跨域访问之CORS

CORS:定义

2014年1月16日，W3C的Web应用工作组（Web Applications Working Group）和Web应用安全工作组（Web AppSec）联合发布了跨源资源共享（Cross-Origin Resource Sharing）的W3C正式推荐标准（W3C Recommendation）。该标准定义了在必须访问跨域资源时，浏览器与服务端应该如何沟通，它提供一种机制，允许客户端（如浏览器）对非源站点的资源发出访问请求。所有提供跨源资源请求的API都可以使用本规范中定义的算法。

出于安全性的考虑，用户代理（如浏览器）通常拒绝跨站的访问请求，但这会限制运行在用户代理的Web应用通过Ajax或者其他机制从另一个站点访问资源、获取数据。跨源资源共享（CORS）扩充了这个模型，通过使用自定义的HTTP响应头部（HTTP
Response Header），通知浏览器资源可能被哪些跨源站点以何种HTTP方法获得。例如，浏览器在访问 http://example.com 站点的Web应用时，Web应用如果需要跨站访问另一站点的资源 http://hello-world.example，就需要使用该标准。http://hello-world.example 在HTTP的响应头部中定义 Access-Control-Allow-Origin:
http://example.org，通知浏览器允许 http://example.org 跨源从 http://hello-world.example上获取资源。

CORS 图解

过程说明：

1 浏览器发出跨域访问请求，request header 携带 origin

2 服务器收到请求，服务器返回response header

3 浏览区检查reponse header ,如果Access-Control-Allow-Origin
: 包含request header的Origin,那么浏览器认为这是安全操作，允许操作服务端返回的数据，否则浏览器认为非同源数据，不允许操作。

目前主流的浏览器基本都支持CORS规范，所以实现CORS跨域访问的关键就是Server 端返回的respone中添加 Access-Control-Allow-Origin：对应的请求origin.

Web API 如何实现跨域请求:

服务端代码

类说明：

1. CorsAttribute：请求跨域的标记

1. CorsMessageHandler:

拦截请求，确认请求是否允许跨域，如果允许，那么给response header 添加跨域请求 Access-Control-Allow-Origin，否则返回

{

"Message": "Cross-origin
request denied"

}

代码：

using System;

using System.Collections.Generic;

using System.Linq;

using System.Net.Http;

using System.Threading;

using System.Threading.Tasks;

using System.Web;

using System.Web.Http;

using System.Web.Http.Controllers;

using System.Net;

using System.Diagnostics;

namespace Hbb0b0.MVC.CORS

{

    /// <summary>

    /// 跨域属性

    /// 此处参考

    /// asp.net webapi 2

    ///

    /// </summary>

    public class CorsAttribute : Attribute

    {

        /// <summary>

        /// 允许的站点列表

        /// </summary>

        public Uri[] AllowOrigins

        {

            get;

            private set;

        }

        /// <summary>

        /// 错误提示

        /// </summary>

        public string ErrorMessage

        {

            get;

            set;

        }

        /// <summary>

        /// 构造函数

        /// </summary>

        /// <param name="allowOrigins"></param>

        public CorsAttribute(params string[] allowOrigins)

        {

            this.AllowOrigins = (allowOrigins ?? new string[]).Select(p => new Uri(p)).ToArray();

        }

        /// <summary>

        /// 获取请求头信息，判断是否请求被允许跨域

        /// </summary>

        /// <param name="request"></param>

        /// <param name="headers"></param>

        /// <returns></returns>

        public bool TryEvaluate(HttpRequestMessage request, out IDictionary<string, string> headers)

        {

            headers = null;

            string origin = request.Headers.GetValues(HttpHeaderKeys.ORIGIN).FirstOrDefault();

            Uri originUrl = new Uri(origin);

            if (this.AllowOrigins.Contains(originUrl))

            {

                headers = this.GenerateResposeHeaders(request);

                return true;

            }

            this.ErrorMessage = "Cross-origin request denied";

            return false;

        }

        /// <summary>

        /// 添加跨域许可请求头

        /// </summary>

        /// <param name="request"></param>

        /// <returns></returns>

        private IDictionary<string, string> GenerateResposeHeaders(HttpRequestMessage request)

        {

            string origin = request.Headers.GetValues("Origin").First();

            Dictionary<string, string> headers = new Dictionary<string, string>();

            headers.Add(HttpHeaderKeys.ACCESS_CONTROL_ALLOW_ORIGIN, origin);

            if (request.IsPreflightRequest())

            {

                headers.Add(HttpHeaderKeys.ACCESS_CONTROL_ALLOW_ORIGIN, "*");

                string requestHeaders = request.Headers.GetValues(HttpHeaderKeys.ACCESS_CONTROL_ALLOW_REQUEST_HEADERS).FirstOrDefault();

                if (!string.IsNullOrEmpty(requestHeaders))

                {

                    headers.Add(HttpHeaderKeys.ACCESS_CONTROL_ALLOW_REQUEST_HEADERS, requestHeaders);

                }

            }

            return headers;

        }

    }

    /// <summary>

    /// Http Header keys

    /// </summary>

    public class HttpHeaderKeys

    {

        public const string ORIGIN = "Origin";

        public const string ACCESS_CONTROL_ALLOW_ORIGIN = "Access-Control-Allow-Origin";

        public const string ACCESS_CONTROL_ALLOW_REQUEST_METHOD = "Access-Control-Allow-Request-Methods";

        public const string ACCESS_CONTROL_ALLOW_REQUEST_HEADERS = "Access-Control-Allow-Request-Headers";

    }

    /// <summary>

    /// 判断是否是非简单跨域请求扩展方法

    /// </summary>

    public static class HttpRequestMessageExtensions

    {

        public static bool IsPreflightRequest(this HttpRequestMessage request)

        {

            return request.Method == HttpMethod.Options &&

                      request.Headers.GetValues(HttpHeaderKeys.ORIGIN).Any() &&

                      request.Headers.GetValues(HttpHeaderKeys.ACCESS_CONTROL_ALLOW_REQUEST_METHOD).Any();

        }

    }

    public class CorsMessageHandler : DelegatingHandler

    {

        protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)

        {

            //Debugger.Break();

            HttpMethod originalMethod = request.Method;

            bool isPreflightRequest = request.IsPreflightRequest();

            if (isPreflightRequest)

            {

                string method = request.Headers.GetValues(HttpHeaderKeys.ACCESS_CONTROL_ALLOW_REQUEST_HEADERS).FirstOrDefault();

                request.Method = new HttpMethod(method);

            }

            HttpConfiguration config = request.GetConfiguration();

            HttpControllerDescriptor description = config.Services.GetHttpControllerSelector().SelectController(request);

            HttpControllerContext context = new HttpControllerContext(request.GetConfiguration(), request.GetRouteData(), request)

            {

                ControllerDescriptor = description

            };

            HttpActionDescriptor actionDescriptor = config.Services.GetActionSelector().SelectAction(context);

            CorsAttribute corsAttribute = actionDescriptor.GetCustomAttributes<CorsAttribute>().FirstOrDefault();

            if (corsAttribute == null)

            {

                return base.SendAsync(request, cancellationToken);

            }

            IDictionary<string, string> headers;

            request.Method = originalMethod;

            HttpResponseMessage response = null;

            //判断请求是否被授权

            bool authorized = corsAttribute.TryEvaluate(request, out headers);

            //处理非简单跨域请求

            if (isPreflightRequest)

            {

                if (authorized)

                {

                    response = new HttpResponseMessage(HttpStatusCode.OK);

                }

            }

            //简单跨域请求

            else

            {

                //如果授权，返回请求资源

                if (authorized)

                {

                    response = base.SendAsync(request, cancellationToken).Result;

                }

                //非授权，返回错误信息

                else

                {

                    response = request.CreateErrorResponse(HttpStatusCode.BadRequest, corsAttribute.ErrorMessage);

                }

            }

            //添加header

            if (headers!=null)

            {

                foreach (var item in headers)

                {

                    response.Headers.Add(item.Key, item.Value);

                }

            }

            return Task.FromResult<HttpResponseMessage>(response);

        }

    }

}

客户端代码：

@{

    Layout = null;

}

<!DOCTYPE html>

<html>

<head>

    <meta name="viewport" content="width=device-width" />

    <title>View</title>

    <script type="text/javascript" src="~/Scripts/jquery-1.10.2.js"></script>

    <script type="text/javascript">

        $(function () {

            $.ajax({

                type: "POST",

                async: false,

                url: "http://localhost/Hbb0b0.mvc.API/api/StudentAPI/",

                beforeSend: function (xmlHttpRequest) {

                    //xmlHttpRequest.setRequestHeader("Origin", "http://hbb0b0notebook:51929/");

                },

                success: function (data) {

                    //alert(data.name);

                    console.log(data);

                    getStudentList(data);

                },

                error: function () {

                    alert('fail');

                }

            });

        });

        function getStudentList(list) {

            console.debug("GetStudenListCors", list);

            $.each(list, function (index, student) {

                var html = "<ul>";

                html += "<li> name:" + student.Name + "</li>"

                html += "</ul>";

                $("#divStudentList").append(html);

            });

        }

    </script>

</head>

<body>

    <div id="divStudentList">

    </div>

</body>

</html>

运行结果

Request Header:

Response header:

结论

CORS 作为W3C官方正式的跨域资源访问方案，不像JSONP 是一种临时方案。

CORS 不对请求类型做限制，get, post 都支持，JSONPzhi只支持get. 所以 CORS比JSONP更通用，更是主流的跨域资源访问请求解决方案。

本文参考了<<asp.net  webapi 2  框架解密>>,此书的确是深入剖析asp.net 框架的好书。