Unable to connect to DNS at pdns99.ultradns.net

[AFirstenberg]

We have a GCE instance in us-central1 that seems to be having problems doing DNS lookups against pdns99.ultradns.net. DNS lookups from outside GCP work fine.

A traceroute to pdns99.ultradns.net from the GCE instance never indicates any hops succeed.

This was working fine and suddenly started failing with no configuration changes on our side. Google status is showing everything is ok.

We have weird networking issues like this with Google periodically. What is the best way to diagnose and resolve them?

[diannemcm]

Hi @AFirstenberg,

Welcome to Google Cloud Community!

Here are the breakdowns of possible causes why you are encountering the issue:

• Firewall rules might be blocking traffic to the DNS server on GCE instance, VPC or Google Cloud network layer. It is possible that a restrictive rule was put in place.

- Verify firewall rules, there should be no rules blocking outbound traffic to pdns99.ultradns.net on port 53 (DNS). DNS uses UDP port 53 or TCP port 53, depending on the size of the request or response. See Best practices for Cloud DNS.

• Other possible cause is the DNS server issue and a Network routing issue since the traceroute shows that no hop succeeded.

 - Utilize Google Cloud's monitoring tools to track network performance, latency, and DNS-related problems. This will be useful information for troubleshooting.

• Run connectivity test under Network Intelligence on GCP console on port 53 and/or create a test VM using a different region other than us-central1 and test if still facing DNS lookup failing.
• This also could happen if there was a recent change on pdns99.ultradns.net side but your GCE instance hasn't updated its cache. Try flushing the DNS cache. Check the FAQ section for guidance. Please be aware of the requirements and troubleshooting steps on how to do this. This document also provides other workarounds for more types of DNS issues.

If the issue still persists and you need further assistance, you can file a ticket with our Google Cloud support.

I hope the above information is helpful.