android是如何 DNS解析
2014/02/10 13:22 | by ipaddr ]
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 access to the Internet Domain Name System (DNS). The resolver configuration file contains information that is read by the resolver routines the first time they are invoked by a process. The file is designed to be human readable and contains a list of keywords with values that provide various 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：
DNS 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。
(tomzhou注：经测试，系统主要使用net.dns1, net.dns2两个配置进行解析，wifi & 3g网络在启用时，都会调用相关脚本设置这两个属性，同时还会修改net.change， net.dnschange等属性）。
实际代码中调用关系如下：
gethostbyaddr
_dns_gethtbyaddr
_dns_gethtbyaddr
_dns_gethtbyaddr
res_nquery
_dns_gethtbyaddr
__res_get_state
__res_get_state
_res_thread_get
_res_thread_get
res_ninit
res_ninit
__res_vinit()
