Tuesday, September 28, 2021

Linux - TCP traffic to IP without ARP entry vanishes

I've spent some time today trying to debug IP traffic that wasn't going anywhere. Our application was sending it out, but tcpdump wasn't showing the traffic leaving the interface (Not a routing issue).

Turns out, ARP was "broken", and there was no entry for the IP we were sending too. I've manually added an entry and asked someone to investigate our ARP issue however....

Is there anywhere in linux to help me debug this? What happens with traffic that gets dispatched from an application but doesn't reach the network? Any logs or "tcpdump" like commands to capture traffic like this in the future?



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