9. Host-Based Authentication: "hostbased"

Some sites wish to allow authentication based on the host that the user is coming from and the user name on the remote host. While this form of authentication is not suitable for high-security sites, it can be very convenient in many environments. This form of authentication is OPTIONAL. When used, special care SHOULD be taken to prevent a regular user from obtaining the private host key.

The client requests this form of authentication by sending the following message. It is similar to the UNIX "rhosts" and "hosts.equiv" styles of authentication, except that the identity of the client host is checked more rigorously.

This method works by having the client send a signature created with the private key of the client host, which the server checks with that host's public key. Once the client host's identity is established, authorization (but no further authentication) is performed based on the user names on the server and the client, and the client host name.

byte      SSH_MSG_USERAUTH_REQUEST
string    user name
string    service name
string    "hostbased"
string    public key algorithm for host key
string    public host key and certificates for client host
string    client host name expressed as the FQDN in US-ASCII
string    user name on the client host in ISO-10646 UTF-8 encoding [RFC3629]
string    signature

Public key algorithm names for use in 'public key algorithm for host key' are defined in the transport layer specification [SSH-TRANS]. The 'public host key and certificates for client host' may include certificates.

The value of 'signature' is a signature with the private host key of the following data, in this order:

string    session identifier
byte      SSH_MSG_USERAUTH_REQUEST
string    user name
string    service name
string    "hostbased"
string    public key algorithm for host key
string    public host key and certificates for client host
string    client host name expressed as the FQDN in US-ASCII
string    user name on the client host in ISO-10646 UTF-8 encoding [RFC3629]

The server MUST verify that the host key actually belongs to the client host named in the message, that the given user on that host is allowed to log in, and that the 'signature' value is a valid signature on the appropriate value by the given host key. The server MAY ignore the client 'user name', if it wants to authenticate only the client host.

Whenever possible, it is RECOMMENDED that the server perform additional checks to verify that the network address obtained from the (untrusted) network matches the given client host name. This makes exploiting compromised host keys more difficult. Note that this may require special handling for connections coming through a firewall.