[TriLUG] Understanding Traceroute

[Alexey Toptygin]

On Tue, 1 Mar 2011, James Jones wrote:

> All,
>
> I attempted to use traceroute ( ubuntu 10.04 ) today and expected
> certain results which didn't happen. I expected to see:
> 1. my router's ip address
> 2. my cable modem's ip address
> 3. my gateway's ip address
> 4. finally the route to destination
> I saw number 1 & 4, but not number 2 or 3.
>
> command was simple  traceroute destination
>
> Was I expecting too much?

Traceroute will show you the IP on the interface facing you on every 
device along the path to destination that decrements the IP TTL field 
(i.e. acts as a router), with the caviat that some devices will not send 
the ICMP messages that traceroute relies on, in which case you will see 
stars for that hop.

IME cable modems never have an IP address assigned, they are purely layer 
2 devices.

You should be seeing the IP of the gateway upstream from the router on 
your premises. If you don't and you see a row of stars that means it's not 
sending ICMP TTL exceeded messages. If its IP is missing and you don't see 
a row of stars, it's possible that it's configured to send all ICMP errors 
from a single IP, which is not the one on the interface facing you (i.e. 
you are seeing it, but not with the address you expect); another 
possibility in this case is that the gateway is misconfigured and is not 
decrementing TTL for some reason (this is very rare, but I have seen it at 
least once).

 			Alexey