Question 81763 in Docker Question posted in
The official documentation can be found here.

Is it possible to launch a new Docker container in the network of an already running container?

Can I start a new Docker container, from the command line, in the same network of an already running container?

The original container was not run with a defined network (no --network parameter).

Here is an example of what I would like to happen:

# Start a 1st container named host1.
$ docker run --name host1 -dit alpine ash
…

# Start a 2nd container and open a shell.
$ docker run -it --rm alpine

# Try to ping the 1st container from inside the 2nd container.
/ ~ ping host1
ping: bad address 'host1'

How could the second command be changed so that the second container sees host1?

Tags:

3

Answers


  1. 0

    Yes. if you don’t specify a network the default network is used. So starting a new container without specifying a network will put it on the default.

    Login or Signup to reply.
  2. 0

    Can I start a new Docker container, from the command line, in the same
    network of an already running container?

    Yes. Actually, in your example, you’re probably already doing just that: you created both containers in the default bridge network – even though you didn’t specify --network, Docker created them in this default network.

    You can’t use host1 hostname to reach your first container from your second container because DNS resolution in the default bridge network won’t resolve container’s hostname in this default network. (it would work in a non-default network specified with --network because DNS resolution is different)

    However you can reach it directly via it’s IP, for example:

    # Start a 1st container named host1.
$ docker run --name host1 -dit alpine ash

# Get host1 IP
$ docker inspect host1 --format='{{range .NetworkSettings.Networks}}{{.IPAddress}}{{end}}'

172.17.0.2

# Start a 2nd container and open a shell.
$ docker run -it --rm alpine

# Try to ping the 1st container from inside the 2nd container.
# Won't work with hostname
/ ~ ping host1
ping: bad address 'host1'

# Will work with IP
/ ~ ping 172.17.0.2
PING 172.17.0.2 (172.17.0.2): 56 data bytes
64 bytes from 172.17.0.2: seq=0 ttl=64 time=0.322 ms
...

    It looks like that what you really want is to contact container by their hostname in default bridge network. Your question may be reformulated as

    Can I start a container in the default bridge network and resolve its hostname from another container in the same network?

    You can specify host1 IP with --add-host such as:

    $ docker run -it --rm 
    --add-host host1:$(docker inspect host1 --format='{{range .NetworkSettings.Networks}}{{.IPAddress}}{{end}}') 
    alpine

/ ~ ping host1
PING host1 (172.17.0.2): 56 data bytes
64 bytes from 172.17.0.2: seq=0 ttl=64 time=0.322 ms

    Which will pass host1s IP as available hostname to new container

    That being said, you’ll probably be better of using a custom network, for example:

    docker network create mynetwork
docker run --name host3 -dit --network mynetwork alpine ash
docker run -it --network mynetwork alpine ping host3
    Login or Signup to reply.
  3. 0

    It is possible. You have to add host resolution to /etc/hosts in each container:

    # docker exec -it host2 ash -c "echo `docker inspect host1 --format='{{range .NetworkSettings.Networks}}{{.IPAddress}}{{end}}'` host1 >> /etc/hosts"
# docker exec -it host2 ping -c 1 host1
PING host1 (192.168.128.2): 56 data bytes
    Login or Signup to reply.
Please signup or login to give your own answer.

Back To Top
Search