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           #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);
           int rcmd_af(char **ahost, int inport, const char *locuser,
                       const char *remuser, const char *cmd, int *fd2p,
                       sa_family_t af);
           int rresvport_af(int *port, sa_family_t af);
           int iruserok_af(uint32_t raddr, int superuser,
                           const char *ruser, const char *luser, sa_family_t af);
           int ruserok_af(const char *rhost, int superuser,
                          const char *ruser, const char *luser, sa_family_t af);
       Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
           rcmd(),    rcmd_af(),    rresvport(),    rresvport_af(),    iruserok(),
           iruserok_af(), ruserok(), ruserok_af(): _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  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
           1023.  Only a privileged process (CAP_NET_BIND_SERVICE) is  allowed  to
           bind  to a privileged port.  In the glibc implementation, this function
           restricts its search to the ports from 512 to 1023.  The port  argument
           is  value-result:  the  value  it  supplies  to the call is used as the
           starting point for a circular search of the port range; on (successful)
           return, it contains the port number that was bound to.
       iruserok() and ruserok()
           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 gethost-
           name(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.
       *_af() variants
           All  of the functions described above work with IPv4 (AF_INET) sockets.
           The "_af" variants take  an  extra  argument  that  allows  the  socket
           address  family  to be specified.  For these functions, the af argument
           can be specified as AF_INET or AF_INET6.  In addition,  rcmd_af()  sup-
           ports the use of AF_UNSPEC.


           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 over-
           loaded to mean "All network ports in use."
           For information on the return from ruserok() and iruserok(), see above.


           The    functions    iruserok_af(),   rcmd_af(),   rresvport_af(),   and
           ruserok_af() functions are provide in glibc since version 2.2.



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