rcmd, rresvport, iruserok, ruserok, rcmd_af, rresvport_af, iruserok_af, ruserok_af - routines for

       For an explanation of the terms used in this section, see attributes(7).
       ┌──────────────────────────────────────────────────────────────────────┬───────────────┬────────────────┐
       │ Interface                                                            │ Attribute     │ Value          │
       ├──────────────────────────────────────────────────────────────────────┼───────────────┼────────────────┤
       │ 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 │
       └──────────────────────────────────────────────────────────────────────┴───────────────┴────────────────┘

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

       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).

   rcmd()
       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).

   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 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()andruserok()
       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.

iruserok_af()
       rcmd_af()
       rresvport_af()
       ruserok_af()
              glibc 2.2.

       Solaris, 4.2BSD.  The "_af" variants are more recent additions, and are not present on as wide a range of
       systems.

       Standard C library (libc, -lc)

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

       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.

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

Linux man-pages 6.9.1                              2024-05-02                                            rcmd(3)

       BSD.

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

       intrcmd(char**restrictahost,unsignedshortinport,constchar*restrictlocuser,constchar*restrictremuser,constchar*restrictcmd,int*restrictfd2p);intrresvport(int*port);intiruserok(uint32_traddr,intsuperuser,constchar*ruser,constchar*luser);intruserok(constchar*rhost,intsuperuser,constchar*ruser,constchar*luser);intrcmd_af(char**restrictahost,unsignedshortinport,constchar*restrictlocuser,constchar*restrictremuser,constchar*restrictcmd,int*restrictfd2p,sa_family_taf);intrresvport_af(int*port,sa_family_taf);intiruserok_af(constvoid*restrictraddr,intsuperuser,constchar*restrictruser,constchar*restrictluser,sa_family_taf);intruserok_af(constchar*rhost,intsuperuser,constchar*ruser,constchar*luser,sa_family_taf);

   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