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3.10 What is a `single point of failure', and how do I avoid having one?

  An architecture whose security hinges upon one mechanism has a single point of failure. Software that runs bastion hosts has bugs. Applications have bugs. Software that controls routers has bugs. It makes sense to use all of these components to build a securely designed network, and to use them in redundant ways.

If your firewall architecture is a screened subnet, you have two packet filtering routers and a bastion host. (See question 3.2 from this section.) Your Internet access router will not permit traffic from the Internet to get all the way into your private network. However, if you don't enforce that rule with any other mechanisms on the bastion host and/or choke router, only one component of your architecture needs to fail or be compromised in order to get inside. On the other hand, if you have a redundant rule on the bastion host, and again on the choke router, an attacker will need to defeat three mechanisms.

Further, if the bastion host or the choke router needs to invoke its rule to block outside access to the internal network, you might want to have it trigger an alarm of some sort, since you know that someone has gotten through your access router.

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