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     rcmd, rresvport, iruserok, ruserok - routines for returning a stream to a
     remote command


     #include <netdb.h>   /* Or <unistd.h> on some systems */

     rcmd(char **ahost, int inport, const char *locuser, const char *remuser,
             const char *cmd, int *fd2p);

     rresvport(int *port);

     iruserok(u_int32_t raddr, int superuser, const char *ruser,
             const char *luser);

     ruserok(const char *rhost, int superuser, const char *ruser,
             const char *luser);


     The rcmd() function is used by the superuser to execute a command on a
     remote machine using an authentication scheme based on reserved port
     numbers.  The rresvport() function returns a descriptor to a socket with
     an address in the privileged port space.  The iruserok() and ruserok()
     functions are used by servers to authenticate clients requesting service
     with rcmd().  All four functions are present in the same file and are
     used by the rshd(8) server (among others).

     The rcmd() function looks up the host *ahost using gethostbyname(3),
     returning -1 if the host does not exist.  Otherwise *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
     stdin and stdout.  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 (unit 2) on this channel, and will also accept bytes on this
     channel as being UNIX signal numbers, to be forwarded to the process
     group of the command.  If fd2p is 0, then the stderr (unit 2 of the
     remote command) will be made the same as the stdout 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(8).

     The rresvport() function is used to obtain a socket with a privileged
     address bound to it.  This socket is suitable for use by rcmd() and
     several other functions.  Privileged Internet ports are those in the
     range 0 to 1023.  Only the superuser is allowed to bind an address of
     this sort to a socket.

     The iruserok() and ruserok() functions take a remote host’s IP address or
     name, respectively, two user names and a flag indicating whether the
     local user’s name is that of the superuser.  Then, if the user is NOT the
     superuser, it checks the /etc/hosts.equiv file.  If that lookup is not
     done, or is unsuccessful, the .rhosts in the local user’s home directory
     is checked to see if the request for service is allowed.

     If this file does not exist, is not a regular file, is owned by anyone
     other than the user or the superuser, or is writeable by anyone other
     than the owner, the check automatically fails.  Zero is returned if the
     machine name is listed in the “hosts.equiv” file, or the host and remote
     user name are found in the “.rhosts” file; otherwise iruserok() and
     ruserok() return -1.  If the local domain (as obtained from
     gethostname(2)) is the same as the remote domain, only the machine name
     need be specified.

     If the IP address of the remote host is known, iruserok() should be used
     in preference to ruserok(), as it does not require trusting the DNS
     server for the remote host’s domain.


     The rcmd() function returns a valid socket descriptor on success.  It
     returns -1 on error and prints a diagnostic message on the standard

     The rresvport() function 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 ‘‘All network ports in use.’’


     rlogin(1), rsh(1), intro(2), rexec(3), rexecd(8), rlogind(8), rshd(8)


     These functions appeared in 4.2BSD.