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socket_activation.md

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Podman socket activation

Socket activation conceptually works by having systemd create a socket (e.g. TCP, UDP or Unix socket). As soon as a client connects to the socket, systemd will start the systemd service that is configured for the socket. The newly started program inherits the file descriptor of the socket and can then accept the incoming connection (in other words run the system call accept()). This description corresponds to the default systemd socket configuration Accept=no that lets the service accept the socket.

Podman supports two forms of socket activation:

  • Socket activation of the API service
  • Socket activation of containers

Socket activation of the API service

The architecture looks like this

stateDiagram-v2
    [*] --> systemd: first client connects
    systemd --> podman: socket inherited via fork/exec
Loading

The file /usr/lib/systemd/user/podman.socket on a Fedora system defines the Podman API socket for rootless users:

$ cat /usr/lib/systemd/user/podman.socket
[Unit]
Description=Podman API Socket
Documentation=man:podman-system-service(1)

[Socket]
ListenStream=%t/podman/podman.sock
SocketMode=0660

[Install]
WantedBy=sockets.target

The socket is configured to be a Unix socket and can be started like this

$ systemctl --user start podman.socket
$ ls $XDG_RUNTIME_DIR/podman/podman.sock
/run/user/1000/podman/podman.sock
$

The socket can later be used by for instance docker-compose that needs a Docker-compatible API

$ export DOCKER_HOST=unix://$XDG_RUNTIME_DIR/podman/podman.sock
$ docker-compose up

When docker-compose or any other client connects to the UNIX socket $XDG_RUNTIME_DIR/podman/podman.sock, the service podman.service is started. See its definition in the file /usr/lib/systemd/user/podman.service.

Socket activation of containers

Since version 3.4.0 Podman supports socket activation of containers, i.e., passing a socket-activated socket to the container. Thanks to the fork/exec model of Podman, the socket will be first inherited by conmon and then by the OCI runtime and finally by the container as can be seen in the following diagram:

stateDiagram-v2
    [*] --> systemd: first client connects
    systemd --> podman: socket inherited via fork/exec
    state "OCI runtime" as s2
    podman --> conmon: socket inherited via double fork/exec
    conmon --> s2: socket inherited via fork/exec
    s2 --> container: socket inherited via exec
Loading

This type of socket activation can be used in systemd services that are generated from container unit files (see podman-systemd.unit(5)) (Note, quadlet requires the use of cgroup v2) or from the command podman generate systemd. The container must also support socket activation. Not all software daemons support socket activation but it's getting more popular. For instance Apache HTTP server, MariaDB, DBUS, PipeWire, Gunicorn, CUPS all have socket activation support.

Example: socket-activated echo server container in a systemd service

This example shows how to run the socket-activated echo server socket-activate-echo in a systemd user service. Podman version 4.4.0 or higher is required.

Enable lingering for your regular user

$ loginctl enable-linger $USER

The command has these effects on your enabled systemd user units:

  • the units are automatically started after a reboot
  • the units are not automatically stopped after you log out

Create directories

$ mkdir -p ~/.config/systemd/user
$ mkdir -p ~/.config/containers/systemd

Create the file ~/.config/containers/systemd/echo.container with the file contents:

[Unit]
Description=Example echo service
Requires=echo.socket
After=echo.socket

[Container]
Image=ghcr.io/eriksjolund/socket-activate-echo
Network=none

[Install]
WantedBy=default.target

The file follows the syntax described in podman-systemd.unit(5).

The [Install] section is optional. If you remove the two last lines, the echo.service will not be automatically started after a reboot. Instead, the echo.service is started when the first client connects to the socket.

The line Network=none is optional. It improves security by removing network connectivity for the container. The container can still be serving the internet because Network=none has no effect on activated sockets.

A socket-activated service also requires a systemd socket unit. Create the file ~/.config/systemd/user/echo.socket that defines the sockets that the container should use

[Unit]
Description=Example echo socket

[Socket]
ListenStream=127.0.0.1:3000
ListenDatagram=127.0.0.1:3000
ListenStream=[::1]:3000
ListenDatagram=[::1]:3000
ListenStream=%h/echo_stream_sock

# VMADDR_CID_ANY (-1U) = 2^32 -1 = 4294967295
# See "man vsock"
ListenStream=vsock:4294967295:3000

[Install]
WantedBy=sockets.target

%h is a systemd specifier that expands to the user's home directory.

After editing the unit files, systemd needs to reload its configuration.

$ systemctl --user daemon-reload

While reloading its configuration systemd generates the unit echo.service from the file ~/.config/containers/systemd/echo.container by executing the unit generator /usr/lib/systemd/system-generators/podman-system-generator.

Optional: View the generated echo.service to see the podman run command that will be run.

$ systemctl --user cat echo.service

Configure systemd to automatically start echo.socket after reboots.

$ systemctl --user enable echo.socket

Pull the container image beforehand

$ podman pull ghcr.io/eriksjolund/socket-activate-echo

Start the socket unit

$ systemctl --user start echo.socket

Test the echo server with the program socat

$ echo hello | socat -t 30 - tcp4:127.0.0.1:3000
hello
$ echo hello | socat -t 30 - tcp6:[::1]:3000
hello
$ echo hello | socat -t 30 - udp4:127.0.0.1:3000
hello
$ echo hello | socat -t 30 - udp6:[::1]:3000
hello
$ echo hello | socat -t 30 - unix:$HOME/echo_stream_sock
hello
$ echo hello | socat -t 30 - VSOCK-CONNECT:1:3000
hello

The option -t 30 configures socat to use a timeout of 30 seconds when socat reads from the socket awaiting to get an EOF (End-Of-File). As the container image has already been pulled, such a long timeout is not really needed.

The echo server works as expected. It replies "hello" after receiving the text "hello".

Example: socket activate an Apache HTTP server with systemd-socket-activate

Instead of setting up a systemd service to test out socket activation, an alternative is to use the command-line tool systemd-socket-activate.

Let's build a container image for the Apache HTTP server that is configured to support socket activation on port 8080.

Create a new directory ctr and a file ctr/Containerfile with this contents

FROM docker.io/library/fedora
RUN dnf -y update && dnf install -y httpd && dnf clean all
RUN sed -i "s/Listen 80/Listen 127.0.0.1:8080/g" /etc/httpd/conf/httpd.conf
CMD ["/usr/sbin/httpd", "-DFOREGROUND"]

Build the container image

$ podman build -t socket-activate-httpd ctr

In one shell, start systemd-socket-activate.

$ systemd-socket-activate -l 8080 podman run --rm --network=none localhost/socket-activate-httpd

The TCP port number 8080 is given as an option to systemd-socket-activate. The --publish (-p) option for podman run is not used.

In another shell, fetch a web page from localhost:8080

$ curl -s localhost:8080 | head -6
<!doctype html>
<html>
  <head>
<meta charset='utf-8'>
<meta name='viewport' content='width=device-width, initial-scale=1'>
<title>Test Page for the HTTP Server on Fedora</title>
$

Disabling the network with --network=none

If the container only needs to communicate over the socket-activated socket, it's possible to disable the network by passing --network=none to podman run. This improves security because the container then runs with less privileges.

Native network performance over the socket-activated socket

When using rootless Podman, network traffic is normally passed through slirp4netns. This comes with a performance penalty. Fortunately, communication over the socket-activated socket does not pass through slirp4netns so it has the same performance characteristics as the normal network on the host.

Starting a socket-activated service

There is a delay when the first connection is made because the container needs to start up. To minimize this delay, consider passing --pull=never to podman run and instead pull the container image beforehand. Instead of waiting for the start of the service to be triggered by the first client connecting to it, the service can also be explicitly started (systemctl --user start echo.service).

Stopping a socket-activated service

Some services run a command (configured by the systemd directive ExecStart) that exits after some time of inactivity. Depending on the restart configuration for the service (systemd directive Restart), it may then be stopped. An example of this is podman.service that stops after some time of inactivity. The service will be started again when the next client connects to the socket.