Question

is it easy to write a script to test whether the network is ever down for the next 24 or 48 hours? I can use ssh to connect to a shell and come back 48 hours later to see if it is still connected to see if the network has ever been down, but can i do it programmatically easily?

Answer 1

The Internet (and your ethernet) is a packet-switched network, which makes the definition of ‘down’ difficult.

Some things are obvious; for example, if your ethernet card doesn’t report a link, then it’s down (unless you have redundant connections). But having a link doesn’t mean its up.

The basic, 100 mile view of how the Internet works is that your computer splits the data it wants to send into ~1500-byte segments called packets. It then, pretty much blindly, sends them on their way, however your routing table says to. Then that machine repeats the process. Eventually, through many repetitions, it reaches the remote host.

So, you may be tempted to define up as the packet reached its destination. But, what happens if the packet gets corrupted, e.g., due to faulty hardware or interference? The next router will just discard it. Ok, that’s fine, you may well want to consider that down. What if a router on the path is too busy, or the link it needs to be sent on is full? The packet will be dropped. You probably don’t want to count that as down.

Packets routinely get lost; higher-level protocols (e.g., TCP) deal with it and retransmit the packet. In fact, TCP uses the packet drop on link full behavior to discover the link bandwidth.

You can monitor packet loss with a tool like ping, as the other answer states.