What is the default action for packets not matching a firewall filter?
What is the default action for packets not matching a firewall filter?
The default action for packets not matching a firewall filter is to silently discard them. Firewalls generally follow a policy where any packet that does not explicitly match a rule in the filter is denied access as a security measure to prevent unwanted or malicious traffic from passing through. This ensures only explicitly allowed traffic moves forward, maintaining network security.
Ref: Understanding How Firewall Filters Are Evaluated | Junos OS | Juniper Networks "Understanding How Firewall Filters Are Evaluated ... When a firewall filter consists of more than one term, the firewall filter is evaluated sequentially: 1. ... 2. ... 3. ... 4. If a packet passes through all the terms in the filter without a match, the packet is discarded. ..."
Consequently, if a packet passes through all the terms in a filter without matching any conditions, the packet is discarded. If you configure a firewall filter that has no terms, all packets that pass through the filter are discarded. https://www.juniper.net/documentation/us/en/software/junos/routing-policy/topics/concept/firewall-filter-ex-series-evaluation-understanding.html
B. The packet will be accepted. Explanation: In Junos OS, the default action for packets not matching any term in a firewall filter is to accept the packet. If you want to specify a different action for unmatched packets, you can add a catch-all term at the end of the filter with the desired action (e.g., discard, reject, or a custom action).
what is the point of have a firewall then if you accept all other traffic?
B. The default action for packets not matching a firewall filter is to be accepted. If a packet does not match any of the defined filter rules, it is allowed to pass through the firewall.
In JUNOS, the default action for packets that do not match any firewall filter rule is to deny the packets. This means that if there is no explicit rule that permits the traffic, the default behavior is to block it. In JUNOS firewall filters, the last rule in the filter is typically an implicit "deny" rule. This rule denies any packet that doesn't match any of the preceding rules. Here's an example of a simple JUNOS firewall filter: