How to convert xinetd service to systemd?

Environment

• Red Hat Enterprise Linux (RHEL) 7, 8 and 9
• xinetd
• systemd

Issue

• Need to convert xinetd to systemd unit file.

Resolution

• Let's assume an xinetd service file like the following:

# cat /etc/xinetd.d/appld 
service application
{
        flags        = REUSE
        socket_type  = stream
        wait         = no
        user         = example
        server       = /opt/appl/bin/appld
        type         = UNLISTED
        port         = 24146
        protocol     = tcp
        disable      = no
}

• The name appld, the path to the code which gets to handle the incoming request (/opt/appl/bin/appld) and the other values are examples and would likely have other values in the real world. Above service file would be translated into the two unit files below :

# cd /etc/systemd/system

# cat application.socket

[Unit]
Description=Application Socket

[Socket]
ListenStream=24146
Accept=yes                          

[Install]
WantedBy=sockets.target



# cat application@.service

[Unit]
Description=Application Per-Connection Server

[Service]
ExecStart=-/usr/bin/appld     # the binary to start
User=example
Group=example
StandardInput=socket

• Enable, Start and check the status of the application socket.

# systemctl enable application.socket
# systemctl start application.socket
# systemctl status -l application.socket

• To see which services are now active :

# systemctl --full | grep application

appld@172.31.0.52:24146-172.31.0.4:47779.service  loaded active running       App Per-Connection Server
appld@172.31.0.52:24146-172.31.0.54:52985.service loaded active running       App Per-Connection Server
appld.socket                                   loaded active listening     App Socket for Per-Connection Servers

• As expected, there are now two service instances running, for the two connections, and they are named after the source and destination address of the TCP connection as well as the port numbers. (For AF_UNIX sockets the instance identifier will carry the PID and UID of the connecting client.) This allows us to individually introspect or kill specific App instances.

• To terminate the session of a specific client :

# systemctl stop appld@172.31.0.52:24146-172.31.0.4:47779.service

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