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I think when a hop (including destination hop) reply a UDP request (by traceroute) always has ttl=1 as in this link

But I see ttl isn't equal 1 in ICMP Port Unreachable message responses from destination hop. like this

8.8.8.8 > 192.168.1.6: ICMP 8.8.8.8 udp port 33466 unreachable, length 36
    IP (tos 0x80, ttl 2, id 10956, offset 0, flags [none], proto UDP (17), length 60, bad cksum 8d66 (->bba7)!)
    192.168.1.6.33995 > 8.8.8.8.33466: UDP, length 32
19:03:42.346002 IP (tos 0x0, ttl 46, id 0, offset 0, flags [none], proto ICMP (1), length 56)
    8.8.8.8 > 192.168.1.6: ICMP 8.8.8.8 udp port 33467 unreachable, length 36
    IP (tos 0x80, ttl 3, id 10957, offset 0, flags [none], proto UDP (17), length 60, bad cksum 8c65 (->baa6)!)
    192.168.1.6.49358 > 8.8.8.8.33467: UDP, length 32
19:03:42.348871 IP (tos 0x0, ttl 46, id 0, offset 0, flags [none], proto ICMP (1), length 56)
    8.8.8.8 > 192.168.1.6: ICMP 8.8.8.8 udp port 33468 unreachable, length 36
    IP (tos 0x80, ttl 2, id 10959, offset 0, flags [none], proto UDP (17), length 60, bad cksum 8d63 (->bba4)!)
    192.168.1.6.44693 > 8.8.8.8.33468: UDP, length 32
19:03:42.362155 IP (tos 0x0, ttl 46, id 0, offset 0, flags [none], proto ICMP (1), length 56)
    8.8.8.8 > 192.168.1.6: ICMP 8.8.8.8 udp port 33469 unreachable, length 36
    IP (tos 0x80, ttl 2, id 10962, offset 0, flags [none], proto UDP (17), length 60, bad cksum 8d60 (->bba1)!)
    192.168.1.6.43577 > 8.8.8.8.33469: UDP, length 32
19:03:42.362185 IP (tos 0x20, ttl 47, id 0, offset 0, flags [none], proto ICMP (1), length 56)
    8.8.8.8 > 192.168.1.6: ICMP 8.8.8.8 udp port 33470 unreachable, length 36
    IP (tos 0x80, ttl 3, id 10963, offset 0, flags [none], proto UDP (17), length 60, bad cksum 8c5f (->baa0)!)
    192.168.1.6.60249 > 8.8.8.8.33470: UDP, length 32
19:03:42.382381 IP (tos 0x0, ttl 46, id 0, offset 0, flags [none], proto ICMP (1), length 56)
    8.8.8.8 > 192.168.1.6: ICMP 8.8.8.8 udp port 33471 unreachable, length 36
    IP (tos 0x80, ttl 3, id 10965, offset 0, flags [none], proto UDP (17), length 60, bad cksum 8c5d (->ba9e)!)
    192.168.1.6.33430 > 8.8.8.8.33471: UDP, length 32

Could you explain why is that?

Thanks for reading !

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  • Did any answer help you? if so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer.
    – Ron Maupin
    Aug 12, 2017 at 3:21

1 Answer 1

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The ICMP response is an entirely separate unicast message from a router to the original source used in the traceroute. The formation of the ICMP error packet is independent of the original traceroute packet headers that triggered the error except in determining where to send the error message.

If TTL=1 for each of the ICMP messages trying to get back to your traceroute client, only the message from the first hop would make it back.

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