Linux DHCP sending wrong parameters in offer packet

Fri Jan 25 16:29:15 UTC 2019

Class should be global, I believe. That is my understanding anyway. You shouldn't have them inside of a subnet {} or shared-network {} statement. The limiting occurs with the match if statement. That is my understanding anyways. 

> From: "Gaikwad Yogesh" <Yogesh.Gaikwad at tieto.com>
> To: "Users of ISC DHCP" <dhcp-users at lists.isc.org>
> Sent: Friday, January 25, 2019 9:43:48 AM
> Subject: RE: Linux DHCP sending wrong parameters in offer packet

> Hi Glenn,

> I followed Cisco documentation on below link to configure the option 43 for
> Cisco AP's:

> https://www.cisco.com/c/en/us/support/docs/wireless-mobility/wireless-lan-wlan/97066-dhcp-option-43-00.html

> There are two parts to this config, one is in the global config, and other part
> sits inside the subnet.

> Below lines are in the global configuration

> # CISCO OPTION 43
> option space Cisco_LWAPP_AP;
> option Cisco_LWAPP_AP.server-address code 241 = array of ip-address;

> And then below is the subnet configuration

> subnet 10.56.20.0 netmask 255.255.255.0 {
> option routers 10.56.20.1;
> option subnet-mask 255.255.255.0;
> option broadcast-address 10.56.20.255;
> default-lease-time 43200;

> class "Cisco AP c3800" {

> match if option vendor-class-identifier = "Cisco AP c3800";
> option vendor-class-identifier "Cisco AP c3800";
> vendor-option-space Cisco_LWAPP_AP;
> option Cisco_LWAPP_AP.server-address 10.140.1.1; }

> pool {
> failover peer "te-failover";
> deny dynamic bootp clients;
> range 10.56.20.10 10.56.20.250;
> }
> }

> We have several such subnets configured which have this 'class "Cisco AP c3800"'
> configuration inside the subnets, the above mentioned subnet sits at the end of
> the dhcpd.conf file.

> I understood your concern to have it configured globally, but then what if I
> want to have a different 'Cisco_LWAPP_AP.server-address' for different subnets?

> Best regards,

> Yogesh Gaikwad, Technical Architect - TECA Network

> Tieto Corporation, TSM

> -----Original Message-----
> From: dhcp-users [mailto:dhcp-users-bounces at lists.isc.org] On Behalf Of Glenn
> Satchell
> Sent: Friday, January 25, 2019 7:45 PM
> To: Users of ISC DHCP <dhcp-users at lists.isc.org>
> Subject: Re: Linux DHCP sending wrong parameters in offer packet

> Hi Yogesh,

> Without seeing hyour config file I can guess that the option 43 setting should
> be set in the global configuration rather than inside one of your subnets.
> Setting it inside the subnet causes inheritance of those subnet values.

> To confirm, could you please post your dhcpd.conf? If it is is very large
> perhaps you might post the relevant parts.

> regards,
> -glenn
> --
> Glenn Satchell mailto:glenn.satchell at uniq.com.au | Today is the past
> Uniq Advances Pty Ltd http://www.uniq.com.au | that people in
> Mobile 0409 458 580 | the future will
> Member IT Professionals Association www.itpa.org.au | dream about.

> On Fri, January 25, 2019 11:41 pm, Gaikwad Yogesh wrote:
> > Hi,

> > There is issue that linux DHCP is sending wrong parameters while
> > sending offer packet for Cisco AP's in one of our office locations.

> > I have taken packet captures on the DHCP server side.

> > As you can see in below packet capture there is DHCP DISCOVER message
> > from Cisco AP requesting for IP and other parameters.

> > Then in DHCP offer packet, it offers IP from correct subnet, but with
> > wrong router IP, subnet mask and broadcast address.

> > Basically, my observation is that it is giving wrong parameters in
> > offer from the topmost subnet (as linux works from top to bottom
> > approach) that has cisco option 43 configured. If I remove the Cisco
> > option 43 from that subnet then it provides parameters from next
> > subnet which has the Cisco option 43 in it.

> > The correct subnet from which I am expecting that the AP should get
> > all its parameters is configured at the end of the DHCP conf file.

> > DHCP DISCOVER PACKET

> > [cid:image001.png at 01D4B4D9.6F320900]

> > DHCP OFFER PACKET

> > [cid:image002.png at 01D4B4D9.6F320900]

> > Best regards,

> > Yogesh Gaikwad, Technical Architect - TECA Network

> > Tieto Corporation, TSM

> > email Yogesh.Gaikwad at tieto.com<mailto:Yogesh.Gaikwad at tieto.com>,
> > direct
> > +91 (0)2067303120
> > Wing 1, Cluster D, EON Free Zone, MIDC Kharadi Knowledge Park, Pune -
> > 411 014, India

> > [Tieto_logo_for_email_signature_127x50]<http://www.tieto.com/>

> > Please note: The information contained in this message may be legally
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