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From: securitylist at (Abe Usher)
Subject: Sending remote procedure calls through e-mail (RPC-Mail)

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 <> provides an 
interesting network authentication mechanism for establishing a 
connection to a networked computer that has no open ports (as advertised 

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
(4) When 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:

Abe Usher, CISSP

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