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407 Proxy Authentication Required

407 Proxy Authentication Required is an error a HTTP proxy returns if it requires authentication. It’s really similar to 401 Unauthorized.

Example

HTTP/1.1 407 Proxy Authentication Required
Proxy-Authenticate: Basic; realm="Secured area"

As you can see above, the response looks pretty similar to 401 and can use the same authentication schemes.

This is an example of a client authenticating with a proxy:

GET / HTTP/1.1
Proxy-Authorization: Basic d2VsbCBkb25lOllvdSBmb3VuZCB0aGUgc2Vjb25kIGVhc3RlciBlZ2cK

If a clients connects to a server that requires authentication via a proxy that also requires authentication, both headers might appear in a request:

GET / HTTP/1.1
Proxy-Authorization: Basic ZWFzdGVyIGVnZzpudW1iZXIgdGhyZWUK
Authorization: Bearer c2VuZCBtZSBhIHR3ZWV0IG9yIHNvbWV0aGluZwo

Usage

HTTP proxies used to be more common. They can act like local caches or provide an additional layer of security in a corporate environment. These days they are less common for those purposes. Caching is not as needed, and the security layer is more often implemented with a VPN. Most HTTP proxies these days are reverse proxies, which is mostly transparent for HTTP clients.

Basic and Digest authentication for proxies is widely supported by browsers and other HTTP clients.

References

HTTP series

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Informational 1xx

Successful 2xx

Redirection 3xx

Client Error 4xx

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