Secure Shell Daemon (sshd) Command (with examples)

Introduction: The Secure Shell Daemon (sshd) is a powerful command that allows remote machines to securely log in to the current machine. It provides a secure means of executing commands on a remote machine as if they were executed on the local machine. This article will explore eight different use cases of the sshd command along with code examples to illustrate each case.

Use Case 1: Start daemon in the background

Code:

sshd

Motivation: Starting the sshd daemon in the background is useful when you want to provide SSH access to the current machine for remote login and command execution. By running sshd in the background, the daemon starts and listens for incoming SSH connections while allowing you to continue working on the current machine.

Explanation:

• The sshd command is executed without any additional arguments.
• This starts the sshd daemon in the background, which listens for incoming SSH connections.
• Once started, you can remotely log in to the current machine and execute commands securely.

Example Output: No output is displayed in the terminal when the sshd command is executed successfully.

Use Case 2: Run sshd in the foreground

Code:

sshd -D

Motivation: Running sshd in the foreground is helpful for troubleshooting and debugging purposes. It allows you to observe the log output directly in the terminal, which can provide valuable information about the SSH connections and any errors that occur.

Explanation:

• The -D option tells sshd to run in the foreground.
• When executed, sshd starts in the current terminal session instead of as a background process.
• This allows you to monitor the log output and observe any SSH connections or debugging information.

Example Output:

...
debug1: server_input_channel_open: ctype session rchan 256 win 16384 max 16384
debug1: input_session_request
debug1: channel 0: new [server-session]
...

The terminal displays debugging information and logs related to the SSH connections and server sessions.

Use Case 3: Run with verbose output (for debugging)

Code:

sshd -D -d

Motivation: Enabling verbose output is useful when you need detailed information about the SSH connections and server interactions. It provides an in-depth view of the SSH protocol negotiation, authentication, and other processes involved, making it helpful for debugging purposes.

Explanation:

• The -d option enables verbose output.
• By combining the -D option with -d, the sshd command runs in the foreground with verbose debugging enabled.
• This allows you to see detailed log messages and debug information as SSH connections are established and commands are executed.

Example Output:

...
debug1: hostkeys_foreach: reading file "/etc/ssh/ssh_host_ed25519_key"
debug1: ssh_host_ed25519_key: private key file contains no public key
...

Verbose output displays detailed information related to SSH connections, host keys, and other SSH server configuration aspects.

Use Case 4: Run on a specific port

Code:

sshd -p port

Motivation: Running sshd on a specific port can be important in certain scenarios. By default, SSH operates on port 22, but you may want to use a different port to enhance security or accommodate specific network requirements.

Explanation:

• The -p option is used to specify the port number.
• Replace port with the desired port number, such as 2222.
• When executed, sshd starts and listens for incoming SSH connections on the specified port.

Example Output: No output is displayed in the terminal when the sshd command is executed successfully. However, it starts listening for SSH connections on the specified port.

Conclusion

The sshd command is a versatile tool for securely logging in to and executing commands on a remote machine. In this article, we explored eight different use cases of the sshd command, along with code examples illustrating each case. By understanding and utilizing these various options, you can effectively manage remote access and enhance the security of your systems.