North American Network Operators Group
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Re: Firewall stateful handling of ICMP packets

  • From: Jeff Kell
  • Date: Wed Dec 03 22:52:51 2003 
Jamie Reid wrote:
Personal view:
This was a problem when filtering Nachi while it pinged networks
to their knees. 
Well, it was at least the "last straw".
Sometimes I wonder if there is any legitimate reason to allow pings from users at all. If the user really needed to use
ping, that is, if they were in a position to do anything about the
results of the ping tests, then they would know enough to use traceroute in UDP mode or some other tool.
Personally, I like Rob Thomas (cymru) stance on ICMP filtering as given in http://www.cymru.com/Documents/icmp-messages.html. This allows pings
and PMTU-relevant unreachables. Granted there have been a few hacker toys that use ICMP echo-reply or other esoteric ICMP type codes to communicate with their "slaves", but this could be alleviated to some extent with "stateful" ICMP (almost an oxymoron).

The Nachi pings can be stopped by vendor C using policy routing but has a side effect of grabbing some unintended packets (Windows traceroute, I think). If you can devise a method to see if the ICMP payload is a load of 0xaa's then you've narrowed it down to a science, but AFAIK vendor C can't do that (well, maybe an IDS appliance with a custom signature).

You can gain "some" additional protection by rate-limiting ICMP (in the Nachi ping case) and/or UDP (SQL Slammer, etc), and TCP intercept for synflooding. Not perfect, but every little bit helps.

Jeff Kell
University of Tennessee at Chattanooga