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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 */

       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 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
       nonzero, 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 usernames 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 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; 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 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 4.2BSD.


       iruserok() is not declared in glibc headers.


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


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