--- title: Free Reverse DNS Lookup description: Turn an IP address into its hostname — the PTR record for any IPv4 or IPv6 address, plus the in-addr.arpa or ip6.arpa name being queried. Check a mail server's reverse DNS before a provider rejects it, identify who a suspicious address belongs to, and confirm forward and reverse agree. canonical: https://watchfor.io/reverse-dns-lookup --- [Back to all free tools](/free-tools) # Reverse DNS Lookup Turn an IP address into its hostname — the PTR record for any IPv4 or IPv6 address, plus the in-addr.arpa or ip6.arpa name being queried. Check a mail server's reverse DNS before a provider rejects it, identify who a suspicious address belongs to, and confirm forward and reverse agree. Result No result yet Enter the inputs above and press Run. Results appear here in a moment. Monitor this 24/7 — get alerts on the first failure. [Start free](/auth/sign-up) Only run these tools against systems you own or are authorized to test. Using them to scan or probe systems without permission violates our [Terms](/legal/terms). ## What is Reverse DNS Lookup ? Reverse DNS turns an IP address back into a name. For 1.1.1.1 the resolver asks for the PTR record of 1.1.1.1.in-addr.arpa and gets one.one.one.one ; for an IPv6 address it asks under ip6.arpa with the address reversed one hex digit at a time. Unlike forward DNS, the reverse zone belongs to whoever holds the IP block — your ISP or cloud provider — so you don't set a PTR in your domain's DNS; you set it (or ask for it) where the address came from. This tool looks up the PTR record for any IPv4 or IPv6 address, from the resolver of your choice, and shows the reversed name that was actually queried. Mail servers are the reason most people need it: receiving MTAs check that the sending IP has a PTR and that the PTR's name resolves back to the same IP (forward-confirmed reverse DNS). Without that, mail is greylisted or rejected. It is also the quickest way to put a name on an address in your logs. ## How it works A PTR lookup is a normal DNS query on an unusual name: - 1 Reverse the address — 203.0.113.7 becomes 7.113.0.203.in-addr.arpa . 2001:db8::1 is expanded to 32 hex digits, reversed, dotted, and suffixed with ip6.arpa . The tool builds the name for you and shows it. - 2 Query `QTYPE=PTR` — The resolver walks the reverse tree, which is delegated by IP block from the regional registries (ARIN, RIPE, APNIC…) down to the ISP or cloud provider that holds the addresses. - 3 Report the name(s) — Most addresses have zero or one PTR; some have several. NXDOMAIN means no reverse record exists for that address. To check forward confirmation, look up the A/AAAA record of the returned name and compare it with the IP you started from — the SMTP server test does this automatically for a mail host. ## When you'd use it ### Setting up a mail server Before the first message leaves, confirm the sending IP's PTR names the server ( mail.example.com ) and that mail.example.com resolves back to that IP. Gmail, Outlook.com and most corporate gateways reject or heavily penalise mail from addresses without matching reverse DNS. ### Identifying an address in the logs Repeated hits from 185.220.x.x ? The PTR often names the network ( tor-exit-… , crawl-…googlebot.com , ec2-…compute.amazonaws.com ). Combined with the IP lookup's ASN, it tells you who to talk to. ### Verifying a crawler Googlebot and Bingbot document their verification as a reverse lookup followed by a forward lookup: the PTR must end in googlebot.com / search.msn.com and resolve back to the same IP. A bot claiming to be Googlebot from an address with a different PTR is lying. ### After moving to a new IP The old PTR still names the old server; the provider's control panel or a support ticket sets the new one. Until it's updated, mail from the new address is suspicious to receivers. ## Reading the result ### The PTR name A hostname. A generic one ( static-203-0-113-7.isp.example ) says the block owner never set a custom name — fine for a laptop, a red flag for a mail server. ### NXDOMAIN No PTR record. Common on consumer and cloud addresses by default. For an outbound mail server it is the single most common reason for rejected mail. ### Multiple PTRs Allowed but unhelpful — receivers may pick any one. Keep one PTR per mail server address. ### The queried name Shown so you can see the reversal was right — especially for IPv6, where the nibble format is easy to get wrong by hand. ## Check it yourself dig -x 1.1.1.1 @1.1.1.1 +noall +answer dig -x builds the in-addr.arpa / ip6.arpa name for you. host 1.1.1.1 does the same with less output. ## Common pitfalls ### Trying to add a PTR in your own DNS zone The reverse zone for your IP is delegated to your ISP or cloud provider, not to example.com . AWS, Google Cloud, Hetzner, DigitalOcean and OVH all have a reverse-DNS setting per address; residential ISPs usually won't set one at all. ### PTR that doesn't resolve forward The PTR says mail.example.com but mail.example.com has no A record, or a different one. That fails forward-confirmed reverse DNS and is treated much like no PTR at all. ### Reverse DNS behind NAT or a shared IP The address your traffic leaves from is the one that matters. A server behind a NAT gateway sends mail from the gateway's IP, whose PTR belongs to the gateway. ## Run Reverse DNS Lookup on every change, not just once. Get alerts the moment something breaks — across HTTP, DNS, SSL, RDAP, ping, blacklist and more. Free forever for 10 monitors. No card. [Start free](/auth/sign-up) ## Frequently asked questions How do I set a reverse DNS / PTR record? In the control panel of whoever assigned you the IP address: cloud providers have a field per IP or elastic IP, hosting providers have it in the server settings, business ISPs set it on request. It cannot be set in your domain's zone. Why is reverse DNS important for email? Receiving servers use it as a basic identity check: a legitimate mail server has a PTR that names it and resolves back. Missing or generic PTRs are characteristic of compromised consumer machines, so mail from them is rejected or scored as spam. What is forward-confirmed reverse DNS (FCrDNS)? PTR of the IP gives a name; A/AAAA of that name gives the same IP back. Both directions agree, so the name's owner and the address's owner have both vouched for the link. Can an IP have more than one PTR? Technically yes. Practically, receivers may pick any of them, and some treat multiple PTRs as suspicious. Use one. How does reverse DNS work for IPv6? The 128-bit address is written as 32 hex digits, reversed, each digit separated by a dot, under ip6.arpa . 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 . The IPv6 subnet calculator generates these names. ## Related dns tools - [DNS LookupResolve A, AAAA, MX, TXT, NS, SOA, CNAME, PTR, CAA, SRV records for any domain.](/dns-checker) - [DNS PropagationCheck how a DNS record looks from every prober region simultaneously — spot stale anycast nodes and propagation gaps on a world map.](/dns-propagation-checker) - [Whois LookupModern RDAP-based domain registration lookup: registrar, expiry, nameservers, status flags.](/whois-lookup) - [A Record LookupFind the IPv4 address a domain resolves to — every A record with its TTL, from the resolver of your choice. Spot a stale IP after a migration, a missing record behind a 'site can't be reached', or a round-robin set that is one address short.](/a-record-lookup) --- Canonical page: https://watchfor.io/reverse-dns-lookup · Site guide: https://watchfor.io/llms.txt