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RCMD(3) OpenBSD Programmer's Manual RCMD(3)
rcmd, rresvport, iruserok, ruserok - routines for returning a stream to a
rcmd(char **ahost, int inport, const char *locuser, const char *remuser,
const char *cmd, int *fd2p);
iruserok(u_int32_t raddr, int superuser, const char *ruser,
const char *luser);
ruserok(const char *rhost, int superuser, const char *ruser,
const char *luser);
The rcmd() function is used by the super-user to execute a command on a
remote machine using an authentication scheme based on reserved port num-
bers. 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 super-user, rcmd() will
invoke rcmdsh(3) to run the command via rsh(1). 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 au-
thenticate clients requesting service with rcmd(). All four functions
are present in the same file and are used by the rshd(8) server (among
The rcmd() function looks up the host *ahost using gethostbyname(3), re-
turning -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 re-
siding at the well-known Internet port inport. If the user is not the su-
per-user, 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 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. Note that if the user is not
the super-user, fd2p must be 0.
The protocol is described in detail in rshd(8).
The rresvport() function is used to obtain a socket with a privileged ad-
dress 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 super-user is allowed to bind an address of this sort to
a socket. rresvport() needs to be seeded with a port number; if that
port is not available it will find another.
The iruserok() and ruserok() functions take a remote host's IP address or
name, respectively, two user names and a flag indicating whether the lo-
cal user's name is that of the super-user. Then, if the user is not the
super-user, 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 super-user, 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 iruserok() and
ruserok() return -1. If the local domain (as obtained from
gethostname(3)) 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 serv-
er for the remote host's domain.
The rcmd() function returns a valid socket descriptor on success. It re-
turns -1 on error and prints a diagnostic message on the standard error.
The rresvport() function returns a valid, bound socket descriptor on suc-
cess. 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''.
rlogin(1), rsh(1), intro(2), bindresvport(3), rcmdsh(3), rexec(3),
rexecd(8), rlogind(8), rshd(8)
These functions appeared in 4.2BSD.
OpenBSD 2.6 June 4, 1993 2
Source: OpenBSD 2.6 man pages. Copyright: Portions are copyrighted by BERKELEY
SOFTWARE DESIGN, INC., The Regents of the University of California, Massachusetts
Institute of Technology, Free Software Foundation, FreeBSD Inc., and others.
(Corrections, notes, and links courtesy of RocketAware.com)
FreeBSD Sources for rcmd(3) functions
OpenBSD sources for rcmd(3)
Up to: Remote Process Communication - Methods of communicating to remote processes. Remote Procedure Calls, sockets, data format translation, et al
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