From: Rob Sherwood (capveg@cs.umd.edu)
Date: Thu Dec 28 2006 - 04:40:48 EST
On Wed, Dec 27, 2006 at 05:36:58PM -0800, Becky Nelson wrote:
> I am running a traceroute and have two hops that report the same
> address. Could someone please explain what would cause this? I
> suspect that this is some type of firewall?
There are a number of things that can cause this, but the most common
is a buggy IP implementation on the first router, which it forwards
packets with a ttl=0.
http://www.freesoft.org/CIE/Topics/54.htm
[snip]
Buggy TCP/IP implementations
Traceroute depends on a rather obscure feature that often doesn't work
correctly. Some of the problems people have found: code that fails
to decrement TTL, code that incorrectly forwards packets with zero
TTL, code that does not generate ICMP Timeouts, and code that sends
ICMPs with the same TTL as the original packet. This last problem,
of course, results in our ICMP Timeouts being sent with zero TTL -
guaranteed not to make it back to us.
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