Umbraco-CMS/src/Umbraco.Web/HttpContextExtensions.cs

using System.Web;

namespace Umbraco.Core
{
    public static class HttpContextExtensions
    {
        public static string GetCurrentRequestIpAddress(this HttpContextBase httpContext)
        {
            if (httpContext == null)
            {
                return "Unknown, httpContext is null";
            }

            HttpRequestBase request;
            try
            {
                // is not null - throws
                request = httpContext.Request;
            }
            catch
            {
                return "Unknown, httpContext.Request is null";
            }

            if (request.ServerVariables == null)
            {
                return "Unknown, httpContext.Request.ServerVariables is null";
            }

            // From: http://stackoverflow.com/a/740431/5018

            try
            {
                var ipAddress = request.ServerVariables["HTTP_X_FORWARDED_FOR"];

                if (string.IsNullOrEmpty(ipAddress))
                    return request.UserHostAddress;

                var addresses = ipAddress.Split(',');
                if (addresses.Length != 0)
                    return addresses[0];

                return request.UserHostAddress;
            }
            catch (System.Exception ex)
            {
                //This try catch is to just always ensure that no matter what we're not getting any exceptions caused since
                // that would cause people to not be able to login
                return string.Format("Unknown, exception occurred trying to resolve IP {0}", ex);
            }
        }
    }
}