DNS 缓存（TTL）
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DNS 缓存是对 DNS 查询结果的临时存储，由以秒为单位的 TTL（Time-To-Live，生存时间）值控制。缓存通过复用近期查询结果而非每次请求都重新查询，减少了 DNS 查找时间和网络流量。
示例
A TTL of 3600 means DNS resolvers will cache the record for 1 hour before querying the authoritative server again.