localhost mapping detection [host.containers.internal]

[Thirumalai]

podman currently does not add localhost mapping (i.e. host.containers.internal) on various conditions (e.g. when running inside a VM). This makes it less portable especially when the same Dockerfile file is being used to build locally and CI/CD pipelines and it includes a step that requires communication with a localhost on the host platform. Is there anyway to detect if the localhost can be reached at host.containers.internal before running podman build so that we can optimise the build logics to use 127.0.0.1/host.containers.local depending on the availability of host mapping?

[Luap99]

It is not clear for me what you mean? host.containers.internal was never mapped to localhost. A container generally can never access the real localhost on the host. It is has its own private network namespace with means it has its own localhost, unless you run with --network host.

[Thirumalai]

It is not clear for me what you mean? host.containers.internal was never mapped to localhost. A container generally can never access the real localhost on the host. It is has its own private network namespace with means it has its own localhost, unless you run with --network host.

I meant it it maps to the Internal IP of the Host, which indirectly allows us to access localhost/127.0.0.1 on the host. However, the behaviour is highly inconsistent. For example, when I run podman in my mac locally, I need to replace 127.0.0.1 with host.containers.internal in internal urls to access the resources from the host system, but the same is not working when podman runs inside EC2.

[Luap99]

Access to the actual localhost (127.0.0.1) is generally not allowed. Podman machine is an exception and does allow it but this is more of an implementation detail of gvproxy, we definitely do not want to allow such access on linux.

If you run podman as root there is simply no way to reach localhost as normal routing cannot do that. For rootless there are a few pasta options that can be used (i.e. --map-host-loopback/--map-gw/--tcp-ns)