Android8.0网络DNS

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 <netid> <domain> <dns1> <dns2> ... 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