Systemd sockets
You might have heard about inetd which is often called a super-server according to Wikipedia. But did you know the systemd can do the same thing using something called systemd sockets. I will show you how to use them.
At first we are going to start with how to actually set them up and how they work.
The first thing to do is to create the socket unit which looks like this
testsocket.socket
[Unit]
Description=Test socket
[Socket]
ListenStream=1024
Accept=yes
[Install]
WantedBy=default.target
To create a unit you can use the command
systemctl --user edit --full --force testsocket.socket # For user unit
sudo systemctl edit --full --force testsocket.socket # For system unit
Which will open up an editor like nano but if you have not selected an editor I recommend using nano which you can set using
select-editor
Except this unit you also need to define a second unit that is a service
testsocket@.service
[Unit]
Description=Test socket service
[Service]
ExecStart=/home/yeen/blep.sh
And do note that there is an @ in the name just like in the Failure handler guide from a while ago.
Not to take a look what the started script actually does using lsof -p $$ inside the script. As you can probably see a socket is open in file descriptor 3 and I have no idea what the pipe in 4 is but that does not matter much.
COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME
blep.sh 9877 yeen cwd DIR 8,2 4096 50298626 /home/yeen
blep.sh 9877 yeen rtd DIR 8,2 4096 2 /
blep.sh 9877 yeen txt REG 8,2 125688 262951 /usr/bin/dash
blep.sh 9877 yeen mem REG 8,2 2220400 263025 /usr/lib/x86_64-linux-gnu/libc.so.6
blep.sh 9877 yeen mem REG 8,2 240936 263020 /usr/lib/x86_64-linux-gnu/ld-linux-x86-64.so.2
blep.sh 9877 yeen 0r CHR 1,3 0t0 5 /dev/null
blep.sh 9877 yeen 1u unix 0x0000000000000000 0t0 120353 type=STREAM
blep.sh 9877 yeen 2u unix 0x0000000000000000 0t0 120353 type=STREAM
blep.sh 9877 yeen 3u IPv6 121878 0t0 TCP localhost:1024->localhost:42958 (ESTABLISHED)
blep.sh 9877 yeen 4r FIFO 0,13 0t0 117586 pipe
blep.sh 9877 yeen 10r REG 8,2 81 50333753 /home/yeen/blep.sh
The script itself used to test it is here
``blep.sh`
#!/bin/sh
a="$(lsof -p $$)"
echo "$a" > ~/sdfdfg.txt
echo "$a" >&3
env > env.txt
What the >&3 does is tell it to write to file descriptor 3 so the lsof output from above is emitted from netcat connecting to the systemd socket.
The reason for env to be there is that some variables set there might be very useful and we can take a look at them here filtered from what usually exists in the env vars of systemd units being run.
12,13c12,13
< INVOCATION_ID=2106c6f489634605b77ec4c53b18e6ad
< JOURNAL_STREAM=8:139232
---
> INVOCATION_ID=10f97a5493074bf8b914b8bbbcd421c2
> JOURNAL_STREAM=8:120353
23a24,26
> LISTEN_FDNAMES=connection
> LISTEN_FDS=1
> LISTEN_PID=9877
29a33,34
> REMOTE_ADDR=127.0.0.1
> REMOTE_PORT=42958
32c37
< SHLVL=1
---
> SHLVL=0
34c39
< SYSTEMD_EXEC_PID=11197
---
> SYSTEMD_EXEC_PID=9877
36c41
< _=/usr/bin/env
---
> _=/usr/bin/dbus-update-activation-environment
As you see there are some things to take note of like REMOTE_ADDR=127.0.0.1 and REMOTE_PORT=42958 which were from a run where I just ran nc localhost 1024 which is netcat.
If you wonder how to get this diff do something like
systemd-run --user bash -c 'env > enva.txt'
diff <(sort enva.txt) <(sort env.txt)
And you will get a good diff.
Now we can test with python too by making a script in it that receives the invocation.
blep.py
#!/usr/bin/python3
import socket
s = socket.fromfd(3, socket.AF_INET, socket.SOCK_STREAM, proto=0)
s.send(b"Blep I am cat meow\n")
s.close()
And as you can guess it will send Blep I am cat meow to anyone who connects to the socket.
One last note is that you might have to use a command like
systemctl --user status testsocket@\*
and scroll a bit up and down to find the status as it creates a temporary service for every invocation that you can find in the logs.
Anyway now you know the basics of systemd sockets so you can have fun with them.
Added later
2024-10-11 23:30
So apparently you can set stdin and stdout to be used to write to the socket. Here is an example. Start by changing your
testsocket@.service
[Unit]
Description=Test socket
[Service]
ExecStart=/home/yeen/blep.sh
StandardInput=socket
StandardOutput=socket
StandardError=journal
It is possible to set all 3 to socket but you should generally set the error one specifically to journal since otherwise it will write stderr to the socket too instead of into the journal.
Now we can update the
blep.sh
#!/bin/sh
lsof -p $$
echo log something here >&2
And now you can log things while also using stdout to write to the socket.
It is also very useful to start another terminal and use the following command to follow everything that happens in the log.
journalctl --user -f
*Mweeoops*
By ellietheyeen 11 October 2024 Permalink Tags: python shell sockets systemd