FYCore/GatewayAPI/HostReplaceMiddleware.cs

using Ocelot.Authentication.Middleware;
using Ocelot.Authorization.Middleware;
using Ocelot.Cache.Middleware;
using Ocelot.Claims.Middleware;
using Ocelot.DownstreamPathManipulation.Middleware;
using Ocelot.DownstreamRouteFinder.Middleware;
using Ocelot.DownstreamRouteFinder.UrlMatcher;
using Ocelot.DownstreamUrlCreator.Middleware;
using Ocelot.Errors.Middleware;
using Ocelot.Headers.Middleware;
using Ocelot.LoadBalancer.Middleware;
using Ocelot.Logging;
using Ocelot.Middleware;
using Ocelot.Multiplexer;
using Ocelot.QueryStrings.Middleware;
using Ocelot.RateLimit.Middleware;
using Ocelot.Request.Middleware;
using Ocelot.Requester.Middleware;
using Ocelot.RequestId.Middleware;
using Ocelot.Responder.Middleware;
using Ocelot.Security.Middleware;
using Ocelot.WebSockets.Middleware;

namespace GatewayAPI
{
    public static class OcelotPipelineExtensions
    {
        public static RequestDelegate BuildOcelotPipeline(this IApplicationBuilder app, OcelotPipelineConfiguration pipelineConfiguration)
        {
            app.UseDownstreamContextMiddleware();
            app.UseExceptionHandlerMiddleware();
            app.MapWhen((HttpContext httpContext) => httpContext.WebSockets.IsWebSocketRequest, delegate (IApplicationBuilder wenSocketsApp)
            {
                wenSocketsApp.UseDownstreamRouteFinderMiddleware();
                wenSocketsApp.UseMultiplexingMiddleware();
                wenSocketsApp.UseDownstreamRequestInitialiser();
                wenSocketsApp.UseLoadBalancingMiddleware();
                wenSocketsApp.UseMiddleware<HostReplaceMiddleware>();
                wenSocketsApp.UseDownstreamUrlCreatorMiddleware();
                wenSocketsApp.UseWebSocketsProxyMiddleware();
            });
            app.UseIfNotNull(pipelineConfiguration.PreErrorResponderMiddleware);
            app.UseResponderMiddleware();
            app.UseDownstreamRouteFinderMiddleware();
            app.UseMultiplexingMiddleware();
            app.UseSecurityMiddleware();
            if (pipelineConfiguration.MapWhenOcelotPipeline != null)
            {
                foreach (KeyValuePair<Func<HttpContext, bool>, Action<IApplicationBuilder>> item in pipelineConfiguration.MapWhenOcelotPipeline)
                {
                    app.MapWhen(item.Key, item.Value);
                }
            }

            app.UseHttpHeadersTransformationMiddleware();
            app.UseDownstreamRequestInitialiser();
            app.UseRateLimiting();
            app.UseRequestIdMiddleware();
            app.UseIfNotNull(pipelineConfiguration.PreAuthenticationMiddleware);
            if (pipelineConfiguration.AuthenticationMiddleware == null)
            {
                app.UseAuthenticationMiddleware();
            }
            else
            {
                app.Use(pipelineConfiguration.AuthenticationMiddleware);
            }

            app.UseClaimsToClaimsMiddleware();
            app.UseIfNotNull(pipelineConfiguration.PreAuthorizationMiddleware);
            if (pipelineConfiguration.AuthorizationMiddleware == null)
            {
                app.UseAuthorizationMiddleware();
            }
            else
            {
                app.Use(pipelineConfiguration.AuthorizationMiddleware);
            }

            app.UseClaimsToHeadersMiddleware();
            app.UseIfNotNull(pipelineConfiguration.PreQueryStringBuilderMiddleware);
            app.UseClaimsToQueryStringMiddleware();
            app.UseClaimsToDownstreamPathMiddleware();
            app.UseLoadBalancingMiddleware();
            app.UseMiddleware<HostReplaceMiddleware>();
            app.UseDownstreamUrlCreatorMiddleware();
            app.UseOutputCacheMiddleware();
            app.UseHttpRequesterMiddleware();
            return app.Build();
        }

        private static void UseIfNotNull(this IApplicationBuilder builder, Func<HttpContext, Func<Task>, Task> middleware)
        {
            if (middleware != null)
            {
                builder.Use(middleware);
            }
        }
    }

    public class HostReplaceMiddleware : OcelotMiddleware
    {
        private readonly RequestDelegate _next;

        public HostReplaceMiddleware(RequestDelegate next, IOcelotLoggerFactory loggerFactory)
            : base(loggerFactory.CreateLogger<HostReplaceMiddleware>())
        {
            _next = next;
        }

        public async Task Invoke(HttpContext httpContext)
        {
            DownstreamRequest downstreamRequest = httpContext.Items.DownstreamRequest();
            List<PlaceholderNameAndValue> list = httpContext.Items.TemplatePlaceholderNameAndValues();
            list.ForEach((v) => {
                downstreamRequest.Host = downstreamRequest.Host.Replace(v.Name, v.Value);
            });
            await _next(httpContext);
        }
    }
}