What is Reverse DNS?
Reverse DNS allows you to find the hostname associated with an IP address. It's the inverse operation of standard DNS resolution.
Common use cases:
- Email deliverability: Verify your mail server has a correct PTR
- Network diagnostics: Identify machines in logs with readable names
- Security: Verify the identity of incoming connections
- Auditing: Validate consistency between forward and reverse zones
How does reverse DNS work?
For IPv4 (in-addr.arpa)
The address is reversed octet by octet, then .in-addr.arpa is added:
IP: 203.0.113.10
Query: 10.113.0.203.in-addr.arpa → PTR → mail.example.com
For IPv6 (ip6.arpa)
Each hexadecimal character is separated and reversed:
IP: 2001:db8::1
Query: 1.0.0.0...8.b.d.0.1.0.0.2.ip6.arpa → PTR → server.example.com
The importance of forward/reverse consistency
For credibility, DNS configuration must be consistent:
| Direction | Query | Result |
|---|---|---|
| Forward | mail.example.com → A → | 203.0.113.10 |
| Reverse | 203.0.113.10 → PTR → | mail.example.com |
If both match, consistency is validated. This is what mail servers and many security services verify.
Reverse DNS and email deliverability
Why it's essential
Mail servers perform several checks:
- PTR exists: The IP must have a PTR record
- PTR is consistent: The PTR name must resolve back to the original IP
- Professional name: Avoid generic names like "static-203-0-113-10.isp.net"
Example of correct configuration
Server IP: 203.0.113.25
PTR: 203.0.113.25 → smtp.yourdomain.com
A: smtp.yourdomain.com → 203.0.113.25
FAQ - Frequently asked questions
Q: What is a PTR record?
A: A PTR resolves an IP address to a hostname. It's the reverse of an A/AAAA record which resolves a name to an IP.
Q: Why is reverse DNS important for emails?
A: Mail servers verify that the sending IP has a valid PTR. Without a correct PTR, your emails may be marked as spam.
Q: How do I configure reverse DNS for my server?
A: Contact your hosting provider or IP provider. Only the IP block owner can configure PTR records.
Q: Why doesn't my IP have a PTR?
A: Residential IPs often don't have custom PTR records. Professional hosting providers typically allow you to configure one.
Complementary tools
| Tool | Purpose |
|---|---|
| Whois IP | Identify the owner of an IP |
| My IP Address | Display your public IP |
| Netmask Calculator | Calculate network ranges |