How to disable IPv6 on OpenBSD?
You must build a custom kernel that lacks the INET6 option. But the OpenBSD maintainers seem to advise against this.
-inet6 Disable inet6(4) on the given interface and remove all
configured inet6(4) addresses, including the link-local
ones. To turn it on again, assign any inet6 address or
this if from `man ifconfig'.
family Specify which type of Internet protocol family to prefer, if
a host is reachable using different address families. By
default IPv4 addresses are queried first, and then IPv6
addresses. The syntax is:
family family1 [family2]
A maximum of two families can be specified, where family can
be any of:
inet4 IPv4 queries.
inet6 IPv6 queries.
this is from `man resolv.conf'
Custom kernel is not OpenBSD way to go! Do not ever ask for help with custom kernel, you don't understand technical details anyway :)