Esto lo usamos en su día y funcionó (gracias Bulma!). Estábamos ahora rehaciéndolo y no iba. Leyendo el manual del ssh (RTFM!) hemos visto esto:

“The file ~/.ssh/authorized_keys lists the public keys that are permitted for logging in. When the user logs in, the ssh program tells the server which key pair it would like to use for authentication. The client proves that it has access to the private key and the server checks that the corresponding public key is authorized to accept the account.”

¡y efectivamente, en vez de añadir la clave pública a known_hosts la hemos añadido en authorized_keys y ya va!