I have a site where I record links to user-provided web pages. I only want to record them if they are valid.
I was using checkdnsrr($domain, 'A') to check domain validity, and I noticed this was always returning true. I’m on shared hosting (so I can’t change anything). I reported a bug to the server admin, and he replied:
We use OpenDNS for our resolvers on the servers. Their resolvers will return an IP/DNS record even for invalid domains as they have a wildcard set up to point any invalid domains to a default IP. Because of this all domains you enter will show as having an A record.
Okay then.
I want to avoid using cURL on an invalid domain as this would be a waste, but there is actually a bigger problem. I believe this resolver returns some kind of error page like “This domain does not exist,” and emits a 200 or some other supposedly valid return status. I use this status to confirm that the page is indeed valid, so now any invalid domain will be recorded as valid by my site.
(Humorously, a valid domain with an invalid URL that leads to a page emitting 404 or some other error status will not be recorded).
Anyway, is there some other way to check domain validity? I don’t mind using some third party site.
OpenDNS (and others that do this) usually return the same IP for all domains that should come up NXDOMAIN (invalid). If the result of your check is that IP, consider it invalid. It’s one extra check, but it should be a valid workaround.