DNS Diagnostics
역방향 DNS 및 FCrDNS
Check PTR records, verify forward-confirmed reverse DNS, and explain whether a mail server or outbound IP has a trustworthy reverse DNS setup.
PTR Lookup
Fetches reverse DNS hostnames directly from the resolver path.
FCrDNS Check
Confirms whether PTR hostnames resolve back to the same IP.
Operational Context
Helps explain failures that affect mail, abuse desks, and server trust.
Examples
Use this before troubleshooting SMTP reputation, outbound relay issues, or host identity mismatches.
Why FCrDNS matters
Mail receivers, security tools, and abuse workflows often expect the PTR hostname to round-trip back to the same IP.
Common failure mode
A PTR may exist but point at a hostname that resolves to a different IP, usually after renumbering or DNS drift.
What “good” looks like
One intentional PTR hostname, a matching forward lookup, and a hostname pattern that reflects the server role.
이 도구에 대해
사용 방법
- Enter an IPv4 or IPv6 address that you want to verify.
- Run the reverse DNS check to fetch PTR hostnames.
- Review the forward-confirmation panel to see whether those hostnames resolve back to the same IP.
- Use the warnings section to identify missing PTR records or hostname mismatches.
- Cross-check the result with your MTA, cloud IP allocation, or DNS provider if the verification fails.
기능
- Reverse DNS lookup for IPv4 and IPv6 addresses
- Forward-confirmed reverse DNS verification
- Mismatch detection for stale or incorrect PTR hostnames
- Human-readable verdicts for mail and operations teams
- Landing-page style explanations for SEO and educational use
일반적인 사용 사례
- Checking outbound SMTP relay identity before production rollout
- Auditing cloud instances after IP renumbering
- Troubleshooting spam filtering or poor mail reputation
- Validating provider-managed PTR delegation
- Explaining host identity during incident response or abuse review
기술 세부사항
Reverse DNS stores hostnames in PTR records under reverse-mapping zones such as in-addr.arpa for IPv4 and ip6.arpa for IPv6.
Forward-confirmed reverse DNS means:
- The IP resolves to one or more PTR hostnames.
- Those hostnames resolve forward with A or AAAA lookups.
- At least one forward address matches the original IP.
This check does not prove trust by itself, but it is a widely used hygiene signal in mail systems, abuse processing, and infrastructure operations.
Reverse DNS FAQ
What is the difference between reverse DNS and FCrDNS?
Reverse DNS only asks for PTR records. FCrDNS goes one step further and checks whether the returned hostname resolves back to the same IP address.
Does a missing PTR always break mail delivery?
Not always, but it often hurts reputation and can trigger stricter filtering. Many receivers expect outbound mail IPs to have sensible reverse DNS.
Can one IP have multiple PTR records?
Yes. It is allowed, but multiple PTRs should be intentional because they can confuse identity-based checks and operational debugging.
Why can PTR exist while forward confirmation fails?
The hostname may have been moved, deleted, or updated incorrectly. Reverse and forward DNS often drift apart during migrations or IP reassignments.