What is a packet filtering firewall and where does it fall short?

A packet filtering firewall decides whether to pass or drop each packet by checking its source and destination address, protocol and port against a ruleset. It does not track sessions or read content. That makes it fast and simple, and it also means it cannot spot attacks hidden inside allowed traffic.

What does it inspect?

NIST SP 800-41 Rev. 1 describes the packet filter as the most basic feature of a firewall. Working at the network layer, it makes decisions from a short list of fields:

• the source IP address
• the destination IP address
• the network or transport protocol, such as TCP, UDP or ICMP
• possibly the source and destination ports of the session
• the interface the packet crosses and its direction

NIST adds that packet filters are not concerned with the content of packets, and that their behavior is governed by a ruleset. The most common example of a pure packet filtering device, it notes, is a router using access control lists.

Why is it called stateless?

NIST also calls these devices stateless inspection firewalls. They do not keep track of the state of each flow, so they cannot associate multiple requests within one session. Every packet is judged alone.

Compare stateful inspection, which NIST describes as tracking connections in a state table, usually with source and destination addresses, ports and connection state, and blocking packets that deviate from the expected state. NIST observes that packet filtering sits at the core of most modern firewalls, but few firewalls sold today do only stateless filtering.

What is it good at?

• Speed. Checking a few header fields is cheap, so it adds little delay.
• Simplicity. The rules are easy to read and audit.
• Availability. Routers and operating systems include it already.
• Coarse control. Blocking whole address ranges or unused ports is exactly its job.

Where does it fall short?

Because it ignores state, a rule that allows replies must be broad enough to let any packet through that looks like a reply, which an attacker can imitate. Because it ignores content, it cannot see that an allowed web request carries an attack. And a ruleset grows with every exception, so long lists get hard to reason about.

Constructed example: the reply problem

An office wants staff to browse the web. The network team writes two stateless rules: allow outbound traffic to port 443, and allow inbound traffic from port 443 back to the office range. The second rule is the weak one. Anything outside that sets its source port to 443 can send packets to internal hosts, whether or not an internal user asked for them. A stateful firewall would admit only packets that belong to a connection the inside host opened. This scenario is constructed, but it is the textbook reason stateful inspection replaced stateless filtering at the edge.

Where does it still make sense?

As a first, cheap layer. Use it on router interfaces to drop traffic from address ranges that should never appear, to restrict management ports to a few administrator addresses, and to apply ingress and egress filtering. NIST describes ingress filtering as filtering inbound traffic and egress filtering as filtering outbound, for example blocking internal machines from launching denial of service traffic outward or using external file transfer servers.

Treat it as a base and add stateful inspection, an application aware proxy or a web application firewall where you need to see more.

How to check or apply this

1. List your ruleset in order and note what each rule is for.
2. End with an explicit default deny.
3. Add egress rules, not only ingress ones.
4. Replace broad "established" style rules with stateful rules where your device supports them.
5. Remove rules with no owner or no hits in your logs.
6. Review the ruleset when a system is retired or an address range changes.
7. Test changes in a lab or at a quiet time, with a rollback plan.

Quick check

Sources

• NIST SP 800-41 Rev. 1, Guidelines on Firewalls and Firewall Policy

Last reviewed: 18 October 2026.