1 Linux DNS规范
Linux上并没有一个单独的方法可以完成DNS查询工作；没有一个有这样的明确接口的核心系统调用system call。不过，glibc (nss)的getaddrinfo (3), gethostbyname (3)等相关API (RFC3493)提供了DNS查询功能。
1）不支持nscd（name service cache daemon）服务，那么这2个函数接口通过文件/etc/resolv.conf获取DNS服务器的IP地址，然后通过socket通信发送请求到该DNS IP地址去获取待查询域名的IP地址。
2）支持nscd服务（or dnsmasq），这2个函数接口发送请求给nscd，nscd再返回响应给请求者。
3）ping uses glibc gethostbyname()。

2 Android netd DNS
2.1 Android 4.3 DNS
Android 4.3 incorporated a somewhat confusing series of commits under the heading of "dns cache per interface," which effectively causes all Bionic DNS requests to be proxied through netd.

ANDROID_DNS_MODE=local
dumpsys connectivity
dumpsys netd
ndc tether dns set
ndc tether dns list

iptables -t nat -I OUTPUT -p udp \
--dport 53 -j DNAT \
--to-destination 192.168.1.5:53
iptables -t nat -I OUTPUT -p tcp \
--dport 53 -j DNAT \
--to-destination 192.168.1.5:53

This works by intercepting the DNS requests meant for the remote nameserver and redirecting it to the local DNS server. 

2.2 Android netd DNS
libcore/ojluni/src/main/java/java/net/InetAddress.java
netd中dns server设置文件：ResolverController.cpp
netd中域名解析文件：DnsProxyListener.cpp

Figure 2-1 Android netd DNS

2.3 libc DNS解析函数
Android bionic: based on OpenBSD, b means BSD
bionic/libc/dns/net/getaddrinfo.c
getaddrinfo(): IPv4 and IPv6
bionic/libc/dns/net/getnameinfo.c
getnameinfo()
bionic/libc/dns/net/gethnamaddr.c
gethostbyname(): only IPv4, gethostbyname_r(), r means reentry

通过netd获取DNS服务器的IP地址（Ubuntu通过进程dnsmasq和配置文件/etc/resolv.conf），然后构建DNS请求包，查询域名对应的IP地址，并且按照netId的分类缓存到netd中：
bionic/libc/dns/net/gethnamaddr.c
gethostbyname_internal_real()

2.4 Android三个网络基础库DNS解析
1) JDK HttpURLConnection
2) Apache HttpClient，Android 6.0之后不再支持HttpClient
3) OkHttp, third-party library

以上3个类库都会调用下面的JDK函数
InetAddress.getByName()
getaddrinfo() - libc DNS parser API

2.5 DHCP
DHCP Offer报文Options中包含了DNS Server地址

Android GB system/core/libnetutils/dhcp_utils.c
int dhcp_do_request(...,
in_addr_t *dns1, in_addr_t *dns2...)

3 Android netId
3.1 获取网络接口的netId
netd中将接口名（wlan0、eth0等）转化为netId的函数：
server/NetworkController.cpp
NetworkController::getNetworkForInterface(
const char* interface)

3.2 代码示例
server/CommandListener.cpp
函数CommandListener::NetworkCommand::runCommand()的最后添加如下的代码：
//     0            1                2
// network query_netid if_name
if (!strcmp(argv[1], "query_netid")) {
    if (argc < 3) {
        return syntaxError(client,
            "Missing argument");
    }
    int _netId =
        gCtls->netCtrl.getNetworkForInterface(
            argv[2]);
    char msg[16] = {0};
    int retval = snprintf(msg, 15, "%d", _netId);
    if (retval > 0) {
        client->sendMsg(
            ResponseCode::CommandOkay,
            msg, false);
        return 0;
    } else {
        return operationError(client,
            "Can not get netId", retval);
    }
}
用法：ndc network query_netid wlan0

4 ndc
4.1 查询可用命令表
ndc interface list

4.2 清除netd DNS缓存
ndc resolver flushif eth0
ndc resolver flushdefaultif -- flush default DNS resolver

Android 8.0没有导出刷新DNS缓存的接口，可以调用如下API添加。
bionic/libc/dns/resolv/res_cache.c
_resolv_flush_cache_for_net(netId)

4.3 设置netd DNS
ndc resolver setnetdns ...
ndc resolver setnetdns eth0 "" 8.8.8.8 192.168.1.1
ndc resolver setnetdns eth0 localdomain 8.8.8.8 192.168.1.1

5 Abbreviations
dnsmasq: DNS masquerade
mdnsd: multicast DNS，组播DNS
ndc：Native Daemon Connector
ojluni：Android libcore中的模块，包含OpenJDK、Language、Util、Net、IO