31.2 Firewall Concepts

There are two basic ways to create firewall rulesets: “inclusive” or “exclusive”. An exclusive firewall allows all traffic through except for the traffic matching the ruleset. An inclusive firewall does the reverse. It only allows traffic matching the rules through and blocks everything else.

An inclusive firewall offers much better control of the outgoing traffic, making it a better choice for systems that offer services to the public Internet. It also controls the type of traffic originating from the public Internet that can gain access to your private network. All traffic that does not match the rules, is blocked and logged by design. Inclusive firewalls are generally safer than exclusive firewalls because they significantly reduce the risk of allowing unwanted traffic to pass through them.

Note: Unless noted otherwise, all configuration and example rulesets in this chapter, create inclusive type firewalls.

Security can be tightened further using a “stateful firewall”. This type of firewall keeps track of which connections are opened through the firewall and will only allow traffic through which either matches an existing connection or opens a new one. The disadvantage of a stateful firewall is that it can be vulnerable to Denial of Service (DoS) attacks if a lot of new connections are opened very fast. With most firewalls it is possible to use a combination of stateful and non-stateful behavior to make an optimal firewall for the site.