DHCP binding ignored

[dan_miles86]

First of all, yes I know DHCP on a router is not a great idea, however needs must.

I'm creating manual pools linked to the client identifier. Sometimes these work, sometimes they don't and just get given an IP out of the main pool. I've tried with the 01 prefix and without.

For example;

ip dhcp pool Sonoff1Test

   host 192.168.20.90 255.255.255.0

   client-identifier 0168.c63a.8125.15

   lease infinite

ip dhcp pool Sonoff1

   host 192.168.20.91 255.255.255.0

   client-identifier 68c6.3a81.2515

   lease infinite

Then when checking bindings given;

192.168.20.90        0168.c63a.8125.15       Infinite                              Manual

192.168.20.91        68c6.3a81.2515            Infinite                              Manual

192.168.20.170      68c6.3a81.2515            Nov 19 2017 08:01 PM    Automatic

You can see the identifiers match but gets given .170 instead of .90 or .91.

Any ideas?

[Flavio Miranda]

Hi @dan_miles86

Your client identifier may be not correct. 

Make sure by running the command:

debug ip dhcp server packet

-If I helped you somehow, please, rate it as useful.- 

[dan_miles86]

If the client id was wrong, surely I wouldn't see the same one in the active binding list? None the less, here's the debug showing the same ID and wrong IP.

*Nov 19 20:46:45.783: DHCPD: DHCPDISCOVER received from client 68c6.3a81.2515 on interface BVI1.
*Nov 19 20:46:45.783: DHCPD: Allocate an address without class information (192.168.20.0)
*Nov 19 20:46:47.783: DHCPD: Sending DHCPOFFER to client 68c6.3a81.2515 (192.168.20.202).
*Nov 19 20:46:47.783: DHCPD: creating ARP entry (192.168.20.202, 68c6.3a81.2515).
*Nov 19 20:46:47.783: DHCPD: unicasting BOOTREPLY to client 68c6.3a81.2515 (192.168.20.202).
*Nov 19 20:46:47.783: DHCPD: DHCPDISCOVER received from client 68c6.3a81.2515 on interface BVI1.
*Nov 19 20:46:47.783: DHCPD: Sending DHCPOFFER to client 68c6.3a81.2515 (192.168.20.202).
*Nov 19 20:46:47.783: DHCPD: creating ARP entry (192.168.20.202, 68c6.3a81.2515).
*Nov 19 20:46:47.783: DHCPD: unicasting BOOTREPLY to client 68c6.3a81.2515 (192.168.20.202).
*Nov 19 20:46:48.035: DHCPD: DHCPREQUEST received from client 68c6.3a81.2515.
*Nov 19 20:46:48.035: DHCPD: Sending DHCPACK to client 68c6.3a81.2515 (192.168.20.202).
*Nov 19 20:46:48.035: DHCPD: creating ARP entry (192.168.20.202, 68c6.3a81.2515).
*Nov 19 20:46:48.035: DHCPD: unicasting BOOTREPLY to client 68c6.3a81.2515 (192.168.20.202)

[Georg Pauwen · Accepted solution]

Hello,

on a side note, be sure that you have not configured the addresses used in the manual bindings in the DHCP excluded address scope.

Either way, does it work when you use 'hardware-address' instead of 'client-identifier' ?

What are your clients ? I seem to recall that e.g. Linux clients don't work well with client identifiers, while Windows clients do...

[dan_miles86]

Thanks Georg!

FYI is a microprocessor so likely to be Linux there somewhere.

The info helped me find;

https://twigleaf.wordpress.com/2008/12/06/cisco-ios-dhcp-manual-bindings-for-dummies/

which explains this well.

[Georg Pauwen]

Hello,

as far as I recall, 'hardware-address' uses BOOTP, wheres 'client-identifier' uses DHCP. Linux clients use BOOTP, so that would be the underlying reason...

I guess that is exactly similar to what the document in the link you posted says...