[LUG] Subnet Masks - discovery

[Simon Waters <Simon@xxxxxxxxxxxxxxxxxxxx>]

A little known feature of the ICMP protocol is that it allows
remote discovery of subnet masks.

This is one of the many reasons why when firewalling data
centers and corporate networks, you may allow ICMP echo/reply,
but you probably want to restrict some other types of ICMP
packet. Redirects and other ICMPs are potentially nastier in
that they may allow an attack rather than just probing, but
probing is one of the things firewalls are suppose to stop.

However I now find I need a handy tool for discovering the
subnet masks of interfaces of remote systems.

Theory is fine but is there a small, lightweight, easy to
configure, Linux tool to check the subnet mask of a specific
host? Is it likely to be an option of some of the tools I
already have?

There are some heavyweight topology discovering programs, and
plenty of tools for probing firewalls, but I just want to check
out the machines listed in a specific traceroute or two, and all
the machines listed under a specific RRset (multiple IP
addresses for the same name).


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