rcmd(3) — Linux manual page


rcmd(3)                 Library Functions Manual                 rcmd(3)

NAME         top

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

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

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

       int rcmd(char **restrict ahost, unsigned short inport,
                   const char *restrict locuser,
                   const char *restrict remuser,
                   const char *restrict cmd, int *restrict 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 **restrict ahost, unsigned short inport,
                   const char *restrict locuser,
                   const char *restrict remuser,
                   const char *restrict cmd, int *restrict fd2p,
                   sa_family_t af);

       int rresvport_af(int *port, sa_family_t af);

       int iruserok_af(const void *restrict raddr, int superuser,
                   const char *restrict ruser, const char *restrict 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

       rcmd(), rcmd_af(), rresvport(), rresvport_af(), iruserok(),
       iruserok_af(), ruserok(), ruserok_af():
           Since glibc 2.19:
           glibc 2.19 and earlier:

DESCRIPTION         top

       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

       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.

RETURN VALUE         top

       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

ATTRIBUTES         top

       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()  │               │                │

STANDARDS         top


HISTORY         top

              glibc 2.2.

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

BUGS         top

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

SEE ALSO         top

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

Linux man-pages (unreleased)     (date)                          rcmd(3)

Pages that refer to this page: rexec(3)