// Plugins // Domain Check

F.A.Q.

The top-level domain (TLD), for example ".com", is the highest level in the hierarchical domain name system, it is the last characters after the last dot. The second-level domain (SLD) is one level below a top-level domain, for example ".com.de".

Domains are managed by a network information center (NIC), where also information about the domain owner is stored. For every TLD or SLD there is such a NIC and they offer a whois server to check the registration status of a domain. In most cases this whois server is public and could be accessed by the plugin.

To add a custom whois server follow these steps (example for TLD .com):

• Find the TLD in this list or directly open: https://www.iana.org/domains/root/db/com.html
• The whois server is displayed at the end of the page: WHOIS Server: whois.verisign-grs.com
• Add TLD and whois server to the custom whois servers, use "status_free_unknown" for status free
• Enable whois link in the general settings, if not already done
• Do a check for a domain name that is surely free ("zaqcryuosa")
• Open the whois details a look for a key sentence confirming the domain is free: No match for "ZAQCRYUOSA.COM"
• Use that sentence or words ("no match for") as status free

By allowing the check of unsupported TLDs it is possible to check every TLD, even if there is no whois server available for that TLD. The plugin then checks if the domain is assigned to an IP address, which means it is linked to a webserver. Nevertheless, this check is not 100% reliable, because a domain could be registered and not assigned to an IP address (yet).

If the check sometimes or randomly results in an error this could occur when using a caching plugin. To resolve this problem disable the option "Use nonces" in the advanced settings. The problem is caused because the so called nonce (number used once) is cached and not regenerated when expired. More informations about nonces could be found here: https://codex.wordpress.org/WordPress_Nonces.

An image visualizing the structure could be found here.