Interact with podman docker via socket in Redhat 9 - docker

Question

I'm trying to migrate one of my dev boxes over from centos 8 to RHEL9. I rely heavily on docker and noticed when I tried to run a docker command on the RHEL box it installed podman-docker. This seemed to go smoothly; I was able to pull an image, launch, build, commit a new version without problem using the docker commands I knew already.
The problem I have encountered though is I can't seem to interact with it via the docker socket (which seems to be a link to the podman one).
If I run the docker command:
[#rhel9 ~]$ docker images
Emulate Docker CLI using podman. Create /etc/containers/nodocker to quiet msg.
REPOSITORY TAG IMAGE ID CREATED SIZE
docker.io/redhat/ubi9 dev_image de371523ca26 6 hours ago 805 MB
docker.io/redhat/ubi9 latest 9ad46cd10362 6 days ago 230 MB
it has my images listed as expected. I should be able to also run:
[#rhel9 ~]$ curl --unix-socket /var/run/docker.sock -H 'Content-Type: application/json' http://localhost/images/json | jq .
% Total % Received % Xferd Average Speed Time Time Time Current
Dload Upload Total Spent Left Speed
100 3 100 3 0 0 55 0 --:--:-- --:--:-- --:--:-- 55
[]
but as you can see, nothing is coming back. The socket is up and running as I can ping it without issue:
[#rhel9 ~]$ curl -H "Content-Type: application/json" --unix-socket /var/run/docker.sock http://localhost/_ping
OK
I also tried the curl commands using the podman socket directly but it had the same results. Is there something I am missing or a trick to getting it to work so that I can interact with docker/podman via the socket?

Answer 1

Podman isn't implemented using a client/server model like Docker. By default there is no socket, because there's no equivalent to the docker daemon. Podman does provide a compatibility interface that you can use by enabling the podman.socket unit:
$ systemctl enable --now podman.socket
This exposes a Unix socket at /run/podman/podman.sock that responds to Docker API commands. But!
The socket connects you to podman running as root, whereas you've been running podman as a non-root user: so you won't see the same list of images, containers, networks, etc.
Some random notes:
Podman by default runs "rootless": you can run it as an unprivileged user, and all of its storage, metadata, etc, is stored in your home directory.
You can also run Podman as root, in which case the behavior is more like Docker.
If you enable the podman socket, you can replace podman-docker with the actual Docker client (and use things like docker-compose), although I have run into occasional issues with this. Mostly I just use podman, and run docker engine in a VM). You will need to configure Docker to look at the podman socket in /run/podman/podman.sock.
I have podman.socket enabled on my system, so this works:
$ curl --unix-socket /run/podman/podman.sock -H 'content-type: application/json' http://localhost/_ping
OK
Or:
$ curl --unix-socket /run/podman/podman.sock -H 'content-type: application/json' -sf http://localhost/containers/json | jq
[
{
"Id": "f0d9a880c45bb5857b24f46bcb6eeeca162eb68d574c8ba16c4a03703c2d60f4",
"Names": [
"/sleeper"
],
"Image": "docker.io/library/alpine:latest",
"ImageID": "14119a10abf4669e8cdbdff324a9f9605d99697215a0d21c360fe8dfa8471bab",
"Command": "sleep inf",
"Created": 1655418914,
"Ports": [],
"Labels": {},
"State": "running",
"Status": "Up 3 days",
"NetworkSettings": {
"Networks": {
"podman": {
"IPAMConfig": null,
"Links": null,
"Aliases": null,
"NetworkID": "podman",
"EndpointID": "",
"Gateway": "10.88.0.1",
"IPAddress": "10.88.0.2",
"IPPrefixLen": 16,
"IPv6Gateway": "",
"GlobalIPv6Address": "",
"GlobalIPv6PrefixLen": 0,
"MacAddress": "06:55:82:1b:1a:41",
"DriverOpts": null
}
}
},
"Mounts": null,
"Name": "",
"Config": null,
"NetworkingConfig": null,
"Platform": null,
"AdjustCPUShares": false
}
]

Answer 2

I managed to solve my problem although I'm not entirely sure how the scenario came about. I was looking through the output of docker info and podman info and noticed that they both had the remote socket set as:
remoteSocket:
exists: true
path: /run/user/1000/podman/podman.sock
rather than /run/podman/podman.sock which is where I thought it was (this socket does actually exist on my machine). Looking at the systemd file for podman.socket I can see that the socket was specified as %t/podman/podman.sock and checking the man page for podman-system-service it specified the rootless socket as unix://$XDG_RUNTIME_DIR/podman/podman.sock (where my $XDG_RUNTIME_DIR=/run/user/1000.
To get it all working with my software I just needed to make sure the DOCKER_HOST env variable was correctly set e.g. export DOCKER_HOST=unix:///run/user/1000/podman/podman.sock