# What is a Proxy Firewall?

> “Proxy firewall” is a broad, non-standard label for a firewall architecture that uses a proxy as an intermediary between communicating endpoints.

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## Content

In the application-proxy-gateway model described by NIST, the gateway accepts a connection on one side, applies policy, and creates a separate connection on the other side for traffic that is allowed.

Because the proxy acts as a protocol endpoint, it can validate supported application syntax, restrict commands or content, authenticate a requester, normalize traffic, and hide some internal addressing. These capabilities depend on protocol-specific understanding. Generic relaying, unsupported protocols, or encrypted content that is not lawfully terminated and inspected may receive only lower-layer controls.

### Key points

- **Protocol support:** Identify which application protocols and versions the proxy understands, which features it permits, and how unsupported or upgraded traffic is handled.
- **Trust boundary:** Define where transport encryption terminates and resumes, how destination and client identity are validated, and how keys, certificates, content, and logs are protected.
- **Operations:** Govern policy, authentication, updates, capacity, timeouts, error behavior, high availability, and bypass paths; test protocol compatibility and failure modes before enforcement.
- **Important limitation:** Proxying does not guarantee safe content or correct authorization. The proxy becomes a security and availability dependency, and parsing differences, unknown protocols, tunnels, direct backend paths, or excessive generic pass-through can defeat the intended control.

### Related terms

[Firewall](<https://yellowcube.eu/glossary/firewall/>) · [Proxy server](<https://yellowcube.eu/glossary/proxy-server/>) · [Reverse proxy](<https://yellowcube.eu/glossary/reverse-proxy/>) · [Web application firewall (WAF)](<https://yellowcube.eu/glossary/web-application-firewall/>) · [Secure web gateway (SWG)](<https://yellowcube.eu/glossary/secure-web-gateway/>)

### Sources

[NIST SP 800-41 Rev. 1: Guidelines on Firewalls and Firewall Policy](https://csrc.nist.gov/pubs/sp/800/41/r1/final) · [NIST glossary: Application-Proxy Gateway](https://csrc.nist.gov/glossary/term/application_proxy_gateway)

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