反向 DNS（PTR 记录）
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反向 DNS 查找使用 PTR 记录将 [[ip-address|IP 地址]] 映射回域名，与标准 [[dns|DNS]] 查找相反。反向 DNS 通常用于电子邮件服务器验证——许多邮件服务器会拒绝来自没有有效 PTR 记录的 IP 的邮件。
示例
Mail servers check that the IP 203.0.113.1 has a PTR record matching mail.example.com before accepting email from it.