What Is Reverse DNS Lookup?
Reverse DNS lookup is the process of resolving an IP address back to its associated hostname by querying PTR (Pointer) records. Unlike a standard DNS lookup that translates domain names to IP addresses, a reverse DNS query does the opposite — it takes an IP address like 8.8.8.8 and returns the hostname dns.google.
Our free reverse DNS lookup tool queries PTR records from multiple DNS servers including Google DNS, Cloudflare, Quad9, and OpenDNS. Enter any IPv4 or IPv6 address to instantly discover its hostname, verify PTR record configuration, and check reverse DNS for email deliverability. No installation required — results in milliseconds.
How Reverse DNS Lookup Works (Step by Step)
Understanding how a reverse DNS lookup works helps you troubleshoot PTR record issues and configure reverse DNS correctly for email servers. Here is the complete reverse DNS resolution process:
8.8.8.8) or IPv6 address (e.g., 2001:4860:4860::8888) into the reverse DNS lookup tool..in-addr.arpa for IPv4. So 8.8.8.8 becomes 8.8.8.8.in-addr.arpa. For IPv6, each hex nibble is reversed and appended to .ip6.arpa.dns.google for 8.8.8.8). For email servers, best practice is to verify the hostname resolves back to the original IP (forward-confirmed reverse DNS).Understanding PTR Records — The Heart of Reverse DNS
PTR records (Pointer records) are the DNS record type used exclusively for reverse DNS lookups. They map IP addresses to hostnames and are stored in the special in-addr.arpa and ip6.arpa namespaces. Use our PTR record checker above to look up PTR records for any IP address.
IPv4 PTR records are stored in the in-addr.arpa zone. The IP octets are reversed: 192.168.1.1 becomes 1.1.168.192.in-addr.arpa. This reversal allows the DNS hierarchy to delegate reverse zones efficiently by IP range.
IPv6 PTR records use ip6.arpa. Each hex digit (nibble) is reversed and dot-separated. A 128-bit IPv6 address produces a 63-character ARPA name. Our tool handles IPv6 reverse DNS lookup automatically.
FCrDNS verifies that the PTR hostname resolves back to the original IP. Example: 8.8.8.8 → dns.google → 8.8.8.8. This bidirectional check is required by many anti-spam systems. Use our Domain to IP tool to verify the forward lookup.
PTR records have TTL (Time-to-Live) values that control how long DNS resolvers cache the result. Typical PTR TTL values range from 3600s (1 hour) to 86400s (24 hours). Changes to PTR records propagate after the old TTL expires.
Reverse DNS and Email Deliverability
Reverse DNS is one of the most critical factors for email deliverability. Mail servers worldwide check PTR records to verify the identity of sending servers and filter spam. Here is why every email administrator must configure reverse DNS correctly.
Why Mail Servers Check Reverse DNS
- Spam Prevention: IPs without PTR records are often spam sources. Gmail, Outlook, and Yahoo reject or flag emails from IPs with no reverse DNS configured.
- Sender Identity: PTR records confirm the sending server identity. The PTR hostname should match or be related to the domain in the email's HELO/EHLO greeting.
- FCrDNS Validation: Anti-spam systems verify the PTR hostname resolves back to the original IP. This forward-confirmed reverse DNS check prevents IP spoofing.
- RFC 5321 Compliance: The SMTP specification recommends that receiving servers verify PTR records for connecting clients.
PTR Record Best Practices for Email
- Match HELO Identity: Set the PTR hostname to match your mail server's HELO/EHLO name (e.g.,
mail.example.com). - Use Meaningful Hostnames: Avoid generic ISP hostnames like
host-192-168-1-1.isp.com. Use descriptive names under your domain. - Verify FCrDNS: After setting a PTR record, use our reverse DNS tool to confirm it resolves, then use Domain to IP to verify the forward lookup.
- Complete Email Auth: Combine PTR records with SPF, DKIM, and DMARC for full email authentication.
How to Check PTR Records (3 Methods)
There are three ways to perform a reverse DNS lookup and check PTR records for an IP address. Our online tool is the fastest method, but knowing command-line options is valuable for system administrators.
1Online Reverse DNS Lookup (Recommended)
Enter an IP address in our reverse DNS lookup tool above and click "Lookup". Choose from multiple DNS servers (Google, Cloudflare, Quad9, OpenDNS, AdGuard) to query PTR records. Results include hostname, ARPA address, IP version, and response time.
2Reverse DNS Using nslookup (Windows/Mac/Linux)
nslookup 8.8.8.8nslookup 8.8.8.8 1.1.1.1nslookup -type=PTR 8.8.8.8.in-addr.arpanslookup 2001:4860:4860::88883Reverse DNS Using dig (Mac/Linux)
dig -x 8.8.8.8dig -x 8.8.8.8 +shortdig @1.1.1.1 -x 8.8.8.8dig -x 2001:4860:4860::8888dig PTR 8.8.8.8.in-addr.arpadig -x 8.8.8.8 +traceWhen to Use Reverse DNS Lookup
Reverse DNS vs Forward DNS
Translates a domain name to an IP address. Uses A (IPv4) and AAAA (IPv6) records. Example: google.com → 142.250.80.46. Performed by our DNS Lookup and Domain to IP tools.
Translates an IP address to a hostname. Uses PTR records in the in-addr.arpa (IPv4) or ip6.arpa (IPv6) namespace. Example: 8.8.8.8 → dns.google. Performed by this reverse DNS lookup tool.
Forward DNS is managed by domain owners via their registrar. Reverse DNS is managed by IP owners (ISPs/hosting providers). An IP can have one PTR record but a domain can have multiple A records. Not all IPs have PTR records configured.
The gold standard: verify both directions match. Use our reverse DNS tool to find the PTR hostname, then use Domain to IP to confirm the hostname resolves back to the same IP.
Reverse DNS Lookup for Security and Network Administration
Security Use Cases
- Intrusion Detection: Reverse DNS helps identify the source of malicious traffic. Security tools resolve IPs in alerts to hostnames for faster analysis and attribution.
- Firewall Log Analysis: Transform raw IP addresses in firewall logs into readable hostnames. Identify patterns like multiple connections from the same organization.
- Access Control: Some systems use reverse DNS to restrict access. For example, allowing connections only from IPs with PTR records matching
*.trusted-org.com. - Phishing Investigation: When analyzing phishing emails, reverse DNS on the sending IP can reveal the true origin server and whether it belongs to a legitimate provider.
Common PTR Record Issues
- No PTR Record: The most common issue. Many ISPs don't configure PTR records for dynamic IPs. Contact your provider to set one up for your server IPs.
- Generic ISP Hostname: PTR records like
host-1-2-3-4.isp.netmay trigger spam filters. Request a custom PTR hostname from your provider. - FCrDNS Failure: The PTR hostname doesn't resolve back to the original IP. This breaks forward-confirmed reverse DNS and fails anti-spam checks.
- Stale PTR Records: After migrating servers, old PTR records may still point to outdated hostnames. Use our reverse DNS checker to audit your PTR records regularly.
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Why Use an Online Reverse DNS Lookup Tool?
While command-line tools like nslookup and dig -x can perform reverse DNS lookups, our online tool offers several advantages. Query PTR records from multiple DNS servers (Google, Cloudflare, Quad9, OpenDNS, AdGuard) with a single click, see real-time response times, and get the complete ARPA address — all without opening a terminal. This makes it ideal for quick PTR record checks, verifying email server configuration, and troubleshooting reverse DNS issues.
Reverse DNS lookup is essential for email administrators, security analysts, network engineers, and anyone managing server infrastructure. Whether you're configuring PTR records for a new mail server, investigating suspicious IP addresses, or auditing your network's reverse DNS configuration, our tool provides instant results. For a complete IP analysis, combine reverse DNS with our IP Lookup (geolocation, ISP, ASN) and IP Blacklist Checker (spam/RBL status).