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Ncat proposal: new environment variables
From: Jacek Wielemborek <wielemborekj1 () gmail com>
Date: Wed, 10 Jul 2013 16:49:22 +0200

Hi guys,

During the development of the coming --lua-extensions Ncat switch,
David came up with the idea of transforming some of the functionality
that was starting to form. Instead of making it some Lua API that's
inaccessible to --sh-exec and --exec scripts, he proposed to add a few
environment variables so that not only Lua programs could take
advantage of that, but every program ran with the --exec command-line.

There are two types of proposed environment variables. First of them
is associated to connection parameters - the most obvious of which
would be the remote address. The advantage of that would be that the
scripts ran by Ncat could know the IP address/domain name of the
server they're talking to. For example, a script talking to the HTTP
server could alter its "Host: " header according to the target domain
- see example:

ncat  --sh-exec 'echo -e "OPTIONS / HTTP/1.1\r\nHost:
$NCAT_HOST\r\n\r\n" ; cat /dev/stdin > /dev/stderr' insecure.org 80

In the listen mode, NCAT_HOST could be set to the incoming
connection's address. There could be probably more stuff a script
could be interested in though - for example, whether it's running over
SSL or not. For example, a tiny web server could use this information
while generating links to decide whether the URL's should start with
"http" or "https".

The second group of the environment variables I'd like to suggest
would be currently made of file descriptors to two special Ncat
streams - ncatin and ncatout. ncatin is basically Ncat's standard
input and ncatout's for stdout. This would allow your scripts to not
only control the connection, but also the terminal. I believe it would
be especially useful when used in a pipe of chains, for example like

some_command | ncat --sh-exec "some-wrapper" example.com > output.txt

The some_wrapper could for example implement the websocket protocol -
one of the jobs that was originally meant for Ncat Lua scripts.

So, what do you think of these environment variables? Would you use them?

Jacek Wielemborek
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