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 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 super-user
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 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
Pa hosts.equiv
file, or the host and remote user name are found in the
Pa .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.