Thanks for the clarification Peter. You're right, I failed to show that
I have configuration in each of the subnet definitions to match on the
client-class (and to restrict to the IP address of the expected relay)
"client-class": "VLAN_84",
"relay": { "ip-address": "10.11.12.13" },
Conveniently, the behavior I have seen over the years is that a
well-behaved client gets a NAK if it tries to rebind an address that is
isn't assigned to, such as after adding a reservation for a host that
has been using the subnet's pool. I didn't recall reading documentation
that stated that this was intentional behavior. That is behind my
hesitation in suggesting this for someone's environment that may be more
sensitive than mine.
Coming back to this many years later, I checked the current
documentation at
>https://kea.readthedocs.io/en/latest/arm/dhcp4-srv.html#how-the-dhcpv4-server-selects-a-subnet-for-the-client>. There it states that
> The rule [using the interface on which the packet was received] does
not apply when the client unicasts its message, i.e. is trying to renew
its lease; such a message is accepted through any interface. The
renewing client sets ciaddr to the currently used IPv4 address, and the
server uses this address to select the subnet for the client (in
particular, to extend the lease using this address).
At least as I interpret this, the "convenient behavior" of checking the
ciaddr against the potential subnets is documented behavior of Kea itself.
Jeff
On 4/4/24 12:54 AM, Peter Davies wrote:
Hi Jeff,
As you have discovered, classes based on option 82 values will only
match relayed traffic.
Therefore, all things being equal, Kea will not be able to select a
subnet for renewing clients.
Have you considered using host reservations with "circuit-id" as the
identifier?
Kind Regards Peter
On 03/04/2024 21.09, Jeff Kletsky wrote:
I had hoped that someone would post a better "solution" than what
I've been using.
My topology is a Cisco SG-series switch in Level 3 mode that is
supplying DHCP (v4) relay to a dedicated subnet with the Kea hosts.
With the caveat that I have not tested this approach for robustness
under attack, what I do is check to see if the request appears to be
a valid REBIND and then select a client class based on either the
VLAN from the circuit ID or that it appears to be a directly sent rebind.
I don't recall how I decided that Kea would select the proper subnet
on these direct rebind requests. I am probably relying on
undocumented behavior. I recall not performing a match against the IP
range for a given VLAN as I didn't want to have to keep the subnet
information in sync across different files.
If anyone can improve on this, I'd appreciate the feedback.
Jeff
// Renew prefers to go direct to the issuing server
// so there is no circuit identifier or topology
//
// Kea doesn't check the existing leases and its
// KNOWN selector appears to be related to the client
// having a reservation.
//
// Select based on it being a Request (renew) packet
// that went direct with matching Ip addresses
{
"name": "is_request",
"test": "option[53].hex == 0x3"
},
{
"name": "is_direct",
"test": "pkt4.giaddr == 0.0.0.0"
},
{
"name": "addresses_match",
"test": "pkt4.ciaddr == pkt.src"
},
{
"name": "is_direct_rebind",
"test": "member('is_request') and member('is_direct') and
member('addresses_match')"
},
// Try just the combination of relay circuit check or rebind
{
"name": "VLAN_84",
"test": "member('circuit_84') or member('is_direct_rebind')"
},
[continues for other VLANs in use]
On 3/28/24 10:40 AM, brazda.li...@seznam.cz wrote:
Hi, I am trying to start kea dhcp with client classification using
option 82 through dhcp relay server.
When client tries to do renew of ip address, tries to prolongate his
lease, kea response with NAK.
The problem is that when client makes simple dhcp discover, the
packet goes broadcast through the router, router acts like dhcp
relay and relays packet to dhcp server kea with added option 82.
But when client makes dhcp renewal-packet goes unicast directly to
dhcp server without option 82. As I understand, this causes kea to
response with NAK, because packet doesn't match to subnet rule
criteria. In the logs I see message "ailed to select a subnet for
incoming packet, src 100.64.1.1, type DHCPREQUEST"
Is there a way to configure kea to accept renewal requests if lease
already exists and mac address of a client corresponds to stores lease?
Full log - https://pastebin.com/yviEFneL
Full config - https://pastebin.com/2DxfQKb6
Thanks for any advice
Libor
--
Peter Davies
Support Engineer
Internet Systems Corporation
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