Host Keys and Certificates

Server Box verifies every SSH server before it sends a password or key.

First connection

The first time you connect to a server, Server Box shows the host key type and its fingerprint (SHA256:...) and asks whether to trust it. Compare the fingerprint with the one the server reports:

ssh-keygen -lf /etc/ssh/ssh_host_ed25519_key.pub

After you accept, Server Box remembers the key for that server. If the server later presents a different key, Server Box warns you before it connects.

Host certificates

A server can present an OpenSSH host certificate ([email protected] and similar) instead of a plain host key. Server Box supports Ed25519, ECDSA and RSA certificates.

Without a trusted certificate authority, Server Box handles a certificate as OpenSSH does: it uses the key inside the certificate and asks about it on the first connection, as described above. The fingerprint is the one of that key, which matches the Server host certificate: ... SHA256:... line of ssh -v.

Trusted certificate authorities

If your servers’ host keys are signed by a certificate authority (CA), you can trust that CA once instead of confirming each server. This is the equivalent of an @cert-authority line in OpenSSH’s known_hosts.

Open Settings → SSH → General → Host certificate authorities and select Add:

• Key: the CA’s public key line, for example the contents of ca.pub: ssh-ed25519 AAAA... [email protected]. Ed25519, ECDSA and RSA keys are supported. You can also paste a whole known_hosts line, @cert-authority *.example.com ssh-ed25519 AAAA...; the hosts are then taken from it.
• Host: which servers the CA vouches for. The default * matches every server. It cannot be empty.

A server connects without a prompt when its certificate is signed by a matching CA and is valid: a host certificate, within its validity period, without critical options, not signed with SHA-1 (ssh-rsa), and listing the server among its principals. Principals are compared with the address Server Box connected to and with the address configured for the server; a certificate without principals is valid for any host, and a principal may contain * and ?.

Otherwise, the server is asked about on the first connection as described above, using the key inside its certificate. This includes a certificate that a matching CA signed but that is expired or names another host, which is what OpenSSH does too.

Host patterns

The Host field uses the known_hosts pattern syntax:

Pattern	Matches
*	Every server
*.example.com	web.example.com, db.example.com
web-?.example.com	web-1.example.com, not web-10.example.com
10.0.0.*	Addresses from 10.0.0.0 to 10.0.0.255
*.example.com,!bastion.example.com	Every example.com host except bastion

Separate several patterns with commas. A pattern that starts with ! excludes the hosts it matches. Matching ignores case.

A server on a port other than 22 is matched as [host]:port, as in OpenSSH. For example, [*.example.com]:2222 matches web.example.com on port 2222, and *.example.com does not.