The RPC protocol includes a slot for authentication parameters on every call. The contents of the authentication parameters are determined by the type of authentication used by the server and client. A server may support several different flavors of authentication at once. The AUTH_NONE flavor provides null authentication, that is, no authentication information is passed. The AUTH_UNIX flavor provides UNIX-style user ID, group ID, and groups with each call. The AUTH_DES flavor provides DES-encrypted authentication parameters based on a network-wide name, with session keys exchanged via a public key scheme. The AUTH_KERB flavor provides DES encrypted authentication parameters based on a network-wide name with session keys exchanged via Kerberos secret keys.
The NFS server checks permissions by taking the credentials from the RPC authentication information in each remote request. For example, using the AUTH_UNIX flavor of authentication, the server gets the user's effective user ID, effective group ID and groups on each call, and uses them to check access. Using user ids and group ids implies that the client and server either share the same ID list or do local user and group ID mapping. Servers and clients must agree on the mapping from user to uid and from group to gid, for those sites that do not implement a consistent user ID and group ID space. In practice, such mapping is typically performed on the server, following a static mapping scheme or a mapping established by the user from a client at mount time.
The AUTH_DES and AUTH_KERB style of authentication is based on a network-wide name. It provides greater security through the use of DES encryption and public keys in the case of AUTH_DES, and DES encryption and Kerberos secret keys (and tickets) in the AUTH_KERB case. Again, the server and client must agree on the identity of a particular name on the network, but the name to identity mapping is more operating system independent than the uid and gid mapping in AUTH_UNIX. Also, because the authentication parameters are encrypted, a malicious user must know another users network password or private key to masquerade as that user. Similarly, the server returns a verifier that is also encrypted so that masquerading as a server requires knowing a network password.
The NULL procedure typically requires no authentication.