Introduction to troubleshooting Linux firewalls
We believe it's important to discuss the topics of security and risk management, as they relate to firewalls, before we can jump into the process of troubleshooting. After all, your firewall problem might be something fundamental, as opposed to just a misconfiguration that's affecting your security in a very negative way. We strongly believe that you cannot manage a firewall without first thoroughly understanding risk management and computer security.
By Scott Shinn and Michael Shinn at InformIT.
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