[Kea-users] Struggling with "failed to select a subnet for incoming packet" - Kea 3.0.3

Thu Apr 9 21:31:54 UTC 2026

On 4/9/26 15:15, Tim Harman wrote:
> On 10/04/2026 5:10 am, Doug Freed wrote:
>> On 4/9/26 03:39, Darren Ankney wrote:
>>> Hi Tim,
>>>
>>> Could you attach the pcap of the client traffic?  Nothing is jumping
>>> out from the included log messages.  It would be ideal if the pcap
>>> included both unsuccessful renewal and successful DORA exchanges.
>>>
>>> Thank you,
>>> Darren Ankney
> 
>>
>> It would also be good to crank the logging up to DEBUG level 99 during 
>> the pcap.  There's a lot of useful debug logging around subnet 
>> selection that would likely provide insight here.
>>
>> -Doug
> 
> A small snippet.
> tcpdump capture and full log outputs are attached.
> 
> Apr 10 08:01:36 kea-dhcp4[297513]: 2026-04-10 08:01:36.703 DEBUG [kea- 
> dhcp4.packets/297513.140641718414336] DHCP4_BUFFER_RECEIVED received 
> buffer from 192.168.0.174:68 to 192.168.0.1:67 over interface eth1
> Apr 10 08:01:36 kea-dhcp4[297513]: 2026-04-10 08:01:36.704 DEBUG [kea- 
> dhcp4.options/297513.140641684354752] DHCP4_BUFFER_UNPACK parsing buffer 
> received from 192.168.0.174 to 192.168.0.1 over interface eth1
> Apr 10 08:01:36 kea-dhcp4[297513]: 2026-04-10 08:01:36.704 INFO [kea- 
> dhcp4.dhcp4/297513.140641684354752] DHCP4_QUERY_LABEL received query: 
> [hwtype=1 6c:f7:34:ef:8f:f1], cid=[01:6c:f7:34:ef:8f:f1], tid=0x8c6f352f
> Apr 10 08:01:36 kea-dhcp4[297513]: 2026-04-10 08:01:36.705 INFO [kea- 
> dhcp4.packets/297513.140641684354752] DHCP4_PACKET_RECEIVED [hwtype=1 
> 6c:f7:34:ef:8f:f1], cid=[01:6c:f7:34:ef:8f:f1], tid=0x8c6f352f: 
> DHCPREQUEST (type 3) received from 192.168.0.174 to 192.168.0.1 on 
> interface eth1
> Apr 10 08:01:36 kea-dhcp4[297513]: 2026-04-10 08:01:36.705 DEBUG [kea- 
> dhcp4.packets/297513.140641684354752] DHCP4_QUERY_DATA [hwtype=1 
> 6c:f7:34:ef:8f:f1], cid=[01:6c:f7:34:ef:8f:f1], tid=0x8c6f352f, packet 
> details: local_address=192.168.0.1:67, remote_address=192.168.0.174:68,
> Apr 10 08:01:36 kea-dhcp4[297513]: msg_type=DHCPREQUEST (3), 
> trans_id=0x8c6f352f,
> Apr 10 08:01:36 kea-dhcp4[297513]: options:
> Apr 10 08:01:36 kea-dhcp4[297513]:   type=053, len=001: 3 (uint8)
> Apr 10 08:01:36 kea-dhcp4[297513]:   type=055, len=012: 1(uint8) 
> 3(uint8) 6(uint8) 15(uint8) 26(uint8) 28(uint8) 51(uint8) 58(uint8) 
> 59(uint8) 43(uint8) 114(uint8) 108(uint8)
> Apr 10 08:01:36 kea-dhcp4[297513]:   type=057, len=002: 1500 (uint16)
> Apr 10 08:01:36 kea-dhcp4[297513]:   type=060, len=015: "android- 
> dhcp-11" (string)
> Apr 10 08:01:36 kea-dhcp4[297513]:   type=061, len=007: 
> 01:6c:f7:34:ef:8f:f1
> Apr 10 08:01:36 kea-dhcp4[297513]: 2026-04-10 08:01:36.705 DEBUG [kea- 
> dhcp4.dhcpsrv/297513.140641684354752] 
> DHCPSRV_SUBNET4_SELECT_NO_RAI_OPTIONS No RAI options found to use for 
> subnet selection.
> Apr 10 08:01:36 kea-dhcp4[297513]: 2026-04-10 08:01:36.705 DEBUG [kea- 
> dhcp4.dhcpsrv/297513.140641684354752] 
> DHCPSRV_SUBNET4_SELECT_NO_RELAY_ADDRESS Relay address (giaddr) in client 
> packet is empty.
> Apr 10 08:01:36 kea-dhcp4[297513]: 2026-04-10 08:01:36.705 DEBUG [kea- 
> dhcp4.dhcpsrv/297513.140641684354752] 
> DHCPSRV_SUBNET4_SELECT_NO_USABLE_ADDRESS No subnet selected because no 
> suitable address to use for subnet selection was found.
> Apr 10 08:01:36 kea-dhcp4[297513]: 2026-04-10 08:01:36.705 DEBUG [kea- 
> dhcp4.packets/297513.140641684354752] DHCP4_SUBNET_SELECTION_FAILED 
> [hwtype=1 6c:f7:34:ef:8f:f1], cid=[01:6c:f7:34:ef:8f:f1], 
> tid=0x8c6f352f: failed to select subnet for the client
> Apr 10 08:01:36 kea-dhcp4[297513]: 2026-04-10 08:01:36.705 DEBUG [kea- 
> dhcp4.dhcp4/297513.140641684354752] DHCP4_CLASS_ASSIGNED [hwtype=1 
> 6c:f7:34:ef:8f:f1], cid=[01:6c:f7:34:ef:8f:f1], tid=0x8c6f352f: client 
> packet has been assigned to the following class: UNKNOWN
> Apr 10 08:01:36 kea-dhcp4[297513]: 2026-04-10 08:01:36.705 DEBUG [kea- 
> dhcp4.dhcp4/297513.140641684354752] 
> DHCP4_CLASSES_ASSIGNED_AFTER_SUBNET_SELECTION [hwtype=1 
> 6c:f7:34:ef:8f:f1], cid=[01:6c:f7:34:ef:8f:f1], tid=0x8c6f352f: client 
> packet has been assigned to the following classes: ALL, 
> VENDOR_CLASS_android-dhcp-11, UNKNOWN
> Apr 10 08:01:36 kea-dhcp4[297513]: 2026-04-10 08:01:36.705 DEBUG [kea- 
> dhcp4.leases/297513.140641684354752] DHCP4_REQUEST [hwtype=1 
> 6c:f7:34:ef:8f:f1], cid=[01:6c:f7:34:ef:8f:f1], tid=0x8c6f352f: server 
> is processing DHCPREQUEST with hint=192.168.0.174
> Apr 10 08:01:36 kea-dhcp4[297513]: 2026-04-10 08:01:36.705 ERROR [kea- 
> dhcp4.bad-packets/297513.140641684354752] DHCP4_PACKET_NAK_0001 
> [hwtype=1 6c:f7:34:ef:8f:f1], cid=[01:6c:f7:34:ef:8f:f1], 
> tid=0x8c6f352f: failed to select a subnet for incoming packet, src 
> 192.168.0.174, type DHCPREQUEST
> Apr 10 08:01:36 kea-dhcp4[297513]: 2026-04-10 08:01:36.705 DEBUG [kea- 
> dhcp4.callouts/297513.140641684354752] HOOKS_CALLOUTS_BEGIN begin all 
> callouts for hook leases4_committed
> Apr 10 08:01:36 kea-dhcp4[297513]: 2026-04-10 08:01:36.706 DEBUG [kea- 
> dhcp4.callouts/297513.140641684354752] HOOKS_CALLOUT_CALLED hooks 
> library with index 1 has called a callout on hook leases4_committed that 
> has address 0x7fe9b1a7e860 (callout duration: 0.058 ms)
> Apr 10 08:01:36 kea-dhcp4[297513]: 2026-04-10 08:01:36.706 DEBUG [kea- 
> dhcp4.callouts/297513.140641684354752] HOOKS_CALLOUTS_COMPLETE completed 
> callouts for hook leases4_committed (total callouts duration: 0.058 ms)
> Apr 10 08:01:36 kea-dhcp4[297513]: 2026-04-10 08:01:36.706 DEBUG [kea- 
> dhcp4.options/297513.140641684354752] DHCP4_PACKET_PACK [hwtype=1 
> 6c:f7:34:ef:8f:f1], cid=[01:6c:f7:34:ef:8f:f1], tid=0x8c6f352f: 
> preparing on-wire format of the packet to be sent
> Apr 10 08:01:36 kea-dhcp4[297513]: 2026-04-10 08:01:36.706 INFO [kea- 
> dhcp4.packets/297513.140641684354752] DHCP4_PACKET_SEND [hwtype=1 
> 6c:f7:34:ef:8f:f1], cid=[01:6c:f7:34:ef:8f:f1], tid=0x8c6f352f: trying 
> to send packet DHCPNAK (type 6) from 192.168.0.1:67 to 192.168.0.174:68 
> on interface eth1
> Apr 10 08:01:36 kea-dhcp4[297513]: 2026-04-10 08:01:36.706 DEBUG [kea- 
> dhcp4.packets/297513.140641684354752] DHCP4_RESPONSE_DATA [hwtype=1 
> 6c:f7:34:ef:8f:f1], cid=[01:6c:f7:34:ef:8f:f1], tid=0x8c6f352f: 
> responding with packet DHCPNAK (type 6), packet details: 
> local_address=192.168.0.1:67, remote_address=192.168.0.174:68,
> Apr 10 08:01:36 kea-dhcp4[297513]: msg_type=DHCPNAK (6), 
> trans_id=0x8c6f352f,
> Apr 10 08:01:36 kea-dhcp4[297513]: options:
> Apr 10 08:01:36 kea-dhcp4[297513]:   type=053, len=001: 6 (uint8)
> Apr 10 08:01:36 kea-dhcp4[297513]:   type=054, len=004: 192.168.0.1 
> (ipv4-address)
> Apr 10 08:01:36 kea-dhcp4[297513]:   type=061, len=007: 
> 01:6c:f7:34:ef:8f:f1
> Apr 10 08:01:38 kea-dhcp4[297513]: 2026-04-10 08:01:38.901 DEBUG [kea- 
> dhcp4.packets/297513.140641718414336] DHCP4_BUFFER_RECEIVED received 
> buffer from 0.0.0.0:68 to 255.255.255.255:67 over interface eth1
> Apr 10 08:01:38 kea-dhcp4[297513]: 2026-04-10 08:01:38.902 DEBUG [kea- 
> dhcp4.options/297513.140641675962048] DHCP4_BUFFER_UNPACK parsing buffer 
> received from 0.0.0.0 to 255.255.255.255 over interface eth1
> Apr 10 08:01:38 kea-dhcp4[297513]: 2026-04-10 08:01:38.902 INFO [kea- 
> dhcp4.dhcp4/297513.140641675962048] DHCP4_QUERY_LABEL received query: 
> [hwtype=1 6c:f7:34:ef:8f:f1], cid=[01:6c:f7:34:ef:8f:f1], tid=0xe07f740b
> Apr 10 08:01:38 kea-dhcp4[297513]: 2026-04-10 08:01:38.902 DEBUG [kea- 
> dhcp4.dhcpsrv/297513.140641675962048] 
> DHCPSRV_SUBNET4_SELECT_NO_RAI_OPTIONS No RAI options found to use for 
> subnet selection.
> Apr 10 08:01:38 kea-dhcp4[297513]: 2026-04-10 08:01:38.902 DEBUG [kea- 
> dhcp4.dhcpsrv/297513.140641675962048] 
> DHCPSRV_SUBNET4_SELECT_NO_RELAY_ADDRESS Relay address (giaddr) in client 
> packet is empty.
> Apr 10 08:01:38 kea-dhcp4[297513]: 2026-04-10 08:01:38.902 DEBUG [kea- 
> dhcp4.dhcpsrv/297513.140641675962048] 
> DHCPSRV_SUBNET4_SELECT_BY_INTERFACE_NO_MATCH No subnet matches 
> interface: eth1
> Apr 10 08:01:38 kea-dhcp4[297513]: 2026-04-10 08:01:38.902 DEBUG [kea- 
> dhcp4.dhcpsrv/297513.140641675962048] DHCPSRV_CFGMGR_SUBNET4_ADDR 
> selected subnet 192.168.0.0/24 for packet received by matching address 
> 192.168.0.1
> Apr 10 08:01:38 kea-dhcp4[297513]: 2026-04-10 08:01:38.902 DEBUG [kea- 
> dhcp4.packets/297513.140641675962048] DHCP4_SUBNET_SELECTED [hwtype=1 
> 6c:f7:34:ef:8f:f1], cid=[01:6c:f7:34:ef:8f:f1], tid=0xe07f740b: the 
> subnet with ID 1 was selected for client assignments
> Apr 10 08:01:38 kea-dhcp4[297513]: 2026-04-10 08:01:38.902 DEBUG [kea- 
> dhcp4.packets/297513.140641675962048] DHCP4_SUBNET_DATA [hwtype=1 
> 6c:f7:34:ef:8f:f1], cid=[01:6c:f7:34:ef:8f:f1], tid=0xe07f740b: the 
> selected subnet details: 192.168.0.0/24
> Apr 10 08:01:38 kea-dhcp4[297513]: 2026-04-10 08:01:38.902 INFO [kea- 
> dhcp4.packets/297513.140641675962048] DHCP4_PACKET_RECEIVED [hwtype=1 
> 6c:f7:34:ef:8f:f1], cid=[01:6c:f7:34:ef:8f:f1], tid=0xe07f740b: 
> DHCPDISCOVER (type 1) received from 0.0.0.0 to 255.255.255.255 on 
> interface eth1
> Apr 10 08:01:38 kea-dhcp4[297513]: 2026-04-10 08:01:38.902 DEBUG [kea- 
> dhcp4.packets/297513.140641675962048] DHCP4_QUERY_DATA [hwtype=1 
> 6c:f7:34:ef:8f:f1], cid=[01:6c:f7:34:ef:8f:f1], tid=0xe07f740b, packet 
> details: local_address=255.255.255.255:67, remote_address=0.0.0.0:68,
> Apr 10 08:01:38 kea-dhcp4[297513]: msg_type=DHCPDISCOVER (1), 
> trans_id=0xe07f740b,
> Apr 10 08:01:38 kea-dhcp4[297513]: options:
> Apr 10 08:01:38 kea-dhcp4[297513]:   type=053, len=001: 1 (uint8)
> Apr 10 08:01:38 kea-dhcp4[297513]:   type=055, len=012: 1(uint8) 
> 3(uint8) 6(uint8) 15(uint8) 26(uint8) 28(uint8) 51(uint8) 58(uint8) 
> 59(uint8) 43(uint8) 114(uint8) 108(uint8)
> Apr 10 08:01:38 kea-dhcp4[297513]:   type=057, len=002: 1500 (uint16)
> Apr 10 08:01:38 kea-dhcp4[297513]:   type=060, len=015: "android- 
> dhcp-11" (string)
> Apr 10 08:01:38 kea-dhcp4[297513]:   type=061, len=007: 
> 01:6c:f7:34:ef:8f:f1
> Apr 10 08:01:38 kea-dhcp4[297513]:   type=080, len=000: ''
> Apr 10 08:01:38 kea-dhcp4[297513]: 2026-04-10 08:01:38.902 DEBUG [kea- 
> dhcp4.dhcpsrv/297513.140641675962048] 
> DHCPSRV_SUBNET4_SELECT_NO_RAI_OPTIONS No RAI options found to use for 
> subnet selection.
> Apr 10 08:01:38 kea-dhcp4[297513]: 2026-04-10 08:01:38.903 DEBUG [kea- 
> dhcp4.dhcpsrv/297513.140641675962048] 
> DHCPSRV_SUBNET4_SELECT_NO_RELAY_ADDRESS Relay address (giaddr) in client 
> packet is empty.
> Apr 10 08:01:38 kea-dhcp4[297513]: 2026-04-10 08:01:38.903 DEBUG [kea- 
> dhcp4.dhcpsrv/297513.140641675962048] 
> DHCPSRV_SUBNET4_SELECT_BY_INTERFACE_NO_MATCH No subnet matches 
> interface: eth1
> Apr 10 08:01:38 kea-dhcp4[297513]: 2026-04-10 08:01:38.903 DEBUG [kea- 
> dhcp4.dhcpsrv/297513.140641675962048] DHCPSRV_CFGMGR_SUBNET4_ADDR 
> selected subnet 192.168.0.0/24 for packet received by matching address 
> 192.168.0.1
> Apr 10 08:01:38 kea-dhcp4[297513]: 2026-04-10 08:01:38.903 DEBUG [kea- 
> dhcp4.packets/297513.140641675962048] DHCP4_SUBNET_SELECTED [hwtype=1 
> 6c:f7:34:ef:8f:f1], cid=[01:6c:f7:34:ef:8f:f1], tid=0xe07f740b: the 
> subnet with ID 1 was selected for client assignments
> 
> Kind Regards,
> Tim Harman

This is a VyOS-specific bug, which happens in this exact situation. VyOS 
is patching Kea to fix another issue, but the patch is incorrect. The 
patch was submitted to us here:

https://gitlab.isc.org/isc-projects/kea/-/merge_requests/2811

One of our developers noticed the problem, and there's a second commit 
in the MR which resolves it, but that has not been applied to VyOS's 
patch.  If you build VyOS yourself, you can update the patch or remove 
it if the issue it's fixing doesn't apply to you.

-Doug