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rcmd(3)

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

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

     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:
             _DEFAULT_SOURCE
         Glibc 2.19 and earlier:
             _BSD_SOURCE

Description
     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()  func-
     tions are used by servers to authenticate clients requesting
     service with rcmd().  All four functions  are  used  by  the
     rshd(8) server (among others).

  rcmd()
     The rcmd() function looks up the host *ahost using  gethost-
     byname(3),  returning -1 if the host does not exist.  Other-
     wise, *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 for-
     warded 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).

  rresvport()
     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 capa-
     bility   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 argu-
     ment 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.

Return Value
     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 glo-
     bal 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.

Versions
     The functions iruserok_af(), rcmd_af(), rresvport_af(),  and
     ruserok_af()  functions  are  provide in glibc since version
     2.2.

Attributes
     For an explanation of the terms used in  this  section,  see
     attributes(7).     allbox;    lbw27    lb    lb   l   l   l.
     Interface Attribute Value T{ rcmd(),  rcmd_af()  T}   Thread
     safety  MT-Unsafe T{ rresvport(), rresvport_af() T}   Thread
     safety  MT-Safe T{ iruserok(), ruserok(),
     iruserok_af(),  ruserok_af()   T}   Thread   safety  MT-Safe
     locale

Conforming to
     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.

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

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

Colophon
     This page is part of release 5.02  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 https://www.kernel.org/doc/man-pages/.
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