Packet Filtering Concept Dashboard

Core Concept & Walkthrough

What Packet Filtering Means

Packet filtering checks packet-header fields against firewall rules and decides PASS, BLOCK, or FLAG.

Step-By-Step Logic

Receive packet at inspection node.
Read fields: source, destination, protocol/next-header, TTL/hop-limit.
Evaluate conditions (Exact, GT, LT, Between, Bit).
Resolve action priority: BLOCK > FLAG > PASS.

Mini Input Example

Rule A: BLOCK Source=2001:db8:1::10. Rule B: FLAG Next Header=UDP.

P1 (source 2001:db8:1::10, TCP) => BLOCK
P2 (source 2001:db8:1::11, UDP) => FLAG
P3 (source 2001:db8:1::11, TCP) => PASS

Purpose & Real Use

Enforce security policy and limit unauthorized traffic.
Enterprise and campus firewalls
Cloud virtual network security groups
ISP and carrier edge filtering
Linux hosts with netfilter/nftables

Key Parameters

Source and destination IP addresses
Protocol (IPv4) or Next Header (IPv6)
TTL (IPv4) or Hop Limit (IPv6)
Traffic Class and Flow Label (IPv6)
Rule operators and AND/OR logic
Rule action: PASS / BLOCK / FLAG

Video Explainer

How Packet Filtering Works In Firewalls

https://www.youtube.com/watch?v=kDEX1HXybrU

References & Extended Reading

Textbook Reference

Kurose & Ross, Computer Networking: A Top-Down Approach (8th Ed), topic portal:https://gaia.cs.umass.edu/kurose_ross/index.php

Official Technical Sources

IETF RFC 8200 (IPv6):https://www.rfc-editor.org/rfc/rfc8200
Linux netfilter project:https://www.netfilter.org/
NIST Cybersecurity guidance:https://csrc.nist.gov/

Educational Sources

Quick overview: Packet filtering:https://en.wikipedia.org/wiki/Packet_filter