Issue / bug with DHCP INFORM requests

Randall C Grimshaw rgrimsha at syr.edu
Sun Dec 11 04:04:15 UTC 2011


It is a known 'limitation' that the dhcp server does not record the client specifics in the lease. At the time of the inform request the server will provide the global and subnet specific responses... but nothing from group related configurations as originally provided.
Initially this was a small limitation, but over time more clients are asking for info packets as part of normal device discovery.
It is quietly conceeded that the feature of providing group based configuration is deprecated by default. We still do it. Somehow it seems to still be helpful for the initial directory server binding... ymmv.

Randall Grimshaw rgrimsha at syr.edu
________________________________________
From: dhcp-users-bounces+rgrimsha=syr.edu at lists.isc.org [dhcp-users-bounces+rgrimsha=syr.edu at lists.isc.org] on behalf of Guillaume [sirocco at gmx.fr]
Sent: Saturday, December 10, 2011 7:03 PM
To: dhcp-users at lists.isc.org
Subject: Issue / bug with DHCP INFORM requests

Hi,

I setup a dhcp server with release 4.2.3-P1 and I got problems with DHCP
INFORM requests.
it seems that DHCP INFORM does not take care of host configuration.

I made a specific configuration where a host does not have the same
parameters as the subnet :

ddns-domainname "home.dns";
ddns-update-style none;
ddns-updates off;

option domain-name "home.dns";

default-lease-time  21600;
max-lease-time      43200;


subnet 10.0.0.0 netmask 255.255.255.0 {
         authoritative;
         allow unknown-clients;
         option subnet-mask              255.255.255.0;
         option broadcast-address        10.0.0.255;
         option routers                  10.0.0.1;
         option domain-name-servers      10.0.0.1;
         option ntp-servers              10.0.0.1;
         option time-offset              1;
         range                           10.0.0.240 10.0.0.254;
}

host mypc {
         hardware ethernet               1c:11:11:11:11:11;
         fixed-address                   10.0.0.208;
         option subnet-mask              255.255.255.0;
         option broadcast-address        10.0.0.255;
         option routers                  10.0.0.2;
         option domain-name-servers      10.0.0.2;
         option ntp-servers              10.0.0.2;
         option time-offset              1;
}

- - -

Here is the issue :
When I boot "mypc" it sends "DHCP Request" to the snmp server and it
gets the good ip address (10.0.0.208) and good gateway, dns and ntp
server (10.0.0.2)
After some time "mypc" sends "DHCP Inform" but the dhcp server provides
wrong gateway, dns and ntp server (10.0.0.1)

Then, if I manually force a "DHCP Request" mypc gets good ip address
(10.0.0.208) and good gateway, dns and ntp server (10.0.0.2)
but as soon as mypc sends "DHCP Inform" it gets wrong gateway, dns and
ntp server (10.0.0.1) from the DHCP server

Do you have any idea why this happen ?

Thanks,

Guillaume
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