Re: How to make a "telnetd" using tcpsvd and bash ?

[Laurent Bercot <ska-supervision@skarnet.org>]

> During the preparation of some material that should try to explain my
> collegues about the wonderful world of easy daemon-making using either
> "runit/ipsvd" or "daemontools/ucspi-tcp" I wanted to make a "sample telnetd"
> using runsv, tcpsvd and bash...
> 
> That sounds easy and I have it almost working, except that all of my typing
> on the terminal goes into the log instead of going back to the termainal the
> same goes for the "prompt" showing working-dir and username...

 The problem is that the shell on the server is not running in a terminal.
telnetd (and sshd), when used interactively, creates a pty, and runs its
child in it, so that the child runs exactly as if it wasn't invoked by
some network server.
 Telnet-like protocols in interactive sessions are not so easily faked
by just a tcpserver and a shell: they do something more, i.e. pseudo-
terminal allocation and handling. This is one of the hairiest and
ugliest parts of Unix - welcome to the machine.

 I had a try at it some time ago, using DJB's ptyget package to perform
the server-side pseudo-terminal work. It kind of worked, but wasn't in
the least cool, maintainable nor portable. Besides, I had no client
support (you need to put the client terminal in raw mode, transmit
SIGWINCH information, and so on). I gave up. But I still have the idea
in the back of my mind.
 If you want to have a go at it, the ttymodes library, from skalibs
(http://www.skarnet.org/software/skalibs/libttymodes.html), inspired
from the ptyget package, may help you. Good luck, Jim.

-- 
 Ska