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An example inetd-like socket-activated service. #systemd #inetd #systemd.socket

README

This is an example of a socket-activated per-connection service (which is usually referred to as inetd-like service). A thorough explanation can be found at http://0pointer.de/blog/projects/inetd.html.

Define a socket unit

The key point here is to specify Accept=yes, which will make the socket accept connections (behaving like inetd) and pass only the resulting connection socket to the service handler.

Create /etc/systemd/system/baz.socket:

[Unit]
Description=Baz Socket

[Socket]
ListenStream=127.0.0.1:9999
Accept=yes

[Install]
WantedBy=sockets.target

Define a service template unit

We now create a service template from which the actual service will be instantiated on-demand. The lifetime of this service is usually very short (thus it may not appear at systemctl).

Create /etc/systemd/system/baz@.service:

[Unit]
Description=Baz Service
Requires=baz.socket

[Service]
Type=simple
ExecStart=/usr/bin/python /opt/baz-service/serve.py %i
StandardInput=socket
StandardError=journal
TimeoutStopSec=5
#RuntimeMaxSec=10

[Install]
WantedBy=multi-user.target

An example service handler would be (located at /opt/baz-service/serve.py as specified at the service unit file):

#!/usr/bin/python
import sys
import logging
logging.basicConfig(level=logging.INFO)

instance = sys.argv[1]

# The connected socket is duplicated to stdin/stdout
data = sys.stdin.readline().strip()
logging.info('baz-service: at instance %s, got request: %s', instance, data)
sys.stdout.write(data.upper() + '\r\n')

Test

Start (or enable it to start at boot time) the socket (verify it via systemctl status baz.socket), and make a request to the service:

echo Hello| nc localhost 9999

You 'll notice that the socket's status has changed (i.e. Accepted: 1). Also, any log message generated by the on-demand service should be present at the journal (e.g. journalctl --all| grep -e baz).

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