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FORWARD PROXY

Core Concept:

  • Server between clients and the internet
  • Forwards client requests to external servers on behalf of clients
  • Hides client identity (external servers see proxy IP)
  • Provides access control and content filtering

Why It Matters​

  • Client anonymity - external servers see proxy IP, not client IP
  • Access control - centralized point to enforce organizational policies
  • Content filtering - block access to specific websites/content
  • Bandwidth optimization - cache frequently accessed content

When to Use​

  • Need to control internet access in corporate network
  • Want to cache external content to reduce bandwidth
  • Need to anonymize client requests for privacy
  • Want to bypass geo-restrictions or censorship
  • Need centralized logging of outbound traffic
  • Require malware/content filtering before content reaches clients

When Not to Use​

  • Direct internet access needed for performance (gaming, real-time apps)
  • Client needs direct connection for protocol reasons (P2P, WebRTC)
  • Small network where overhead exceeds benefits
  • Cloud environments (typically use NAT gateway instead)

Trade-offs​

Benefits:

  • Enhanced privacy (hides client IP)
  • Centralized access control
  • Bandwidth savings (caching)
  • Content filtering and security
  • Simplified network management

Drawbacks:

  • Additional latency for requests
  • Single point of failure
  • Privacy concerns (proxy sees all traffic)
  • Can break client applications expecting direct access
  • SSL interception raises security concerns

Key Distinctions​

Forward Proxy vs Reverse Proxy:

  • Forward Proxy: Works for clients, servers see proxy IP
  • Reverse Proxy: Works for servers, clients see proxy IP
  • Forward proxy = client-side; reverse proxy = server-side

Forward Proxy vs VPN:

  • Proxy: Application-level (HTTP/HTTPS), per-app configuration
  • VPN: Network-level (all traffic), system-wide
  • VPN more comprehensive; proxy more targeted

This contrasts with reverse_proxy which proxies for servers rather than clients.

Common Use Cases​

Corporate Network Access Control​

Organizations use forward proxies to control which websites employees can access.

Content Caching​

ISPs and large organizations cache popular content to reduce bandwidth costs.

Anonymity/Privacy​

Users route traffic through proxy to hide identity from destination servers.

Bypassing Restrictions​

Access geo-blocked content or bypass firewalls by routing through proxy in different location.

Common Pitfalls​

WARNING

SSL/TLS interception issues: If proxy intercepts HTTPS, must install custom CA certificate on all clients. Otherwise, SSL errors occur.

DANGER

Proxy auto-configuration (PAC) complexity: Incorrect PAC files can break internet access. Keep PAC logic simple and well-tested.

Quick Reference​

SolutionTypeStrengthsUse Case
SquidOpen sourceMature, cachingCorporate networks
PrivoxyOpen sourcePrivacy-focusedAd-blocking, privacy
TinyproxyOpen sourceLightweightResource-constrained
CCProxyCommercialEasy Windows setupWindows networks
ShadowsocksOpen sourceCircumventionBypassing censorship

Proxy Types​

HTTP Proxy:

  • Handles HTTP/HTTPS traffic
  • Can cache content
  • Most common type

SOCKS Proxy:

  • Protocol-agnostic (works with any TCP/UDP)
  • No caching
  • Lower-level than HTTP proxy

Transparent Proxy:

  • Intercepts traffic without client configuration
  • Client unaware of proxy
  • Set up at network level

Anonymous Proxy:

  • Hides client IP but identifies as proxy
  • Reveals proxy IP to server

Elite/High Anonymity Proxy:

  • Hides client IP and doesn't identify as proxy
  • Server doesn't know request came through proxy

Client Configuration​

Manual Proxy Configuration:

HTTP Proxy: proxy.company.com
Port: 8080
HTTPS Proxy: proxy.company.com
Port: 8080
Bypass proxy for: localhost, 127.0.0.1, *.internal.company.com

PAC (Proxy Auto-Configuration):

function FindProxyForURL(url, host) {
    // Direct access for local networks
    if (isPlainHostName(host) || 
        shExpMatch(host, "*.internal.company.com") ||
        isInNet(host, "10.0.0.0", "255.0.0.0"))
        return "DIRECT";
    
    // Use proxy for everything else
    return "PROXY proxy.company.com:8080; DIRECT";
}

Squid Basic Configuration​

# Port to listen on
http_port 3128

# Access control lists
acl localnet src 10.0.0.0/8
acl SSL_ports port 443
acl Safe_ports port 80 443 21 22

# Deny access to blocked sites
acl blocked_sites dstdomain .facebook.com .twitter.com
http_access deny blocked_sites

# Allow local network
http_access allow localnet

# Deny everything else
http_access deny all

# Cache settings
cache_dir ufs /var/spool/squid 10000 16 256
maximum_object_size 100 MB

Request Flow​

Client App → Forward Proxy → [Access Control] → [Cache Check] → Internet Server
                          ↓
                    [Content Filter]
                          ↓
                      [Logging]
                          ↓
Client App ← [Cache Store if needed] ← Forward Proxy ← Internet Server

References​