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

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

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!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.

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  • Why those ports? – Ken Sharp Jan 3 '16 at 23:38
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    @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

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