Can't receive packets on UDP/53

Hi,

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.:
`iptables -S`:
```
-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?