Many registrars that move their SIDN account to Openprovider suddenly bump into the 15-per-day limit on whois queries. Their software often uses the practically unlimited 'registrar whois' feature and now they've stopped their SIDN accreditation they must rely on the limited public whois.
Luckily, SIDN provides an availability check with much higher limits as well (500 per IP address per day), the so-called "is" check. Calling the "is" check is as simple as calling a regular whois command and returns just the domain's status:
$ whois -h whois.domain-registry.nl 'is openprovider.nl'
openprovider.nl is active
$ whois -h whois.domain-registry.nl 'is this-domain-is-available.nl'
this-domain-is-available.nl is free
Other statuses that can be returned are requested, inactive, excluded, withdrawn or in quarantine.
This functionality can be used in your own availability checker, but also in the configuration of third party software like WHMCS or HostFact.
If these 500 queries a day are still insufficient, then we strongly advise to implement the checkDomainRequest from our API. This function provides you with practically unlimited availability checks; not only for .nl, but for all extensions that Openprovider delivers.