How to Use the Command 'sshd' (with examples)

The Secure Shell Daemon (sshd) is an integral part of ensuring secure communications between machines. It provides a means for remote systems to securely log in and execute commands on the host machine. This capability is integral to tasks such as remote server management, automated deployments, and accessing services in a secure manner. In this article, we’ll explore some practical use cases of the sshd command, which involve starting the daemon in different modes and configurations.

Use case 1: Start Daemon in the Background

Code:

sshd

Motivation:

Running sshd without any additional flags or arguments initiates the daemon in a standard, detached mode that runs in the background. This is the most common usage scenario and is part of the standard server initialization process. By running in the background, sshd can continuously listen for incoming SSH connection requests without tying up a terminal session, which is ideal for servers dedicated to handling remote connections.

Explanation:

• sshd: The basic invocation of the sshd command starts the SSH daemon. By default, it detaches from the terminal and runs in the background, becoming a persistent service on the server. This background execution is crucial for allowing sshd to continuously accept and manage incoming connection requests without interruption.

Example Output:

Since sshd runs as a daemon in the background, there is no direct output to the terminal upon execution. Instead, verifying its operation is typically done by checking processes with ps commands or ensuring that SSH clients can connect successfully.

Use case 2: Run sshd in the Foreground

Code:

sshd -D

Motivation:

Running sshd in the foreground is particularly useful during the setup or debugging phase of a server. It allows administrators to monitor the daemon’s output directly within the terminal, which includes messages about incoming connections, errors, and also system notifications. This can be invaluable when ensuring that sshd is configured correctly or troubleshooting specific issues related to daemon initialization or connection handling.

Explanation:

• -D: This flag stands for “Don’t detach” and keeps sshd running in the foreground, sending output directly to the terminal. By preventing the daemon from detaching, administrators can have interactive oversight and manual control over its execution, making real-time problem evaluation and correction more straightforward.

Example Output:

In the terminal, sshd will output logs directly, such as “Server listening on 0.0.0.0 port 22” indicating that it has started successfully and is ready to accept connections.

Use case 3: Run with Verbose Output (for Debugging)

Code:

sshd -D -d

Motivation:

For an in-depth analysis of sshd's behavior, enabling verbose output is essential. This mode supports debugging by providing detailed logs that outline each step the daemon takes in setting up and managing connections. This level of detail can be instrumental in diagnosing complex issues and understanding the precise actions sshd performs, such as key exchanges, authentication mechanisms, and connection states.

Explanation:

• -D: As previously explained, this keeps the process in the foreground.
• -d: The lowercase ’d’ flag enables debug mode, increasing the verbosity of the output. This adds layers of information crucial for debugging and solving intricate configuration errors or unexpected behaviors in connection management.

Example Output:

With verbose output, messages like “debug1: sshd version OpenSSH_X.X” and details about authentication attempts appear in the terminal, offering comprehensive insights into the SSH daemon’s inner workings.

Use case 4: Run on a Specific Port

Code:

sshd -p 2222

Motivation:

Changing the default port (22) on which SSH listens can be a security measure to protect against malicious attacks that target well-known ports on a server. By configuring sshd to listen on a non-standard port, administrators can obscure the SSH service, effectively reducing automated attack attempts. This tactic is part of a layered security strategy that includes other measures such as firewalls and intrusion detection systems.

Explanation:

• -p port: This option specifies the port number on which sshd should listen for incoming connections. In this example, replacing ‘port’ with 2222 tells sshd to listen on port 2222 instead of the default port 22. Custom port assignments must be coordinated with the firewall settings and client configurations to ensure accessibility.

Example Output:

When checking active listening services, using a command like netstat -tuln, you would see output indicating that sshd is now listening on the specified port: “tcp 0 0 0.0.0.0:2222 0.0.0.0:* LISTEN”.

Conclusion:

The sshd command’s versatility is demonstrated through various deployment and configuration scenarios that align with security, monitoring, and debugging needs. Whether you’re starting the daemon for everyday operation, engaging in detailed analysis, or securing your system through port reconfiguration, sshd provides a critical foundation for secure remote interactions. Familiarity and adeptness in managing this daemon are vital skills for system administrators focused on maintaining robust and secure server environments.