Android: 获取dns IP地址

在Android系统中，想获取dns server的IP地址并不是一件容易的事，在bionic中，有关于获取dns IP地址的相关代码，但是你在native代码中却不能接调用相关的函数获取。

如在bionic/libc/include/resolv.h中有如下声明：

__BEGIN_DECLS
#pragma GCC visibility push(default)

struct res_state;

extern struct __res_state *__res_state(void);
#define _res (*__res_state())

#define b64_ntop __b64_ntop
#define b64_pton __b64_pton
extern int b64_ntop(u_char const*, size_t, char*, size_t);
extern int b64_pton(char const*, u_char*, size_t);

#define dn_comp __dn_comp
extern int dn_comp(const char*, u_char*, int, u_char**, u_char**);
extern int dn_expand(const u_char*, const u_char*, const u_char*, char*, int);

#pragma GCC visibility pop
__END_DECLS

文件中设置了编译条件：visibility为default, 而在bionic/libc/Android.mk文件中，将visibility设置为了hidden, 就是使得你在native代码中使用了_res或者是__res_state()函数时，在进行编译链接时，会报如下错误：

: error: undefined reference to '__res_state'
collect2: error: ld returned 1 exit status

既然这条路行不通，那我们还有什么别的办法吗，先看一下与dns相关的系统属性：

$ adb shell getprop | grep dns
[dhcp.wlan0.dns1]: [10.0.0.1]
[dhcp.wlan0.dns2]: []
[dhcp.wlan0.dns3]: []
[dhcp.wlan0.dns4]: []
[net.change]: [net.dns1]
[net.dns1]: [10.0.0.1]

这里我们可以看到当前连接的wifi网络的dns以及系统全局使用的dns(net.dns1)，也许，我们只能从系统属性获取dns了。

在java代码中，我们可以通过LinkProperties即调用ConnectivityManager.getLinkProperties()得到特定网络的相关信息，当然这其中也包含了dns server的信息(LinkProperties.getDnsServers())：

Describes the properties of a network link. A link represents a connection to a network. It may have multiple addresses and multiple gateways, multiple dns servers but only one http proxy and one network interface. Note that this is just a holder of data. Modifying it does not affect live networks.

相关的代码如下：

        // ...
        ConnectivityManager cm = (ConnectivityManager)getSystemService(CONNECTIVITY_SERVICE);
        NetworkInfo activeNetworkInfo = cm.getActiveNetworkInfo();
        for (Network network : cm.getAllNetworks()) {
            NetworkInfo networkInfo = cm.getNetworkInfo(network);
            if (networkInfo.getType() == activeNetworkInfo.getType()) {
                LinkProperties lp = cm.getLinkProperties(network);
                for (InetAddress addr : lp.getDnsServers()) {
                    // Get DNS IP address here:
                }
            }
        }
        // ...