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When traceroute to a host IP, it gives message as

Tracing the route to 192.168.3.11 , min ttl = 1, max ttl = 30 .

  1  172.16.11.210      16 ms   0 ms    16 ms

  2  *  *       *

  3  192.168.30.1       !A      *       !A

What does !A mean ?

  • What happen if you ping that host? – Sobrique Jan 14 '16 at 9:42
  • But ping gives continuous reply – Shyamkkhadka Jan 14 '16 at 9:58
  • ping uses icmp_echo which is a different thing to the UDP used by traceroute. usually both are blocked, because someone doesn't want you explore their network. – Sobrique Jan 14 '16 at 17:06
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!A means administratively prohibited. It's a slightly friendly way of telling you you're "not allowed" than just dropping traffic. (And is usually because a firewall is configure to deny this traffic for some reason).

  • But ping gives continuous reply – Shyamkkhadka Jan 14 '16 at 9:59
  • Linux or Windows tracert? Linux tracert uses UDP packets by default, not ICMP. Both Linux and Windows ping use ICMP. So my assumption here is that they have blocked UDP. If using Linux tracert, add the -I flag to force ICMP usage and the tracert will probably complete without any administratively prohibited ICMP replies. – parkamark Jan 14 '16 at 10:47
  • I also note that the host you are tracerouting to and the host that is actually replying with !A have different IP addresses. It may be the case that during the traceroute, once the TTL is terminating at 192.168.30.1, that host is configured to return !A (firewall/router or similar device). But when sending pings directly to 192.168.3.11 (with enough TTL), the TTL does not terminate at 192.168.30.1 and the packets are forwarded to 192.168.3.11 and a normal ping reply is sent back. Try leaving the traceroute to continue past hop 3 and see what happens. – parkamark Jan 14 '16 at 11:04
  • Right, the host I am tracerouting to (192.168.3.11) and the host that is actually replying with !A have different IP addresses (192.168.30.1). The second one being IP address of router. The traceroute command terminates after above mentioned third step itself. And I am doing tracert from Cisco Switch and router. – Shyamkkhadka Jan 15 '16 at 2:21

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