Communicating between Docker containers in different networks on the same host

Question

Any possibility to make containers in different networks within the same host to communicate? Please note that I am not using docker-compose at the moment.

The following is a summary of what I did. I created two networks using the following commands

docker network create --driver bridge mynetwork1   
docker network create --driver bridge mynetwork2

Then I ran two containers on each of these created networks using the commands:

docker run --net=mynetwork1 -it name=mynet1container1 mycontainerimage
docker run --net=mynetwork1 -it name=mynet1container2 mycontainerimage
docker run --net=mynetwork2 -it name=mynet2container1 mycontainerimage
docker run --net=mynetwork2 -it name=mynet2container2 mycontainerimage

I then identified the IP Addresses of each of the containers from the networks created using

docker network inspect mynetwork1
docker network inspect mynetwork2

Using those I was able to communicate between the containers in the same network, but I could not communicate between the containers across the networks. Communication was possible only by adding the containers to the same network.

Much thanks...

Answer 1

Containers in different networks can not communicate with each other because iptables drop such packets. This is shown in the DOCKER-ISOLATION-STAGE-1 and DOCKER-ISOLATION-STAGE-2 chains in the filter table.

    sudo iptables -t filter -vL

Rules can be added to DOCKER-USER chain to allow communication between different networks. In the above scenario, the following commands will allow ANY container in mynetwork1 to communicate with ANY containers in mynetwork2.

The bridge interface names of the network (mynetwork1 and mynetwork2) need to be found first. Their names are usually look like br-07d0d51191df or br-85f51d1cfbf6 and they can be found using command "ifconfig" or "ip link show". Since there are multiple bridge interfaces, to identify the correct ones for the networks of interest, the inet address of the bridge interface (shown in ifconfig) should match the subnet address shown in command 'docker network inspect mynetwork1'

    sudo iptables -I DOCKER-USER -i br-########1 -o br-########2 -j ACCEPT
    sudo iptables -I DOCKER-USER -i br-########2 -o br-########1 -j ACCEPT

The rules can be fine tuned to allow only communications between specific IPs. E.g,

    sudo iptables -I DOCKER-USER -i br-########1 -o br-########2 -s 172.17.0.2 -d 172.19.0.2 -j ACCEPT
    sudo iptables -I DOCKER-USER -i br-########2 -o br-########1 -s 172.19.0.2 -d 172.17.0.2 -j ACCEPT