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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ATTRIBUTES | STANDARDS | HISTORY | BUGS | SEE ALSO | COLOPHON |
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rcmd(3) Library Functions Manual rcmd(3)
rcmd, rresvport, iruserok, ruserok, rcmd_af, rresvport_af,
iruserok_af, ruserok_af - routines for returning a stream to a
remote command
Standard C library (libc, -lc)
#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
feature_test_macros(7)):
rcmd(), rcmd_af(), rresvport(), rresvport_af(), iruserok(),
iruserok_af(), ruserok(), ruserok_af():
Since glibc 2.19:
_DEFAULT_SOURCE
glibc 2.19 and earlier:
_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 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).
rcmd()
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).
rresvport()
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 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
above.
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() │ │ │
└───────────────────────────────┴───────────────┴────────────────┘
BSD.
iruserok_af()
rcmd_af()
rresvport_af()
ruserok_af()
glibc 2.2.
Solaris, 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 glibc 2.12.
rlogin(1), rsh(1), rexec(3), rexecd(8), rlogind(8), rshd(8)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
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man-pages@man7.org
Linux man-pages 6.15 2025-05-17 rcmd(3)
Pages that refer to this page: rexec(3)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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