如何使用HttpWebRequest.Credentials属性进行基本身份验证？

Question

How can I use the Webrequest Credentials Property to send an basic authentication header? Why isn't the Authorization header send with the request even when PreAuthenticate is set to true?

WebRequest request = (HttpWebRequest)WebRequest.Create("https://api.github.com/user");
request.Credentials = new NetworkCredential("githubUsername", "githubPassword");
request.PreAuthenticate = true;
var response = request.GetResponse();

Answer 1

The server should send a HTTP 401 Not Authorized response code containing a WWW-Authenticate HTTP header.

WWW-Authenticate: Basic realm="example"

The PreAuthenticate property only works after authentication has taken place. From MSDN:

true to send an HTTP Authorization header with requests after authentication has taken place; otherwise, false. The default is false.

See other answers for more in depth explanation.

Answer 2

I've done some additional research based on Måns Tånneryd `s answer. And the link he posted in his comment: PreAuthenticate Property of WebRequest - Problem .

First of all as described in his link the HttpWebRequest.PreAuthenticate Property does NOT send the Authentication header PRE authentication, but pre sends it in following requests, after Authentication. From MSDN:

true to send an HTTP Authorization header with requests after authentication has taken place; otherwise, false. The default is false.

So even with the PreAuthenticate property set to true, we still need an WWW-Authenticate challenge with a 401 Unauthorized before anything happens. Now if we try to authenticate against github with the following code:

WebRequest request = (HttpWebRequest)WebRequest.Create("https://api.github.com/user");
request.Credentials = new NetworkCredential("githubUsername", "githubPassword");

var response = request.GetResponse();

An WebException will be thrown, because we don't get a WWW-Authenticate challenge. If we capture this in Fiddler we will get the following:

However if we try this, with the exact same code, against a website that does return a WWW-Authenticate challenge we will see the following in Fiddler:

And the response will have the result as expected.

Answer 3

I can successfull run this code accessing another server that also requires both SSL and authentication. This server differs from the github in that the github returns a json result saying that it requires authentication and the other server returns a "classic" 401 html page. Sniffing the network you can see that the .net code tries to do anonymous auth even if you do set preauth to true which I think is rather confusing. However, upon receiving a regular 401-page it tries again, and this time with the auth info and everything works. It seems to me though as if .net reacts differently upon receiving the json version of a 401, not making a second try.

I guess this is not the answer you are looking for but hopefully it sheds some more light on the situation.