When it comes to DNS errors triggered during web monitoring, it’s not typically easy to quickly identify and understand the exact issue that leads to connection errors. In this article, we’ll walk you through some DNS error troubleshooting steps.
For common recommendations on troubleshooting errors, please visit the Troubleshooting Monitoring Errors article.
Why Does DNS Error Occur?
In general, the most common reasons for DNS errors, like DNS timeouts or DNS misconfiguration, occurs with the DNS provider.
For example, DNS tree propagation can be considerably long. The DNS chain to the authoritative DNS nameserver where the host’s IP address is stored is long, which increases the DNS resolution time. In this case, the DNS timeout error can be received due to inconsistency between a monitoring timeout limit and time it actually takes to complete DNS resolving.
If DNS resolving takes more than 11 seconds (allowed by default), the DNS timeout error will be generated. Note that this is not necessarily a website availability issue, but a long timeout that was aborted.
One of the first troubleshooting steps we recommend is checking the DNS tree and understand at which point a DNS server issue or timeout occurred. Pull an Online Report for the device and on the Log tab use the built-in DNS Trace option.
To build a DNS tree in real-time, use the Dotcom-Monitor DNS trace tool. On the DNS Trace tab, enter the IP and location and start the test. The propagation is started from the root servers as it is executed for the Device Cached or Non-Cached DNS modes.
DNS errors troubleshooting
To troubleshoot a DNS error, review the error description provided for each server node of the DNS tree as shown in the picture above (step 2).
If you experience DNS errors frequently, you can set up a separate DNS task for the domain in question (the Verify Response On option set to First Responding) and set up a specific monitoring frequency to check the domain name resolution into an IP.
DNS timeout troubleshooting
If a DNS server timeout is the issue, we recommended to contact your provider to check if any DNS servers on their side have performance degradation. Also, an overloaded DNS server can slow down the server response time to a name request.
If there are no DNS servers performance issues, use one of the suggested approaches:
- Change the DNS resolve mode to TTL Cached.
- Use an External DNS Server.
To deal with DNS timeout errors, you can change the resolving mode from Device Сashed to TTL Сached.
By default, in the Device Cached mode, Dotcom-Monitor makes a full resolution from root DNS servers without any caching on every check. In other words, this is the most reliable approach because the entire DNS chain, starting from the root DNS server, is checked. However, the disadvantage of this approach is that it increases device execution time and in the case of a long DNS tree, can lead to timeout issues.
On the other hand, the TTL Cached mode allows for mimicking a DNS lookup as it is executed on an actual user’s computer. Generally, to resolve a website IP address, the DNS lookup information is cached locally on a user’s computer. On the first request for a domain’s IP address, the DNS record is saved to the cache and used on subsequent requests to the domain. This speeds up the process by skipping all the lookup steps from the DNS resolution process. Similarly, a local DNS server with an installed monitoring agent is used to pre-cache lookup information in the TTL Cached mode. The DNS records are saved in the local server’s cache during TTL, and depending on what the TTL is for a specific host (usually around a day), it will rarely be requested. Therefore, the possibility of getting a timeout is reduced significantly.
To use a public caching service, such as Google (220.127.116.11, 18.104.22.168) or Cloudfare (22.214.171.124), change the DNS mode to External DNS Server. Dotcom-Monitor will fetch IP addresses from a public service’s DNS cache. This option will reduce DNS resolving time and can help to troubleshoot a timeout issue.