check-domain | A simple component to check the status of a domain | DNS library

If you do not have specific access (like being a registrar), and if you do not target a specific TLD (as some TLDs do have a specific public service called domain availability), the only tool that makes sense is to query whois servers.

You have then at least the following two problems:

1. use the appropriate whois server based on the given domain name
2. taking into account that whois servers are rate-limited so if you are bulk querying them without care you will first hit delays and then even risk your IP to be blacklisted, for some time.

For the second point the usual methods apply (handling delays on your side, using multiple endpoints, etc.)

For the first point, in another of my reply here: https://unix.stackexchange.com/a/407030/211833 you could find some explanations of what you observe depending on the wrapper around whois you use and some counter measures. See also my other reply here: https://webmasters.stackexchange.com/a/111639/75842 and specifically point 2.

Note that depending on your specific requirements and if you are able to at least change part of them, you may have other solutions. For example, for gTLDs, if you tolerate 24 hours delay, you may use the published zonefiles of registries to find domain names registered (those published so not all of them).

Also, why you are right in a generic sense that using a third party has its weaknesses, if you find a worthy registrar that both has access to many registries and that provides you with an API, you could then use it for your needs.

In short, I do not believe you can achieve this task with all cases (100% reliability, 100% TLDs, etc.). You will need some compromises but they depend on your initial needs.

Also very important: do not shell out to run a whois command, this will create many security and performance problems. Use the appropriate libraries from your programming language to do whois queries or just open a TCP socket on port 43 and send your queries on one line terminated by CR+LF, reading back a blob of text, this is basically only what is defined in RFC3912.