Internet Protocol version 6 (IPv6) has been enabled by default in NetBSD for so long (since at least 1.5) that there is little point in describing how to enable it.
IPv6 temporary addresses have been supported in NetBSD releases since version 7.0. The command to enable them is:
sysctrl –w net.inet6.ip6.use_tempaddr=1
and the command to disable them is:
sysctrl –w net.inet6.ip6.use_tempaddr=0
IPv6 temporary addresses are supported in dhcpcd. See the man page for configuration details.
The NetBSD Project maintains an IPv6 Frequently Asked Questions (IPv6 FAQ) about configuring IPv6 in NetBSD.
The recommended practice is to block all IPv6 inbound and outbound traffic in the npf packet filter, rather than attempting to disable it in the kernel.
1. Edit the file /etc/npf.conf
2. Add the following lines to the (default) group
block in inet6
block out inet6
3. Then, reload and start npf.
For more details, please look at the NetBSD npf.conf manual page.