The Fn rcmd function looks up the host Fa *ahost using gethostbyname(3), returning -1 if the host does not exist. Otherwise Fa *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 Fa 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 Fa 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 Fa *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 Fa 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 Fn rresvport function is used to obtain a socket with a privileged address bound to it. This socket is suitable for use by Fn 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 Fn iruserok and Fn 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 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 Fn iruserok and Fn 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, Fn iruserok should be used in preference to Fn ruserok , as it does not require trusting the DNS server for the remote host's domain.
The Fn 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.''