The manpage of traceroute says that the "-U" parameter (UDP probing) is the default, but I'm getting different results every time.

With "-U":

traceroute -U www.univ-paris1.fr
traceroute to www.univ-paris1.fr (, 30 hops max, 60 byte packets
13  rap-vl165-te3-2-jussieu-rtr-021.noc.renater.fr (  59.445 ms  56.924 ms  56.651 ms
18  * paris1web.univ-paris1.fr (  23.797 ms  23.603 ms

but the normal traceroute gives me another result (never reaches the final node) - it's either "!X" or just exits after the maximum of 30 hops:

traceroute www.univ-paris1.fr
traceroute to www.univ-paris1.fr (, 30 hops max, 60 byte packets
11  te1-1-paris1-rtr-021.noc.renater.fr (  28.147 ms  28.250 ms  28.538 ms
[... non-responding nodes ...]
28  site-1.03-jussieu.rap.prd.fr (  85.941 ms !X * *

Note: I tried this very often and always get the same results. The path in my local network is always the same.

So what does the "-U" parameter actually change here? I'm especially interested what the reason for "!X" could be (communication administratively prohibited).

EDIT: If that helps, paris-traceroute gives me the following for the last hop:

14 P(1, 6) site-1.03-jussieu.rap.prd.fr ( 34.938 ms !5 !T2

which means that node discards the packet with TTL=2 and returns an unknown message (not "destination unreachable" or the like).


In my man page, " -U Use UDP to particular destination port for tracerouting (instead of increasing the port per each probe). Default port is 53 (dns).

So with -U, traceroute use 53/udp port, and without a random port. Then, this port is maybe blacklisted on the firewall it is why you have the !X

  • You are totally right - checked it with Wireshark. Seems like the manpage I referenced differs. The blacklisting also makes sense as the last hop seems to drop the packet already with TTL=2. Thanks! – AndiDog Jan 17 '11 at 13:37
  • also, try tracepath command if you having trouble with traceroute – The Unix Janitor Feb 18 '11 at 14:37

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