The
rcmd() function is available for use by anyone to run commands on a remote system. It acts like the
orcmd() command, with the exception that it makes a call out to the
rcmd(1) command, or any other user-specified command, to perform the actual connection (thus not requiring that the caller be running as the super-user), and is only available for the “shell/tcp” port. The
orcmd() function is used by the super-user to execute a command on a remote machine using an authentication scheme based on reserved port numbers. While
rcmd() and
orcmd() can only handle IPv4 address in the first argument,
rcmd_af() and
orcmd_af() can handle other cases as well. The
rresvport() function returns a descriptor to a socket with an address in the privileged port space. The
rresvport_af() function is similar to
rresvport(), but you can explicitly specify the address family to use. Calling
rresvport_af() with
AF_INET has the same effect as
rresvport(). The
iruserok() and
ruserok() functions are used by servers to authenticate clients requesting service with
rcmd(). All six functions are present in the same file and are used by the
rshd(8) server (among others).
iruserok_sa() is an address family independent variant of
iruserok().
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.
rcmd_af() and
orcmd_af() take address family in the last argument. If the last argument is
PF_UNSPEC, interpretation of
*ahost will obey the underlying address resolution like DNS.
The protocol is described in detail in
rshd(8).
The
rresvport() and
rresvport_af() functions are 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 writable 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(3)) 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.
While
iruserok() can handle IPv4 addresses only,
iruserok_sa() and
ruserok() can handle other address families as well, like IPv6. The first argument of
iruserok_sa() is typed as
void * to avoid dependency between
<unistd.h> and
<sys/socket.h>.