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on one server we encountered an issue where all kinds of UDP connections are failing. The same application and tests work on other servers with the same OS and similar setup.

nc -vz -u 83.61.**.** 7092
Connection to 83.61.**.** 7092 port [udp/*] succeeded!

With the go app:

read udp 5.9.**.**:49696->83.61.**.**:7092: i/o timeout

We tested it with a php and golang app.

We have tried it with a fully disabled firewall as well.

What could be the cause if netcat succeeds but other applications can't.

user2693017
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  • UDP is a _connectionless_ protocol, so there are no UDP connections. – Ron Maupin Jun 12 '20 at 22:29
  • The UDP test from openbsd-netcat is pretty worthless. It doesn't actually tell you anything useful, like whether the other end received the UDP traffic. In particular it will report succeeded! if the UDP packets were dropped! – Michael Hampton Jun 12 '20 at 22:30

1 Answers1

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Connection to [...] port [udp/*] succeeded!

is a missleading message from netcat. netcat can't know if the port is open with UDP because this is not a connected protocol unlike TCP.

use the good old way: look at the other side if your UDP packets comes in with tcpdump

exeral
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