AUTH_SYS Authentication

The client can use AUTH_SYS (called AUTH_UNIX in previous releases) style authentication by setting clnt->cl_auth after creating the RPC client handle:

clnt->cl_auth = authsys_create_default();

This causes each RPC call associated with clnt to carry with it the following credentials-authentication structure shown in Example 4-26.

Example 4-26 AUTH_SYS Credential Structure

/*
 * AUTH_SYS flavor credentials.
 */
struct authsys_parms {
	u_long aup_time;					/* credentials creation time */
	char *aup_machname;				/* client's host name */
	uid_t aup_uid;						/* client's effective uid */
	gid_t aup_gid;						/* client's current group id */
	u_int aup_len;						/* element length of aup_gids*/
	gid_t *aup_gids;					/* array of groups user is in */
};

rpc.broadcast defaults to AUTH_SYS authentication.

Example 4-27 shows a server, with procedure RUSERPROC_1(), that returns the number of users on the network. As an example of authentication, it checks AUTH_SYS credentials and does not service requests from callers whose uid is 16.

Example 4-27 Authentication Server

nuser(rqstp, transp)
	struct svc_req *rqstp;
	SVCXPRT *transp;
{
	struct authsys_parms *sys_cred;
	uid_t uid;
	unsigned int nusers;

	/* NULLPROC should never be authenticated */
	if (rqstp->rq_proc == NULLPROC) {
		if (!svc_sendreply( transp, xdr_void, (caddr_t) NULL))
			fprintf(stderr, "can't reply to RPC call\n");
		return;
	}

	/* now get the uid */
	switch(rqstp->rq_cred.oa_flavor) {
		case AUTH_SYS:
			sys_cred = (struct authsys_parms *) rqstp->rq_clntcred;
			uid = sys_cred->aup_uid;
			break;
		default:
			svcerr_weakauth(transp);
			return;
	}
	switch(rqstp->rq_proc) {
		case RUSERSPROC_1:
			/* make sure caller is allowed to call this proc */
			if (uid == 16) {
				svcerr_systemerr(transp);

				return;
			}
			/*
			 * Code here to compute the number of users and assign it
			 * to the variable nusers
			 */
			if (!svc_sendreply( transp, xdr_u_int, &nusers))
				fprintf(stderr, "can't reply to RPC call\n");
			return;
		default:
			svcerr_noproc(transp);
			return;
	}
}

Note the following:

• The authentication parameters associated with the NULLPROC (procedure number zero) are usually not checked.

• The server calls svcerr_weakauth() if the authentication parameter's flavor is too weak; there is no way to get the list of authentication flavors the server requires.

• The service protocol should return status for access denied; in Example 4-27, the protocol calls the service primitive svcerr_systemerr(), instead.

The last point underscores the relation between the RPC authentication package and the services: RPC deals only with authentication and not with an individual service's access control. The services themselves must establish access-control policies and reflect these policies as return statuses in their protocols.