ssh-keysign Command in Linux
ssh-keysign is a helper program that plays an important role in the OpenSSH package. It is mainly used to provide host-based authentication, a security feature that enables systems to authenticate against one another using host keys instead of user passwords.
ssh-keysign runs silently in the background, generating digital signatures from host keys stored locally to provide authentication between computers. It is called internally by the ssh command within host-based authentication processes and should not be used directly by end-users.
Host-based authentication streamlines secure communication between machines by removing the requirement for repetitive user-based authentication for every connection. Instead, ssh-keysign produces cryptographically secure digital signatures from the host private keys, making inter-machine interactions seamless and secure.
Table of Contents
Here is a comprehensive guide to the options available with the ssh-keysign command −
Configuration of ssh-keysign
To enable ssh-keysign, you need to modify the global SSH client configuration file (/etc/ssh/ssh_config) and set the following parameter −
EnableSSHKeysign yes
This makes sure that ssh-keysign has access to host keys and can create signatures on authentication.
Files Used by ssh-keysign
- Host Key Files − /etc/ssh/ssh_host_rsa_key, /etc/ssh/ssh_host_ecdsa_key, /etc/ssh/ssh_host_ed25519_key These are the private host keys used to create digital signatures. They must be owned by root and readable only by root.
- Host Certificate Files − /etc/ssh/ssh_host_rsa_key-cert.pub, /etc/ssh/ssh_host_ecdsa_key-cert.pub, /etc/ssh/ssh_host_ed25519_key-cert.pub If these files are present, they hold the public certificate information matching the private keys.
Examples of ssh-keysign Command in Linux
Since ssh-keysign is not directly invoked by users, practical examples focus on enabling and configuring it for host-based authentication.
- Enable Host-Based Authentication
- Verify Host Key Permissions
- Test Host-Based Authentication
- Debug Host-Based Authentication
- Restrict Host-Key Access
Enable Host-Based Authentication
In order to employ ssh-keysign for host-based authentication, edit the SSH client configuration file.
sudo nano /etc/ssh/ssh_config
Insert the following line −
EnableSSHKeysign yes
Verify Host Key Permissions
Make sure host key files are owned by root and have restricted permissions.
sudo chmod 600 /etc/ssh/ssh_host_rsa_key sudo chown root:root /etc/ssh/ssh_host_rsa_key
The permissions and owner of the host key file are configured for security.
Test Host-Based Authentication
After enabling ssh-keysign, test host-based authentication by connecting to a remote server.
ssh -o HostbasedAuthentication=yes user@remote-server
The ssh command uses ssh-keysign to generate a digital signature for authentication.
Debug Host-Based Authentication
If authentication fails, use verbose mode to debug the process.
ssh -vvv -o HostbasedAuthentication=yes user@remote-server
Displays detailed debugging information about the authentication process, including interactions with ssh-keysign.
Restrict Host Key Access
To enhance security, ensure that host key files are accessible only to ssh-keysign.
sudo chmod 700 /etc/ssh sudo chmod 600 /etc/ssh/ssh_host_rsa_key
Conclusion
The ssh-keysign command is a specialized tool for securely implementing host-based SSH authentication. Through the use of local host keys to digitally sign data, it removes unnecessary user-based authentication, making secure communication between systems easy and efficient.
Efficient configuration and management of ssh-keysign provides solid security and hassle-free authentication processes. Though not called directly by users, its function in host-based authentication renders it a crucial part of the OpenSSH package.