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Security Fundamentals & Crypto · Intermediate

Application-Level Gateway (ALG)

Proxy firewall that terminates connections and inspects traffic at the application layer for specific protocols.

Definition

An application-level gateway (application proxy) acts as an intermediary for protocols such as HTTP, FTP, or SMTP, enforcing protocol-specific rules and hiding internal hosts. It offers deep inspection but adds latency and requires protocol support. The term ALG is also used for NAT helpers that fix embedded addresses in protocols like FTP and SIP.

Example

An HTTP proxy firewall rejects requests with malformed headers before they reach the internal server.

Exam tip

Proxy firewalls break the direct connection; packet filters do not.

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