I have a few fundamental conceptual issues due to which i am not able to forward the traffic on a docker container containing a network.

Docker container is listening on 2123/udp

Approach 0 Publish all argument does not seem to work. There was no mapping/change in iptables before and after this command.

 docker run -d -P image


Approach 1 I want to send traffic from any interface/ip/port to a docker container. I found that most of my traffic is on eno1, ip = x.x.x.x and port 22. I tried binding this port 22 to docker using

docker run -d -p 22:2123 image
As the port is active so it could not bind with port 22

Approach 2 I chose a random port 80 and tried to publish it

docker run -d -p x.x.x.x:80:2123

I did not print any error. However, when i send packets to this ip and port, connection is refused.

Packets sent as follows.
dd if=/dev/zero bs=9000 count=1000 > /dev/tcp/x.x.x.x/80

Therefore i added a port forwarding rule to forward all traffic received on port 22 to x.x.x.x:80.

echo 1 > /proc/sys/net/ipv4/ip_forward
iptables -t nat -A PREROUTING -p tcp --dport 22 -j DNAT --to-destination x.x.x.x:80

Still connection refused. Even with this refused connection, i saw two transactions

 tcpdump -i any port 2123
 IP <dockerNWip>.37042 > <host>.2123: Flags [S], seq 1289990254, win 29200, options [mss 1460,sackOK,TS val 437161 ecr 0,nop,wscale 7], length 0
 IP <dockerNWip>.37042 > <host>.2123: Flags [S], seq 1289990254, win 29200, options [mss 1460,sackOK,TS val 437161 ecr 0,nop,wscale 7], length 0
 IP  <host>.2123 > <dockerNWip>.37042: Flags [R.], seq 0, ack 1289990255, win 0, length 0
 IP <host>.2123 > <dockerNWip>.37042: Flags [R.], seq 0, ack 1, win 0, length 0

I am not sure where the problem lies. My question

 1) Do i need to bind x.x.x.x and port 80? I thought the rule would do that.
 2) Do i need anything else besides the publishing the ports?
 3) Is my way to tcpdumping the port correct to monitor the traffic?

My strategy/assumption was that binding host port 80 with container port 2123 should automatically route the traffic.

  • 2
    You say the container exposes port 2123 over UDP, so you should set your docker port bindings to be UDP: docker run -d -p $UNUSED_HOST_PORT:2123/udp $IMAGE and see if that works any better. If that still does not work, make sure the Docker network is in a functioning state. Also, your test to check if it is working appears to be using TCP, you would want to switch to a test using UDP. – GracefulRestart Apr 28 '18 at 17:24
  • I have followed all of these instructions. However still don't see any traffic on port 2123. The binding is not successful. – RNA Apr 29 '18 at 7:25

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