逆引きDNS（PTR）検索

このツールの使い方

1. 1

   Type or paste an IP address into the input box, for example 8.8.8.8 or an IPv6 address like 2001:4860:4860::8888.

2. 2

   Click Lookup to send the address to our server, which runs a reverse DNS (PTR) query against the public DNS system.

3. 3

   Read the returned hostnames in the results list, or the 'No PTR record found' notice if the address has no PTR record or the lookup fails.

4. 4

   Try another address to compare results, for example a mail server IP versus a residential or cloud IP.

逆引きDNS（PTR）検索とは何ですか？

よくある活用例

• Confirm a sending mail server's IP has a valid PTR record before going live, since Gmail and Microsoft 365 often reject or junk mail from IPs with no reverse DNS.

• Read web server or firewall access logs and turn raw IP addresses into hostnames to spot which networks or providers are connecting.

• Verify that a cloud or VPS provider correctly applied the custom PTR record you requested for an outbound IP.

• Investigate a suspicious IP during incident response to see whether it points back to a known ISP, hosting company, or CDN.

• Sanity-check a newly delegated IP block by confirming PTR records resolve as expected after a network migration.

• Troubleshoot SSH or service connections where the daemon does a reverse lookup on the client IP and a missing PTR causes slow or failed logins.

よくある質問