这两天把家里 DNS 改成 RA 下发, 遇到了标题上的问题
使用 radvdump, 输出为:

interface switch0
{
        AdvSendAdvert on;
        # Note: {Min,Max}RtrAdvInterval cannot be obtained with radvdump
        AdvManagedFlag off;
        AdvOtherConfigFlag off;
        AdvReachableTime 0;
        AdvRetransTimer 0;
        AdvCurHopLimit 64;
        AdvDefaultLifetime 1800;
        AdvHomeAgentFlag off;
        AdvDefaultPreference medium;
        AdvSourceLLAddress on;

        prefix [GUA Prefix]
        {
                AdvValidLifetime 2592000;
                AdvPreferredLifetime 604800;
                AdvOnLink on;
                AdvAutonomous on;
                AdvRouterAddr off;
        }; # End of prefix definition


        prefix [ULA Prefix]
        {
                AdvValidLifetime 2592000;
                AdvPreferredLifetime 604800;
                AdvOnLink on;
                AdvAutonomous on;
                AdvRouterAddr off;
        }; # End of prefix definition


        RDNSS [DNS]
        {
                AdvRDNSSLifetime 600;
        }; # End of RDNSS definition

}; # End of interface definition

默认为 600 秒, 显然是太短了, 息屏超过十分钟 DNS 过期, 导致唤醒时优先使用移动数据. EdgeOS 没有提供内置的 RDNSS Lifetime 节点, 但是可以设置 radvd-options "RDNSS [DNS] { AdvRDNSSLifetime 86400; };" 来解决这个问题, 注意要删除 name-server 节点.
修正后的 radvdump 输出:

interface switch0
{
        AdvSendAdvert on;
        # Note: {Min,Max}RtrAdvInterval cannot be obtained with radvdump
        AdvManagedFlag off;
        AdvOtherConfigFlag off;
        AdvReachableTime 0;
        AdvRetransTimer 0;
        AdvCurHopLimit 64;
        AdvDefaultLifetime 1800;
        AdvHomeAgentFlag off;
        AdvDefaultPreference medium;
        AdvSourceLLAddress on;

        prefix [GUA Prefix]
        {
                AdvValidLifetime 2592000;
                AdvPreferredLifetime 604800;
                AdvOnLink on;
                AdvAutonomous on;
                AdvRouterAddr off;
        }; # End of prefix definition


        prefix [ULA Prefix]
        {
                AdvValidLifetime 2592000;
                AdvPreferredLifetime 604800;
                AdvOnLink on;
                AdvAutonomous on;
                AdvRouterAddr off;
        }; # End of prefix definition


        RDNSS [DNS]
        {
                AdvRDNSSLifetime 86400;
        }; # End of RDNSS definition

}; # End of interface definition

如果你最近遇到了类似的问题, 可以参考一下.