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 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

       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 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).

       │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 5.08 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)