Reverse DNS (rDNS) is the opposite of a normal DNS lookup: it starts with an IP address and returns the hostname assigned to it, using a PTR record. It matters most for email, where receiving servers routinely distrust or reject mail from IP addresses that have no valid reverse DNS.
A regular, or "forward", DNS lookup answers the question "what IP address does mail.example.com have?". Reverse DNS answers "what name belongs to 203.0.113.25?". The two are maintained separately, often by different organizations, which is why they can disagree and why checking both is a useful trust signal.
How reverse DNS works
DNS can only look up names, so reverse lookups turn an IP address into a special name inside a reserved domain, then query that name for a PTR (pointer) record. The PTR record's value is the hostname.
IPv4 and in-addr.arpa
For IPv4, the four octets of the address are written in reverse order and followed by in-addr.arpa. The address 203.0.113.25 becomes:
25.113.0.203.in-addr.arpa. 3600 IN PTR mail.example.com.The reversal exists because DNS names get more specific from right to left, while IP addresses get more specific from left to right. Flipping the octets lets the DNS hierarchy follow the way IP address blocks are allocated: the zone 113.0.203.in-addr.arpa covers the whole 203.0.113.0/24 network and can be delegated to whoever holds that block.
IPv6 and ip6.arpa
IPv6 uses the ip6.arpa domain and goes one step further: the full 128-bit address is expanded to all 32 hexadecimal digits ("nibbles"), each digit becomes its own label, and the order is reversed. The address 2001:db8::25 becomes:
5.2.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.8.b.d.0.1.0.0.2.ip6.arpa. IN PTR mail.example.com.Nobody types these by hand; tools such as dig -x build them for you.
Who controls PTR records?
This is the part that confuses most people. Your domain's DNS provider controls forward records like A and MX for example.com. But PTR records live in the reverse zone for the IP address, and that zone belongs to whoever the address block is assigned to: your hosting company, cloud provider or ISP.
- Cloud servers and VPS: most providers let you set the reverse DNS name in their control panel or API, often requiring that the matching forward record exists first.
- Dedicated servers and business internet: you usually request the PTR through a support ticket, or the provider delegates a reverse zone to your own name servers if you have a large enough block.
- Home broadband: the ISP sets a generic name such as
203-0-113-25.dynamic.example.netand normally will not change it.
Forward-confirmed reverse DNS (FCrDNS)
Anyone who controls an IP block can set its PTR record to any name, including a domain they do not own. A PTR record on its own therefore proves very little. Forward-confirmed reverse DNS closes the gap by checking both directions:
- Look up the PTR for the IP address:
203.0.113.25returnsmail.example.com. - Look up the A (or AAAA) record for that name:
mail.example.comreturns203.0.113.25. - If the forward lookup includes the original IP, the result is forward-confirmed.
Because the second step depends on the domain owner's DNS, a match shows that both the IP holder and the domain owner agree on the name. Our reverse DNS lookup tool performs this forward confirmation automatically.
Why reverse DNS matters for email
Email is where reverse DNS really earns its keep. Spam often comes from compromised home computers and short-lived servers whose IPs have no PTR record or only a generic ISP-assigned one. Receiving mail servers use rDNS as an early, cheap filter:
- Many servers reject or heavily penalize connections from IPs that have no PTR record at all.
- Some check that the PTR is forward-confirmed.
- Generic-looking names that embed the IP address or words like "dynamic" or "dsl" are commonly scored as likely spam sources.
Google's published sender guidelines, for example, require sending servers to have valid forward and reverse DNS records. If you run your own mail server, set the PTR for its sending IP to a hostname such as mail.example.com, make sure that name has an A record pointing back to the same IP, and use the same name in the server's SMTP greeting (HELO/EHLO). The PTR does not have to match the domain in the From address, but it must be a real, forward-confirmed name.
Reverse DNS is only one part of deliverability. Pair it with proper authentication records, covered in SPF, DKIM and DMARC explained, and confirm your MX records with our MX lookup.
Other uses of reverse DNS
- Logs and diagnostics: web server logs, firewalls and tools like
traceroutedisplay hostnames instead of bare IPs, which often reveal the network or city of a router, such asae1.edge1.fra.example.net. - Verifying crawlers: major search engines recommend confirming their bots with a reverse lookup followed by a forward lookup, because the user agent string is trivial to fake.
- Identifying networks: a PTR name can hint at which provider runs an address, complementing what an IP lookup or WHOIS query shows.
Checking reverse DNS yourself
From a terminal, both of these work on macOS and Linux, and nslookup also works on Windows:
$ dig -x 203.0.113.25 +short
mail.example.com.
$ nslookup 203.0.113.25
25.113.0.203.in-addr.arpa name = mail.example.com.Then confirm the forward direction with dig mail.example.com A +short and check that the same IP comes back. An answer of NXDOMAIN on the reverse lookup means no PTR record exists for that address.
Frequently asked questions
Can an IP address have more than one PTR record?
Technically yes, but it is discouraged. Many programs only read the first answer, and mail servers expect a single, consistent name. Use one PTR per IP address.
Do I need reverse DNS for a website?
No. Browsers never check reverse DNS when loading a site, and many web servers on shared or CDN infrastructure have generic PTR names. It is primarily important for servers that send email.
Why does my home IP have a strange reverse DNS name?
Your ISP assigns generic PTR names to its customer addresses, usually containing the IP and a word like "dynamic" or the region. You cannot change it on a residential connection, which is one reason home connections are poorly suited to sending email directly.
Does reverse DNS reveal my identity?
It reveals your ISP and sometimes a rough region, the same kind of information an IP lookup provides. It does not include your name or street address.