1 - Abstract

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To enable certificate based login on a host, the public rsa key 'ssh_host_rsa_key.pub' needs to be signed. The resulting certificate is called 'ssh_host_rsa_key-cert.pub'. To enable ssh based login two things are required on the host:

• host certificate: 'ssh_host_rsa_key-cert.pub'
• CA public key: 'yubikeyX.pub'

Step 1 - Host Certifiacte

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

As there are two different types of certificates, there are two individual tasks to setup a host.

• Host Certificate
• Client Certificate

Here at NetDEF we only use Client Certificates.

2 - Client Certificate

To setup the client certificate, the public key of the certificate authority is needed. There are three public keys called  'yubikey1.pub', 'yubikey2.pub' and 'yubikey3.pub'.

Step 1 - Verify client certificates

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

Step 2 - Trust User CA Certificate

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

### User CA certificate
TrustedUserCAKeys /etc/ssh/yubikeyXyubikey1.pub
TrustedUserCAKeys /etc/ssh/yubikey2.pub
TrustedUserCAKeys /etc/ssh/yubikey3.pub

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Step 2 - Principals

Now, Next we 'll configure one of our 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:file. For each user we need to create a file.

mkdir -p

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

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

Step

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3 - Restart

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the SSH daemon