What is sshd in linux/unix environment ?

In Linux & Networking: Secure Shell Daemon

In computer networks and Linux/Unix systems, sshd is a crucial background software program:

• What it does: The letters stand for Secure Shell daemon (in Linux, a "daemon" is simply a background process).
• How it works: It runs continuously on a server, listening for incoming network connections via the Secure Shell (SSH) Protocol.
• Purpose: When you want to securely log into your server or transfer files from a remote location, sshd is the program that catches your request, handles user authentication, and encrypts the entire communication tunnel.

Managing the sshd Service

On modern Linux distributions (such as Ubuntu, Debian, CentOS, RHEL, and Fedora), you manage the background process using systemctl. You will need root or sudo privileges to run these.

• Check the service status: See if the daemon is currently running, stopped, or experiencing errors.

sudo systemctl status sshd

• Start the service: Turn the SSH server on
sudo systemctl start sshd
• Stop the service: Turn the SSH server off (Warning: This will disconnect active remote sessions once they close and prevent new ones).

sudo systemctl stop sshd


• Reload the service: Applies configuration changes without dropping active user connections (safer than a full restart).


sudo systemctl reload sshd


• Enable on boot: Ensure that sshd automatically starts up whenever the server reboots.


sudo systemctl enable sshd


• Disable on boot: Prevents the SSH server from launching automatically when the system starts.

                sudo systemctl disable sshd

Testing Configuration Files

Before you restart your SSH server after making changes, you should always test your configuration file for syntax errors. If there is a typo, the server won't start, and you could get locked out.

Test syntax: Run the daemon in test mode. If it returns no output, your configuration is valid.

                    sudo sshd -t

Extended test mode: Checks the validity of the configuration file, prints the effective configuration options, and then exits.

                    sudo sshd -T

Locating Important Files

While these aren't execution commands, these file paths are critical when working with sshd:

• Main Configuration File: /etc/ssh/sshd_config (Where you change ports, disable root logins, or configure key authentication).

• System Logs: /var/log/auth.log (Ubuntu/Debian) or /var/log/secure (RHEL/CentOS). You can view unauthorized access attempts here using:

               sudo tail -f /var/log/auth.log

Checking Active Connections

•     If you want to see who is currently connected to your server via sshd, you can use network monitoring tools:

                ps aux | grep sshd

• See active network connections on the SSH port (usually 22):

                sudo ss -tulpn | grep sshd