NAME
rcmd, rcmd_af,
rresvport, rresvport_af,
ruserok —
routines for returning a stream to a
remote command
SYNOPSIS
#include
<unistd.h>
int
rcmd(char
**ahost, int
inport, const char
*locuser, const char
*remuser, const char
*cmd, int
*fd2p);
int
rcmd_af(char
**ahost, int
inport, const char
*locuser, const char
*remuser, const char
*cmd, int *fd2p,
int af);
int
rresvport(int
*port);
int
rresvport_af(int
*port, int af);
int
ruserok(const
char *rhost, int
superuser, const char
*ruser, const char
*luser);
DESCRIPTION
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. If the
calling process is not setuid, the RSH environment
variable is set, and inport is
“shell/tcp”,
rcmdsh(3) is called instead with the value of
RSH. Alternately, if the user is not the superuser,
rcmd() will invoke
rcmdsh(3) to run the command via
ssh(1). While rcmd() can handle IPv4 cases
only, the rcmd_af() function can handle other cases
as well.
The
rresvport()
and
rresvport_af()
functions return a descriptor to a socket with an address in the privileged
port space. The ruserok() function is used by
servers to authenticate clients requesting service with
rcmd().
The
rcmd()
function looks up the host *ahost using
getaddrinfo(3) and, if the host exists,
*ahost is set to the canonical name of the host. A
connection is then established to a server residing at the well-known
Internet port inport. If the user is not the
superuser, the only valid port is “shell/tcp” (usually port
514).
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
NULL, then the standard error (unit 2 of the remote
command) will be made the same as the standard output 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. Note that if the
user is not the superuser, fd2p must be
NULL.
rcmd_af()
takes address family in the last argument. If the last argument is
AF_UNSPEC, interpretation of
*ahost will obey the underlying address resolution
like DNS.
The
rresvport()
and
rresvport_af()
functions are 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 IPPORT_RESERVED - 1, which happens to be 1023. Only
the superuser is allowed to bind an address of this sort to a socket.
rresvport() and
rresvport_af() need to be seeded with a port number;
if that port is not available these functions will find another.
The
ruserok()
function takes a remote host's name, two user names, 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 writeable 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 user name are found in the
.rhosts file; otherwise
ruserok()
returns -1. If the local domain (as obtained from
getaddrinfo(3)) is the same as the remote domain, only the
machine name need be specified.
ruserok()
implicitly requires trusting the DNS server for the remote host's
domain.
DIAGNOSTICS
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() and
rresvport_af() functions return 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”.
SEE ALSO
ssh(1), intro(2), bindresvport(3), bindresvport_sa(3), rcmdsh(3)
HISTORY
These functions appeared in 4.2BSD.
The iruserok() and
iruserok_sa() functions, IP address based versions
of ruserok(), were removed in
OpenBSD 6.0.