I can’ receive packets on UDP/53 on a VPS using netcat. There’s nothing related to 53 in
ss -lptun. Any other port seems to work fine, including similar low port numbers.
I’m running this on the machine:
nc -u -l 53
If I run
nc -u localhost 53 or
nc -u $EXTERNAL_IP 53 on this VPS, it works. It doesn’t work from another machine (for this port, it works for others).
OVH told me that they don’t process UDP/53 any differently from other packets.
I haven’t customized anything related to iptables etc.:
-P INPUT ACCEPT -P FORWARD ACCEPT -P OUTPUT ACCEPT -N f2b-sshd -A INPUT -p tcp -m multiport --dports 22 -j f2b-sshd -A f2b-sshd -j RETURN
What can I do to debug this further?