rcmd, rresvport, ruserok - routines for returning a stream to a remote
int rcmd(ahost, inport, locuser, remuser, cmd, fd2p)
unsigned short inport;
char *locuser, *remuser, *cmd;
ruserok(rhost, super-user, ruser, luser)
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(8C) server (among others).
rcmd() looks up the host *ahost using gethostbyname (see gethos-
tent(3N)), 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 its standard input (file descriptor 0) and standard output (file
descriptor 1). If fd2p is non-zero, then an auxiliary channel to a con-
trol process will be set up, and a descriptor for it will be placed in
*fd2p. The control process will return diagnostic output from the com-
mand (file descriptor 2) on this channel, and will also accept bytes on
this channel as signal numbers, to be forwarded to the process group of
the command. If fd2p is 0, then the standard error (file descriptor 2)
of the remote command will be made the same as its 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-
The protocol is described in detail in rshd(8C).
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
(see gethostent(3N)) 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 /etc/hosts.equiv
file, or the host and remote user name are found in the .rhosts file;
otherwise ruserok() returns -1. If the super-user flag is 1, the
checking of the /etc/hosts.equiv file is bypassed.
rlogin(1C), rsh(1C), intro(2), gethostent(3N), rexec(3N), rexecd(8C),
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."
22 November 1987 RCMD(3N)