Docker Compose default network is not accessible in docker-in-docker on Ubuntu 26.04

[halftomato]

Ubuntu version: 26.04

Docker version: 29.5.3

Problem: Docker Compose default network connectivity is broken in docker-in-docker environment - containers cannot communicate with each other

Root cause: iptables-legacy FORWARD chain has default policy set to DROP

Workaround: sudo iptables-legacy -P FORWARD ACCEPT

[Kaniska244]

Hi halftomato

Thank you for reporting this issue. Would you kindly help me with a sample dev container container to check this problem further?

[halftomato]

Kaniska (@Kaniska244)
Thank you for your time!

devcontainer.json sample

{
    "name": "ubuntu-resolute",
    // Or docker hub ubuntu:resolute, it is the same situation.
    "image": "mcr.microsoft.com/devcontainers/base:ubuntu26.04",
    "features": {
        "ghcr.io/devcontainers/features/common-utils:2": {
            "installZsh": true,
            "configureZshAsDefaultShell": true,
            "installOhMyZsh": true,
            "installOhMyZshConfig": true,
            "username": "codespace",
            "userUid": "1001",
            "userGid": "1001"
        },
        "ghcr.io/devcontainers/features/docker-in-docker:3": {
            "version": "latest",
            "moby": false
        },
        "ghcr.io/devcontainers/features/sshd:1": {}
    },
    "overrideFeatureInstallOrder": [
        "ghcr.io/devcontainers/features/common-utils",
        "ghcr.io/devcontainers/features/sshd",
        "ghcr.io/devcontainers/features/docker-in-docker"
    ],
    "remoteUser": "codespace",
    "containerUser": "codespace"
}

Here is an example of compose.yml.
In fact, most of the compose files I tested have the same issue: the network is unreachable.

services:
  filebrowser:
    image: filebrowser/filebrowser:latest
    container_name: filebrowser
    restart: no
    ports:
      - '801:80'