Traceroute and meaning of outputs

Van Jacobson is a prominent person in networking, especially for TCP/IP. What I didn’t know was (according to wikipedia) original traceroute was also written by him. As this tool is the swiss knife of a Tech Support Engineer, I would like to share the meaning of some of the outputs. If you have any other error, please do share here to improve the list.

!N

This sample output indicates that network that 10.1.2.4 IP belongs doesn’t exist on the 10.11.1.1 host if you take a packet capture when you see this error, you will see that you receive, ICMP destination unreachable “Network Unreachable” message from this host.

root@LAB1021-PC10:~# traceroute -n 10.1.2.4
traceroute to 10.1.2.4 (10.1.2.4), 30 hops max, 60 byte packets
 1  10.11.6.1  0.620 ms  0.522 ms  0.450 ms
 2  10.11.1.1  1.260 ms !N  1.205 ms !N *      <----

!H

This error however indicates that IP network is available but the individual host 10.11.2.4 can't be reached. It isn't available. The last host (10.11.1.1) which is supposed to provide connectivity to the destination device returns ICMP destination unreachable "Host unreachable" to the source host.

root@LAB1021-PC10:~# traceroute -n 10.11.2.4 
traceroute to 10.11.2.4 (10.11.2.4), 30 hops max, 60 byte packets
 1  10.11.6.1  0.763 ms  0.744 ms  0.993 ms
 2  10.11.1.1  1.979 ms  2.017 ms  2.003 ms
 3  10.11.1.1  2999.349 ms !H  2999.359 ms !H  2999.397 ms !H   <----

!F

This error is received if you are trying some PMTU discovery. Intermediate host which can't deliver this oversized packet returns ICMP Destination unreachable "Fragmentation needed and DF bit set"

root@LAB1021-PC10:~# traceroute -n 10.11.5.2 -F 1400
traceroute to 10.11.5.2 (10.11.5.2), 30 hops max, 1400 byte packets
 1  10.11.6.1  1.574 ms  1.534 ms  1.492 ms
 2  10.11.1.1  2.497 ms  2.649 ms  2.570 ms
 3  10.11.1.1  2.546 ms !F-1000  2.532 ms !F-1000  2.441 ms !F-1000    <----It informs that Next-hop MTU is 1000 on this host 10.11.1.1 

Any other error letter you have seen? Drop your comment here!

About: rtoodtoo

Worked for more than 10 years as a Network/Support Engineer and also interested in Python, Linux, Security and SD-WAN, currently living in the Netherlands and works as a Network Support Engineer. // JNCIE-SEC #223 / RHCE / PCNSE


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