The firewall features that matter most are stateful inspection, a well maintained default deny rule set, logging, and application awareness. Deep packet inspection, TLS decryption and intrusion prevention add depth but cost performance and operational effort, so enable them where the risk justifies it.
How did firewall features grow?
NIST SP 800-215 traces the evolution in feature sets. Packet filters and network address translation monitor and control packets at a network interface and apply preset rules. Stateful inspection, also known as dynamic packet filtering, monitors connection state and decides which packets belong to a known active connection. Deep packet inspection examines the content of packets, header and payload, which lets it find hidden threats in a data stream such as exfiltration attempts, content policy violations and malware. Threat detection and response features gather data across packets and sessions to find incidents.
The same document notes that a next-generation firewall adds application data awareness, looking at layer 7 as well as layers 3 and 4, and can include TLS decryption, intrusion prevention and deployment as an appliance, VM or cloud service.
What is the core you cannot skip?
Default deny. NIST SP 800-41 says deny by default policies should be used for incoming TCP and UDP traffic, and notes that outgoing policies are usually less strict because users reach a wide range of external hosts. A firewall with a permissive rule set has the features but not the effect.
Stateful inspection. It is the baseline for most boundaries, as discussed in NIST's guidance, because it drops packets that do not belong to a tracked connection.
Logging. Without logs you cannot tune rules, investigate or prove what happened.
Management. Rules need owners, reviews and a change process. Old rules accumulate quietly and widen exposure.
Which features are situational?
Deep packet inspection and application control. Useful when you need to block a file type, command or application over a common port. NIST SP 800-41 describes application firewalls that can recognize unexpected command sequences and enforce input checks.
TLS decryption. Needed for inspection of encrypted traffic, but it introduces key handling, privacy and performance questions, and some applications break when intercepted.
Intrusion prevention. Valuable in front of exposed services, but needs tuning to avoid blocking legitimate traffic.
VPN support. SP 800-41 discusses VPNs alongside firewalls, since encapsulated traffic can interact with filtering. Consider whether remote access belongs on the same device as your perimeter filtering.
Host based firewalls. The same guidance describes host based and personal firewalls, which protect a single machine wherever it sits.
One more feature deserves attention: central management. If you run several firewalls, a single place to define policy, push changes and compare configurations prevents drift, where each device slowly differs from the others. It also gives auditors one place to see who changed what and when.
A constructed example
A clinic with 40 staff buys a firewall with every feature enabled. The device slows down at peak, and the administrator turns off inspection to fix it, never noticing that an any-to-any outbound rule was never removed. A better plan would have been a simple stateful device with default deny inbound, outbound limited to what each system needs, such as an allow list of destinations for the billing system, and application inspection reserved for the public appointment portal.
How to check or apply this
- Write your policy as allowed flows: who may talk to what, on which ports.
- Set default deny inbound, tighten outbound where you can, then add only the allowed flows.
- Confirm stateful inspection is on and logs are shipped off the device.
- Decide where deeper inspection is worth its cost, usually in front of public services.
- Test performance with inspection enabled under peak load.
- Review rules every quarter and remove ones with zero hits.
- Add host firewalls on laptops and servers.
Quick check
Is a next-generation firewall always better?
No. It does more, but more features means more tuning and more to go wrong. A well run stateful firewall can beat a poorly run feature rich one.
Which single setting gives the most benefit?
Default deny for inbound traffic with a short, reviewed allow list, and outbound rules tightened wherever the business allows.
Sources
- NIST SP 800-215, Guide to a Secure Enterprise Network
- NIST SP 800-41 Rev. 1, Guidelines on Firewalls and Firewall Policy
Last reviewed: 18 October 2026.
