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  5. Proxy-Authenticate

Proxy-Authenticate header

Baseline Widely available

This feature is well established and works across many devices and browser versions. It’s been available across browsers since ⁨July 2015⁩.

The HTTP Proxy-Authenticate response header defines the authentication method (or challenge) that should be used to gain access to a resource behind a proxy server. It is sent in a 407 Proxy Authentication Required response so a client can identify itself to a proxy that requires authentication.

Header type Response header
Forbidden request header Yes

Syntax

http
Proxy-Authenticate: <challenge>, …

The value is a comma-separated list of challenges, where a <challenge> is comprised of an <auth-scheme>, followed by an optional <token68> or a comma-separated list of <auth-params>:

challenge = <auth-scheme> <auth-param>, …, <auth-paramN>
challenge = <auth-scheme> <token68>

For example:

http
Proxy-Authenticate: <auth-scheme>
Proxy-Authenticate: <auth-scheme> token68
Proxy-Authenticate: <auth-scheme> auth-param1=param-token1
Proxy-Authenticate: <auth-scheme> auth-param1=param-token1, …, auth-paramN=param-tokenN

The presence of a token68 or authentication parameters depends on the selected <auth-scheme>. For example, Basic authentication requires a <realm>, and allows for optional use of charset key, but does not support a token68:

http
Proxy-Authenticate: Basic realm="Dev", charset="UTF-8"

Directives

<auth-scheme>

A case-insensitive token indicating the Authentication scheme used. Some of the more common types are Basic, Digest, Negotiate and AWS4-HMAC-SHA256. IANA maintains a list of authentication schemes, but there are other schemes offered by host services.

<auth-param> Optional

An authentication parameter whose format depends on the <auth-scheme>. <realm> is described below as it's a common authentication parameter among many auth schemes.

<realm> Optional

The string realm followed by = and a quoted string describing a protected area, for example realm="staging environment". A realm allows a server to partition the areas it protects (if supported by a scheme that allows such partitioning). Some clients show this value to the user to inform them about which particular credentials are required — though most browsers stopped doing so to counter phishing. The only reliably supported character set for this value is us-ascii. If no realm is specified, clients often display a formatted hostname instead.

<token68> Optional

A token that may be useful for some schemes. The token allows the 66 unreserved URI characters plus a few others. It can hold a base64, base64url, base32, or base16 (hex) encoding, with or without padding, but excluding whitespace. The token68 alternative to auth-param lists is supported for consistency with legacy authentication schemes.

Generally, you will need to check the relevant specifications for the authentication parameters needed for each <auth-scheme>.

Note: See WWW-Authenticate for more details on authentication parameters.

Examples

Proxy-Authenticate Basic auth

The following response indicates a Basic auth scheme is required with a realm:

http
Proxy-Authenticate: Basic realm="Staging server"

Specifications

Specification
HTTP Semantics
# field.proxy-authenticate

Browser compatibility

See also

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