A firewall can implement security rules that dictate specific rules for the network and the Internet. However, several different methods exist to implement these rules.
First, security rules can be defined in a filter configuration file in the network interface card (NIC) firmware, or in a firewall control program.
Next, security rules can be implemented using static rules in the firewall configuration file.
Lastly, security rules can be enforced by dynamically loading dynamic rules and storing them in the file hierarchy at the application level.
Most web application firewall frameworks require a comprehensive set of security rules and allow the firewall to manage network and user access. For example, a web application firewall filter configuration must be created so that all user’s actions are blocked and access denied to web application resources that the security rule specifies, and there are services like Fortinet which can help business understand this rules and apply them.
The source code for this example firewall firewall, available here.
The First Filter in the Firewall
Before the firewall can apply security rules, a rule must be defined. The firewall filter configuration file must define all security rules needed. This configuration can be seen here.
For the web application that we are creating, let’s define the initial firewall rule:
allow localhost:8000 inbound HTTP request access from “firstname.lastname@example.org” and any matching host.
The rule uses the common definition notation for a configuration file, where all of the file’s fields are numbered starting from 0. In this case, the rule is the second rule in the file, as we can see in the definition statement. We also defined the source, action, path, and argument types. In the example, we have defined the host attribute to match the host domain given as a parameter in the path, and we have added a url parameter to match any URL.
This rule allows both TCP and HTTP traffic, so it will filter out incoming connections to the example.com host. This rule blocks TCP and HTTP traffic on the local port 8000 from a local host called “email@example.com”. In other words, the firewall will match incoming requests to the above host, and block access to all HTTP resources.
The firewall defines a number of actions to apply to each individual request. In this example, we defined the path value to match the entire host, and the inbound action to deny HTTP requests from the whole host. We also define a number of protocol and source ports to match the HTTP protocol requests from the example.com domain. We will discuss these ports in more detail later.
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The next rule in the file applies a rule of the same form to the incoming requests for all ports on all interfaces. In this case, we defined both the source port and protocol to match all incoming requests on all interfaces, and source port of the initial connection.
The filter provides for the future implementation of this firewall rule by adding multiple rules. For example, let’s create a rule to allow HTTP requests on all interfaces on the LAN and all interfaces on the Internet:
# Define the inbound rule to match incoming HTTP requests from port 443 on interface eth0 to any matching host. from filter 0 action accept on filter 0 return status 200 allow tcp from all any to 443 match host address “all” port 443 protocol tcp match path “/” interface eth0/0/1/0 match transport all
The firewall filter configuration file can now be looked at and created by a web browser as a web page containing all of the required firewall rules.