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Most modern load balancers, including Amazon’s Elastic Load Balancer (ELB), allow you to configure them to handle SSL. Although they can forward the request to your web nodes using SSL, it is more efficient to offload the SSL processing to the load balancer and forward requests from there to your web servers using plain HTTP on port 80. Load balancers that support offloading SSL generally inject a “X-Forwarded-Proto” header into the request with the value “http” or “https” to indicate the protocol of the original request. This approach is quite secure as the load balancer typically replaces any “X-Forwarded-Proto” header present in the original request. This is true for ELB.

You can use this header in ASP.NET Web API to make sure a request is secure. For example, here’s an attribute you can put on any controller or controller method to require SSL. It supports SSL terminated at the load balancer as well as plain old SSL straight to the server:

using System;
using System.Configuration;
using System.Linq;
using System.Net.Http;
using System.Web.Http.Controllers;
using System.Web.Http.Filters;

namespace AspNetApiHelpers
{
    public class RequireHttpsAttribute : AuthorizationFilterAttribute
    {
        public override void OnAuthorization(HttpActionContext actionContext)
        {
            if (actionContext.Request.RequestUri.Scheme != Uri.UriSchemeHttps && !IsForwardedSsl(actionContext))
            {
                actionContext.Response = new HttpResponseMessage(System.Net.HttpStatusCode.Forbidden)
                {
                    ReasonPhrase = "HTTPS Required"
                };
            }
            else
            {
                base.OnAuthorization(actionContext);
            }
        }

        private static bool IsForwardedSsl(HttpActionContext actionContext)
        {
            var xForwardedProto = actionContext.Request.Headers.FirstOrDefault(x => x.Key == "X-Forwarded-Proto");
            var forwardedSsl = xForwardedProto.Value != null &&
                xForwardedProto.Value.Any(x => string.Equals(x, "https", StringComparison.InvariantCultureIgnoreCase));
            return forwardedSsl;
        }
    }
}