RCMD(3) Linux Programmer's Manual RCMD(3)
rcmd, rresvport, iruserok, ruserok - routines for returning a stream to
a remote command
#include <<netdb.h>> /* Or <unistd.h> on some systems */
int rcmd(char **ahost, int 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);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
rcmd(), rresvport(), ruserok(): _BSD_SOURCE
The rcmd() function is used by the superuser to execute a command on a
remote machine using an authentication scheme based on reserved port
numbers. The rresvport() function returns a 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 present in the
same file and 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 non-zero, then an auxiliary channel to
a control process will be set up, and a 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 provi-
sion is made for sending arbitrary signals to the remote process,
although you may be able to get its attention by using out-of-band
The protocol is described in detail in rshd(8).
The rresvport() function is used to obtain a socket with a privileged
address bound to it. This socket is suitable for use by rcmd() and
several other functions. Privileged Internet ports are those in the
range 0 to 1023. Only the superuser is allowed to bind an address of
this sort to a socket.
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, or is writable by anyone other
than the owner, 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 gethost-
name(2)) is the same as the remote domain, only the machine name need
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.
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 over-
loaded to mean "All network ports in use."
Not in POSIX.1-2001. Present on the BSDs, Solaris, and many other sys-
tems. These functions appeared in 4.2BSD.
iruserok() is not declared in glibc headers.
rlogin(1), rsh(1), intro(2), rexec(3), rexecd(8), rlogind(8), rshd(8)
This page is part of release 3.05 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.
Linux 2007-12-28 RCMD(3)