Back Doors

1) Run through existing software
Operating systems
Commercial software
Open Source Software
Spy ware

2) Different than vulnerability
Are intentionally put into software
Regularly used to spy or sabotage

3) Back-door patches can be applied to any software
And by anyone with administrator access to the machine

4) Detecting backdoors
Monitor traffic through a firewall
Monitor traffic with a packet sniffer
Restrict usage to a known good network
Format & install only trusted software
Determine protocols used by your ISP
Log abnormal activity
Monitor 24/7 in case of timed attack.

5) Run a test-case
Isolate traffic from outside servers.
Minimize software connecting to the internet.
Connect with another computer, running different programs.

6) Embedded Hardware
Your firewall may be vulnerable
Embedded hardware is more secure
Embedded network tools are available
Use existing Unix boot disk(s)
http://bengross.com/smallunix.htm