Routing Policies and Firewall Filters

• 1. Filter name: A unique name that identifies the filter.

• 2. Filter term: A set of match conditions and actions that are evaluated together.

• 3. Match condition: A statement that specifies the criteria that must be met for the filter term to be applied.

• 4. Action: A statement that specifies what to do if the match conditions are met. Actions can include accepting the traffic, discarding the traffic, or modifying the traffic.

• 5. Next term: A statement that specifies what to do if the match conditions are not met and the next term in the filter should be evaluated.

• 6. Filter chain: A set of filter terms that are evaluated in order until a match is found or all terms have been evaluated.

The filter structure allows you to specify complex processing rules for network traffic by chaining together multiple filter terms with specific match conditions and actions. The filter terms are evaluated in order until a match is found or all terms have been evaluated, and the appropriate action is taken based on the filter term that matches the network traffic. The filter chain structure allows you to control the flow of network traffic and ensure that only authorized traffic is allowed through the network.

Example Configuration:

Here's an example of a firewall filter in Junos:

filter example-filter {
    term allow-ssh {
        from {
            protocol tcp;
            destination-port 22;
        }
        then {
            accept;
        }
    }
    term deny-all {
        then {
            reject;
        }
    }
}

This filter, named "example-filter", contains two terms: "allow-ssh" and "deny-all". The first term, "allow-ssh", specifies a match condition that only allows TCP traffic to destination port 22 (i.e. SSH traffic). If this match condition is met, the traffic is accepted. The second term, "deny-all", does not specify any match conditions and serves as a catch-all for all remaining traffic. If the first term does not match, the traffic is rejected by the "deny-all" term.

In this example, the filter allows SSH traffic while blocking all other traffic. This is a simple example, but the filter structure and terms can be used to specify much more complex processing rules for network traffic.