PTR レコード
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定義
ポインターレコード（Pointer Record）。IPアドレスをドメイン名に逆引きする逆引きDNSルックアップに使用されるDNSレコードです。メールサーバーの検証とネットワーク診断に不可欠です。
Reverse DNS Architecture
Forward DNS maps names to IPs. PTR records enable reverse DNS (rDNS): mapping an IP back to a name. Reverse DNS uses a special domain: in-addr.arpa for IPv4Internet Protocol version 4. The fourth revision of IP using 32-bit addresses (e.g., 192.168.1.1), providing approximately 4.3 billion unique addresses. Still the most widely used internet protocol despite address exhaustion. and ip6.arpa for IPv6Internet Protocol version 6. The successor to IPv4 using 128-bit addresses (e.g., 2001:0db8::1), providing a virtually unlimited address space of 3.4 x 10^38 addresses. Designed to solve IPv4 address exhaustion.. An IP like 203.0.113.45 is queried as 45.113.0.203.in-addr.arpa — the octets reversed, with .in-addr.arpa appended.
Authority over reverse zones follows IP allocation. IANA delegates in-addr.arpa to RIRs, which delegate sub-zones to ISPs, which delegate to their customers. If your organization owns a /24 block, your ISP can delegate the corresponding /24 reverse zone (e.g., 113.0.203.in-addr.arpa) to your Authoritative DNS ServerA DNS server that holds the original, definitive DNS records for a domain zone. It provides answers directly from its zone data rather than querying other servers, serving as the source of truth for that domain. servers, allowing you to manage PTR records for your own IPs.
Why PTR Records Matter
Email servers validate PTR records to reduce spam. Many mail transfer agents (MTAs) perform a reverse lookup on the connecting server's IP and check that the resulting hostname forward-resolves back to the same IP (forward-confirmed rDNS, or FCrDNS). Mail from IPs lacking PTR records or failing FCrDNS is often rejected or scored as spam. If you run a mail server, configuring a PTR record on your Public IP AddressA globally unique IP address assigned by an ISP that is routable on the public internet. Every device directly accessible from the internet must have a public IP address. is essential.
PTR records also improve log readability. Network logs that record IP addresses become far more interpretable when reverse resolution provides meaningful hostnames like mail.company.com instead of bare numbers.
IPv6 PTR Records
IPv6Internet Protocol version 6. The successor to IPv4 using 128-bit addresses (e.g., 2001:0db8::1), providing a virtually unlimited address space of 3.4 x 10^38 addresses. Designed to solve IPv4 address exhaustion. PTR records use ip6.arpa with all 32 hex nibbles reversed. The address 2001:db8::1 becomes 1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.8.b.d.0.1.0.0.2.ip6.arpa. Use Reverse DNS Lookup to look up PTR records for any IP address.