From: Abe Usher (securitylistsharp-ideas.net)
Date: Tue Oct 19 2004 - 21:27:38 CDT

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Have you ever had the need to remotely send a command to a system, but
you could not access it directly via ssh or telnet because the firewall
is blocking all inbound connections?

The practice of portknocking <http://www.portknocking.org/> provides an
interesting network authentication mechanism for establishing a
connection to a networked computer that has no open ports (as advertised
on portknocking.org).

While I find portknocking ingenious, it is somewhat cumbersome and
overly complex for most users. I propose an alternative - send remote
procedure calls via e-mail. I've coded an application that fits the
bill: RPC-Mail.

The premise of RPC-Mail is simple:
(1) Construct an e-mail message that has a command that you want one of
your remote PCs to execute.
(2) Send the e-mail to a special account that is only used by RPC-Mail.
(3) Have the remote PC set up with a scheduled task or cron job to
periodically execute the application RPC-Mail.py.
(4) When RPC-Mail.py executes, it parses all of the subject lines and
message bodies of e-mail messages that it finds. If the message body
contains a special passphrase, RPC-Mail executes the subject line as a
command, and returns standard output as an e-mail message.

For more information check out my full write up on:
http://www.sharp-ideas.net/

Cheers,
Abe Usher, CISSP

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