The Serpent

// Cursing the Internet since 1998

Ping Isn't Always Your Friend

It’s a common mistake amongst network engineers to use Ping as a test for global, unimpeded connectivity. In my early days, I was guilty of this too, clinging to the overly used explanation of “but Ping works”. Ping will confirm ICMP connectivity, but little else. For example, take a look at the following packet capture showing a failed web service on server 10.91.15.15, which should be accessible via port 8085, the client is 10.91.1.148. This is a classic example of a problems that comes with the tag line… “but ping works”:

Failed Ping capture

The engineer was convinced the network was at fault, because the server easily responded to Ping OK. However a quick capture on the network revealed that no TCP connection came back from the server, after a quick check – the firewall was configured to drop traffic on port 8085.

Ping will confirm layer 3 connectivity (remember the OSI Model) but will not help you with the layers above. In this case TCP (layer 4) was at fault, so different troubleshooting techniques are required to investigate further. A packet capture will help you get to the bottom of it much more quickly.