DDNS Access Issues Why External Works but Internal Doesn't

Introduction

When setting up a home server or NAS with DDNS (Dynamic DNS), users often encounter a puzzling situation: while the service works perfectly over 4G/5G networks, it fails when accessed from within the local network using the same domain name. This seemingly contradictory behavior has logical technical explanations and solutions, which we'll explore in detail.

The Problem

A common scenario:

DDNS domain works perfectly through external networks (4G/5G)
The same domain fails when accessed from the local network
Direct access via local IP works without issues

Technical Background

Understanding NAT and DNS Resolution

The core issue lies in NAT Loopback (also known as Hairpin NAT). When accessing an external domain from within the local network, the following process occurs:

Device requests DNS resolution for DDNS domain
DNS server returns public IP address
Device attempts to access the public IP
Router needs to redirect this traffic back internally (NAT Loopback)

Network Flow Comparison

External Access Path:
Mobile (4G) -> ISP Network -> Public IP -> Router -> Local Server

Expected Internal Path:
Device (WiFi) -> Router -> Local Server

Actual Internal Path Without NAT Loopback:
Device (WiFi) -> Router -> [Fails to Return] -> Local Server

Solutions

1. Modify Local Hosts File

The simplest solution is to add a local DNS entry:

For Windows (C:\Windows\System32\drivers\etc\hosts):

192.168.1.x    your-ddns-domain.com

For Linux/Mac (/etc/hosts):

192.168.1.x    your-ddns-domain.com

2. Set Up Local DNS Server

Deploy a local DNS server (like Pi-hole or dnsmasq) with custom records:

# dnsmasq configuration example
address=/your-ddns-domain.com/192.168.1.x

3. Enable NAT Loopback on Router

Steps vary by router manufacturer:

Check router settings for "NAT Loopback", "NAT Reflection", or "Hairpin NAT"
Enable the feature if available
Save and restart network services

4. Split DNS Configuration

Implement a split-horizon DNS setup:

External DNS: Points to public IP
Internal DNS: Points to local IP
Requires a local DNS server

Best Practices

Documentation
- Document your network configuration
- Keep records of all custom DNS entries
- Note router settings changes
Security Considerations
- Ensure proper firewall rules
- Use strong authentication
- Regularly update access credentials
Testing
- Verify access from multiple devices
- Test both internal and external connections
- Monitor for potential conflicts

Troubleshooting Guide

Verify DNS Resolution
```
nslookup your-ddns-domain.com
```

Check Network Connectivity

ping your-ddns-domain.com
traceroute your-ddns-domain.com

Validate Service Availability

telnet your-ddns-domain.com port
curl -v your-ddns-domain.com

Conclusion

Understanding why DDNS behaves differently for internal and external access is crucial for proper network configuration. While the issue may seem complex, the solutions are straightforward once you understand the underlying mechanics. Choose the solution that best fits your network architecture and security requirements.