Hello All,
I am unable to set up a localhost netcat listener on UDP port 4500 that
responds to a client on that same host. I encountered this issue
attempting to test whether UDP 4500 was open on our departmental firewall.
Simple test case: Fresh build of OpenBSD 6.8. No local network, no
packet filter, no iked running.
# netstat -na -f inet | grep 4500
[empty]
# fstat | grep 4500
[empty]
$ nc -ul localhost 4501 &
[1] 72638
$ nc -u localhost 4501
ZZZZZ
ZZZZZ
^C
$ pkill nc
[1]+ Stopped nc -ul localhost 4501
$ nc -ul localhost 4500 &
[2] 70181
$ nc -u localhost 4500
ZZZZZ
^C
$ pkill nc
[2]- Terminated nc -ul localhost 4500
The server running on port 4500 does not echo. Why not? Is there
something obvious that I'm missing?
I've tried this on three different OpenBSD 6.8 systems (all amd64). Is
UDP 4500 reserved in some way? Other ports I've tried work fine. Linux
and MacOS systems work fine on this port.
Cheers,
Chris
