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Judging by the specific output Connection to Connection to 10.1.0.100 53 port [udp/domain] succeeded!0 53 port [udp/domain] succeeded! you are using openbsd-netcat.

Judging by the specific output Connection to Connection to 10.1.0.100 53 port [udp/domain] succeeded! you are using openbsd-netcat.

So while the other suggestion may be valid, I don't think that's happening in this particular case.

udptest issues around 3 writes to the open socket. There is a note that this doesn't work for IPv6 and fails after around 100 ports checked.

So while the other suggestion may be valid, I don't think that's happening in this particular case.

Looking at the code for that there the test is that there you can bind to the UDP socket, i.e. there is an open connection:

Looking at the code for that the test is to bind to the UDP socket, i.e. there is an open connection: