1 Abstract

As there are two different types of certificates, there are two individual tasks as well in order to setup a hosts. If one would like to use only one certificate, execute the corresponding task.

2 Host Certificate

2.1 Sign host's public key

To enable certificate based login on a host, its public rsa key needs to be singed by the CA authority. To do so copy the public rsa key 'ssh_host_rsa_key.pub' to the CA, signed it and copy the certificate back to the host. The resulting certificate is called 'ssh_host_rsa_key-cert.pub'. The instructions how to singed a hosts public key can be found here.

2.1 Tell the SSH daemon about the certificate

To tell the SSH daemon about the certificate add the following configuration lines to the file '/etc/ssh/sshd_config'. The host sends this certificate to the client to identify itself as a trusted host

### Host certificate
HostCertificate /etc/ssh/ssh_host_rsa_key-cert.pub

2.3 Restart the SSH daemon

3 Client Certificate

To setup the client certificate, the public key of the certificate authority is needed. The file is called 'yubikeyX.pub' depending on the CA server.

3.1 Verify client certificates

Add the following lines to the file '/etc/ssh/sshd_config' to tell the SSH daemon about the public key to verify client certificates. The host trusts all certificates the are signed by our CA.

### User CA certificate
TrustedUserCAKeys /etc/ssh/yubikeyX.pub

3.2 Principals

Next we configure the hosts to accept only certain principals. To do so, add this line to '/etc/ssh/sshd_config'

### Auth Principals
AuthorizedPrincipalsFile /etc/ssh/auth_principals/%u

Then we need to populate the principals file. For each user we need to create a file.

mkdir -p /etc/ssh/auth_principals
echo -e 'host.netdef.org\nroot-everywhere' > /etc/ssh/auth_principals/root

This allows to all users to login as root that have either host.netdef.org or root-everywhere specified in the list of principals within their certificate.

You can control access to any other local user by creating the corresponding files under '/etc/ssh/auth_principals'.

3.3 Restart the SSH daemon