ASP.NET - Get IPv4 Address, even if the user is using a proxy or an intranet user

Today I was working on some PayPal integration for an internal web application when part of the API suggested I needed an IP Address for the user to enhance security.

Realising this was an intranet application which could be used externally, I had to cater for all aspects. Getting an IP Address for an internal users involves looking up the host addresses using the System.Net.Dns namespace, rather than a simple peek in a server variable. As well as this, I needed to make sure I could identify real IP Addresses for users behind a proxy. If that wasn;t enough, the PayPal API doesn't support IPv6, so I had to search the host list for a IPv4 address and use that.... PayPal requires an IP or it will fail, so if everything else fails, it will return the loopback address (ack!). here's what I came up with...

UPDATE - Feb 2013
I have provided a couple of updates to this code in light of the comments. Thanks for the input!

Code Snippet

1.  
2. // Invoker
3. string IPAddress = IPHelper.GetIPAddress(Request.ServerVariables["HTTP_VIA"],
4.                                                 Request.ServerVariables["HTTP_X_FORWARDED_FOR"],
5.                                                 Request.ServerVariables["REMOTE_ADDR"]);
6.  
7.  
8.  
9. public class IPHelper
10. {
11.     /// <summary>
12.     /// Gets the user's IP Address
13.     /// </summary>
14.     /// <param name="httpVia">HTTP_VIA Server variable</param>
15.     /// <param name="httpXForwardedFor">HTTP_X_FORWARDED Server variable</param>
16.     /// <param name="RemoteAddr">REMOTE_ADDR Server variable</param>
17.     /// <returns>user's IP Address</returns>
18.     public static string GetIPAddress(string HttpVia, string HttpXForwardedFor, string RemoteAddr)
19.     {
20.         // Use a default address if all else fails.
21.         string result = "127.0.0.1";
22.  
23.         // Web user - if using proxy
24.         string tempIP = string.Empty;
25.         if (HttpVia != null)
26.             tempIP = HttpXForwardedFor;
27.         else // Web user - not using proxy or can't get the Client IP
28.             tempIP = RemoteAddr;
29.  
30.         // If we can't get a V4 IP from the above, try host address list for internal users.
31.         if (!IsIPV4(tempIP) || tempIP == "127.0.0.1 ")
32.         {
33.             try
34.             {
35.                 string hostName = System.Net.Dns.GetHostName();
36.                 foreach (System.Net.IPAddress ip in System.Net.Dns.GetHostAddresses(hostName))
37.                 {
38.                     if (IsIPV4(ip))
39.                     {
40.                         result = ip.ToString();
41.                         break;
42.                     }
43.                 }
44.             }
45.             catch { }
46.         }
47.         else
48.         {
49.             result = tempIP;
50.         }
51.  
52.         return result;
53.     }
54.  
55.     /// <summary>
56.     /// Determines weather an IP Address is V4
57.     /// </summary>
58.     /// <param name="input">input string</param>
59.     /// <returns>Is IPV4 True or False</returns>
60.     private static bool IsIPV4(string input)
61.     {
62.         bool result = false;
63.         System.Net.IPAddress address = null;
64.  
65.         if (System.Net.IPAddress.TryParse(input, out address))
66.             result = IsIPV4(address);
67.  
68.         return result;
69.     }
70.  
71.     /// <summary>
72.     /// Determines weather an IP Address is V4
73.     /// </summary>
74.     /// <param name="address">input IP address</param>
75.     /// <returns>Is IPV4 True or False</returns>
76.     private static bool IsIPV4(System.Net.IPAddress address)
77.     {
78.         bool result = false;
79.  
80.         switch (address.AddressFamily)
81.         {
82.             case System.Net.Sockets.AddressFamily.InterNetwork:   // we have IPv4
83.                 result = true;
84.                 break;
85.             case System.Net.Sockets.AddressFamily.InterNetworkV6: // we have IPv6
86.                 break;
87.             default:
88.                 break;
89.         }
90.  
91.         return result;
92.     }
93. }

End of Code Snippet