when trying to modify ipv6 DNS on an up-to-date pre-installed FreeBSD 11.2-RELEASE system per bsdconfig, it seems, that this tool only allows ipv4 entries. A valid ipv6 address like 2a03:b0c0:0:1010::e9a:3001 produces following error message from bsdconfig:
[...]ERROR! One or more individual octets within the IPv4 address
(separated by dots) contains one or more invalid characters.
Octets must contain only the characters 0-9.
Invalid IP Address: 2a03:b0c0:0:1010::e9a:3001[...]
This error is reproducible.
A solution for me is to use DHCP and writing this entries into my router configuration.
But maybe this report helps people, who need a static network configuration on their FreeBSD system.
I'll go ahead and add the missing bits to support IPv6. That includes CIDR notation and all the various different representations of IPv6 that are valid.