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OCR H446 · Component 1 · 1.3.4

Firewalls, Proxies
& Internet Security

OCR A Level Computer Science · cszone.co.uk
H446 SpecA Level
Learning Objectives

By the end of this topic you will be able to:

Explain how firewalls work: packet filtering, stateful inspection, application-level gateways
Describe the role of proxy servers
Explain the purpose of VPNs
Understand how HTTPS and TLS secure web communication
Firewalls

Firewalls

A firewall monitors and controls incoming and outgoing network traffic based on security rules. It acts as a barrier between a trusted internal network and untrusted external networks (internet).
Packet filtering: examines packet headers (source IP, destination IP, port, protocol). Allows or blocks based on rules. Fast but cannot inspect packet content — cannot detect malicious payloads in allowed traffic.
Stateful inspection: tracks the state of active connections. Only allows return traffic for established sessions initiated from inside. More secure than simple packet filtering.
Application-layer gateway (proxy firewall): inspects traffic at the application layer. Can understand HTTP, FTP etc. and block specific content. Slowest but most thorough.
Proxy Servers

Proxy Servers

A proxy server acts as an intermediary between clients and the internet. The client sends requests to the proxy, which forwards them on behalf of the client. The origin server sees the proxy's IP, not the client's.
Benefits
Anonymity: hides internal client IP addresses from external servers
Caching: frequently requested pages are cached — reduces bandwidth and improves response time
Content filtering: can block access to specific URLs or categories (schools, businesses)
Forward vs Reverse
Forward proxy: sits in front of clients — used by organisations to control outbound access
Reverse proxy: sits in front of servers — distributes load, provides SSL termination, hides backend servers
VPN and HTTPS

VPNs and HTTPS/TLS

VPN (Virtual Private Network): creates an encrypted tunnel between the client and a VPN server. All traffic is encrypted end-to-end. Allows secure remote access to corporate networks and masks the user's real IP. ISP sees only encrypted traffic to the VPN server.
HTTPS / TLS: TLS (Transport Layer Security) encrypts the HTTP connection between browser and web server. Uses asymmetric encryption to exchange keys, then symmetric encryption for data. Certificate from a CA authenticates the server's identity.
Difference: HTTPS encrypts one specific connection to a website; a VPN encrypts all network traffic from the device, routing it through the VPN server before reaching any destination.
Exam Practice
OCR H446 Style · 4 marks
Describe two ways a proxy server can improve the security and efficiency of a corporate network.
[4 marks]
2
Content filtering: the proxy can inspect URLs and block access to malicious or inappropriate websites, preventing users from visiting harmful domains or downloading malware. This reduces the attack surface without needing individual device configuration.
2
Caching: the proxy stores local copies of frequently accessed web pages. When multiple users request the same resource, the proxy serves the cached version rather than fetching from the internet — reducing bandwidth consumption and improving response time.
Common Mistakes

Don't Lose Marks

!
Saying a firewall encrypts traffic — a firewall filters/blocks traffic based on rules; it does not encrypt it. Encryption is provided by TLS/HTTPS or a VPN. These are different security mechanisms.
!
Saying a proxy server makes you anonymous — a proxy hides your IP from the destination server, but the proxy itself logs all your traffic. True anonymity requires more layers. VPNs also don't make you fully anonymous.
!
Confusing stateful and stateless inspection — stateless (packet filtering) checks each packet independently with no memory of previous packets; stateful inspection tracks connection state and knows which packets belong to established sessions.
1.3.4d Complete
Well done! ✓
Firewalls, Proxies and Internet Security
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