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Absolute OpenBSD, 2nd Edition by Michael W. Lucas

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Firewalls

The word firewall has been tortured beyond recognition over the past 20 or so years, until it has ceased to mean much of anything in particular. In general, a firewall sits between a private and public network, and controls the traffic between the two.

You can buy a firewall for your cable modem for under $100, and you can purchase an enterprise firewall cluster for $1 million. What’s the difference? They’re all firewalls, much as rats and cats and elephants are all mammals, but some are welcome in your home and most are not.[45] Which you permit, of course, is your personal preference. And firewalls are much the same.

Some firewalls filter application-level traffic. Some only filter based on protocol or ports. Some firewalls inspect ...

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