OpenBSD as a single-stack DHCPv6 client

I wanted to try running OpenBSD as a single-stack IPv6 host on a network where dnsmasq provides stateful DHCPv6. This is a less common deployment case than using SLAAC with or without stateless DHCPv6, but it has the advantage that it allows dnsmasq to register a FQDN for the client in its local DNS, so that the client can be accessed by name on the local LAN without further configuration. My relevant dnsmasq config is this:

enable-ra
dhcp-range=::10,::ffff,constructor:br6,64,12h

The dhcp-range line implicitly has it serving stateful DHCPv6 on a range derived from the prefix delegated to interface br6, with a prefix length of 64 and a 12 hour lease.

I tried a few linux clients and they all seemed to pick it up without further configuration, but OpenBSD did not. It seems like as of 7.9-current, there is no support for requesting IA_NA addressing (a non-temporary v6 address for the host) using only base. dhcp6leased is specialized for requesting IA_PD (prefix delegation) only.

However, dhcpcd in ports can request IA_NA. So I temporarily assigned an address by hand using ifconfig just long enough to do pkg_add dhcpcd and came up with the following for /etc/dhcpcd.conf

ipv6only
hostname "obsd"
interface vio0
ipv6rs
ia_na 1

At first this only partially worked. It was getting a v6 address and a DNS server from DHCPv6, but actual outbound access was failing. The reason was, DNS lookups were still returning A records, and something was assigning an APIPA v4 (169.254 autoconfigured link local) address to the NIC (vio0). route show was indeed showing broken v4 routes related to this.

The solution was disabling the start of both dhcpleased and dhcp6leased and adding ipv6only to the dhcpcd config, preventing anything from trying v4 configuration.

After that things are working fine. OpenBSD is now pulling a v6 address via DHCPv6 and working well as a single stack client.