使用共享IP地址从IIS站点进行出站Web请求失败

Question

• IIS 7.5 / Windows Server 2008 R2
• Multiple IIS sites bound to the same IP address, using host names.
• Inbound traffic to sites working fine.
• Outbound web requests made by the back-end site code fail. Remote site returns 404 (NotFound).
• Verified via a network trace that the traffic is making it to the remove server.
• Same requests work fine if done from a site using a dedicated IP address (ie not shared w/ any other sites).

Anyone have any ideas on how to make this work or what could be going wrong?

Network trace on hosting server:

Successful request from site w/ non-shared IP address:

No.     Time            Source                Destination           Protocol Info
   6366 15:54:35.590463 192.168.1.76          173.194.77.121        HTTP     GET /key/value/one/two HTTP/1.1 
   6369 15:54:35.599879 173.194.77.121        192.168.1.76          TCP      http > 55407 [ACK] Seq=1 Ack=110 Win=344 Len=0
   6370 15:54:35.621587 173.194.77.121        192.168.1.76          HTTP     HTTP/1.1 200 OK  (application/json)
   6608 15:54:35.815774 192.168.1.76          173.194.77.121        TCP      55407 > http [ACK] Seq=110 Ack=357 Win=509 Len=0

Failed request from site using a shared IP address:

No.     Time            Source                Destination           Protocol Info
   9720 15:54:39.244192 192.168.1.80          173.194.77.121        HTTP     GET /key/value/one/two HTTP/1.1 
   9760 15:54:39.256958 173.194.77.121        192.168.1.80          TCP      [TCP segment of a reassembled PDU]
   9761 15:54:39.256962 173.194.77.121        192.168.1.80          HTTP     HTTP/1.1 404 Not Found  (text/html)
   9762 15:54:39.257027 192.168.1.80          173.194.77.121        TCP      55438 > http [ACK] Seq=212 Ack=1676 Win=512 Len=0

Code:

public static HttpWebRequest CreateWebRequest(string url, string method = "GET", string referer = null, string contentType = null, int timeout = 100000, string authentication = null, string bindToIpAddress = null, string host = null)
{
    var request = (HttpWebRequest)WebRequest.Create(url);

    if (!string.IsNullOrWhiteSpace(bindToIpAddress))
    {
        IPAddress bindIp;
        if (!IPAddress.TryParse(bindToIpAddress, out bindIp))
        {
            throw new ArgumentException("bindToIpAddress");
        }

        request.ServicePoint.BindIPEndPointDelegate = ((sp, rep, rc) =>
        {
            return new IPEndPoint(bindIp, 0);
        });
    }

    request.Accept = "*/*";
    request.ContentType = contentType;
    request.Referer = referer;
    request.Method = method;
    request.Timeout = timeout;

    if (!string.IsNullOrWhiteSpace(host))
    {
        request.Host = host;
    }

    return request;
}

string GetData()
{
    try
    {
        string result;

        var request = CreateWebRequest("http://jsonplaceholder.typicode.com/posts/1", 
                                       "GET", 
                                       "somedomain.com", 
                                       timeout: (10 * 1000), 
                                       bindToIpAddress: "192.168.27.133" /*site IP*/);

        request.Accept = "application/json";

        using (var response = request.GetResponse())
        {
            using (var sr = new StreamReader(response.GetResponseStream()))
            {
                result = sr.ReadToEnd();
            }
        }

        return result;
    }
    catch (Exception ex)
    {
        return null;
    }
}

Answer 1

The 404 response comes back from the remote site, so the remote site is failing to process your request, and it has nothing to do with what's going on on your local server.

The only difference from the remote site's point of view is the sender's IP address, so it must be configured to accept requests only from certain IP addresses. This restriction could be in the remote server, or any firewall, router or proxy in between.

Answer 2

This turned out to be a bug in our application. In some cases, the host header (Host property on the request) was being set improperly (to the hosting/source site's host name). The stripped down code sample in the question doesn't show it. It was fine for web services that ignored the header and was an issue (404 response) for others that did not ignore the header. The issue had nothing to do w/ IIS or shared IP address. Thanks for all of the responses.