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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | VERSIONS | ATTRIBUTES | CONFORMING TO | BUGS | SEE ALSO | COLOPHON |
RCMD(3) Linux Programmer's Manual RCMD(3)
rcmd, rresvport, iruserok, ruserok, rcmd_af, rresvport_af,
iruserok_af, ruserok_af - routines for returning a stream to a remote
command
#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:
_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. 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
above.
The functions iruserok_af(), rcmd_af(), rresvport_af(), and
ruserok_af() functions are provide in glibc since version 2.2.
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() │ │ │
└────────────────────────────┴───────────────┴────────────────┘
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
systems.
iruserok() and iruserok_af() are declared in glibc headers only since
version 2.12.
rlogin(1), rsh(1), intro(2), rexec(3), rexecd(8), rlogind(8), rshd(8)
This page is part of release 4.12 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
https://www.kernel.org/doc/man-pages/.
Linux 2016-10-08 RCMD(3)
Pages that refer to this page: rexec(3)