I have set up a brand new Ubuntu server, and I'm encountering a very strange network related issue - the processes running on the server cannot 'see' packets originating from external networks, but they do show up when I run a tcpdump on the server's only network interface. However, when I send the exact same kind of traffic from other server on the same subnet, it goes all the way through.
I originally encountered the problem with SNMP traps (UDP 162/10162) coming to the server (it's supposed to be a logging server), but the same issue persists on more ports (tried a couple random high-ports, as well as a few privileged ports).
I have written two short python scripts, one is running on the 'target' server, and dumps traffic comming to one of the ports, and two identical scripts are run on two other servers (one on the local network, the other in an external network), generating the traffic - I can see traffic from both servers when running tcpdump, but the listening python script only dumps traffic from the local server.
I have checked iptables multiple times, and flushed all rules in all chains and tables.
I'm pretty stumped by this, if anyone has a hint as to what I can check next, I'd be very grateful.
Thanks in advance.