How can I get the IP of a container in a specific network_mode from another container outside of that mode?

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In a project there are three services in the docker compose yml:

A VPN.

A container (named first) connected to that VPN using network_mode.

A container (named second) not connected to that VPN.

From first I can get second's IP using the container name (second), but the oposite does not work.

"first" and "second" are simple python scripts sending data to each other using socket.

I can send data from "second" to "first" if I use the IP address instead of the container name, but that is not a solution I can use in the project.

This is the .yml I'm using:

version: '3.9'

services:
    vpn:
        build: ./vpn
        container_name: vpn
        env_file:
            - ss.env
        cap_add: 
            - NET_ADMIN
            - NET_RAW
        devices:
            - /dev/net/tun:/dev/net/tun
        dns:
            - 1.1.1.1

    first:
        build: ./first
        container_name: first
        depends_on: 
            - vpn
        network_mode: service:vpn

    second:
        build: ./second
        container_name: second
        depends_on: 
            - vpn

The relevant part of the python scripts:

#first.py
client.sendto(bytes('message from second',encoding='utf8'), ('second', 37021))

#second.py
client.sendto(bytes('message from second',encoding='utf8'), ('first', 37020))

Also, the vpn log:

vpn       | 2021-10-20 00:44:21 TUN/TAP device tun0 opened
vpn       | 2021-10-20 00:44:21 /sbin/ip link set dev tun0 up mtu 1500
vpn       | 2021-10-20 00:44:21 /sbin/ip link set dev tun0 up
vpn       | 2021-10-20 00:44:21 /sbin/ip addr add dev tun0 10.8.8.2/24
vpn       | 2021-10-20 00:44:21 /sbin/ip route add 104.111.100.109/32 via 192.168.144.1
vpn       | 2021-10-20 00:44:21 /sbin/ip route add 0.0.0.0/1 via 10.8.8.1
vpn       | 2021-10-20 00:44:21 /sbin/ip route add 128.0.0.0/1 via 10.8.8.1
docker-compose docker
1 Answers

Your problems come from a misconfiguration of the networks.

First of all, when you're starting services with docker-compose up -d you're creating a default network with the name of the folder where that Compose file is located. You can check that with docker network ls.

Well, all your services will connect by default to that network, except if you define a different default or you change the network mode, as it's your case for first.

Basically, let's suppose you have your Compose file under a directory called myapp.

  1. When you start your containers, Docker Compose is creating a network with the name myapp-default.
  2. Your services vpn and second will join that network, but first will work with the network stack from vpn.
  3. Since first is using the same network namespace of vpn, it can discover second without any problem using the service name.
  4. Since first isn't in the default network, second cannot discover it.

If you want first to be discoverable by second you shouldn't use the same network stack of vpn but just let it join the default network created by Compose (or create another network by yourself and make the three of them to join that network).

user_89515710
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