A packet-filtering firewall operates at the Network layer (Layer 3) of the OSI model. It examines the header of each packet passing through it and makes a filtering decision based on a set of rules. These rules typically use information found in the IP header (source and destination addresses) and the TCP/UDP header (source and destination ports), which corresponds to the Transport layer (Layer 4). This type of firewall is stateless, meaning it treats each packet as an isolated event without considering the context of previous packets.

A. Circuit-level gateway: This firewall operates at the Session layer (Layer 5). It validates TCP handshakes but does not inspect the actual packet contents or headers beyond establishing the session.

B. Application gateway: This firewall, also known as a proxy firewall, operates at the Application layer (Layer 7). It understands application-specific protocols and can filter traffic based on content.

C. Proxy server: This is a general term for an intermediary server, and in the context of firewalls, it typically refers to an application-level gateway that operates at the Application layer (Layer 7).

1. National Institute of Standards and Technology (NIST). (2009). Guidelines on Firewalls and Firewall Policy (Special Publication 800-41 Revision 1).

Section 2.1.1

"Packet Filter Firewalls

" Page 2-2: "Packet filter firewalls work at the network layer of the Open Systems Interconnection (OSI) model... Packet filters can filter traffic based on the packet header information: source and destination IP addresses

source and destination ports

and the protocol type (e.g.

TCP

UDP

ICMP)."

2. Stallings

W.

& Brown

L. (2018). Computer Security: Principles and Practice (4th ed.). Pearson.

Chapter 20.2

"Firewall Characteristics and Access Policy

" Section "Types of Firewalls": Describes packet filtering firewalls as applying a set of rules to each incoming and outgoing IP packet

making decisions based on fields in the IP and transport (TCP or UDP) headers

including source/destination IP addresses and port numbers.

3. Al-Shaer

E.

& Hamed

H. (2004). A Survey on Firewall Technologies. IEEE Communications Surveys & Tutorials

6(1)

24-41.

Section II.A

"Packet Filtering Firewalls

" Page 26: "Packet filtering firewalls operate at the network layer of the OSI model... The filtering decision is based on the information contained in the packet header

such as source and destination IP addresses

source and destination ports..." (DOI: https://doi.org/10.1109/COMST.2004.5342224)