When you see "!Z" or "!X" in a traceroute, what does that mean?

Where is it coming from, and how do I fix it?


!X means "communication administratively prohibited" and !Z "communication with destination host administratively prohibited" As far as I remember, you get !X on ipv4 and !Z on ipv6 and it should be documented in the man (8) pages.

Since Linux uses UDP for trace-routes, this can originate from a --reject-with icmp-host-prohibited rule at the destination. Some Linux distros have this as a default configuration. To fix this you need to reply with --reject-with icmp-port-unreachable on UDP ports 33434 through 33534.

  • Why those ports? – Ken Sharp Jan 3 '16 at 23:38
  • 1
    @KenSharp Because those are the ports traceroute uses. See the man page for a detailed description of the algorithm. – Michael Hampton Mar 10 '16 at 15:24

protected by Michael Hampton Mar 10 '16 at 15:19

Thank you for your interest in this question. Because it has attracted low-quality or spam answers that had to be removed, posting an answer now requires 10 reputation on this site (the association bonus does not count).

Would you like to answer one of these unanswered questions instead?

Not the answer you're looking for? Browse other questions tagged or ask your own question.