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);
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).
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.
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.
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.
The functions
iruserok_af(),
rcmd_af(),
rresvport_af(), and
ruserok_af() functions are provided since glibc 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 (), iruserok_af (), ruserok_af () |
Thread safety |
MT-Safe locale |
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 glibc 2.12.
rlogin(1),
rsh(1),
rexec(3),
rexecd(8),
rlogind(8),
rshd(8)