Junos Enterprise Routing, 2nd Edition

Firewall Filters

To protect the router, you can deploy packet filters to allow only certain traffic into the router’s control plane (Routing Engine [RE]). These filters have different names on each router OS, but they still operate in the same stateless manner. On a Cisco device, these filters are called access lists, and on a Juniper router, they are called firewall filters. These filters look similar to the policy we discussed in Chapter 5; however, filters operate on the actual data-forwarding plane. Table 8-2 provides a comparison of the two features.

Table 8-2. Firewall filters versus routing policies

Feature	Firewall filter	Routing policy
Operates in...	Forwarding plane	Control plane
Match keyword	from	from
Action keyword	then	then
Match attributes	Packet fields	Route attributes
Default action	Discard	Depends on default policy
Applied to...	Interfaces	Routing protocols/tables
Named terms required	Yes	No
Chains allowed	Yes	Yes
Absence of from statement	Match all	Match all

Firewall filter syntax takes a human-friendly, intuitive form:

firewall {
    family inet {
        filter filter-1 {
            term term-1 {
                from {
                    protocol tcp;
                    destination-port telnet;
                }
                then {
                    accept;
                }
            }
        }
    }
}

This filter matches on Telnet traffic and accepts the packets. As observed, the syntax is very similar to a routing policy with the match conditions in the from term and the actions specified in a then term.

Filter Processing

Similar to a policy, a filter is made up of multiple terms, and each term is examined in the order listed. If there is a match in ...