29.2. Firewall Concepts

A firewall ruleset can be either exclusive or inclusive. An exclusive firewall allows all traffic through except for the traffic matching the ruleset. An inclusive firewall does the reverse as it only allows traffic matching the rules through and blocks everything else.

An inclusive firewall offers 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 a private network. All traffic that does not match the rules is blocked and logged. Inclusive firewalls are generally safer than exclusive firewalls because they significantly reduce the risk of allowing unwanted traffic.

Note:

Unless noted otherwise, all configuration and example rulesets in this chapter create inclusive firewall rulesets.

Security can be tightened further using a stateful firewall. This type of firewall keeps track of open connections and only allows traffic which either matches an existing connection or opens a new, allowed connection. 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. Most firewalls use a combination of stateful and non-stateful behavior.

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