int rcmd(char **ahost, unsigned short inport, char *luser, char *ruser, char *cmd, int *fd2p);
int rresvport(int *port);
int ruserok(char *rhost, int super-user, char *ruser, char *luser);
rresvport is a routine which returns a descriptor to a socket with an address in the privileged port space.
ruserok is a routine used by servers to authenticate clients requesting service with rcmd.
All three functions are present in the same file and are used by the rshd server (among others).
rcmd looks up the host *ahost using gethostbyname (see gethostent(3N)). If the host exists, *ahost is set to the standard name of the host and a connection is established to a server residing at the well-known Internet port inport.
If the connection succeeds, a socket in the Internet domain of type SOCK_STREAM is returned to the caller, and given to the remote command as its standard input (file descriptor 0) and standard output (file descriptor 1). If fd2p is non-zero, then an auxiliary channel to a control process will be set up, and a descriptor for it will be placed in *fd2p. The control process will return diagnostic output from the command (file descriptor 2) on this channel, and will also accept bytes on this channel as signal numbers, to be forwarded to the process group of the command. If fd2p is 0, then the standard error (file descriptor 2) of the remote command will be made the same as its standard output and no provision is made for sending arbitrary signals to the remote process, although you may be able to get its attention by using out-of-band data.
The protocol is described in detail in rshd (see rshd(1Mtcp)).
The rresvport routine is used to obtain a socket with a privileged address bound to it. This socket is suitable for use by rcmd and several other routines. Privileged Internet ports are those in the range 0 to 1023. Only a user with appropriate privileges is allowed to bind an address of this sort to a socket.
ruserok takes a remote host's name, two user names and a flag indicating whether the local user's name is that of the privileged user. If the privileged user flag is 0, the routine then checks the file /etc/hosts.equiv. If this check fails, then the routine checks .rhosts in the local user's home directory to see if the request for service is allowed. If .rhosts is writeable by group or other, then access is denied.
rresvport returns a valid, bound socket descriptor on success. It returns -1 on error with the global value errno set according to the reason for failure. The error code EAGAIN is overloaded to mean that all network ports are in use.
ruserok returns a 0 if the machine name is listed in the /etc/hosts.equiv file or if the host and remote user name are found in the .rhosts file; otherwise ruserok returns -1.