Why can't ifupdown find dhcpcd?

So I have an embedded system I've built with yocto. I'm trying to enable DHCPv6. But when I try to bring the interface online, ifup says it can't find DHCPv6 software, even though dhcpcd is installed. And dhcpcd works if I run it manually.

root@1234:~# cat /etc/network/interfaces
auto lo
iface lo inet loopback

auto eth0
iface eth0 inet static
        address 192.168.5.80
        netmask 255.255.255.0
        gateway 192.168.5.1

iface eth1 inet6 dhcp

root@1234:~# ip a show dev eth1
8: eth1:  mtu 1500 qdisc noqueue qlen 1000
    link/ether ce:e6:83:59:f1:93 brd ff:ff:ff:ff:ff:ff
    inet 169.254.23.45/16 brd 169.254.255.255 scope global noprefixroute eth1
       valid_lft forever preferred_lft forever
    inet6 fe80::468b:6249:e50b:a437/64 scope link
       valid_lft forever preferred_lft forever

root@1234:~# which dhcpcd
/sbin/dhcpcd

root@1234:~# ifup eth1
No DHCPv6 client software found!
ifup: failed to bring up eth1

root@1234:~#

Note that eth1 is just a dummy interface for now, so that I don't break my ssh connection messing with eth0, which is really where I want to add DHCPv6.