RCMD(3) BSD Programmer's Manual RCMD(3)
rcmd, rresvport, ruserok - routines for returning a stream to a remote
rcmd(char **ahost, int inport, const char *locuser, const char *remuser,
const char *cmd, int *fd2p);
iruserok(u_long 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. The rresvport() function returns 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(). All
three functions are present in the same file and are used by the rshd(8)
server (among others).
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 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.
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
The iruserok() and ruserok() functions take a remote host's IP address or
name, as returned by the gethostbyname(3) routines, two user names and a
flag indicating whether the local 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
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 re-
mote user name 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.
The iruserok() function is strongly preferred for security reasons. It
requires trusting the local DNS at most, while the ruserok() function re-
quires trusting the entire DNS, which can be spoofed.
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), rexec(3), rexecd(8), rlogind(8),
These functions appeared in 4.2BSD.
4.2 Berkeley Distribution June 4, 1993 2