今天在curl一个网站的时候遇到一个奇怪的问题，下面是输出：
lxg@lxg-X240:~$ curl -L http://www.yngs.gov.cn/ -v
* Hostname was NOT found in DNS cache
* Trying 116.52.12.163…
* Connected to www.yngs.gov.cn (116.52.12.163) port 80 (#0)
GET / HTTP/1.1
User-Agent: curl/7.38.0
Host: www.yngs.gov.cn
Accept: /< HTTP/1.1 302 Moved Temporarily
< Date: Wed, 04 Nov 2015 14:08:49 GMT
< Transfer-Encoding: chunked
< Location: http://www.yngs.gov.cn/newWeb/template/index.jsp
< Content-Type: text/html; charset=UTF-8
< Set-Cookie: JSESSIONID=SLyTW6RR3R7zPNkkvzvpj12Q1snzzvNFQjYPDbDhYbvgTXWhSnff!-995202664; path=/; HttpOnly
< X-Powered-By: *********
< Set-Cookie: SANGFOR_AD=20111157; path=/
<
* Ignoring the response-body
* Connection #0 to host www.yngs.gov.cn left intact
* Issue another request to this URL: ‘http://www.yngs.gov.cn/newWeb/template/index.jsp’
* Found bundle for host www.yngs.gov.cn: 0xb89840c0
* Re-using existing connection! (#0) with host www.yngs.gov.cn
* Connected to www.yngs.gov.cn (116.52.12.163) port 80 (#0)
GET /newWeb/template/index.jsp HTTP/1.1
User-Agent: curl/7.38.0
Host: www.yngs.gov.cn
Accept: /
……… //上面的输出一直重复
* Ignoring the response-body
* Connection #0 to host www.yngs.gov.cn left intact
* Maximum (50) redirects followed
curl: (47) Maximum (50) redirects followed
最后的错误显示超过了curl设定的最大50次跳转。
从上面的输出来看访问http://www.yngs.gov.cn/的时候返回302跳转，跳转的url为http://www.yngs.gov.cn/newWeb/template/index.jsp，但是接着访问 http://www.yngs.gov.cn/newWeb/template/index.jsp的时候还是返回同样的302跳转，跳转后的地址是目标自身，这样肯定就会一直在 http://www.yngs.gov.cn/newWeb/template/index.jsp这个url上跳转，当超过curl设定的默认最大跳转次数50以后就异常结束了。
既然curl有问题那么试一下wget命令看看吧，看这个命令是否也是会遇到同样的错误结果：
lxg@lxg-X240:~$ wget http://www.yngs.gov.cn/ –debug
—request begin—
GET / HTTP/1.1
User-Agent: Wget/1.16.1 (linux-gnu)
Accept: /
Accept-Encoding: identity
Host: www.yngs.gov.cn
Connection: Keep-Alive
—request end——response begin—
HTTP/1.1 302 Moved Temporarily
Date: Wed, 04 Nov 2015 14:18:51 GMT
Transfer-Encoding: chunked
Location: http://www.yngs.gov.cn/newWeb/template/index.jsp
Content-Type: text/html; charset=UTF-8
Set-Cookie: JSESSIONID=7JJTW6TLpKRF0vyNXtRpQrnZffkgDfB0vh6vDzQ9jhGNvRsmZxyv!-1122044597; path=/; HttpOnly
X-Powered-By: *********
Set-Cookie: SANGFOR_AD=20111151; path=/
—response end—302 Moved Temporarily
Stored cookie www.yngs.gov.cn -1 (ANY) / <session> <insecure> [expiry none] JSESSIONID 7JJTW6TLpKRF0vyNXtRpQrnZffkgDfB0vh6vDzQ9jhGNvRsmZxyv!-1122044597
Stored cookie www.yngs.gov.cn -1 (ANY) / <session> <insecure> [expiry none] SANGFOR_AD 20111151
Registered socket 3 for persistent reuse.
URI content encoding = “UTF-8”
位置：http://www.yngs.gov.cn/newWeb/template/index.jsp [跟随至新的 URL]URI content encoding = None
–2015-11-04 22:23:09– http://www.yngs.gov.cn/newWeb/template/index.jsp
再次使用存在的到 www.yngs.gov.cn:80 的连接。
Reusing fd 3.—request begin—
GET /newWeb/template/index.jsp HTTP/1.1
User-Agent: Wget/1.16.1 (linux-gnu)
Accept: /
Accept-Encoding: identity
Host: www.yngs.gov.cn
Connection: Keep-Alive
Cookie: JSESSIONID=7JJTW6TLpKRF0vyNXtRpQrnZffkgDfB0vh6vDzQ9jhGNvRsmZxyv!-1122044597; SANGFOR_AD=20111151
—request end——response begin—
HTTP/1.1 200 OK
Date: Wed, 04 Nov 2015 14:18:51 GMT
Transfer-Encoding: chunked
Content-Type: text/html; charset=UTF-8
X-Powered-By: *********
—response end—
200 OK
上面是wget的执行结果（去掉了一些多余的输出），我们看到wget能正常的获取到http://www.yngs.gov.cn/的结果，并没有出现curl遇到的死循环跳转的问题。那么也就是说这个网站本身是没有问题的，只是我们访问的时候可能缺少了一些参数。
接着我对比了一下curl和wget的request、response信息，我发现两者在第一次请求http://www.yngs.gov.cn/的时候request、response都是差不多的，不同的可能就是user-agent。但是当再次请求302返回回来的redirect url http://www.yngs.gov.cn/newWeb/template/index.jsp的时候两者的request中的参数就有一些不一样了，wget的请求中是把第一次响应返回Cookie带上了，但是curl却是Ignoring the response-body，忽略了第一次响应返回的数据，第二次请求的时候没有带上第一次请求返回的Cookie。
此时基本可以判断是因为curl访问的时候默认忽略了response返回的数据，redirect url的时候没有设置Cookie导致的，那么怎么来验证呢？
第一种方法就是禁掉wget的Cookie看还能否正常获取内容：
lxg@lxg-X240:~$ wget http://www.yngs.gov.cn/ –debug –no-cookies
Setting –cookies (cookies) to 0
—request begin—
GET / HTTP/1.1
User-Agent: Wget/1.16.1 (linux-gnu)
Accept: /
Accept-Encoding: identity
Host: www.yngs.gov.cn
Connection: Keep-Alive
—request end——response begin—
HTTP/1.1 302 Moved Temporarily
Date: Wed, 04 Nov 2015 14:43:41 GMT
Transfer-Encoding: chunked
Location: http://www.yngs.gov.cn/newWeb/template/index.jsp
Content-Type: text/html; charset=UTF-8
Set-Cookie: JSESSIONID=SDLtW6ZdvQwPpqGR5mBf2N1TxChNlySvTN8lhDBTQpyP3KvDdr0R!-170174379; path=/; HttpOnly
X-Powered-By: *********
Set-Cookie: SANGFOR_AD=20111158; path=/—response end—
–2015-11-04 22:46:33– http://www.yngs.gov.cn/newWeb/template/index.jsp
再次使用存在的到 www.yngs.gov.cn:80 的连接。
Reusing fd 3.—request begin—
GET /newWeb/template/index.jsp HTTP/1.1
User-Agent: Wget/1.16.1 (linux-gnu)
Accept: /
Accept-Encoding: identity
Host: www.yngs.gov.cn
Connection: Keep-Alive
—request end——response begin—
HTTP/1.1 302 Moved Temporarily
Date: Wed, 04 Nov 2015 14:42:16 GMT
Transfer-Encoding: chunked
Location: http://www.yngs.gov.cn/newWeb/template/index.jsp
Content-Type: text/html; charset=UTF-8
Set-Cookie: JSESSIONID=YQJTW6ZLpWhm7pfr3LzL6lkQdQ1XbnBMCHQhjn7vZ2yptMJvsJvW!-1122044597; path=/; HttpOnly
X-Powered-By: *********
Set-Cookie: SANGFOR_AD=20111151; path=/
—response end—…………….
URI content encoding = None
已超过 20 次重定向。
我们看到wget最后也是以尝试20次跳转失败结束。
第二中方法就是开启curl的Cookie：
lxg@lxg-X240:~$ curl -L -b /tmp/curl.cookies http://www.yngs.gov.cn/
* Hostname was NOT found in DNS cache
* Trying 116.52.12.163…
* Connected to www.yngs.gov.cn (116.52.12.163) port 80 (#0)
GET / HTTP/1.1
User-Agent: curl/7.38.0
Host: www.yngs.gov.cn
Accept: /< HTTP/1.1 302 Moved Temporarily
< Date: Wed, 04 Nov 2015 14:55:53 GMT
< Transfer-Encoding: chunked
< Location: http://www.yngs.gov.cn/newWeb/template/index.jsp
< Content-Type: text/html; charset=UTF-8
* Added cookie JSESSIONID=”lswQW6cZzRtvyGkkJm0hL8RscHT98bcC3YD4f4V1RCJvLLwb2ZMJ!-1122044597” for domain www.yngs.gov.cn, path /, expire 0
< Set-Cookie: JSESSIONID=lswQW6cZzRtvyGkkJm0hL8RscHT98bcC3YD4f4V1RCJvLLwb2ZMJ!-1122044597; path=/; HttpOnly
< X-Powered-By: *********
* Added cookie SANGFOR_AD=”20111151” for domain www.yngs.gov.cn, path /, expire 0
< Set-Cookie: SANGFOR_AD=20111151; path=/
<
* Ignoring the response-body
* Connection #0 to host www.yngs.gov.cn left intact
* Issue another request to this URL: ‘http://www.yngs.gov.cn/newWeb/template/index.jsp’
* Found bundle for host www.yngs.gov.cn: 0xb8b74108
* Re-using existing connection! (#0) with host www.yngs.gov.cn
* Connected to www.yngs.gov.cn (116.52.12.163) port 80 (#0)
GET /newWeb/template/index.jsp HTTP/1.1
User-Agent: curl/7.38.0
Host: www.yngs.gov.cn
Accept: /
Cookie: JSESSIONID=lswQW6cZzRtvyGkkJm0hL8RscHT98bcC3YD4f4V1RCJvLLwb2ZMJ!-1122044597; SANGFOR_AD=20111151< HTTP/1.1 200 OK
< Date: Wed, 04 Nov 2015 14:55:53 GMT
< Transfer-Encoding: chunked
< Content-Type: text/html; charset=UTF-8
< X-Powered-By: *********
<
<!DOCTYPE html>
<html xmlns=”http://www.w3.org/1999/xhtml”>
<head>
………………..Connection #0 to host www.yngs.gov.cn left intact
</html>
curl成功获取到了结果。
从上面的结果也就验证了之前的猜想。
curl -h
...
-L/--location Follow Location: hints (H)
--location-trusted Follow Location: and send authentication even
to other hostnames (H)
-m/--max-time <seconds> Maximum time allowed for the transfer
--max-redirs <num> Maximum number of redirects allowed (H)
--max-filesize <bytes> Maximum file size to download (H/F)
...
curl -L --max-redirs 100000 URL