Stealthing port 113 on personal firewalls

One of the things that first caught my eye about the Zone Alarm personal firewall was that it has always been very clever about handling IDENT's port 113. I recall being impressed and thinking "these guys really know what they're doing". When Zone Alarm receives an inbound connection request for port 113, it checks to see whether the computer has recently initiated any outbound connections to the remote server sending the IDENT request. If not, the IDENT packet is simply dropped, stealthing the protected machine.

But if the user does have an existing "relationship" with the sender of the IDENT request, the IDENT packet is allowed to pass through Zone Alarm's firewall protection so that the user's system can respond normally (which usually means immediately returning a closed status for the port). This means that Zone Alarm is a "stateful packet inspecting personal firewall", not just a simpler static packet filter.

At the time of this writing, Zone Alarm is still the only personal firewall to offer this sort of adaptive dynamic IDENT port handling. I hope that other firewalls will follow suit once the benefits are better understood.

The good news is that since IDENT is almost never used, simple "hard stealthing" of port 113, which is available from all personal firewalls, is probably sufficient. It will allow your system to remain completely invisible on the Internet and will almost certainly never cause any connection trouble.