traceroute problems

Patrick Mahan mahan at
Tue Mar 11 02:56:00 UTC 2008

Jonathan Chen presented these words - circa 3/10/08 7:38 PM->
> On Mon, Mar 10, 2008 at 11:30:05PM +0100, Wojciech Puchar wrote:
> [...]
>> traceroute uses UDP packets, no special port numbers.
> traceroute(8) indicates that the default UDP port number used is
> udp/33434, incrementing for each hop out.

The incrementing is the TTL count in the IP header, not the port number.

It works by sending out a UDP packet for a (generally) unused port with
the TTL field to a specific number and looking for ICMP errors to 
indicate how far the packet went (the last node address is contained in
the ICMP error reply).  However, be warned, some network administrators
disable their routers from sending back these types of ICMP messages
to prevent you from learning about their routing paths.  In these cases,
you get back the "1 * * *" type of output from traceroute.  Also, by
default traceroute attempts to do a reverse DNS on the IP address, so
you can speed things up by doing a 'traceroute -n' to avoid this look-up.


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