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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
     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  provi-
     sion  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
     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
     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;  other-
     wise  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()  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.


     For  an  explanation  of  the  terms  used   in   this   section,   see
     |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)


     This page is part of release 4.16 of the Linux  man-pages  project.   A
     description  of  the project, information about reporting bugs, and the
     latest    version    of    this    page,    can     be     found     at

Linux 2017-09-15 RCMD(3)

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