Rsh And Rlogin

> Why does: rlogin -l user host generate two processes (on the
> caller's machine), while: telnet host only generates one?

Rlogin uses one process (parent) to read data from your terminal, and other process (child) to write it to the network. Child and parent exchange SIGUSR and SIGUSR1 in funny ways to syncronize this.

You might want to take a look at ftp://ftp.FreeBSD.org/pub/FreeBSD/FreeBSD-stable/src/usr.bin/rlogin/rlogin.c for details on how this is accomplished.

Telnet doesn't do this. It calls select() to operate on more than one fd. Both ways of doing it are “valid” and “good.” The approach with two processes seems (to me) easier to program and easier to write in a portable manner. Many other utilities (e.g., such as “tip” and “cu”) fork a child to talk to “the other end.” Many more use select() and suffer or require Autoconf if they had to call poll() (on systems without select()).

The source code for telnet can be found in ftp://ftp.FreeBSD.org/pub/FreeBSD/FreeBSD-stable/src/usr.bin/telnet/