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Can't receive packets on UDP/53


#1

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?


#2

Just realized my ISP is dropped packets sent to UDP/53 in certain situations, which explains why I had trouble receiving these packets on the VPS…