Firewall
A network security device that monitors and controls traffic based on security rules.
| Type | How it works | Speed |
|---|---|---|
| Packet filter | Examines IP headers (source/dest IP, port) | Fast |
| Stateful | Tracks connection state (allows return traffic) | Moderate |
| Application (L7) | Inspects payload content (e.g., SQL injection) | Slowest but safest |
Default rule: block all inbound traffic. Allow only specific ports (80, 443) for web servers.
Proxy Servers
An intermediary between clients and servers.
| Type | What it hides | Use case |
|---|---|---|
| Forward proxy | Client identity | Company: all employees browse through it; content filtering, caching, anonymity |
| Reverse proxy | Server identity | Nginx/haproxy in front of web servers: load balancing, SSL termination, caching |
Load Balancer
Distributes incoming traffic across multiple backend servers.
| Algorithm | How it works |
|---|---|
| Round Robin | Each server gets a turn in sequence |
| Least Connections | Send to server with fewest active connections |
| IP Hash | Consistent server based on client IP (session persistence) |
| Weighted | Servers with higher capacity get more traffic |
Benefits: High availability (if one server fails, traffic goes to others), scalability (add more servers), maintenance without downtime.
CDN (Content Delivery Network)
- Distributed servers that cache content geographically close to users
- Static content: images, CSS, JS, video
- Dynamic content acceleration: optimized routes back to origin
- DDoS protection: absorbs attack traffic across the distributed network
- Examples: Cloudflare, Akamai, Fastly
VPN (Virtual Private Network)
Encrypted tunnel between client and a remote network. Extends a private network across a public network (the internet). Common uses: secure remote access to corporate networks, bypassing geo-restrictions.
Q: How does a CDN improve performance?
A: By caching content at edge servers geographically close to users. A user in India gets files from a Mumbai edge server instead of the origin server in California — latency drops from 200ms to 10ms.
Q: Why do we need a Load Balancer?
A: High availability (if a server fails, traffic routes to healthy ones), scalability (add servers behind the balancer without changing DNS), and no-downtime maintenance (take a server out of rotation for updates).
Q: What is a Reverse Proxy?
A: A server in front of web servers that forwards client requests. Used for: load balancing, SSL termination (decrypt HTTPS once, send plain HTTP to backend), caching, security (hides backend topology), and compression.
Q: Difference between REST and HTTP?
A: HTTP is a protocol (transport layer for web). REST is an architectural style for designing APIs using HTTP methods (GET, POST, PUT, DELETE) with stateless resource-oriented operations. REST leverages HTTP but adds design constraints.
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