sshd

Start SSH daemon

$ sshd

$ sshd -t

$ sshd -D

$ sshd -f [/etc/ssh/sshd_config]

$ sshd -d

$ sshd -ddd

$ sshd -p [2222]

sshd [-D] [-d] [-f config] [-p port] [options]

sshd is the OpenSSH server daemon that listens for incoming SSH connections, authenticates users, and provides encrypted remote shell access, file transfer, and port forwarding services. It is the server-side counterpart to the ssh client.
The daemon supports multiple authentication methods including public key, password, keyboard-interactive, and GSSAPI/Kerberos. Access can be restricted by user, group, and source address through configuration directives. Features like X11 forwarding, agent forwarding, and TCP port forwarding are individually controllable.
Debug mode (-d) runs a single connection in the foreground with verbose output for troubleshooting, while -t validates the configuration file for syntax errors before restarting the service. Foreground mode (-D) is commonly used in container environments where the daemon should not detach from the controlling process.

-D

Foreground mode.

Debug mode.

Test configuration.

Config file.

Listen port.

Log to stderr.

Quiet mode.

IPv4 only.

IPv6 only.

/etc/ssh/sshd_config

Main configuration file controlling authentication methods, access restrictions, port, protocol options, forwarding permissions, and logging.

Host key files (RSA, Ed25519, ECDSA) used to identify the server to connecting clients.

Drop-in configuration directory for modular configuration snippets (included via Include directive).

Requires root to bind port 22. Misconfiguration can lock you out. Test changes carefully.

sshd is part of OpenSSH, developed by the OpenBSD project starting in 1999. It replaced insecure protocols like telnet and rsh.

ssh(1), ssh-keygen(1), sshd_config(5)