Traceroute Errors and Messages
Traceroute errors and diagnostic messages provide information about the outbound routing path to a destination. Learning to read traceroute return messages and traceroute output is paramount if you are going to use it to troubleshoot network connections.
EXAMPLES DRAWN FROM CISCO ROUTERS
Traceroutes will also display certain symbols related to other network problems:
| nnn msec | Round Trip Time | This is the number of milliseconds it took for a packet to be sent out, get destroyed when it's time to live expired, and for a response packet to be generated and sent back. Each set of datagrams has it's own response time, and times in prior hops are irrelevant. |
| * | Time Exceeded | The response did not return in the allotted time. The default timeout
is 2 seconds. This response does NOT necessarilly indicate packet loss, merely that the response took longer than 2 seconds to return, which is the default timeout. |
!A |
Administratively denied | An access list is blocking the response. |
| !H | Host Not Available | The host did not respond at all. |
| !T | Timeout | No packet received back. |
| !Q | Source Quench | Throttle back bandwidth usage. |
| !U | Port unreachable | |
| !N | Network Unreachable | |
| !P | Protocol Unreachable | |
| ? | Unknown packet type |
If multiple hosts appear in the trace for a particular hop, it is because there is more than one path to the destination.
On a Cisco router, atraceroute performs a reverse DNS resolution of all IP addresses by default as well as a whois to get the AS number to provide better description in the result for the user.
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