android是如何做DNS解析的

通过ubuntu等Linux系统都是通过resolve.conf文件进行域名解析的，通过man resolve.conf可以查看到：

The resolver is a set of routines in the C library that provide accessto the Internet Domain Name System (DNS). The resolver configurationfile contains information that is read by the resolver routines thefirst time they are invoked by a process. The file is designed to be human readable and contains a list of keywords with values that providevarious types of resolver information.If this file doesn't exist the only name server to be queried will be on the local machine; the domain name is determined from the hostname and the domain search path is constructed from the domain name.

所以DNS解析是由C库函数进行解析的，在Android中bionic/libc/docs/overview.txt文件中我们也可以看到：

NS resolver:
Bionic uses a NetBSD-derived resolver library which has been modified in the following ways:
- don't implement the name-server-switch feature (a.k.a. <nsswitch.h>)
- read /system/etc/resolv.conf instead of /etc/resolv.conf
- read the list of servers from system properties. the code looks for 'net.dns1', 'net.dns2', etc.. Each property should contain the IP address of a DNS server.
these properties are set/modified by other parts of the Android system (e.g. the dhcpd daemon).
the implementation also supports per-process DNS server list, using the properties 'net.dns1.<pid>', 'net.dns2.<pid>', etc... Where <pid> stands for the numerical ID of the current process.
- when performing a query, use a properly randomized Query ID (instead of a incremented one), for increased security.
- when performing a query, bind the local client socket to a random port for increased security.
- get rid of *many* unfortunate thread-safety issues in the original code
Bionic does *not* expose implementation details of its DNS resolver; the content of <arpa/nameser.h> is intentionally blank. The resolver implementation might change completely in the future.

所以android关注的是resolv.conf以及net.dns1等属性。

通过adb shell中通过getprop查看属性，确实有net.dns1、net.dns2以及wifi中的net.tiwlan0.dns1以及net.tiwlan0.dns2，移动网络中的net.rmnet0.dns1以及net.rmnet0.dns2等属性。