Category: Security

In my discussion of per-packet versus per-destination load sharing, I've relied on the "accepted wisdom" that out-of-order TCP packets reduce session performance (as a side note, out-of-order UDP packets are a true performance killer; just try running NFS with out-of-order packets).

Today I've discovered another huge show-stopper: stateful firewalls (read: almost everything in use today) might just drop out-of-order packets, resulting in TCP timeouts and retransmissions (and repeated timeouts will totally wreck the session throughput). Here's how Cisco devices handle this problem:

You could limit terminal access to a router with an access-class in line configuration command for a very long time (since, at least, IOS release 10.0). However, the access-class command only accepted standard access-lists, allowing you to restrict access solely based on source IP addresses. In the meantime, this feature quietly got upgraded to support extended access lists. In the IOS release 12.4, the command even accepts (undocumented !) named access lists.

These new features give you the ability to implement interesting policies, for example:

Cisco has in IOS release 12.3(4)T (integrated into 12.4) finally introduced features (long available in Unix and Windows) that slow down dictionary attacks on a router. On top of logging of login failures, you can also slow down the login process by delaying the router response after a login failure with the login delay seconds command.

On top of that, the you can configure the router to enter quiet mode after several login failures have been detected in specified timeframe with the login block-for seconds attempts tries within seconds configuration command.

Cisco Press has just released my latest book (and my first digital one): Deploying Zone-Based Firewalls. The book covers a completely new way to configure IOS firewall feature set based on security zones you define on a router and inter-zone policies configured using the familiar class-maps and policy-maps.

You can preview this digital book (they call it Digital Short Cut) using the Safari technology at Cisco Press and buy it at Amazon.

Although a bit outdated (and discussing older Cisco IOS releases), the article Cisco IOS from an Attacker's Point of View is still very relevant, as most vulnerabilities discussed in it arise from unsecure router configuration. Highly recommended reading!