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The Book of PF by Peter N.M. Hansteen

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The Bridging Firewall

An Ethernet bridge consists of two or more interfaces that are configured to forward Ethernet frames transparently and are not directly visible to the upper layers, such as the TCP/IP stack. In a filtering context, the bridge configuration is often considered attractive because it means that the filtering can be performed on a machine that does not have any IP addresses of its own. If the machine in question runs OpenBSD or a similarly capable operating system, it is still able to filter and redirect traffic.

The main perceived advantage of such a setup is that attacking the firewall itself is more difficult. The disadvantage is that all admin tasks must be performed at the firewall's console, unless you configure a network ...

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