ruserok(3) — Linux manual page


RCMD(3)                   Linux Programmer's 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

SYNOPSIS         top

       #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:

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

       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.

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

       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

VERSIONS         top

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

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() │               │                │

CONFORMING TO         top

       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

BUGS         top

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

SEE ALSO         top

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

COLOPHON         top

       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)