Podman adds host.docker.internal to /etc/hosts even when not using --add-host=host.docker.internal:host-gateway (docker incompatibility)

[eriksjolund]

Podman adds host.docker.internal to /etc/hosts by default.
Docker does not.
Is this mismatch a bug or a design decision?

(For me it does not matter. I ask because I want to make sure it's not a bug)

podman

$ podman run --rm docker.io/library/fedora cat /etc/hosts | grep host.docker.internal
169.254.1.2	host.containers.internal host.docker.internal
$  

docker

$ docker run --rm docker.io/library/fedora cat /etc/hosts | grep host.docker.internal
$ 

Tested with docker 27.1.2 and podman 5.3.0-dev

For more tests and info about the system:

Click me

more tests

$ docker run --rm --name mytest docker.io/library/fedora cat /etc/hosts 
127.0.0.1	localhost
::1	localhost ip6-localhost ip6-loopback
fe00::0	ip6-localnet
ff00::0	ip6-mcastprefix
ff02::1	ip6-allnodes
ff02::2	ip6-allrouters
172.17.0.2	f64d904d11ca

$ docker run --rm --name mytest --add-host=host.docker.internal:host-gateway docker.io/library/fedora cat /etc/hosts 
127.0.0.1	localhost
::1	localhost ip6-localhost ip6-loopback
fe00::0	ip6-localnet
ff00::0	ip6-mcastprefix
ff02::1	ip6-allnodes
ff02::2	ip6-allrouters
172.17.0.1	host.docker.internal
172.17.0.2	9a4d7f257c63

$ podman run --rm --name mytest docker.io/library/fedora cat /etc/hosts
127.0.0.1	localhost localhost.localdomain localhost4 localhost4.localdomain4
::1	localhost localhost.localdomain localhost6 localhost6.localdomain6
169.254.1.2	host.containers.internal host.docker.internal
10.0.2.15	1f861994d39c mytest

$ podman run --rm --name mytest --add-host=host.docker.internal:host-gateway docker.io/library/fedora cat /etc/hosts
169.254.1.2	host.docker.internal
127.0.0.1	localhost localhost.localdomain localhost4 localhost4.localdomain4
::1	localhost localhost.localdomain localhost6 localhost6.localdomain6
169.254.1.2	host.containers.internal
10.0.2.15	117ee91769d7 mytest

about the system

$ podman version
Client:       Podman Engine
Version:      5.3.0-dev
API Version:  5.3.0-dev
Go Version:   go1.22.6
Git Commit:   649730c9a1dffd1af170dd85e88bd3daecd1b1a8
Built:        Fri Sep  6 01:48:04 2024
OS/Arch:      linux/arm64

$ podman info
host:
  arch: arm64
  buildahVersion: 1.37.0
  cgroupControllers:
  - cpu
  - memory
  - pids
  cgroupManager: cgroupfs
  cgroupVersion: v2
  conmon:
    package: conmon-2.1.12-2.fc41.aarch64
    path: /usr/bin/conmon
    version: 'conmon version 2.1.12, commit: '
  cpuUtilization:
    idlePercent: 99.73
    systemPercent: 0.11
    userPercent: 0.16
  cpus: 2
  databaseBackend: sqlite
  distribution:
    distribution: fedora
    variant: coreos
    version: "42"
  eventLogger: file
  freeLocks: 2048
  hostname: localhost.localdomain
  idMappings:
    gidmap:
    - container_id: 0
      host_id: 1004
      size: 1
    - container_id: 1
      host_id: 786432
      size: 65536
    uidmap:
    - container_id: 0
      host_id: 1004
      size: 1
    - container_id: 1
      host_id: 786432
      size: 65536
  kernel: 6.11.0-0.rc5.20240829gitd5d547aa7b51.46.fc42.aarch64
  linkmode: dynamic
  logDriver: journald
  memFree: 1015431168
  memTotal: 4081860608
  networkBackend: netavark
  networkBackendInfo:
    backend: netavark
    dns:
      package: aardvark-dns-1.12.1-1.fc41.aarch64
      path: /usr/libexec/podman/aardvark-dns
      version: aardvark-dns 1.12.1
    package: netavark-1.12.2-1.fc42.aarch64
    path: /usr/libexec/podman/netavark
    version: netavark 1.12.2
  ociRuntime:
    name: crun
    package: crun-1.15-2.fc41.aarch64
    path: /usr/bin/crun
    version: |-
      crun version 1.15
      commit: e6eacaf4034e84185fd8780ac9262bbf57082278
      rundir: /run/user/1004/crun
      spec: 1.0.0
      +SYSTEMD +SELINUX +APPARMOR +CAP +SECCOMP +EBPF +CRIU +LIBKRUN +WASM:wasmedge +YAJL
  os: linux
  pasta:
    executable: /usr/bin/pasta
    package: passt-0^20240821.g1d6142f-1.fc42.aarch64
    version: |
      pasta 0^20240821.g1d6142f-1.fc42.aarch64-pasta
      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: false
    path: /run/user/1004/podman/podman.sock
  rootlessNetworkCmd: pasta
  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: true
  serviceIsRemote: false
  slirp4netns:
    executable: /usr/bin/slirp4netns
    package: slirp4netns-1.2.2-3.fc41.aarch64
    version: |-
      slirp4netns version 1.2.2
      commit: 0ee2d87523e906518d34a6b423271e4826f71faf
      libslirp: 4.8.0
      SLIRP_CONFIG_VERSION_MAX: 5
      libseccomp: 2.5.5
  swapFree: 0
  swapTotal: 0
  uptime: 145h 9m 38.00s (Approximately 6.04 days)
  variant: v8
plugins:
  authorization: null
  log:
  - k8s-file
  - none
  - passthrough
  network:
  - bridge
  - macvlan
  - ipvlan
  volume:
  - local
registries:
  search:
  - registry.fedoraproject.org
  - registry.access.redhat.com
  - docker.io
store:
  configFile: /var/home/buildpodman/.config/containers/storage.conf
  containerStore:
    number: 0
    paused: 0
    running: 0
    stopped: 0
  graphDriverName: overlay
  graphOptions: {}
  graphRoot: /var/home/buildpodman/.local/share/containers/storage
  graphRootAllocated: 26238496768
  graphRootUsed: 7142010880
  graphStatus:
    Backing Filesystem: xfs
    Native Overlay Diff: "true"
    Supports d_type: "true"
    Supports shifting: "false"
    Supports volatile: "true"
    Using metacopy: "false"
  imageCopyTmpDir: /var/tmp
  imageStore:
    number: 4
  runRoot: /run/user/1004/containers
  transientStore: false
  volumePath: /var/home/buildpodman/.local/share/containers/storage/volumes
version:
  APIVersion: 5.3.0-dev
  Built: 1725587284
  BuiltTime: Fri Sep  6 01:48:04 2024
  GitCommit: 649730c9a1dffd1af170dd85e88bd3daecd1b1a8
  GoVersion: go1.22.6
  Os: linux
  OsArch: linux/arm64
  Version: 5.3.0-dev

$ docker version
Client:
 Version:           27.1.2
 API version:       1.46
 Go version:        go1.23.0
 Git commit:        1.fc42
 Built:             Wed Aug 21 00:00:00 2024
 OS/Arch:           linux/arm64
 Context:           default

Server:
 Engine:
  Version:          27.1.2
  API version:      1.46 (minimum version 1.24)
  Go version:       go1.23.0
  Git commit:       1.fc42
  Built:            Wed Aug 21 00:00:00 2024
  OS/Arch:          linux/arm64
  Experimental:     false
 containerd:
  Version:          1.7.21
  GitCommit:        1.fc42
 runc:
  Version:          1.1.12
  GitCommit:        
 tini-static:
  Version:          0.19.0
  GitCommit:        
$ rpm -qf /usr/bin/docker
docker-cli-27.1.2-1.fc42.aarch64
$ rpm -q moby-engine
moby-engine-27.1.2-1.fc42.aarch64
$ rpm-ostree status
State: idle
warning: Failed to query journal: couldn't find current boot in journal
Deployments:
● ostree-remote-image:fedora:docker://quay.io/fedora/fedora-coreos:rawhide
                   Digest: sha256:b49001cc46b33e163c44eef55f26a722acf21d1bc0dceac3dac99bcc9a97f34e
                  Version: 42.20240830.91.0 (2024-08-30T14:00:28Z)

[Luap99]

I think host.docker.internal is set by default when using docker-desktop so that is where it originated from AFAIK. From activity in issue it seems a lot of people make use of it so setting it by default make sense as it doesn't do any harm when not needed.

[eriksjolund]

Thanks, good to know.