[Gllug] OT: Traceroute

[Simon A. Boggis]

On Tue, 2003-03-18 at 16:36, Bruce Richardson wrote:
> Now, if any of those routers don't send back a reply (in good time), you
> start seeing those * characters.  So they don't mean 'unreachable', they
> just mean that no ICMP messages are coming back.  As Tethys explained,
> this is usually because of a) a firewall blocking ICMP messages or b)
> the router being configured not to send any ICMP messages.

There is also a third possibility for "seeing stars" - a packet filter
en route or on the receiving host drops the UDP packets that traceroute
uses by default, thus no ICMP messages are generated. Hence not all
un-traceroute-able sites are handling ICMP incorrectly.

Often you will find that doing an ICMP traceroute (using echo request
instead of UDP) will work in such cases these days.

Simon

-- 
----------------------------------------------------------------------
Dr Simon A. Boggis                                  Systems Programmer
Department of Computer Science,                     Tel. 020 7882 7522
Queen Mary, University of London, London E1 4NS UK. 
---- GPG public key <http://www.dcs.qmul.ac.uk/~simon/#publickey> ----


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