Impossible to access the gateway/host from a container using an internal network

[krichprollsch]

Hello there 👋

In its doc, Docker says the internal network mode doesn't block the communication between a container and the gateway/host.

Containers on an internal network may communicate between each other, but not with any other network, as no default route is configured and firewall rules are set up to drop all traffic to or from other networks. Communication with the gateway IP address (and thus appropriately configured host services) is possible, and the host may communicate with any container IP directly.
https://docs.docker.com/reference/cli/docker/network/create/#internal

When I'm testing with Docker, the container can connect to the gateway.

$ docker network create --internal internal-net
$ docker inspect internal-net |grep Gateway
                    "Gateway": "172.18.0.1

$ docker run --network internal-net -ti --rm curlimages/curl -I http://172.18.0.1:8000/
HTTP/1.0 200 OK
Server: SimpleHTTP/0.6 Python/3.11.9
Date: Tue, 02 Jul 2024 12:40:49 GMT
Content-type: text/html; charset=utf-8
Content-Length: 429

But my Podman doesn't allow accessing the gateway host.

$ podman network create --internal internal-net
                    
$ podman run --network internal-net -ti --rm docker.io/curlimages/curl -I http://host.docker.internal:8000/
curl: (7) Failed to connect to host.docker.internal port 8000 after 0 ms: Couldn't connect to server

I'm wondering if the difference could be considered as a bug in Podman?
Or there is another way to achieve the same behavior?

Here is my Podman info:

$ podman info
host:
  arch: amd64
  buildahVersion: 1.33.7
  cgroupControllers:
  - memory
  - pids
  cgroupManager: systemd
  cgroupVersion: v2
  conmon:
    package: conmon_2.1.10+ds1-1+b1_amd64
    path: /usr/bin/conmon
    version: 'conmon version 2.1.10, commit: unknown'
  cpuUtilization:
    idlePercent: 92.21
    systemPercent: 1.5
    userPercent: 6.29
  cpus: 8
  databaseBackend: sqlite
  distribution:
    codename: trixie
    distribution: debian
    version: unknown
  eventLogger: journald
  freeLocks: 2047
  hostname: poum
  idMappings:
    gidmap:
    - container_id: 0
      host_id: 1000
      size: 1
    - container_id: 1
      host_id: 100000
      size: 65536
    uidmap:
    - container_id: 0
      host_id: 1000
      size: 1
    - container_id: 1
      host_id: 100000
      size: 65536
  kernel: 6.8.12-amd64
  linkmode: dynamic
  logDriver: journald
  memFree: 2094129152
  memTotal: 15938809856
  networkBackend: netavark
  networkBackendInfo:
    backend: netavark
    dns:
      package: aardvark-dns_1.4.0-6_amd64
      path: /usr/lib/podman/aardvark-dns
      version: aardvark-dns 1.4.0
    package: netavark_1.4.0-4.1_amd64
    path: /usr/lib/podman/netavark
    version: netavark 1.4.0
  ociRuntime:
    name: runc
    package: containerd.io_1.7.18-1_amd64
    path: /usr/bin/runc
    version: |-
      runc version 1.7.18
      commit: v1.1.13-0-g58aa920
      spec: 1.0.2-dev
      go: go1.21.11
      libseccomp: 2.5.5
  os: linux
  pasta:
    executable: /usr/bin/pasta
    package: passt_0.0~git20240624.1ee2eca-1_amd64
    version: |
      pasta 0.0~git20240624.1ee2eca-1
      Copyright Red Hat
      GNU General Public License, version 2 or later
        <https://www.gnu.org/licenses/old-licenses/gpl-2.0.html>
      This is free software: you are free to change and redistribute it.
      There is NO WARRANTY, to the extent permitted by law.
  remoteSocket:
    exists: true
    path: /run/user/1000/podman/podman.sock
  security:
    apparmorEnabled: false
    capabilities: CAP_CHOWN,CAP_DAC_OVERRIDE,CAP_FOWNER,CAP_FSETID,CAP_KILL,CAP_NET_BIND_SERVICE,CAP_SETFCAP,CAP_SETGID,CAP_SETPCAP,CAP_SETUID,CAP_SYS_CHROOT
    rootless: true
    seccompEnabled: true
    seccompProfilePath: /usr/share/containers/seccomp.json
    selinuxEnabled: false
  serviceIsRemote: false
  slirp4netns:
    executable: /usr/bin/slirp4netns
    package: slirp4netns_1.2.1-1+b1_amd64
    version: |-
      slirp4netns version 1.2.1
      commit: 09e31e92fa3d2a1d3ca261adaeb012c8d75a8194
      libslirp: 4.8.0
      SLIRP_CONFIG_VERSION_MAX: 4
      libseccomp: 2.5.5
  swapFree: 12110000128
  swapTotal: 12110000128
  uptime: 21h 0m 8.00s (Approximately 0.88 days)
  variant: ""
plugins:
  authorization: null
  log:
  - k8s-file
  - none
  - passthrough
  - journald
  network:
  - bridge
  - macvlan
  - ipvlan
  volume:
  - local
registries: {}
store:
  configFile: /home/pierre/.config/containers/storage.conf
  containerStore:
    number: 1
    paused: 0
    running: 0
    stopped: 1
  graphDriverName: overlay
  graphOptions: {}
  graphRoot: /home/pierre/.local/share/containers/storage
  graphRootAllocated: 491000610816
  graphRootUsed: 279057465344
  graphStatus:
    Backing Filesystem: extfs
    Native Overlay Diff: "true"
    Supports d_type: "true"
    Supports shifting: "false"
    Supports volatile: "true"
    Using metacopy: "false"
  imageCopyTmpDir: /var/tmp
  imageStore:
    number: 230
  runRoot: /run/user/1000/containers
  transientStore: false
  volumePath: /home/pierre/.local/share/containers/storage/volumes
version:
  APIVersion: 4.9.4
  Built: 0
  BuiltTime: Thu Jan  1 01:00:00 1970
  GitCommit: ""
  GoVersion: go1.22.1
  Os: linux
  OsArch: linux/amd64
  Version: 4.9.4

Thank you 🙏

[Luap99]

If you run rootless there is extra rootless-netns. A internal network is not routed anywhere as such it cannot reach the host where host.containers.internal would point to. You can only reach the gateway ip of your network and this one is in the rootless-netns not on the host.
see #22943 (comment)

As root it should work like docker though.

[krichprollsch]

I'm running rootless indeed.
Thank you for the clarification and the link to the useful discussion 🙏