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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 privileged 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 nonzero, 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 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 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 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.
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.
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."
Not in POSIX.1-2001. Present on the BSDs, Solaris, and many other systems.
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 3.32 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)
HTML rendering created 2010-12-03 by Michael Kerrisk, author of The Linux Programming Interface