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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 **restrict ahost, unsigned short inport,
                   const char *restrict locuser,
                   const char *restrict remuser,
                   const char *restrict cmd, int *restrict 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 **restrict ahost, unsigned short inport,
                   const char *restrict locuser,
                   const char *restrict remuser,
                   const char *restrict cmd, int *restrict fd2p,
                   sa_family_t af);

       int rresvport_af(int *port, sa_family_t af);

       int iruserok_af(const void *restrict raddr, int superuser,
                   const char *restrict ruser, const char *restrict 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(),
           Since glibc 2.19:
           Glibc 2.19 and earlier:


       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  file 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 file
       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 (on Linux, a process that has
       the CAP_NET_BIND_SERVICE capability in the user namespace governing its network namespace)
       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, is writable by anyone other than the owner, or is hardlinked anywhere,
       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.  On failure,
       it  returns  -1 and sets errno to indicate the error.  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(), iruserok_af(), ruserok_af()      │ Thread safety │ MT-Safe locale │


       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), rexec(3), rexecd(8), rlogind(8), rshd(8)


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