I was sniffing remotely, but using tcpdump on Dipsy has the same result. You can see both the echos and replies. MAC addresses all look fine.

They are both on the same network, connected via a couple of switches. I've power cycled the switches.

This problem has actually been going on for months. I had assumed, wrongly, that Vault's network card was playing up after my RH9 -> Debian upgrade (on the basis of the ping problems from Dipsy). Vault has been sitting gather dust for a while due to this.

It is only when I plugged Vault back in again today to try and get it working that I realised what was going on.

Vault is actually supposed to live 30 miles away in someone else's house, which is where it is going tomorrow. Hopefully putting a chunk of the Internet between the two machines will return Dipsy to sanity (if it doesn't I will be really puzzled).