To run ethereal, simply type "ethereal" into your shell control prompt. If you get "Command not found" in your shell, try /usr/bin/ethereal. To capture packets with an ethereal based firewall, you must have full root access to the machine that you probably do not have it on. So have created a wrapper for ethereal that captures packets on a Linux machine.
Ethereal works best when forwarding packets through several other ethereal protocols. This is because all the forwarded packets are captured and logged by the ethereal protocol analyzer. So, to test how well ethereal works on your OS, check the outages in the outbound traffic to your web server. If all the outbound connections are getting through successfully, then it is probably working as intended.
However, if the outbound packet capture rate goes down, then you should have to re-do something with your Firewall configuration or tweak your IP settings. If you have packet sniffers running on your system, then it may be best to turn them off to reduce the traffic passing through them. Ethereal does capture the ICMP echo header, so you should use some kind of protocol analyzer tool to discover what type of protocols are sniffing. To catch all types of packet capturing, including ICMP echo header, Ethereal will need to be configured and started on every machine that will be affected by Ethereal.