RCMD(3)                    Linux Programmer's Manual                   RCMD(3)

       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():
           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 re-
       questing 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  provi-
       sion  is  made for sending arbitrary signals to the remote process, al-
       though 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 sev-
       eral 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 sup-
       plies 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  di-
       rectory 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; other-
       wise iruserok() and ruserok() return -1.  If the local domain  (as  ob-
       tained  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 ad-
       dress 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 er-

       The rresvport() function returns a valid, bound  socket  descriptor  on
       success.   It  returns  -1 on error with the global value errno set ac-
       cording 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  at-

       |Interface                   | Attribute     | Value          |
       |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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Linux                             2017-09-15                           RCMD(3)
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