The Meaning of -i in SSH

The Secure Shell (SSH) command is a network protocol that provides a secure way to access a remote computer. SSH includes a variety of options, one of which is the -i option. This article will delve into the meaning and usage of -i in SSH.

Understanding SSH

Before we dive into the -i option, it’s important to have a basic understanding of SSH. SSH is a protocol that allows secure remote login and other secure network services over an insecure network. It provides strong host-to-host and user authentication as well as secure encrypted communications over the internet.

The -i Option in SSH

The -i option in SSH is used to specify the identity file (private key) for public key authentication. This option allows the user to provide the path to the private key file that will be used to authenticate the connection to the remote server.

By default, SSH looks for identity files in certain locations. For protocol version 1, the default identity file is ~/.ssh/identity. For protocol version 2, the default identity files are ~/.ssh/id_dsa, ~/.ssh/id_ecdsa, ~/.ssh/id_ed25519, and ~/.ssh/id_rsa.

However, the -i option allows you to explicitly specify a different identity file. This is particularly useful when you have multiple identity files or need to use a specific file for authentication.

Using the -i Option

To use the -i option, you would include it in your SSH command followed by the path to your private key file. For example:

ssh -i /path/to/private/key user@hostname

In this command, ssh is the command to start the SSH client program, -i is the option that specifies the identity file, /path/to/private/key is the path to the private key file, user is the username on the remote system, and hostname is the name of the remote system.

The -i option tells SSH to use the private key located at /path/to/private/key for authentication. The command then logs into the remote system hostname as the user user.

Important Notes

It’s important to note that the identity file must be in the correct format. In this case, the file should be in a PEM format, which is a common format for private key files.

If you are using key authentication and have been provided with a certificate, you can also specify it using the -i option. SSH will try to load certificate information from the filename obtained by appending -cert.pub to the identity filenames.

Conclusion

In summary, the -i option in the SSH command is a powerful tool that provides flexibility in choosing the identity file for authentication. It is particularly useful when dealing with multiple identity files or when a specific file needs to be used for authentication. By understanding and utilizing the -i option, you can enhance the security and versatility of your SSH connections.