How do I get Citadel to answer telnet connections directly, without first requiring users to enter a username on the host system?

When the standard telnet service is enabled on a Linux or Unix host, users who connect are presented with a system login: prompt asking for a local operating system account. To provide a seamless experience, you can configure your telnet daemon to run the Citadel client directly instead of /bin/login.

Note on daemon binaries and flags:

• Depending on your distribution, the daemon may be installed as /usr/sbin/in.telnetd (standard Netkit telnetd) or /usr/sbin/telnetd (such as GNU inetutils).
• Netkit telnetd uses the -L /path/to/program option to specify a custom login program, whereas GNU inetutils uses -E /path/to/program. Check man telnetd on your system if you are unsure which implementation is installed.

1. Using systemd

Modern Linux distributions can manage incoming telnet connections using systemd socket activation without requiring a separate superserver like inetd or xinetd.

Create the socket unit file at /etc/systemd/system/telnet.socket:

[Unit]
Description=Citadel Telnet Server Socket

[Socket]
ListenStream=23
Accept=yes

[Install]
WantedBy=sockets.target

Next, create the service template unit file at /etc/systemd/system/telnet@.service (the @ indicates an instantiated service template):

[Unit]
Description=Citadel Telnet Server Session
StandardInput=socket

[Service]
ExecStart=/usr/sbin/in.telnetd -h -L /usr/local/citadel/citadel

Reload systemd and enable the socket:

systemctl daemon-reload
systemctl enable --now telnet.socket

Note: Only enable and start telnet.socket. Do not attempt to start telnet@.service directly. Systemd will automatically spawn a new service instance for each connection that arrives on the socket.

2. Using xinetd

If your system uses xinetd, create or edit /etc/xinetd.d/telnet:

service telnet
{
    flags          = REUSE
    socket_type    = stream
    wait           = no
    user           = root
    server         = /usr/sbin/in.telnetd
    server_args    = -h -L /usr/local/citadel/citadel
    log_on_failure += USERID
}

Then reload or restart xinetd.

3. Using traditional inetd

If your system runs traditional inetd, edit /etc/inetd.conf and add or update the telnet entry:

telnet stream tcp4 nowait root /usr/sbin/tcpd /usr/sbin/in.telnetd -h -L /usr/local/citadel/citadel
telnet stream tcp6 nowait root /usr/sbin/tcpd /usr/sbin/in.telnetd -h -L /usr/local/citadel/citadel

Then reload inetd.