socket-activate-echo

An echo server that supports systemd socket activation

family	type	support
AF_UNIX	SOCK_STREAM	yes
AF_INET	SOCK_STREAM	yes
AF_INET6	SOCK_STREAM	yes
AF_VSOCK	SOCK_STREAM	yes
AF_UNIX	SOCK_DGRAM	yes (but no support for outgoing traffic when run in container because the client socket path on the host is not accessible from within the container)
AF_INET	SOCK_DGRAM	yes
AF_INET6	SOCK_DGRAM	yes

Requirements

• podman version 3.4.0 (released September 2021) or newer
• container-selinux version 2.183.0 (released April 2022) or newer

If you are using an older version of container-selinux and it does not work, add --security-opt label=disable to podman run.

Installation

1. Install socat

   sudo dnf -y install socat
   

About the container image

The container image ghcr.io/eriksjolund/socket-activate-echo is built by the GitHub Actions workflow .github/workflows/publish_container_image.yml from the file ./Containerfile.

Activate an instance of a templated systemd user service

1. Start the echo server sockets

   git clone https://github.com/eriksjolund/socket-activate-echo.git
   mkdir -p ~/.config/systemd/user
   cp -r socket-activate-echo/systemd/* ~/.config/systemd/user
   systemctl --user daemon-reload
   systemctl --user start echo@demo.socket
   

2. Run the commands

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

3. Try establishing an outgoing connection

   $ podman exec -t echo-demo curl https://podman.io
   curl: (6) Could not resolve host: podman.io
   $
   

   (The command-line option --network=none was added to prevent the container from establishing outgoing connections)

Socket activate SOCK_STREAM sockets with systemd-socket-activate

1. Socket activate the echo server

   systemd-socket-activate -l /tmp/stream.sock \
       -l 4000 -l vsock:4294967295:4000 podman run --rm --name echo2 \
       --network=none ghcr.io/eriksjolund/socket-activate-echo
   

   Instead of VMADDR_CID_ANY (4294967295) we could also have used VMADDR_CID_LOCAL (1), in other words, -l vsock:1:4000 (see man 7 vsock).

2. In another shell

   $ echo hello | socat - unix:/tmp/stream.sock
   hello
   $ echo hello | socat - tcp4:127.0.0.1:4000
   hello
   $ echo hello | socat - tcp6:[::1]:4000
   hello
   $ echo hello | socat - VSOCK-CONNECT:1:4000
   hello
   

3. Try establishing an outgoing connection

   $ podman exec -t echo2 curl https://podman.io
   curl: (6) Could not resolve host: podman.io
   $
   

Socket activate SOCK_DGRAM sockets with systemd-socket-activate

1. Socket activate the echo server

   systemd-socket-activate --datagram \
       -l 5000 podman run --rm --name echo3 \
       --network=none ghcr.io/eriksjolund/socket-activate-echo
   

2. In another shell

   $ echo hello | socat - udp4:127.0.0.1:5000
   hello
   $ echo hello | socat - udp6:[::1]:5000
   hello
   

3. Try establishing an outgoing connection

   $ podman exec -t echo3 curl https://podman.io
   curl: (6) Could not resolve host: podman.io
   $
   

Run the echo container inside a VM and connect over AF_VSOCK (SOCK_STREAM)

1. Install requirements

   sudo dnf install -y qemu butane coreos-installer
   

2. Start the Fedora CoreOS VM by running these commands on the host

   STREAM=next
   CID=20
   mkdir -p ~/.local/share/libvirt/images/
   file=$(coreos-installer download -s "${STREAM}" -p qemu -f qcow2.xz --decompress -C ~/.local/share/libvirt/images/)
   cat vm/echo.butane | butane --strict --pretty --files-dir systemd > file.ign
   qemu-kvm -m 2048 \
     -device "vhost-vsock-pci,id=vhost-vsock-pci0,guest-cid=$CID" \
     -cpu host -nographic -snapshot \
     -drive "if=virtio,file=$file" \
     -fw_cfg name=opt/com.coreos/config,file=file.ign -nic "user,model=virtio"
   

   The Context Identifier (CID) is an arbitrary number that is used to identify the VM (see man vsock).

3. Run on the host

    $ CID=20
    $ echo hello | socat -t 30 - VSOCK-CONNECT:$CID:3000
    hello
   

Troubleshooting

The container takes long time to start

Pulling a container image may take long time. This delay can be avoided by pulling the container image beforehand and adding the command-line option --pull=never to podman run.

socat times out before receiving the reply

If socat does not receive any reply within a certain time limit it terminates before getting the reply. The timeout is 0.5 seconds by default. Symptoms of this could be

$ systemctl --user start echo@demo.socket
$ echo hello | socat - udp4:127.0.0.1:3000
$ echo hello | socat - udp4:127.0.0.1:3000
hello

To configure the timeout to be 30 seconds, add the command-line option -t 30.

$ echo hello | socat -t 30 - udp4:127.0.0.1:3000
hello

Another way to handle the problem is to use the command-line option readline to get an interactive user interface. Type the word hello and see it being echoed back.

$ socat readline udp4:127.0.0.1:3000
hello
hello

In this case there will be no timeout because none of the channels have reached EOF.

A good way to diagnose problems is to look in the journald log for the service:

journalctl -xe --user -u echo@demo.service