I don’t find on the httpv2 settings page any option to decide whether the check must be over ipv4 or ipv6.
Does it take the host’s setting?
But that doesn’t make much sense, let’s say a host (server) hosts 100 sites, and not all of them resolve over ipv6, some have a dedicated ipv4 only (or they are not so critical to be worth checking ipv6 as well)
Should I create a separate host for each site (domain) checked?
No it does a DNS lookup and depending on the returned answer the check will be done.
If you get an IPv6 answer this will be used and the same for IPv4.
DNS lookup for A or AAAA? If it returns both?
It would be useful to be able to decide it, to be sure both are monitored… (it may happen that you move the domain to another IP but you forget to change the AAAA record, for example)
The feature to enforce the IP protocol version will be available in Checkmk 2.4.0.