同源是指同样的协议、域名、port，三者都同样才属于同域。不符合上述定义的请求，则称为跨域。
相信每一个开发者都曾遇到过跨域请求的情况，尽管情况不一样，但问题的本质都能够归为浏览器出于安全考虑下的同源策略的限制。
跨域的情形有非常多，最常见的有Ajax跨域、Socket跨域和Canvas跨域。以下列举一些我们常见的跨域情形下。某些浏览器控制台给出的错误提示：
FireFox下的提示：
已阻止交叉源请求：同源策略不同意读取***上的远程资源。能够将资源移动到同样的域名上或者启用 CORS 来解决问题。
Canvas跨域Chrome下的提示：
UncaughtSecurityError:Failed to execute'getImageData'
on 'CanvasRenderingContext2D':The canvas has been taintedby cross-origin data.
或：
Imagefrom origin 'http://js.xx.com' has been blocked from loading by Cross-OriginResource Sharing policy: No 'Access-Control-Allow-Origin' header is present onthe requested resource. Origin 'http://act.xx.com' is
therefore not allowedaccess.
网上有很多解决跨域的方法，大体上有这几种：
1）document.domain+iframe的设置
2）动态创建script
3）利用iframe和location.hash
4）window.name实现的跨域传输数据
5）使用HTML5 postMessage
6）利用flash
7）通过代理，js訪问代理，代理转到不同的域
http://developer.yahoo.com/javascript/howto-proxy.html
8）Jquery JSONP(不能成为真正的Ajax，本质上仍是动态创建script)
http://www.cnblogs.com/chopper/archive/2012/03/24/2403945.html
9）跨域资源共享（CORS） 这是HTML5跨域问题的标准解决方式
说明：方案1~方案6见Rain Man所写的文章《JavaScript跨域总结与解决的方法》
http://www.cnblogs.com/rainman/archive/2011/02/20/1959325.html
以下主要就我总结的几种解决跨域的方法。展开说一下。
1） 绕开跨域。
适用情形是：动静分离。
example1.com域名下的页面中跨域请求是以JavaScript内联方式实现的，而请求的目标静态资源是放在example2.com域名下，这时能够将运行跨域请求的JavaScript代码块独立出来，放到example2.com上，而example1.com页面通过外联方式引入该静态域名下的js文件。这样，js与请求的图片等资源都在example2.com下。就可以正常訪问到。这样的方法事实上是一种巧妙避开跨域的方法。
2） 后台抓取克隆图片。
适用情形：动静不分离（两个域名均执行訪问静态资源）。
example1.com请求example2.com下的资源图片，能够使用PHP抓取图片并在example2.com下生成一份，这样就能够间接訪问到example1.com的静态资源。
html模板演示样例代码：
$("#scratchpad").wScratchPad({ //刮刮卡演示样例，当前域名http://act.xxx.com
width:283,
height:154,
//color: "#a9a9a7",
image2:"imgdata.php?url=http://js.xxx.com/static/activity/sq/guagua315/images/card_inner.jpg",
scratchMove:function() {
}
});
或：
xi=newXMLHttpRequest();
xi.open("GET","imgdata.php?url="+yourImageURL,true);
xi.send();
xi.onreadystatechange=function() {
if(xi.readyState==4 && xi.status==200) {
img=newImage;
img.onload=function(){
ctx.drawImage(img,
0, 0, canvas.width, canvas.height);
}
img.src=xi.responseText;
}
}
PHP处理代码：
<?php//imgdata.php
$url=$_GET['url'];
$img =file_get_contents($url);
$imgname = substr(strrchr($url,"/"),1);
file_put_contents($fn,$img);
echo $imgname;
?>
上述代码在当前php文件夹下生成了克隆生成了一张图片。
3） 后台程序设置Access-Control-Allow-Origin
适用情形：Ajax获取跨域接口的JSON数据。
example1.com请求example2.com的数据接口，则须要在example2.com的数据接口加入跨域訪问授权。
PHP程序中開始出加入header('HeaderName:HeaderValue'); 这种header标记：
header('Access-Control-Allow-Origin:*');
4）改动server配置启用CORS
适用情形：跨域訪问静态资源。
Access-Control-Allow-Origin是什么作用呢？用于授权资源的跨站訪问。比方,静态资源图片都放在example2.com 域名下, 假设在返回的头中没有设置 Access-Control-Allow-Origin , 那么别的域是没有权限外链你的图片的。
要实现CORS跨域，服务端须要这个一个流程，图片引自html5rocks，附图例如以下
a. 对于简单请求，如GET。仅仅须要在HTTP Response后加入Access-Control-Allow-Origin。
b. 对于非简单请求，比方POST、PUT、DELETE等，浏览器会分两次应答。
第一次preflight（method: OPTIONS）。主要验证来源是否合法。并返回同意的Header等。第二次才是真正的HTTP应答。所以server必须处理OPTIONS应答。
这里是一个nginx启用CORS的參考配置演示样例http://enable-cors.org/server_nginx.html。代码：
#
# A CORS (Cross-Origin Resouce Sharing) config for nginx
#
# == Purpose
#
# This nginx configuration enables CORS requests in the following way:
# - enables CORS just for origins on a whitelist specified by a regular expression
# - CORS preflight request (OPTIONS) are responded immediately
# - Access-Control-Allow-Credentials=true for GET and POST requests
# - Access-Control-Max-Age=20days, to minimize repetitive OPTIONS requests
# - various superluous settings to accommodate nonconformant browsers
#
# == Comment on echoing Access-Control-Allow-Origin
#
# How do you allow CORS requests only from certain domains? The last
# published W3C candidate recommendation states that the
# Access-Control-Allow-Origin header can include a list of origins.
# (See: http://www.w3.org/TR/2013/CR-cors-20130129/#access-control-allow-origin-response-header )
# However, browsers do not support this well and it likely will be
# dropped from the spec (see, http://www.rfc-editor.org/errata_search.php?rfc=6454&eid=3249 ).
#
# The usual workaround is for the server to keep a whitelist of
# acceptable origins (as a regular expression), match the request's
# Origin header against the list, and echo back the matched value.
#
# (Yes you can use '*' to accept all origins but this is too open and
# prevents using 'Access-Control-Allow-Credentials: true', which is
# needed for HTTP Basic Access authentication.)
#
# == Comment on spec
#
# Comments below are all based on my reading of the CORS spec as of
# 2013-Jan-29 ( http://www.w3.org/TR/2013/CR-cors-20130129/ ), the
# XMLHttpRequest spec (
# http://www.w3.org/TR/2012/WD-XMLHttpRequest-20121206/ ), and
# experimentation with latest versions of Firefox, Chrome, Safari at
# that point in time.
#
# == Changelog
#
# shared at: https://gist.github.com/algal/5480916
# based on: https://gist.github.com/alexjs/4165271
# location / { # if the request included an Origin: header with an origin on the whitelist,
# then it is some kind of CORS request. # specifically, this example allow CORS requests from
# scheme : http or https
# authority : any authority ending in ".mckinsey.com"
# port : nothing, or :
if ($http_origin ~* (https?://[^/]*\.mckinsey\.com(:[0-9]+)?)$) {
set $cors "true";
} # Nginx doesn't support nested If statements, so we use string
# concatenation to create a flag for compound conditions # OPTIONS indicates a CORS pre-flight request
if ($request_method = 'OPTIONS') {
set $cors "${cors}options";
} # non-OPTIONS indicates a normal CORS request
if ($request_method = 'GET') {
set $cors "${cors}get";
}
if ($request_method = 'POST') {
set $cors "${cors}post";
} # if it's a GET or POST, set the standard CORS responses header
if ($cors = "trueget") {
# Tells the browser this origin may make cross-origin requests
# (Here, we echo the requesting origin, which matched the whitelist.)
add_header 'Access-Control-Allow-Origin' "$http_origin";
# Tells the browser it may show the response, when XmlHttpRequest.withCredentials=true.
add_header 'Access-Control-Allow-Credentials' 'true';
# # Tell the browser which response headers the JS can see, besides the "simple response headers"
# add_header 'Access-Control-Expose-Headers' 'myresponseheader';
} if ($cors = "truepost") {
# Tells the browser this origin may make cross-origin requests
# (Here, we echo the requesting origin, which matched the whitelist.)
add_header 'Access-Control-Allow-Origin' "$http_origin";
# Tells the browser it may show the response, when XmlHttpRequest.withCredentials=true.
add_header 'Access-Control-Allow-Credentials' 'true';
# # Tell the browser which response headers the JS can see, besides the "simple response headers"
# add_header 'Access-Control-Expose-Headers' 'myresponseheader';
} # if it's OPTIONS, then it's a CORS preflight request so respond immediately with no response body
if ($cors = "trueoptions") {
# Tells the browser this origin may make cross-origin requests
# (Here, we echo the requesting origin, which matched the whitelist.)
add_header 'Access-Control-Allow-Origin' "$http_origin";
# in a preflight response, tells browser the subsequent actual request can include user credentials (e.g., cookies)
add_header 'Access-Control-Allow-Credentials' 'true'; #
# Return special preflight info
# # Tell browser to cache this pre-flight info for 20 days
add_header 'Access-Control-Max-Age' 1728000; # Tell browser we respond to GET,POST,OPTIONS in normal CORS requests.
#
# Not officially needed but still included to help non-conforming browsers.
#
# OPTIONS should not be needed here, since the field is used
# to indicate methods allowed for "actual request" not the
# preflight request.
#
# GET,POST also should not be needed, since the "simple
# methods" GET,POST,HEAD are included by default.
#
# We should only need this header for non-simple requests
# methods (e.g., DELETE), or custom request methods (e.g., XMODIFY)
add_header 'Access-Control-Allow-Methods' 'GET, POST, OPTIONS'; # Tell browser we accept these headers in the actual request
#
# A dynamic, wide-open config would just echo back all the headers
# listed in the preflight request's
# Access-Control-Request-Headers.
#
# A dynamic, restrictive config, would just echo back the
# subset of Access-Control-Request-Headers headers which are
# allowed for this resource.
#
# This static, fairly open config just returns a hardcoded set of
# headers that covers many cases, including some headers that
# are officially unnecessary but actually needed to support
# non-conforming browsers
#
# Comment on some particular headers below:
#
# Authorization -- practically and officially needed to support
# requests using HTTP Basic Access authentication. Browser JS
# can use HTTP BA authentication with an XmlHttpRequest object
# req by calling
#
# req.withCredentials=true, and
# req.setRequestHeader('Authorization','Basic ' + window.btoa(theusername + ':' + thepassword))
#
# Counterintuitively, the username and password fields on
# XmlHttpRequest#open cannot be used to set the authorization
# field automatically for CORS requests.
#
# Content-Type -- this is a "simple header" only when it's
# value is either application/x-www-form-urlencoded,
# multipart/form-data, or text/plain; and in that case it does
# not officially need to be included. But, if your browser
# code sets the content type as application/json, for example,
# then that makes the header non-simple, and then your server
# must declare that it allows the Content-Type header.
#
# Accept,Accept-Language,Content-Language -- these are the
# "simple headers" and they are officially never
# required. Practically, possibly required.
#
# Origin -- logically, should not need to be explicitly
# required, since it's implicitly required by all of
# CORS. officially, it is unclear if it is required or
# forbidden! practically, probably required by existing
# browsers (Gecko does not request it but WebKit does, so
# WebKit might choke if it's not returned back).
#
# User-Agent,DNT -- officially, should not be required, as
# they cannot be set as "author request headers". practically,
# may be required.
#
# My Comment:
#
# The specs are contradictory, or else just confusing to me,
# in how they describe certain headers as required by CORS but
# forbidden by XmlHttpRequest. The CORS spec says the browser
# is supposed to set Access-Control-Request-Headers to include
# only "author request headers" (section 7.1.5). And then the
# server is supposed to use Access-Control-Allow-Headers to
# echo back the subset of those which is allowed, telling the
# browser that it should not continue and perform the actual
# request if it includes additional headers (section 7.1.5,
# step 8). So this implies the browser client code must take
# care to include all necessary headers as author request
# headers.
#
# However, the spec for XmlHttpRequest#setRequestHeader
# (section 4.6.2) provides a long list of headers which the
# the browser client code is forbidden to set, including for
# instance Origin, DNT (do not track), User-Agent, etc.. This
# is understandable: these are all headers that we want the
# browser itself to control, so that malicious browser client
# code cannot spoof them and for instance pretend to be from a
# different origin, etc..
#
# But if XmlHttpRequest forbids the browser client code from
# setting these (as per the XmlHttpRequest spec), then they
# are not author request headers. And if they are not author
# request headers, then the browser should not include them in
# the preflight request's Access-Control-Request-Headers. And
# if they are not included in Access-Control-Request-Headers,
# then they should not be echoed by
# Access-Control-Allow-Headers. And if they are not echoed by
# Access-Control-Allow-Headers, then the browser should not
# continue and execute actual request. So this seems to imply
# that the CORS and XmlHttpRequest specs forbid certain
# widely-used fields in CORS requests, including the Origin
# field, which they also require for CORS requests.
#
# The bottom line: it seems there are headers needed for the
# web and CORS to work, which at the moment you should
# hard-code into Access-Control-Allow-Headers, although
# official specs imply this should not be necessary.
#
add_header 'Access-Control-Allow-Headers' 'Authorization,Content-Type,Accept,Origin,User-Agent,DNT,Cache-Control,X-Mx-ReqToken,Keep-Alive,X-Requested-With,If-Modified-Since'; # build entire response to the preflight request
# no body in this response
add_header 'Content-Length' 0;
# (should not be necessary, but included for non-conforming browsers)
add_header 'Content-Type' 'text/plain charset=UTF-8';
# indicate successful return with no content
return 204;
}
# --PUT YOUR REGULAR NGINX CODE HERE--
}
server解析流程例如以下：
a.首先查看http头部有无origin字段；
b.假设没有，或者不同意，直接当成普通请求处理，结束；
c.假设有而且是同意的。那么再看是否是preflight(method=OPTIONS)；
d.假设是preflight，就返回Allow-Headers、Allow-Methods等，内容为空；
e.假设不是preflight。就返回Allow-Origin、Allow-Credentials等，并返回正常内容。
若server为nginx。能够在nginx的conf文件里增加下面内容：
location / {
add_header Access-Control-Allow-Origin *;
}
若server为Apache，则能够依照例如以下配置：
<IfModule mod_setenvif.c>
<IfModule mod_headers.c>
<FilesMatch "\.(cur|gif|ico|jpe?g|png|svgz?|webp)$">
SetEnvIf Origin ":" IS_CORS
Header set Access-Control-Allow-Origin "*" env=IS_CORS
</FilesMatch>
</IfModule>
</IfModule>
为安全起见，Access-Control-Allow-Origin也可设为特定域名的方式。
在HTML5中，有些HTML元素为CORS提供了支持，如img、video新增了crossOrigin属性。属性值能够为anonymous或use-credentials。
比方，canvas画图要用到跨域图片，在JavaScript中要设置img.crossOrigin="Anonymous";
var img = new Image,
canvas = document.createElement("canvas"),
ctx = canvas.getContext("2d"),
src = "http://example.com/image"; // insert image url here img.crossOrigin = "Anonymous"; img.onload = function() {
canvas.width = img.width;
canvas.height = img.height;
ctx.drawImage( img, 0, 0 );
localStorage.setItem( "savedImageData", canvas.toDataURL("image/png") );
}
img.src = src;
// make sure the load event fires for cached images too
if ( img.complete || img.complete === undefined ) {
img.src = "data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///ywAAAAAAQABAAACAUwAOw==";
img.src = src;
}
上述配置完毕后，重新启动server。CORS启用。
然后我们再刷新页面，查询请求头的參数，能够发现多出一个：Access-Control-Allow-Origin:*
，到此证明server配置已经生效。
同一时候我们的canvas画图也能够正常使用了。
刷新页面返回请求响应结果后，HTTP Request Headers的内容：
Remote Address:222.132.18.xx:80
Request URL:http://js.xx.com/static/activity/sq/guagua315/images/card_inner.jpg
Request Method:GET
Status Code:200 OK
Request Headersview source
Accept:image/webp,*/*;q=0.8
Accept-Encoding:gzip, deflate, sdch
Accept-Language:zh-CN,zh;q=0.8
Cache-Control:no-cache
Connection:keep-alive
Host:js.xx.com
Origin:http://act.xx.com
Pragma:no-cache
RA-Sid:7CCAD53E-20140704-054839-03c57a-85faf2
RA-Ver:2.8.8
Referer:http://act.xx.com/sq/guagua315?
uuid=46124642&fid=2&sign=xxx
User-Agent:Mozilla/5.0 (Windows NT 6.1;WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/40.0.2214.115Safari/537.36
Response Headersview source
Accept-Ranges:bytes
Access-Control-Allow-Origin:*
Connection:close
Content-Length:4010
Content-Type:image/jpeg
Date:Thu, 12 Mar 2015 02:29:43 GMT
ETag:"54f7d1b4-faa"
Last-Modified:Thu, 05 Mar 2015 03:47:00 GMT
Powered-By-ChinaCache:MISS fromCNC-WF-3-3X6
Powered-By-ChinaCache:MISS fromCNC-WF-3-3X5
Server:Tengine
Switch:FSCS
附图：
watermark/2/text/aHR0cDovL2Jsb2cuY3Nkbi5uZXQvZnJlc2hsb3Zlcg==/font/5a6L5L2T/fontsize/400/fill/I0JBQkFCMA==/dissolve/70/gravity/SouthEast" alt="">
參考文章：
CORS enabled image https://developer.mozilla.org/en-US/docs/Web/HTML/CORS_enabled_image
CORS on Nginx http://enable-cors.org/server_nginx.html
Nginx CORS实现JS跨域 http://blog.csdn.net/oyzl68/article/details/18741057
转载请注明出处。文章来自于freshlover的CSDN空间《Ajax跨域、Json跨域、Socket跨域和Canvas跨域等同源策略限制的解决方法》