[NTLUG:Discuss] Getting out past a firewall easiest

[Jack Snodgrass]

On Thu, 05 Jun 2003 08:36:46 -0700, Kipton Moravec wrote:

> I am developing a system that will be a sophisticaded appliance for certain 
> businesses.
> 
> Part of the system monitors other equipment and if an error occurs sends 
> some sort of message to a server on the Internet.  In addition I want it to 
> periodically check in to get software updates automatically.
> 


First of all, make sure that the company that you are doing 
work for knows this and agrees to this. If you were an evil 
person, you could do a lot of harm to their internal network 
if they allow you to do this and you wanted to do evil deeds. 
If you try and do this without them agreeing up front, you
may be in for some serious trouble. 


Now that that is said... you don't know what ports will be 
open for external connections. Some companies let pretty
much any outgoing connections. Other companies block pretty
much everything. 

You'll need to design your software so that it can make an 
outgoing socket connection via different ports. It will have
to try them out until it gets a good connection. Don't assume
that everyone has port 80 open ( web ) becuase they may make 
their users use a proxy server and only the proxy server 
can access port 80 outside of the company. Many companies won't
let users send email ( port 25 ) to anything other than the 
local mail server that can then send mail via port 25. 

On your server side, you'll have to set up your server so 
that it listens to multiple ports, detects an incomming 
connection from your client and is able to handle it on 
any port. 

jack