PTR Record Reverse DNS

The PTR record (Pointer record) is used for reverse DNS lookups — resolving an IP address back to a hostname. While A records map domain names to IP addresses, PTR records do the opposite: they map IP addresses to domain names.

What Is a PTR Record?

PTR records enable reverse DNS (rDNS) lookups. Given an IP address, a PTR record tells you the associated hostname. This is commonly used for:

• Email verification — Mail servers check PTR records to verify sender legitimacy
• Logging — Converting IP addresses to hostnames in log files
• Network troubleshooting — Identifying which server an IP belongs to
• Security — Validating that an IP matches its claimed hostname

PTR Record Format

PTR records use a special format where the IP address is reversed and appended to a special domain:

IPv4 PTR Records

The IP address is reversed and .in-addr.arpa is appended:

1.2.0.192.in-addr.arpa.    3600    IN    PTR    mail.example.com.

IPv6 PTR Records

Each hexadecimal digit is reversed and separated by dots, with .ip6.arpa appended:

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.    PTR    mail.example.com.

How Reverse DNS Works

1. An application needs to find the hostname for IP 192.0.2.1
2. It reverses the IP: 1.2.0.192
3. It appends in-addr.arpa: 1.2.0.192.in-addr.arpa
4. It queries DNS for PTR records at that name
5. The DNS server returns: mail.example.com

PTR Records and Email

PTR records are crucial for email deliverability. When your mail server sends email, receiving servers typically:

1. Note the sending IP address
2. Perform a reverse DNS lookup (PTR query)
3. Verify the hostname matches the HELO/EHLO identity
4. Perform a forward DNS lookup on the hostname
5. Verify the IP matches (Forward-Confirmed reverse DNS)

If any of these checks fail, your email is more likely to be marked as spam or rejected.

Forward-Confirmed Reverse DNS (FCrDNS)

FCrDNS requires that:

IP Address → PTR Record → Hostname → A Record → Same IP Address

Example of valid FCrDNS:

192.0.2.1     →  PTR  →  mail.example.com
mail.example.com  →  A    →  192.0.2.1  ✓

Setting Up PTR Records

Unlike most DNS records, PTR records must be created by whoever controls the IP address block — typically your ISP or hosting provider:

• Dedicated server — Contact your hosting provider to set the PTR record
• VPS — Many providers have a control panel option to set PTR
• Cloud hosting — AWS, GCP, Azure have PTR settings in their consoles
• Own IP block — If you own the IP range, you manage the reverse zone

PTR Record Best Practices

• Always set PTR for mail servers — Essential for email deliverability.
• Ensure FCrDNS matches — PTR hostname should resolve back to the same IP.
• Use meaningful hostnames — "mail.example.com" is better than "server1.host.com".
• Match HELO/EHLO identity — The PTR hostname should match your mail server's identity.
• One PTR per IP — While technically you can have multiple, most systems expect one.

Checking PTR Records

You can look up PTR records with command-line tools:

# Using dig
dig -x 192.0.2.1

# Using host
host 192.0.2.1

# Using nslookup
nslookup 192.0.2.1

PTR vs A Record

Troubleshooting PTR Records

Common issues and solutions:

• Email being rejected — Check that PTR exists and matches your mail server's hostname.
• Cannot create PTR record — You need to contact your IP provider; you can't create PTR in your own DNS.
• PTR doesn't match A record — FCrDNS check failing. Update either PTR or A record to match.
• Generic PTR hostname — Contact your hosting provider to set a custom PTR instead of their default.