: bleys; grep 4500 /etc/services ipsec-nat-t 4500/tcp ipsec-msft # IPsec NAT-Traversal ipsec-nat-t 4500/udp ipsec-msft # IPsec NAT-Traversal : bleys; sysctl net.inet.esp.udpencap net.inet.esp.udpencap=1 : bleys
You're trying to use the ipsec ESP encapsulation port, which is enabled by default. If you're a masochist and likes making your life more difficult, you can use that port for your own purposes by disabling that sysctl. If you're not a masochist, use a different port. Philip Guenther On Tue, Dec 8, 2020 at 4:13 PM Chris Johnson <[email protected]> wrote: > 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 > >
