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：
- CorsAttribute：请求跨域的标记
- 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[0]).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。
