to this type of behaviour. Unix/Linux is a much more stable and secure platform for your Web site. Learn Unix, and replace the Windows box that sits outside your firewall.
• Use a firewall that can maintain state-on connections, determine whether someone initiated a phony session request, and drop those packets without a target host response. If your firewall doesn't maintain state-on connections, your network really just has a "speed bump" — not a firewall.
• Use an ingress filter on your network to ensure that no packets enter your outside boundary with a source address of your internal network.
• Use an egress filter on your network to ensure that no packets leave your network with a source address that isn't a part of your internal network.

If you don't use ingress and egress filtering, you're an easier target for black hats and wannabes on your own network to attack the rest of the Internet.

Final thoughts
New scanning techniques will continue to evolve and bombard your network, probing for holes and weak spots in your security. It's up to you to continue to use best practices to defend your organisation's systems.

Mike Mullins has served as an assistant network administrator and a network security administrator for the US Secret Service and the Defense Information Systems Agency. He is currently the director of operations for the Southern Theater Network Operations and Security Centre.

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