rcmd, rresvport, ruserok - routines for returning a stream to a
remote command

int rcmd(ahost, inport, locuser, remuser, cmd, fd2p);
char **ahost;
int inport;
const char *locuser, *remuser, *cmd;
int *fd2p;

int rresvport(port);
int *port;

int ruserok(rhost, superuser, ruser, luser);
const char *rhost;
int superuser;
const char *ruser, *luser;

Rcmd is a routine used by the super-user to execute a command on a
remote machine using an authentication scheme based on reserved
port numbers. Rresvport is a routine which returns a descriptor to
a socket with an address in the privileged port space. Ruserok is
a routine 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).

Rcmd 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 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 routine is used to obtain a socket with a privileged
address bound to it. This socket is suitable for use by rcmd and
several other routines. 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.

Ruserok takes a remote host’s name, as returned by a
gethostbyaddr(3) routine, two user names and a flag indicating
whether the local user’s name is that of the super-user. It then
checks the files /etc/hosts.equiv and, possibly, .rhosts in the
local user’s home directory to see if the request for service is
allowed. A 0 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
superuser flag is 1, the checking of the "hosts.equiv" file is
bypassed. If the local domain (as obtained from gethostname(2)) is
the same as the remote domain, only the machine name need be

rlogin(1), rsh(1), intro(2), rexec(3), rexecd(8), rlogind(8),

Rcmd returns a valid socket descriptor on success. It returns -1
on error and prints a diagnostic message on the standard error.

Rresvport 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."