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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

       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

       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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