The WWW-Authenticate response-header field MUST be included in 401 (Unauthorized) response messages. The field value consists of at least one challenge that indicates the authentication scheme(s) and parameters applicable to the Request-URI.
The HTTP access authentication process is described in "HTTP Authentication: Basic and Digest Access Authentication". User agents are advised to take special care in parsing the WWW-Authenticate field value as it might contain more than one challenge, or if more than one WWW-Authenticate header field is provided, the contents of a challenge itself can contain a comma-separated list of authentication parameters.
HTTP/1.1 401 Unauthorized
Content-Type: text/html; charset=UTF-8
Content-Length: 5134
WWW-Authenticate: Basic realm="DSL Router"
Date: Fri, 30 Jul 2010 16:54:27 +0000
<!DOCTYPE html PUBLIC "-//W3C//DTD ...
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