I use this command to test the net connectivity in the terminal:
docker run --rm --name test -it -p 9999:9999 busybox nc -l 0.0.0.0:9999
and in another terminal
$ telnet localhost 9999
Trying ::1...
Connected to localhost.
Escape character is '^]'.
Connection closed by foreign host.
seemed it connected by closed immediately, I can not input anything.
it's working fine when I try localy
nc -l 0.0.0.0:9999
and
telnet localhost 9999
Docker version 17.12.1-ce, build 7390fc6 Ubuntu VERSION="18.04.1 LTS (Bionic Beaver)"
There are 2 total different style netcat. The nc in container is not the same series with your host, so host pass, container solution fail.
I guess your host nc is not traditional one, something like follows:
# nc
This is nc from the netcat-openbsd package. An alternative nc is available
in the netcat-traditional package.
usage: nc [-46bCDdhjklnrStUuvZz] [-I length] [-i interval] [-O length]
[-P proxy_username] [-p source_port] [-q seconds] [-s source]
[-T toskeyword] [-V rtable] [-w timeout] [-X proxy_protocol]
[-x proxy_address[:port]] [destination] [port]
You container nc is a different version, it has a total different command syntax:
# docker run --rm --name test -it -p 9999:9999 busybox /bin/sh
/ # nc
BusyBox v1.29.3 (2018-10-01 22:37:18 UTC) multi-call binary.
Usage: nc [OPTIONS] HOST PORT - connect
nc [OPTIONS] -l -p PORT [HOST] [PORT] - listen
-e PROG Run PROG after connect (must be last)
-l Listen mode, for inbound connects
-lk With -e, provides persistent server
-p PORT Local port
-s ADDR Local address
-w SEC Timeout for connects and final net reads
-i SEC Delay interval for lines sent
-n Don't do DNS resolution
-u UDP mode
-v Verbose
-o FILE Hex dump traffic
-z Zero-I/O mode (scanning)
If you use netstat in container, you will find 9999 port was not open with your command, as a result, your client quit immediately.
So, you need to change your command to follows:
docker run --rm --name test -it -p 9999:9999 busybox nc -l -p 9999