[Kea-users] DHCPv4 - respond to option 108 without allocating leases

[Lexi Winter]

hi all,

i have an IPv6-only network which is using SLAAC+DHCPv6 managed
configuration.  the IPv6 side is working fine, but i've noticed that
clients (such as iPhone) that connect to this network continue to send
DHCPv4 requests constantly while connected to the network, even though
there's no DHCPv4 server and they have correctly configured CLAT using
SLAAC PREF64.

i'd like to configure DHCPv4 option 108 in Kea to inform these clients
that IPv4 is not necessary, so they stop sending DHCPv4 requests.  (this
is a wireless network, so reducing unnecessary broadcast/multicast
traffic is important.)

i can see in the DHCP request that option 108 is being requested:

---o<--
06:14:29.940632 IP (tos 0x0, ttl 255, id 42014, offset 0, flags [none], proto UDP (17), length 328)
    0.0.0.0.68 > 255.255.255.255.67: BOOTP/DHCP, Request from fe:bd:56:91:6f:79, length 300, xid 0x6caaff0, secs 2, Flags [none]
	  Client-Ethernet-Address fe:bd:56:91:6f:79
	  Vendor-rfc1048 Extensions
	    Magic Cookie 0x63825363
	    DHCP-Message (53), length 1: Discover
	    Parameter-Request (55), length 9:
	      Subnet-Mask (1), Classless-Static-Route (121), Default-Gateway (3), Domain-Name-Server (6)
	      Domain-Name (15), Unknown (108), URL (114), Unknown (119)
	      Unknown (252)
	    MSZ (57), length 2: 1500
	    Client-ID (61), length 7: ether fe:bd:56:91:6f:79
	    Lease-Time (51), length 4: 7776000
---o<--


on the DHCP server's network interface, i've configured a dummy IPv4
subnet to test:


---o<--
ix0.301: flags=1008943<UP,BROADCAST,RUNNING,PROMISC,SIMPLEX,MULTICAST,LOWER_UP> metric 0 mtu 1500
	options=4600703<RXCSUM,TXCSUM,TSO4,TSO6,LRO,RXCSUM_IPV6,TXCSUM_IPV6,MEXTPG>
	ether 207c.14f3.d8f7
	inet 10.3.1.1/24 broadcast 10.3.1.255
	inet6 fe80::1%ix0.301/64 scopeid 0x2b
	inet6 2001:8b0:aab5:c000::1/64
	groups: vlan
	vlan: 301 vlanproto: 802.1q vlanpcp: 0 parent interface: ix0
	media: Ethernet autoselect (10Gbase-Twinax <full-duplex,rxpause,txpause>)
	status: active
	nd6 options=1<PERFORMNUD>
---o<--


i've then configured Kea to send option 108 based on the documentation
at [0].  however, because i don't want to allocate IPv4 addresses, i've
not configured a pool:


--o<--
        "interfaces-config": {
                "interfaces": [ "ix0.103", "ix0.106", "ix0.301" ]
        },
[...]
                {
                        // VLAN301 - Eden wireless (IPv6-only)
                        "id": 3,
                        "subnet": "10.3.1.0/24",
                        "option-data": [
                            {
                                "name": "v6-only-preferred",
                                "data": "300"
                            }
                        ]
                },
--o<--


however, Kea is not sending any DHCP responses to queries:


--o<--
06:21:46.526939 IP 0.0.0.0.68 > 255.255.255.255.67: BOOTP/DHCP, Request from 2a:d1:19:aa:2f:ca, length 300
06:21:49.132138 IP 0.0.0.0.68 > 255.255.255.255.67: BOOTP/DHCP, Request from fe:bd:56:91:6f:79, length 300
06:21:50.327221 IP 0.0.0.0.68 > 255.255.255.255.67: BOOTP/DHCP, Request from fe:bd:56:91:6f:79, length 300
06:21:51.124746 IP 0.0.0.0.68 > 255.255.255.255.67: BOOTP/DHCP, Request from 2a:d1:19:aa:2f:ca, length 300
--o<--


there's nothing in the Kea log that indicates why it's not sending a
response.

am i doing something wrong here?  do i perhaps need an IPv4 pool
configured in Kea in order for a response to be sent?  if so, is there a
way to configure Kea to only offer leases to option 108 clients, to
ensure they don't actually use the allocated address?

	thanks, lexi.

[0] https://kea.readthedocs.io/en/kea-2.4.1/arm/dhcp4-srv.html#ipv6-only-preferred-networks
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