Troubleshooting with Traceroute

Introduction

The modest UNIX traceroute utility is perhaps the most useful utility to the Network Analyst or Administrator for isolating problems. It's also probably the most misunderstood and underestimated. This reference should help a person to use traceroute to its full potential.

What follows is an example of a healthy traceroute. That example will be used, in various forms, as the examples for all other situations in this reference.

A Healthy Traceroute

% traceroute www.yahoo.com
traceroute to www.yahoo.akadns.net (204.71.202.160), 30 hops max, 40 byte packets
 1  krishna (206.50.17.89)  1.823 ms *  1.862 ms
 2  d1xx-l0.tx.verio.net (199.1.11.47)  24.991 ms  24.567 ms  24.798 ms
 3  fa-1-0-0.a05.dllstx01.us.ra.verio.net (199.1.129.65)  24.805 ms  24.730 ms  24.612 ms
 4  ge-6-0-0.r01.dllstx01.us.bb.verio.net (129.250.28.190)  31.841 ms  24.819 ms  24.645 ms
 5  s11-1-0-45M.ar1.DAL1.gblx.net (206.132.119.125)  25.549 ms  25.332 ms  25.904 ms
 6  pos2-0-155M.cr2.DAL1.gblx.net (206.132.119.117)  25.116 ms  25.219 ms  25.833 ms
 7  pos6-0-2488M.cr1.SNV.gblx.net (208.50.169.62)  70.264 ms  70.299 ms  70.194 ms
 8  ge0-0-1000M.hr8.SNV.gblx.net (206.132.254.37)  70.243 ms  70.222 ms  70.191 ms
 9  208.178.22.58 (208.178.22.58)  70.235 ms  73.100 ms  75.066 ms
10  w1.yahoo.com (204.71.202.160)  70.176 ms  70.571 ms  70.343 ms

Some Things to Note

traceroute performs a DNS lookup.

Before beginning to trace the path to the destination, traceroute will resolve an IP address to its corresponding hostname (if it has a PTR record) and will resolve a hostname to its IP address, and, if applicable, the hostname to which it is CNAMEd.

Output

After each node's name and/or IP address is the latency result of three separate probes, an Asterisk * representing no response, or a special code:

!H - No route to host.

There is no entry in the indicated node's routing table for the specified destination host.

!N - No route to network.

There is no entry in the indicated node's routing table for the specified destination network. this error is less common than !H>

!P - Invalid protocol.

The network path to the destination host/network does not support a traceroute protocol.

!X - Route administratively blocked.

The route to the destination host/network has been explicitly blocked by a router ACL or firewall policy.

!S - Source route failed.

!F - Fragmentation needed.

!# - ICMP error.

Troubleshooting Unreachability

Troubleshooting Network Degradation