CompTIA Linux+ XK0-005 - 2.4 - SSH: Configuration Files - ~/.ssh/known_hosts

In the context of secure remote access using SSH, the known_hosts file plays an important role in establishing trust between the client and server. As part of the CompTIA Linux+ exam objectives, it is important to understand the purpose and functioning of the known_hosts file. This guide will provide you with a detailed explanation of the known_hosts file, its significance, and the key configuration settings within the file.

SSH Known Hosts File: ~/.ssh/known_hosts

The known_hosts file, located in the ~/.ssh/ directory, is used by the SSH client to store the public keys of remote SSH servers that the client has previously connected to. When you connect to an SSH server, the client checks the server's public key against the entries in the known_hosts file to verify its authenticity. Let's explore the key aspects of the known_hosts file:

Host Key Entries: Each line in the known_hosts file represents an entry for a specific SSH server. It consists of the host IP address or hostname, the key type, and the corresponding public key. The format is as follows:
```
hostname_or_ip_address key_type public_key
```
Key Type: The key type indicates the algorithm used for generating the server's public key. Examples of key types include ssh-rsa (RSA algorithm), ecdsa-sha2-nistp256 (ECDSA algorithm with NIST P-256 curve), ssh-ed25519 (Ed25519 algorithm), etc. The key type is an important component in verifying the authenticity and security of the SSH server's public key.
Public Key: The public key is the cryptographic key associated with the SSH server. It is used for encrypting data sent from the client to the server during the SSH handshake process. The public key is generated from the server's private key and can be used by the client to verify the authenticity of the server.

When connecting to an SSH server, the client compares the server's public key, including the key type, against the entries in the known_hosts file to ensure it matches. If a match is found, the client proceeds with the connection. If the key type or public key does not match, the client raises an alert, indicating a potential security issue.
Host Key Fingerprints: The known_hosts file displays the fingerprints of the server's public keys. The fingerprint is a unique identifier generated from the public key and serves as a verification mechanism. It helps detect changes in the server's public key, indicating a potential security issue.
Verification and Trust: When you connect to an SSH server, the client compares the server's public key against the entries in the known_hosts file. If a match is found, the connection proceeds without any warnings. If the server's key is not found or has changed, the client raises an alert, asking for confirmation before proceeding.
Managing Entries: Over time, the known_hosts file may accumulate entries for multiple SSH servers. It is important to regularly review and update this file. If a server's key has changed legitimately (e.g., after a server upgrade), you need to update the entry in the known_hosts file to reflect the new key.
Security Implications: The known_hosts file is a critical component of SSH security. An attacker who successfully compromises a server's private key may attempt to replace the public key in the known_hosts file to facilitate a man-in-the-middle attack. Therefore, securing the known_hosts file and monitoring changes is vital.

Conclusion

Understanding the purpose and functionality of the known_hosts file is crucial for establishing secure SSH connections and ensuring the integrity of remote servers. By maintaining and managing the entries in the known_hosts file, you can protect against unauthorized access and potential security threats. Remember to regularly review and update the file as needed.

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