The sshd command is the server daemon in the OpenSSH software suite. It is responsible for securely allowing remote login sessions to a Linux system. The sshd command listens on a specific port (usually port 22) for incoming connections from clients. Once a connection is established, sshd will authenticate the client and then provide a secure channel for the client to communicate with the server.
Overview
The sshd command is typically started at boot time and runs as a background process. It listens on a specific port (usually port 22) for incoming connections from clients. When a client connects, sshd will perform an authentication process to verify the client’s identity. Once authenticated, sshd will provide a secure channel for the client to communicate with the server.
Starting and Stopping sshd
To start or stop the sshd daemon, you can use the following commands:
# Start sshd
sudo systemctl start sshd
# Stop sshd
sudo systemctl stop sshd
Authentication
sshd provides several methods of authentication, including password authentication and public key authentication. By default, sshd will allow password authentication, but it is recommended to use public key authentication for increased security.
Password Authentication
To use password authentication, the client will need to provide a valid username and password combination. The password is sent over the network in an encrypted form to prevent eavesdropping.
Public Key Authentication
Public key authentication is a more secure method of authentication. It involves the use of a public key and a private key. The client generates a key pair and sends the public key to the server. When the client connects, the server will challenge the client to prove that they have the private key associated with the public key. If the client can prove this, they will be authenticated.
Example
To connect to a remote Linux server using ssh, you can use the following command:
ssh username@remote_server
This will prompt you for a password if password authentication is enabled. If public key authentication is enabled, you will need to have a valid private key on your local machine.
Options
The sshd command has several options that can be used to customize its behavior. The following table lists the available options:
| Option | Description |
|---|---|
| -D | Do not detach and run in foreground |
| -d | Debug mode |
| -e | Log to stderr instead of syslog |
| -f | Specify an alternate configuration file |
| -h | Display help message |
| -p | Specify a different port to listen on |
| -T | Test the configuration file and exit |
Troubleshooting tips
If you are having trouble connecting to a remote Linux server using ssh, here are some troubleshooting tips:
- Make sure that
sshdis running on the remote server. You can check this by running the following command on the remote server:
sudo systemctl status sshd
- Make sure that the remote server is accessible from your local machine. You can test this by pinging the remote server:
ping remote_server
- Make sure that you are using the correct username and password (if using password authentication).
- Make sure that you have a valid private key (if using public key authentication). You can check this by running the following command on your local machine:
ls ~/.ssh/id_rsa
This should output the path to your private key.
Notes
- By default,
sshdlistens on port 22. If you want to change the port thatsshdlistens on, you can use the-poption followed by the desired port number. - It is recommended to use public key authentication instead of password authentication for increased security.
sshdlogs to syslog by default. If you want to log to stderr instead, you can use the-eoption.