====== systemd Cookbook ======

===== Network Proxy (like socat) =====

Sometimes you want to create a simple proxy. For example, you want to forward the local port 3389/tcp to the remote host 192.0.2.100 port 3389/tcp to enable RDP access for other hosts. You maybe stumble upon socat and put together a working command like this:

<code>
socat TCP4-LISTEN:3389,fork,reuseaddr TCP4:192.0.2.100:3389
</code>

Now you want to start this proxy in the background in case of a reboot of the system. systemd offers its own way of creating a proxy like this with [[https://www.freedesktop.org/software/systemd/man/latest/systemd-socket-proxyd.html|systemd-socket-proxyd]].

You have to create a socket and a service file. The socket file defines the listening socket and will trigger the service file on start and stop.

<file systemd /etc/systemd/system/proxy-to-rdphost.socket>
[Socket]
ListenStream=3389

[Install]
WantedBy=sockets.target
</file>

<file systemd /etc/systemd/system/proxy-to-rdphost.service>
[Unit]
Requires=proxy-to-rdphost.socket
After=proxy-to-rdphost.socket

[Service]
ExecStart=/usr/lib/systemd/systemd-socket-proxyd 192.0.2.100:3389
PrivateTmp=yes
PrivateNetwork=no
</file>

After creating/editing the files, systemd has to be reloaded.
<code>
systemctl daemon-reload
</code>

Enable the socket to activate it at boot and immediately start it with "--now".
<code>
systemctl enable --now proxy-to-rdphost.socket
</code>