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       rcmd,  rresvport,  iruserok,  ruserok,  rcmd_af,  rresvport_af,  iruserok_af, ruserok_af -
       routines for returning a stream to a remote command


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

       int rcmd(char **ahost, unsigned short 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, unsigned short 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(const void *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  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 port bound to it.
       This socket is suitable for use by rcmd() and several other functions.   Privileged  ports
       are  those  in  the  range 0 to 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 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.

   *_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() supports 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 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."

       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.


       For an explanation of the terms used in this section, see attributes(7).

       │InterfaceAttributeValue          │
       │rcmd(), rcmd_af()           │ Thread safety │ MT-Unsafe      │
       │rresvport(), rresvport_af() │ Thread safety │ MT-Safe        │
       │iruserok(), ruserok(),      │ Thread safety │ MT-Safe locale │
       │iruserok_af(), ruserok_af() │               │                │


       Not in POSIX.1.  Present on the BSDs, Solaris, and many other  systems.   These  functions
       appeared  in 4.2BSD.  The "_af" variants are more recent additions, and are not present on
       as wide a range of systems.


       iruserok() and iruserok_af() are declared in glibc headers only since version 2.12.


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


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