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rcmd, rresvport, iruserok, ruserok - routines for returning a stream to a
#include <netdb.h> /* Or <unistd.h> on some systems */
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 superuser to execute a command on a
remote machine using an authentication scheme based on reserved port
numbers. 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 authenticate clients requesting service
with rcmd(). All four 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),
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() function is 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 1023. Only the superuser is allowed to bind an address of
this sort to a socket.
The iruserok() and ruserok() functions take a remote host’s IP address or
name, respectively, 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 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.
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
server for the remote host’s domain.
The rcmd() function returns a valid socket descriptor on success. It
returns -1 on error and prints a diagnostic message on the standard
The rresvport() function 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.’’
rlogin(1), rsh(1), intro(2), rexec(3), rexecd(8), rlogind(8), rshd(8)
These functions appeared in 4.2BSD.