[Kea-users] Configuring kea for relayed subnets *not* on its own interface's address

[Jeff Kletsky]

I've been able to get kea to run nicely as a DHCP server in 
"conventional" mode with an interface listening on every one of the 
VLANs that I need to serve.

I'm trying to configure it now so that it only responds to relayed DHCP 
through my Cisco SG300-series switches.

     "dhcp-socket-type": "udp"

is already set.


I've been able to decode the Cisco format and it appears to properly 
assign the client-class based on the VLAN:

kea.conf:

         "client-classes": [
         <?include "/usr/local/etc/kea/client_classes.conf"?>
         ],

client_classes.conf includes:

     {
         "name": "VLAN_1010",
         "test": "substring(relay4[1].hex, 2, 2) == 0x03F2"
     }


Thanks to the debugging instructions in 13.9 (very helpful!), I was able 
to fix my first shot and "test" the above expressions. They match as 
expected, with that match being logged in the kea-dhcp4 log:

     EVAL_RESULT Expression VLAN_1010 evaluated to 1


In my application, the DHCP server and switches are "talking" on 
addresses in the 10.2.87.0/24 range, but the pool is in the 
10.10.10.0/24 range.


Setting the subnet parameter to correspond to the pool results in 
"failed to select a subnet for incoming packet"

The subnet parameter is mandatory, so I can't just use the client-class 
to select the pool

If I set it to 10.2.87.0/24 then kea won't start, complaining that the 
pool does not match the prefix of the subnet

So far, only the rather ugly solution of setting the subnet to one that 
overlaps both the interface's address and the pool is all I've found to 
be functional

     {
         "subnet": "10.0.0.0/8",
         "pools": [ { "pool": "10.10.10.200 - 10.10.10.219" } ],
         "client-class": "VLAN_1010",

     [...]

(similarly <?include "ed"?> in the subnet4 section)


The kea instance will *only* be answering relayed DHCP, never direct 
connections.


Is there a better way to configure this?


TIA,

Jeff