[Kea-users] Global host reservations - unexpected behavior

[Gibbins, John (IM&T, Black Mountain)]

Hi Maria,

In the 1.5.0 Kea Administrator Reference Manual (p69 of the PDF) it says:

"Note You can reserve any ip-address in a global reservation. Just keep in mind that Kea will not do any sanity checking on the
address and for Kea 1.5.0, support for global reservations should be considered experimental."

I believe that by "sanity checking" it means that it will not ensure that the address is appropriate for the subnet as you found.  I suspect this may change in later releases.

Disclaimer: I'm very new to kea.

I'm trying to translate our ISC DHCP config to kea.  I had assumed from this that I needed to put reservations within the subnet as you discovered.  To me it makes more sense to have them associated with the subnet rather than globally as ISC DHCP did.
My intention is to translate ISC DHCP reservations to subnet reservations and subclasses to global reservations.

Regards
johng

-----Original Message-----
From: Kea-users <kea-users-bounces at lists.isc.org> On Behalf Of Maria Hrabosova
Sent: Tuesday, 15 October 2019 1:17 AM
To: kea-users at lists.isc.org
Subject: [Kea-users] Global host reservations - unexpected behavior

Hello,

I found a use case in which KEA DHCP server behaves differently than I 
expected and I would like to ask if it was meant to behave that way or 
there is a problem in my configuration. Here is the description:

Imagine you have a known host client-X with a reserved IP address 
10.0.0.9. client-X usually connects via the Shared-B network, but one 
day it wants to connect via Shared-A. The IP address reserved for 
client-X does not match any of the subnets in Shared-A. Therefore, I 
would expect that it would be given an IP address from the pool in the 
subnet 10.1.1.0/24 (that is how it works in ISC DHCP). However, it is 
given it's reserved IP 10.0.0.9 along with the options from the subnet 
10.1.1.0/24, which does not make sense to me. Is this how it was meant 
to behave?

I found a solution how to make the client-X get an IP address from the 
pool in the subnet 10.1.1.0/24 by moving the host reservation from the 
global scope to the scope of the subnet 10.0.0.0/24. Anyway, I've been 
wondering why it doesn't work the same way with the global reservations. 
I tried the global mode first, as I am using host reservations in groups 
(which are global) in ISC DHCP and it works as expected there.

Could you please give it a look? Thanks in advance. See the 
corresponding configuration and the demonstration of the use case below.

Best regards,

Maria Hrabosova


---

kea-dhcp4.conf:

{
   "Dhcp4": {
     "interfaces-config": {
       "interfaces": [
         "server-eth0"
       ],
       "dhcp-socket-type": "udp"
     },
     "control-socket": {
       "socket-type": "unix",
       "socket-name": "/tmp/kea-dhcp4-ctrl.sock"
     },
     "lease-database": {
       "type": "memfile"
     },
     "shared-networks": [
       {
         "name": "Shared-A",
         "relay": {
           "ip-address": "192.0.2.1"
         },
         "subnet4": [
           {
             "subnet": "10.1.1.0/24",
             "pools": [
               {
                 "pool": "10.1.1.200 - 10.1.1.250"
               }
             ],
             "option-data": [
               {
                 "name": "routers",
                 "data": "10.1.1.1"
               },
               {
                 "name": "domain-name-servers",
                 "data": "10.1.1.2, 10.1.1.3"
               }
             ]
           }
         ]
       },
       {
         "name": "Shared-B",
         "relay": {
           "ip-address": "192.0.1.1"
         },
         "subnet4": [
           {
             "subnet": "10.0.0.0/24",
             "pools": [
               {
                 "pool": "10.0.0.200 - 10.0.0.250"
               }
             ],
             "option-data": [
               {
                 "name": "routers",
                 "data": "10.0.0.1"
               },
               {
                 "name": "domain-name-servers",
                 "data": "10.0.0.2, 10.0.0.3"
               }
             ]
           }
         ]
       }
     ],
     "host-reservation-identifiers": [
       "hw-address"
     ],
     "reservation-mode": "global",
     "reservations": [
       {
         "hw-address": "aa:a0:00:00:11:10",
         "ip-address": "10.0.0.9",
         "hostname": "client-X"
       }
     ]
   }
}

---

dhclient-debug.sh:

#!/bin/sh
env

---

ip add
...
     link/ether aa:a0:00:00:11:10 brd ff:ff:ff:ff:ff:ff link-netnsid 1
...


dhclient client1-eth0 --timeout 2 -sf dhclient-debug.sh -H client-X
...
new_ip_address=10.0.0.9
new_routers=10.1.1.1
new_domain_name_servers=10.1.1.2 10.1.1.3
...

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