The Domain Name System (DNS) maps names such as example.com to network records used to connect to a service. A crawler usually resolves a hostname before opening a connection; the resolver’s answer, cache, and network path can therefore affect where a request goes and how long it takes to start.
DNS in a crawl
When a request fails before an HTTP response arrives, check whether the hostname resolved, which record type was returned, and whether the failure is repeatable from the same network. DNS errors are different from HTTP errors: a 404 came from an HTTP server, while a name-resolution failure means the client did not reach that stage.
Caching matters. A resolver may reuse an answer for the record’s time to live, so a changed DNS record may not appear immediately everywhere. Conversely, resolving the same host for every URL can add needless latency. Most HTTP clients and operating systems already have caching behavior, so diagnose actual resolution patterns before adding another cache.
For reproducible debugging, record the hostname and error class rather than assuming a particular IP address is permanent. DNS answers can legitimately vary by resolver, location, and service configuration.
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