Forward Proxy vs Reverse Proxy

Proxy

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Forward vs Reverse Proxies

If you've ever wondered how corporate networks block certain sites, how CDNs speed up your app, or why airport Wi‑Fi hijacks your browser before granting access—proxies are the invisible stagehands. Here's a developer‑friendly mental model.

Snapshot Mental Model

Forward proxy: speaks outward for internal clients.
Reverse proxy: speaks inward for internal services.
Transparent proxy: silently intercepts traffic without prior client configuration.

Think in terms of representation:

• Forward represents the client(s) to the external world.
• Reverse represents the server/application surface to the outside.
• Transparent represents the network's enforcement layer.

Forward Proxy

Configured explicitly (browser setting, environment variable, PAC file). It receives client requests, decides policy (allow/cache/log), then fetches on behalf of the client.
Use cases:

• Privacy / IP masking (egress appears as proxy IP)
• Outbound filtering & policy enforcement
• Caching (package artifacts, static assets)
• Auditing & compliance logs
• Geo/policy circumvention (subject to legal/ethical limits)

ASCII flow:

[Client] -> [Forward Proxy] -> [Internet Resource]
[Client] <- [Forward Proxy] <- [Internet Resource]

Not a VPN: a VPN tunnels all (lower-layer) traffic; a forward proxy usually handles application protocols like HTTP/SOCKS.

Reverse Proxy

Receives external traffic first; routes, secures, load-balances, or transforms before passing to internal services.
Benefits:

• Load balancing & health checks
• TLS termination / cert centralization
• Routing (path/host/header-based)
• Caching & compression
• Security: WAF, rate limiting, bot filtering
• Observability: structured logs, metrics injection

ASCII flow:

[External Client] -> [Reverse Proxy] -> [Service A]
                                 \-> [Service B]

Reverse proxy vs API gateway: gateways add domain-specific API concerns (auth tokens, versioning, quotas, developer portal).
Reverse proxy vs pure L4 load balancer: the proxy works at L7 (HTTP), enabling richer logic.

Transparent Proxy (Airport Wi‑Fi Captive Portal)

You do NOT configure it. The network redirects or intercepts traffic (policy routing, NAT, WCCP) and silently funnels it through a proxy or gateway.
Captive portal sequence:

1. You request any site (http://example.com or a known probe URL).
2. Interception occurs; request is redirected to a login/acceptance page.
3. After auth or ToS acceptance, your MAC/IP marked as authorized.
4. Subsequent traffic proceeds (still may be filtered or shaped).

Challenges with HTTPS: the portal can't easily inject the login page into an encrypted stream; operating systems probe known HTTP endpoints to trigger the captive portal dialog. Transparent ≠ harmless—traffic may still be inspected or logged.

Quick Comparison

Common Misconceptions

1. Forward proxy = VPN (different layers & scope).
2. Reverse proxy alone “secures” the app (logic flaws still exploitable).
3. Transparent means passive (it can actively filter/inspect).
4. Load balancer always equals reverse proxy (layer differences matter).
5. Local forward proxy can't mask IP (depends on egress IP, not physical placement).

Choosing What to Use

Layering example:
External Client -> CDN -> Reverse Proxy/API Gateway -> Services
Internal Dev builds -> Forward Proxy (artifact caching & audit)

Mini FAQ

Q: Can one component do both forward and reverse?
A: Rarely; their traffic directions & configuration differ. Some tools (like mitmproxy) can be adapted, but production roles are usually separated.

Q: Does TLS termination at the reverse proxy break end-to-end security?
A: It terminates externally; you can re-encrypt internally for compliance.

Q: How does the captive portal stop intercepting?
A: Network marks your identifier (MAC/IP) as authorized, bypassing redirect rules.

Q: Why use a forward proxy for builds?
A: Cache dependency downloads (npm, pip, container layers) and log outbound fetches.

Q: Can a transparent proxy perform TLS inspection?
A: Yes, if it installs a trusted root certificate and re-issues certificates on the fly (common in corporate SSL inspection appliances).

Summary

Anchor concepts:

• forward = client envoy;
• reverse = service façade;
• transparent = network checkpoint.

Mastering these clarifies architecture choices, debugging strange Wi‑Fi behavior, and designing scalable, observable edge layers.

Back Matter

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• Proxy vs Reverse Proxy (Real-World Examples)

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