Exposing a port on a live Docker container

You cannot do this via Docker, but you can access the container's un-exposed port from the host machine.

If you have a container with something running on its port 8000, you can run

wget http://container_ip:8000

To get the container's IP address, run the 2 commands:

docker ps
docker inspect container_name | grep IPAddress

Internally, Docker shells out to call iptables when you run an image, so maybe some variation on this will work.

To expose the container's port 8000 on your localhost's port 8001:

iptables -t nat -A  DOCKER -p tcp --dport 8001 -j DNAT --to-destination 172.17.0.19:8000

One way you can work this out is to setup another container with the port mapping you want, and compare the output of the iptables-save command (though, I had to remove some of the other options that force traffic to go via the docker proxy).

NOTE: this is subverting docker, so should be done with the awareness that it may well create blue smoke.

OR

Another alternative is to look at the (new? post 0.6.6?) -P option - which will use random host ports, and then wire those up.

OR

With 0.6.5, you could use the LINKs feature to bring up a new container that talks to the existing one, with some additional relaying to that container's -p flags? (I have not used LINKs yet.)

OR

With docker 0.11? you can use docker run --net host .. to attach your container directly to the host's network interfaces (i.e., net is not namespaced) and thus all ports you open in the container are exposed.

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Here's what I would do:

Commit the live container.

Run the container again with the new image, with ports open (I'd recommend mounting a shared volume and opening the ssh port as well)

sudo docker ps 
sudo docker commit <containerid> <foo/live>
sudo docker run -i -p 22 -p 8000:80 -m /data:/data -t <foo/live> /bin/bash

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While you cannot expose a new port of an existing container, you can start a new container in the same Docker network and get it to forward traffic to the original container.

# docker run \
  --rm \
  -p $PORT:1234 \
  verb/socat \
    TCP-LISTEN:1234,fork \
    TCP-CONNECT:$TARGET_CONTAINER_IP:$TARGET_CONTAINER_PORT

Worked Example

Launch a web-service that listens on port 80, but do not expose its internal port 80 (oops!):

# docker run -ti mkodockx/docker-pastebin   # Forgot to expose PORT 80!

Find its Docker network IP:

# docker inspect 63256f72142a | grep IPAddress
                    "IPAddress": "172.17.0.2",

Launch verb/socat with port 8080 exposed, and get it to forward TCP traffic to that IP's port 80:

# docker run --rm -p 8080:1234 verb/socat TCP-LISTEN:1234,fork TCP-CONNECT:172.17.0.2:80

You can now access pastebin on http://localhost:8080/, and your requests goes to socat:1234 which forwards it to pastebin:80, and the response travels the same path in reverse.

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IPtables hacks don't work, at least on Docker 1.4.1.

The best way would be to run another container with the exposed port and relay with socat. This is what I've done to (temporarily) connect to the database with SQLPlus:

docker run -d --name sqlplus --link db:db -p 1521:1521 sqlplus

Dockerfile:

FROM debian:7

RUN apt-get update && \
    apt-get -y install socat && \
    apt-get clean

USER nobody

CMD socat -dddd TCP-LISTEN:1521,reuseaddr,fork TCP:db:1521

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Here's another idea. Use SSH to do the port forwarding; this has the benefit of also working in OS X (and probably Windows) when your Docker host is a VM.

docker exec -it <containterid> ssh -R5432:localhost:5432 <user>@<hostip>

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To add to the accepted answer iptables solution, I had to run two more commands on the host to open it to the outside world.

HOST> iptables -t nat -A DOCKER -p tcp --dport 443 -j DNAT --to-destination 172.17.0.2:443
HOST> iptables -t nat -A POSTROUTING -j MASQUERADE -p tcp --source 172.17.0.2 --destination 172.17.0.2 --dport https
HOST> iptables -A DOCKER -j ACCEPT -p tcp --destination 172.17.0.2 --dport https

Note: I was opening port https (443), my docker internal IP was 172.17.0.2

Note 2: These rules and temporrary and will only last until the container is restarted

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I had to deal with this same issue and was able to solve it without stopping any of my running containers. This is a solution up-to-date as of February 2016, using Docker 1.9.1. Anyway, this answer is a detailed version of @ricardo-branco's answer, but in more depth for new users.

In my scenario, I wanted to temporarily connect to MySQL running in a container, and since other application containers are linked to it, stopping, reconfiguring, and re-running the database container was a non-starter.

Since I'd like to access the MySQL database externally (from Sequel Pro via SSH tunneling), I'm going to use port 33306 on the host machine. (Not 3306, just in case there is an outer MySQL instance running.)

About an hour of tweaking iptables proved fruitless, even though:

Step by step, here's what I did:

mkdir db-expose-33306
cd db-expose-33306
vim Dockerfile

Edit dockerfile, placing this inside:

# Exposes port 3306 on linked "db" container, to be accessible at host:33306
FROM ubuntu:latest # (Recommended to use the same base as the DB container)

RUN apt-get update && \
    apt-get -y install socat && \
    apt-get clean

USER nobody
EXPOSE 33306

CMD socat -dddd TCP-LISTEN:33306,reuseaddr,fork TCP:db:3306

Then build the image:

docker build -t your-namespace/db-expose-33306 .

Then run it, linking to your running container. (Use -d instead of -rm to keep it in the background until explicitly stopped and removed. I only want it running temporarily in this case.)

docker run -it --rm --name=db-33306 --link the_live_db_container:db -p 33306:33306  your-namespace/db-expose-33306

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You can use SSH to create a tunnel and expose your container in your host.

You can do it in both ways, from container to host and from host to container. But you need a SSH tool like OpenSSH in both (client in one and server in another).

For example, in the container, you can do

$ yum install -y openssh openssh-server.x86_64
service sshd restart
Stopping sshd:                                             [FAILED]
Generating SSH2 RSA host key:                              [  OK  ]
Generating SSH1 RSA host key:                              [  OK  ]
Generating SSH2 DSA host key:                              [  OK  ]
Starting sshd:                                             [  OK  ]
$ passwd # You need to set a root password..

You can find the container IP address from this line (in the container):

$ ifconfig eth0 | grep "inet addr" | sed 's/^[^:]*:\([^ ]*\).*/\1/g'
172.17.0.2

Then in the host, you can just do:

sudo ssh -NfL 80:0.0.0.0:80 root@172.17.0.2

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Based on Robm's answer I have created a Docker image and a Bash script called portcat.

Using portcat, you can easily map multiple ports to an existing Docker container. An example using the (optional) Bash script:

curl -sL https://raw.githubusercontent.com/archan937/portcat/master/script/install | sudo bash
portcat my-awesome-container 3456 4444:8080

And there you go! Portcat is mapping:

port 3456 to my-awesome-container:3456

port 4444 to my-awesome-container:8080

Please note that the Bash script is optional, the following commands:

ipAddress=$(docker inspect my-awesome-container | grep IPAddress | grep -o '[0-9]\{1,3\}\(\.[0-9]\{1,3\}\)\{3\}' | head -n 1)
docker run -p 3456:3456 -p 4444:4444 --name=alpine-portcat -it pmelegend/portcat:latest $ipAddress 3456 4444:8080

I hope portcat will come in handy for you guys. Cheers!

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There is a handy HAProxy wrapper.

docker run -it -p LOCALPORT:PROXYPORT --rm --link TARGET_CONTAINER:EZNAME -e "BACKEND_HOST=EZNAME" -e "BACKEND_PORT=PROXYPORT" demandbase/docker-tcp-proxy

This creates an HAProxy to the target container. easy peasy.

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Here are some solutions:

https://forums.docker.com/t/how-to-expose-port-on-running-container/3252/12

The solution to mapping port while running the container. docker run -d --net=host myvnc that will expose and map the port automatically to your host

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In case no answer is working for someone - check if your target container is already running in docker network:

CONTAINER=my-target-container
docker inspect $CONTAINER | grep NetworkMode
        "NetworkMode": "my-network-name",

Save it for later in the variable $NET_NAME:

NET_NAME=$(docker inspect --format '{{.HostConfig.NetworkMode}}' $CONTAINER)

If yes, you should run the proxy container in the same network.

Next look up the alias for the container:

docker inspect $CONTAINER | grep -A2 Aliases
                "Aliases": [
                    "my-alias",
                    "23ea4ea42e34a"

Save it for later in the variable $ALIAS:

ALIAS=$(docker inspect --format '{{index .NetworkSettings.Networks "'$NET_NAME'" "Aliases" 0}}' $CONTAINER)

Now run socat in a container in the network $NET_NAME to bridge to the $ALIASed container's exposed (but not published) port:

docker run \
    --detach --name my-new-proxy \
    --net $NET_NAME \
    --publish 8080:1234 \
    alpine/socat TCP-LISTEN:1234,fork TCP-CONNECT:$ALIAS:80

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You can use an overlay network like Weave Net, which will assign a unique IP address to each container and implicitly expose all the ports to every container part of the network.

Weave also provides host network integration. It is disabled by default but, if you want to also access the container IP addresses (and all its ports) from the host, you can run simply run weave expose.

Full disclosure: I work at Weaveworks.

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Read Ricardo's response first. This worked for me.

However, there exists a scenario where this won't work if the running container was kicked off using docker-compose. This is because docker-compose (I'm running docker 1.17) creates a new network. The way to address this scenario would be

docker network ls

Then append the following docker run -d --name sqlplus --link db:db -p 1521:1521 sqlplus --net network_name

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It's not possible to do live port mapping but there are multiple ways you can give a Docker container what amounts to a real interface like a virtual machine would have.

Macvlan Interfaces

Docker now includes a Macvlan network driver. This attaches a Docker network to a "real world" interface and allows you to assign that networks addresses directly to the container (like a virtual machines bridged mode).

docker network create \
    -d macvlan \
    --subnet=172.16.86.0/24 \
    --gateway=172.16.86.1  \
    -o parent=eth0 pub_net

pipework can also map a real interface into a container or setup a sub interface in older versions of Docker.

Routing IP's

If you have control of the network you can route additional networks to your Docker host for use in the containers.

Then you assign that network to the containers and setup your Docker host to route the packets via the docker network.

Shared host interface

The --net host option allows the host interface to be shared into a container but this is probably not a good setup for running multiple containers on the one host due to the shared nature.

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