DNS Fallback on Pronto Router — Troubleshooting & Explanation Guide
1. Purpose
This document explains how DNS fallback works on Pronto routers and provides a structured method to identify, test, and troubleshoot DNS resolution failures.
DNS fallback is a fail-safe mechanism that helps maintain connectivity when configured DNS servers become unreachable. However, it is not a permanent solution and underlying issues must be resolved.
2. Scope
This guide applies to:
Pronto PC61 and similar routers
Sites using custom DNS or ISP-provided DNS
Hospitality, retail, branch office deployments
Wired, Wireless, and LTE uplinks
3. Problem Description
In this scenario:
Internet appears connected
Devices receive valid IP addresses
Gateway is reachable
But:
Websites do not load
POS systems fail
Applications timeout
This indicates a DNS resolution failure, not an internet outage.
4. What is DNS Fallback
DNS fallback is a mechanism where the router switches to alternate DNS servers when primary DNS servers fail.
In Pronto Routers:
Router initially uses:
ISP DNS OR
Configured Custom DNS
If DNS fails:
Router waits (~10–15 minutes)
Then switches to fallback DNS
Fallback DNS may include:
Public DNS (Google, Cloudflare, OpenDNS)
Gateway / ISP DNS (depending on scenario)
Indicators:
Yellow LED on router
Internet connected but no browsing (temporary)
Heartbeat failures followed by recovery
5. DNS Fallback Behavior
DNS fallback depends on DNS configuration mode.
🔹 Scenario 1 — DNS Mode: Use ISP DNS
Router uses DNS from ISP (via DHCP)
If ISP DNS fails:
Router falls back to public DNS servers
(e.g., 8.8.8.8, 8.8.4.4, 1.1.1.1, 208.67.222.222)
📸 Example Log — Fallback to Public DNS
Caption :Router switched to public DNS servers after ISP DNS failure.
🔹 Scenario 2 — DNS Mode: Custom DNS
Router uses configured DNS (Primary/Secondary)
If both DNS servers fail:
Router may fall back to Gateway (ISP) DNS if reachable
Note:
This behavior is not guaranteed and depends on:
DNS reachability
Gateway availability
Network conditions
If gateway DNS is not reachable:
DNS resolution will fail completely
📸 Example Log — Fallback to Gateway DNS
Caption: Router fallback to Gateway (ISP) DNS after custom DNS became unreachable.
Key Behavior Notes
Fallback is automatic
It is a temporary recovery mechanism
Manual correction is recommended
Continuous fallback indicates a problem
6. Symptoms of DNS Failure
Internet connected but websites not opening
POS payment failures
Cloud heartbeat failures
Applications timing out
DNS lookup failures
7. Business Impact
DNS failure can impact:
POS transactions
Online ordering
Guest Wi-Fi
Cloud sync
Device monitoring
Internet may appear “up” but services are unusable
8. How DNS Fallback Works
Case 1 — Custom DNS
Router tries Primary DNS
If timeout → tries Secondary DNS
If both fail → may fallback to Gateway DNS (if reachable)
Case 2 — Use ISP DNS
Router uses ISP DNS
If ISP DNS fails → fallback to public DNS servers
Behavior Summary
Automatic process
Services restore when fallback works
Heartbeat resumes after recovery
9. DNS Fallback Variability
DNS fallback behavior may vary depending on:
DNS mode (ISP vs Custom)
Reachability of configured DNS
Gateway availability
Network topology
Fallback is not always guaranteed and should not be relied upon as a permanent fix.
10. How to Test DNS Fallback
Method 1 — Force DNS Failure
Go to: Device Page → Configure Properties→ Edit Uplink
Set DNS Mode = Custom
Enter invalid DNS (example: 1.1.1.250)
Save configuration (router may reboot)
Monitor Event Logs
Expected:
DNS failure occurs
Heartbeat failures appear
Router switches to fallback DNS
Connectivity restores
Method 2 — DNS Reachability Test
Run DNS lookup
Test with:
Gateway DNS
Public DNS
Interpretation
Result | Meaning |
Public DNS works | ISP/Custom DNS issue |
All fail | Network issue |
11. Troubleshooting Procedure
Step 1 — Check Internet
Open:
Diagnostics → Tools → Ping
Run:
Ping 8.8.8.8
Expected Result
Replies received
Latency displayed
If Ping Fails
This is not a DNS issue.
Possible causes:
WAN outage
ISP problem
➡ Escalate as internet connectivity issue
Step 2 — DNS Lookup
Open:
Diagnostics → DNS Lookup
Test using router DNS/Gateway DNS
Run: www.google.com
Expected Result
DNS returns IP address
If DNS Lookup Fails
Example errors:
Request timed out
Server not found
DNS failure confirmed
Step 3 — Test Public DNS
Run DNS lookup using:
DNS Server: 8.8.8.8
Expected Result
DNS returns IP address
Interpretation
If public DNS works but gateway DNS fails → issue with DNS provided by router/ISP
If both fail → network issue or DNS configuration issue
Step 4 — Identify DNS Mode
Go to:
Configure → Uplink edit configuration
Case A — ISP DNS
Issue: ISP DNS unstable
Fix:
Switch to Custom DNS (8.8.8.8 / 1.1.1.1)
ORAllow fallback to Public DNS
Case B — Custom DNS
Issue: Invalid/unreachable DNS
Fix:
Verify DNS entries
Use reliable DNS
13. Resolution Actions
Configure reliable DNS (8.8.8.8 / 1.1.1.1)
Avoid invalid DNS entries
Ensure gateway reachable
Verify firewall not blocking port 53
Check ISP DNS stability
14. Verification
After fix:
✅ Websites load
✅ POS works
✅ DNS lookup success
✅ No heartbeat failures
✅ Stable logs
Monitor for 10–15 minutes
15. Preventive Best Practices
Use Primary + Secondary DNS
Prefer public DNS in unstable environments
Monitor DNS logs
Avoid frequent config changes
16. Escalation
Escalate if:
Frequent fallback events
Persistent DNS failures
Cloud instability
Provide:
Logs
DNS configuration
ISP details
Time of issue
Final Insight
DNS fallback is a safety mechanism, not a solution
Router attempts:
Configured DNS
Reachable DNS
Fallback DNS (Public or Gateway depending on scenario)
👉 Frequent fallback = underlying DNS problem