Subject: [Softwires] about NAT and ping

From: Francis Dupont <[email-masked]>
Date: Wed, 30 Mar 2011 10:24:46 +0200

NATs (NAT-PTs) are defined to translate TCP and UDP but usually can
translate the ICMP echo service as known as 'ping'. It is an (ab)use
of the ID field of ICMP echo/echo reply packets which is handled
the same way than the TCP/UDP source port.

So in the real world 'ping' works at the same place than TCP or UDP,
at least when it is about NATs (some firewalls blindly drop all ICMPs,
some stacks fail to implement ICMP, etc, but this has nothing to do
with NATs), but this statement applies to the poor guy behind a NAT.

The issue is in the "Internet -> poor guy behind a NAT" way. In fact
it could work exactly the same way than for a TCP/UDP server behind
a NAT, but no standard 'ping' command provides a way to give the ID
to use, so to make it to work you need a modified ping (I can provide
a modified source for Linux if someone wants to experiment).

Regards

[email-masked]

PS: I put this here because it seems someone claims ping doesn't work
with 4rd.