Hi all Accept my apologies if this mail is unreadable -- I've got to bear Outlook's crappiness here..
icmp messages are vital to most traffic on the network and they cannot really be used for hacking. of course, icmp floods (i.e. ping of death etc.) are based on them, but your system should be immune.
Actually, ICMP can be used in many ways to compromise a network and/or hosts. They are very useful in mapping protected networks, reconfiguring hosts and causing all sorts of other mischief. Note also that ICMP packets are generally able to carry a payload, making them attractive as tunneling instruments. In contrast to Kurt, though, I am of the opinion that *some* ICMP should be allowed so as to not infer with the proper operation of the network. Which these are depends on the exact situation -- refer to Chapman, Zwiky and Cooper: "Building Internet Firewalls, 2nd edition", chapter 22: Administrative Services, heading "ICMP and Network Diagnostics" (pgs. 647-654) for details. Regards Tobias