在Android11 版本的rom 产品开发过程中，遇到一个问题，发现分配的DNS不可用的情况，所以需要设置备用DNS。

直接上代码

//frameworks/base/services/core/java/com/android/server/ConnectivityService
//ConnectivityService.java -->
//updateLinkProperties() -->
//updateDnses(newLp, oldLp, netId);private void updateDnses(LinkProperties newLp, LinkProperties oldLp, int netId) {if (oldLp != null && newLp.isIdenticalDnses(oldLp)) {return;  // no updating necessary}final NetworkAgentInfo defaultNai = getDefaultNetwork();final boolean isDefaultNetwork = (defaultNai != null && defaultNai.network.netId == netId);if (DBG) {final Collection<InetAddress> dnses = newLp.getDnsServers();log("Setting DNS servers for network " + netId + " to " + dnses);}try {mDnsManager.noteDnsServersForNetwork(netId, newLp);// TODO: netd should listen on [::1]:53 and proxy queries to the current// default network, and we should just set net.dns1 to ::1, not least// because applications attempting to use net.dns resolvers will bypass// the privacy protections of things like DNS-over-TLS.if (isDefaultNetwork) mDnsManager.setDefaultDnsSystemProperties(newLp.getDnsServers());mDnsManager.flushVmDnsCache();} catch (Exception e) {loge("Exception in setDnsConfigurationForNetwork: " + e);}}

在这里，我们添加如下代码：

 private void updateDnses(LinkProperties newLp, LinkProperties oldLp, int netId) {if (oldLp != null && newLp.isIdenticalDnses(oldLp)) {return;  // no updating necessary}final NetworkAgentInfo defaultNai = getDefaultNetwork();final boolean isDefaultNetwork = (defaultNai != null && defaultNai.network.netId == netId);if (DBG) {final Collection<InetAddress> dnses = newLp.getDnsServers();log("Setting DNS servers for network " + netId + " to " + dnses);}try {//add code hereList<InetAddress> dnsServers = newLp.getDnsServers();dnsServers.add(InetAddress.getByName("xxxxxxxxxx"));dnsServers.add(InetAddress.getByName("xxxxxxxxxx"));//-----------mDnsManager.noteDnsServersForNetwork(netId, newLp);// TODO: netd should listen on [::1]:53 and proxy queries to the current// default network, and we should just set net.dns1 to ::1, not least// because applications attempting to use net.dns resolvers will bypass// the privacy protections of things like DNS-over-TLS.if (isDefaultNetwork) mDnsManager.setDefaultDnsSystemProperties(newLp.getDnsServers());mDnsManager.flushVmDnsCache();} catch (Exception e) {loge("Exception in setDnsConfigurationForNetwork: " + e);}}

添加完，单编services,发现报错，

发现报错原因在于：

   /*** Returns all the {@link InetAddress} for DNS servers on this link.** @return An unmodifiable {@link List} of {@link InetAddress} for DNS servers on*         this link.*/public @NonNull List<InetAddress> getDnsServers() {return Collections.unmodifiableList(mDnses);}

既然不能对原有的List做修改，那就曲线来一把，如下：

  List<InetAddress> oriServers = newLp.getDnsServers();List<InetAddress> dnsServers = new ArrayList<InetAddress>();dnsServers.addAll(oriServers);dnsServers.add(InetAddress.getByName("xxxxxxxxxx"));dnsServers.add(InetAddress.getByName("xxxxxxxxxx"));newLp.setDnsServers(dnsServers);

编译验证，可以看到，如果分配的DNS不可用的时候，系统就会使用备用的DNS！！！