Startup.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Net.Http;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.AspNetCore.Http;
using Microsoft.AspNetCore.HttpsPolicy;
using Microsoft.AspNetCore.Mvc;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Logging;
using Microsoft.Extensions.Options;

namespace image_proxy {
    public class Startup {
        public Startup (IConfiguration configuration) {
            Configuration = configuration;
        }

        public IConfiguration Configuration { get; }

        // This method gets called by the runtime. Use this method to add services to the container.
        public void ConfigureServices (IServiceCollection services) {
            services.AddMvc ().SetCompatibilityVersion (CompatibilityVersion.Version_2_1);
            services.AddCors (o => o.AddPolicy ("MyPolicy", builder => {
                builder.AllowAnyOrigin ()
                    .AllowAnyMethod ()
                    .AllowAnyHeader ();
            }));
        }

        // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
        public void Configure (IApplicationBuilder app, IHostingEnvironment env) {
            app.UseCors("MyPolicy");
            app.UseDeveloperExceptionPage ();
            app.Run (async context => {
                //context.Response.Headers.Add("Cache-Control", "max-age=31536000");
                var url = context.Request.Query.First (x => x.Key == "url");
                using (var httpClient = new HttpClient ()) {
                    var response = await httpClient.GetAsync (url.Value);
                    var contentType = response.Content.Headers.ContentType.MediaType;
                    context.Response.Headers.Add("Content-Type", contentType);
                    var content = await response.Content.ReadAsStreamAsync ();
                    using (content) {
                        await content.CopyToAsync (context.Response.Body);
                    }
                }
            });
        }
    }
}