Why my Http client making 2 requests when I specif

问题:

I created RESTful webservice (WCF) where I check credentials on each request. One of my clients is Android app and everything seems to be great on server side. I get request and if it's got proper header - I process it, etc..

Now I created client app that uses this service. This is how I do GET:

// Create the web request  
            var request = WebRequest.Create(Context.ServiceURL + uri) as HttpWebRequest;

            if (request != null)
            {
                request.ContentType = "application/json";

                // Add authentication to request  
                request.Credentials = new NetworkCredential(Context.UserName, Context.Password);

                // Get response  
                using (var response = request.GetResponse() as HttpWebResponse)
                {
                    // Get the response stream  
                    if (response != null)
                    {
                        var reader = new StreamReader(response.GetResponseStream());

                        // Console application output
                        var s = reader.ReadToEnd();

                        var serializer = new JavaScriptSerializer();
                        var returnValue = (T)serializer.Deserialize(s, typeof(T));

                        return returnValue;
                    }
                }
            }

So, this code get's my resource and deserializes it. As you see - I'm passing credentials in my call.

Then when debugging on server-side I noticed that I get 2 requests every time - one without authentication header and then server sends back response and second request comes bach with credentials. I think it's bad for my server - I'd rather don't make any roundtrips. How should I change client so it doesn't happen? See screenshot of Fiddler

EDIT:

This is JAVA code I use from Android - it doesn't do double-call:

MyHttpResponse response = new MyHttpResponse();
        HttpClient client = mMyApplication.getHttpClient();

        try
        {
            HttpGet request = new HttpGet(serviceURL + url);
            request.setHeader(new BasicHeader(HTTP.CONTENT_TYPE, "application/json"));
            request.addHeader("Authorization", "Basic " + Preferences.getAuthorizationTicket(mContext));

            ResponseHandler<String> handler = new BasicResponseHandler();
            response.Body = client.execute(request, handler);
            response.Code = HttpURLConnection.HTTP_OK;
            response.Message = "OK";
        }
        catch (HttpResponseException e)
        {
            response.Code = e.getStatusCode();
            response.Message = e.getMessage();

            LogData.InsertError(mContext, e);
        }

回答1:

The initial request doesn't ever specify the basic header for authentication. Additionally, since a realm is specified, you have to get that from the server. So you have to ask once: "hey, I need this stuff" and the server goes "who are you? the realm of answering is 'secure area'." (because realm means something here) Just because you added it here:

request.Credentials = new NetworkCredential(Context.UserName, Context.Password);

doesn't mean that it's going to be for sure attached everytime to the request.

Then you respond with the username/password (in this case you're doing BASIC so it's base64 encoded as name:password) and the server decodes it and says "ok, you're all clear, here's your data".

This is going to happen on a regular basis, and there's not a lot you can do about it. I would suggest that you also turn on HTTPS since the authentication is happening in plain text over the internet. (actually what you show seems to be over the intranet, but if you do go over the internet make it https).

Here's a link to Wikipedia that might help you further: http://en.wikipedia.org/wiki/Basic_access_authentication

回答2:

Ok, I got it. I manually set HttpHeader instead of using request.Credentials

request.Headers.Add(HttpRequestHeader.Authorization, "Basic " + Convert.ToBase64String(Encoding.UTF8.GetBytes(Context.UserName + ":" + Context.Password)));

Now I see only single requests as expected..

回答3:

As an option you can use PreAuthenticate property of HttpClientHandler. This would require a couple of lines more

            var client = new HttpClient(new HttpClientHandler
            {
                Credentials = yourCredentials,
                PreAuthenticate = true
            });

With using this approach, only the first request is sent without credentials, but all the rest requests are OK.