Based on the existing code of Checkpoint, I have implemented as an example a 
stateful version of ping.

Stateful ping mean that an ICMP echo-reply will be accepted *only* if the FW-1 
have seen before an ICMP echo-request, if the src<>dst match the dst<>src and 
if the icmp-id and icmp-seq match in both message.
Also, we limit echo-reply to *one* message for each echo-request.

As an option, it can also enable ICMP errors messages to fly through your FW-1 
if they are related to an existing TCP/UDP or ICMP connection. Their size is 
also limited.

See http://yassp.parc.xerox.com/fw1 for more detail.

        jean




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