Due to the way that UDP "connections" work, this is expected behaviour. This is discussed in the nc6(1) man page ("UDP"), but is applicable to socat and nc as well:
UDP support in netcat6 works very well in both connect and in listen
mode. When using UDP in listen mode netcat6 accepts UDP packets from
any source that matches the optional address and/or port specified (if
it was specified). However, once the first packet is received,
netcat6 will only receive packets from that client in future. This is
done by putting the UDP socket into "connected" state (see udp(4) and
connect(2)). Packets from other sources are discarded by the kernel
and an ICMP unreachable response is sent.
When connecting to a remote host using UDP, nc6 will report that the
connection is open regardless of whether a remote server is listening.
This is because UDP is a connectionless protocol, and hence no
connection establishment is actually required. However, after sending
the first packet of data, a server may reply with an ICMP unreachable
response causing nc6 to exit with a 'Connection refused' error
message.
When you connect from the sender side, a random UDP source port is chosen. The receiver then binds to that particular host:port pair and will not listen to any other connection from then on. To solve this, you need to force the sender to always use the same port. I used socat for this example because it was easier to do:
Listener:
# socat -d -d UDP-LISTEN:6666 stdout
2018/01/29 22:02:02 socat[20969] N listening on UDP AF=2 0.0.0.0:6666
2018/01/29 22:02:07 socat[20969] N accepting UDP connection from AF=2 10.100.0.5:39000
2018/01/29 22:02:07 socat[20969] N using stdout for reading and writing
2018/01/29 22:02:07 socat[20969] N starting data transfer loop with FDs [5,5] and [1,1]
hello
bye
hello1
bye1
Sender:
# socat -d -d stdin UDP:listener-host:6666
2018/01/29 22:01:56 socat[8237] N using stdin for reading and writing
2018/01/29 22:01:56 socat[8237] N opening connection to AF=2 10.100.0.3:6666
2018/01/29 22:01:56 socat[8237] N successfully connected from local address AF=2 10.100.0.5:39000
2018/01/29 22:01:56 socat[8237] N starting data transfer loop with FDs [0,0] and [5,5]
hello
bye
2018/01/29 22:02:10 socat[8237] N socket 1 (fd 0) is at EOF
2018/01/29 22:02:10 socat[8237] N exiting with status 0
# socat -d -d stdin UDP:listener-host:6666
2018/01/29 22:02:13 socat[8238] N using stdin for reading and writing
2018/01/29 22:02:13 socat[8238] N opening connection to AF=2 10.100.0.3:6666
2018/01/29 22:02:13 socat[8238] N successfully connected from local address AF=2 10.100.0.5:57125
2018/01/29 22:02:13 socat[8238] N starting data transfer loop with FDs [0,0] and [5,5]
hello
2018/01/29 22:02:16 socat[8238] E read(5, 0x5619f9b09330, 8192): Connection refused
2018/01/29 22:02:16 socat[8238] N exit(1)
# socat -d -d stdin UDP:listener-host:6666,sourceport=39000
2018/01/29 22:05:17 socat[8280] N using stdin for reading and writing
2018/01/29 22:05:17 socat[8280] N opening connection to AF=2 10.100.0.3:6666
2018/01/29 22:05:17 socat[8280] N successfully connected from local address AF=2 10.100.0.5:39000
2018/01/29 22:05:17 socat[8280] N starting data transfer loop with FDs [0,0] and [5,5]
hello1
bye1
2018/01/29 22:05:23 socat[8280] N socket 1 (fd 0) is at EOF
2018/01/29 22:05:24 socat[8280] N exiting with status 0
As you can see, the source port changed on the sender but if I forced it to reuse the same port it worked.
