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RCMD(3)                    Linux Programmer's Manual                   RCMD(3)

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

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

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

       int rresvport(int *port);

       int iruserok(uint32_t raddr, int superuser,
                    const char *ruser, const char *luser);

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

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       rcmd(), rresvport(), ruserok(): _BSD_SOURCE

       The  rcmd()  function is used by the superuser to execute a command on a remote machine using an authentication
       scheme based on privileged 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 authenti-
       cate 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 diag-
       nostic 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 usernames and
       a flag indicating whether the local user's name is that of the superuser.  Then, if the user is not  the  supe-
       ruser, 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  writable  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 username are found in the .rhosts  file;  other-
       wise  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 diagnos-
       tic message on the standard error.

       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."

       Not in POSIX.1-2001.  Present on the BSDs, Solaris, and  many  other  systems.   These  functions  appeared  in

       iruserok() is not declared in glibc headers.

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

       This  page  is part of release 3.22 of the Linux man-pages project.  A description of the project, and informa-
       tion about reporting bugs, can be found at

Linux                             2007-12-28                           RCMD(3)