Maybe I haven't tried since Comcast turned on v6 for everyone, but I just set up /etc/network/interfaces as specified in https://wiki.debian.org/IPv6PrefixDeleg

Maybe I haven't tried since Comcast turned on v6 for everyone, but I just set up /etc/network/interfaces as specified in https://wiki.debian.org/IPv6PrefixDelegation https://wiki.debian.org/IPv6PrefixDelegation and it just worked this time!