What Is a PTR Record

A PTR (Pointer) record provides reverse DNS (rDNS) lookup, mapping an IP address back to a domain name. While A/AAAA records convert domain names to IP addresses, PTR records do the opposite—enabling you to find which domain is associated with a given IP.

How PTR Records Work

PTR records use special reverse lookup zones:

IPv4: IP addresses are reversed and placed under in-addr.arpa

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

This maps 192.0.2.1 → mail.example.com

IPv6: Each nibble is reversed under ip6.arpa

4.3.3.7.0.7.3.e.2.a.8.0.0.0.0.0.0.0.0.0.3.a.5.8.8.b.d.0.1.0.0.2.ip6.arpa.    3600    PTR    mail.example.com.

Why PTR Records Matter

Email Deliverability

Mail servers check PTR records to verify sender legitimacy:

• Many mail servers reject email from IPs without valid PTR records
• The PTR should match the server's HELO/EHLO hostname
• Mismatched or missing PTR records often result in spam filtering

Security and Logging

• Firewall logs can show hostnames instead of raw IPs
• Security tools use rDNS for threat analysis
• Network troubleshooting benefits from readable hostnames

Service Verification

• SSH connections can display the connecting hostname
• Web servers can log visitor hostnames
• Network monitoring tools provide clearer reports

Common Use Cases

Mail Server Configuration

; Forward record
mail.example.com.              3600    A      192.0.2.25
 
; Reverse record (in reverse zone)
25.2.0.192.in-addr.arpa.      3600    PTR    mail.example.com.

Web Server

; Forward record
www.example.com.               3600    A      192.0.2.80
 
; Reverse record
80.2.0.192.in-addr.arpa.      3600    PTR    www.example.com.

Multiple Services on One IP

When one IP hosts multiple services, the PTR typically points to the primary hostname or a generic server name:

25.2.0.192.in-addr.arpa.      3600    PTR    server1.example.com.

Record Format

Adding a PTR Record

Important Note

PTR records are typically managed by whoever controls the IP address block:

• Cloud providers (AWS, GCP, Azure) - Configure via their console
• Hosting providers - Use their control panel or contact support
• ISPs - Contact your ISP for PTR record changes
• Own IP space - Manage in your reverse DNS zone

If DNScale manages your reverse zone, you can add PTR records directly.

Using the Dashboard

1. Navigate to your reverse zone (e.g., 2.0.192.in-addr.arpa)
2. Click Add Record
3. Configure the record:
   • Name: Enter the last octet(s) of the IP (e.g., 1 for 192.0.2.1)
   • Type: Select PTR
   • Value: Enter the fully qualified domain name
   • TTL: Set the cache duration (default: 3600)
4. Click Create Record

Using the API

Create a PTR record:

curl -X POST "https://api.dnscale.eu/v1/zones/{reverse_zone_id}/records" \
  -H "Authorization: Bearer YOUR_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "name": "1",
    "type": "PTR",
    "content": "mail.example.com",
    "ttl": 3600
  }'

Note: The zone must be a reverse zone (e.g., 2.0.192.in-addr.arpa).

API Response:

{
  "status": "success",
  "data": {
    "message": "Record created successfully",
    "record": {
      "id": "encoded-record-id",
      "name": "1.2.0.192.in-addr.arpa.",
      "type": "PTR",
      "content": "mail.example.com.",
      "ttl": 3600,
      "disabled": false
    }
  }
}

IP Address to PTR Name Conversion

IPv4

Reverse the octets and append .in-addr.arpa:

192.0.2.1 → 1.2.0.192.in-addr.arpa

IPv6

Expand fully, reverse each nibble, add dots, append .ip6.arpa:

2001:db8::1
= 2001:0db8:0000:0000:0000:0000:0000:0001
→ 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

Best Practices

1. Match forward and reverse - PTR should resolve to a hostname that resolves back to the same IP (forward-confirmed reverse DNS)

2. Use FQDNs - Always use fully qualified domain names with trailing dot

3. One PTR per IP - Unlike A records, each IP should have only one PTR record

4. Mail server PTR is critical - Email deliverability depends heavily on proper PTR configuration

5. PTR must match HELO - For mail servers, ensure PTR matches the SMTP HELO/EHLO hostname

Verifying Forward-Confirmed Reverse DNS (FCrDNS)

For email deliverability, verify your setup:

# Step 1: Check PTR record
dig -x 192.0.2.1
 
# Step 2: Verify the result resolves back
dig A mail.example.com
 
# Both should match the same IP

Testing PTR Records

# Query PTR for IPv4
dig -x 192.0.2.1
 
# Query PTR for IPv6  
dig -x 2001:db8::1
 
# Using specific nameserver
dig -x 192.0.2.1 @ns1.dnscale.eu
 
# Short output
dig -x 192.0.2.1 +short

Common Issues

Related Record Types

• A - Forward lookup (domain → IPv4)
• AAAA - Forward lookup (domain → IPv6)
• MX - Mail server configuration

Conclusion

PTR records are essential for email deliverability and network operations. While they're often managed by IP address providers rather than DNS hosting providers, understanding PTR records helps you ensure proper configuration and troubleshoot delivery issues. When you control your IP space, DNScale can manage your reverse DNS zones alongside your forward zones.