DHCP capability query

Simon Hobson dhcp1 at thehobsons.co.uk
Tue May 19 08:49:10 UTC 2009

parthiban Thiyagarajakumar wrote:
>         Need some information about the capabilities of DHCP servers 
>as per the following.
>       DHCP servers typically allocate IP address based on the MAC 
>address in the chaddr field. However, RFC 4361 (which updates RFC 
>2131), specifies support for the Client ID option (which can be 
>populated with the NAI).The question is : If the client ID option is 
>present, will the DHCP server use that to allocate IP address?

For the ISC server, yes it does. For other servers you'll have to ask 
in appropriate other places.

>RFC 3993: Subscriber ID option.  NWG stage 3 references this RFC. 
>The RFC itself says the following for DHCP server.  
>DHCP Server Behavior
>This suboption provides additional information to the DHCP server. 
>If it is configured to support this option, the DHCP server may use 
>this information in addition to other relay agent option data and 
>other options included in the DHCP client messages in order to 
>assign an IP address and/or other configuration parameters to the 
>client. There is no special additional processing for this suboption.

With a 'simple' config, the ISC server uses the ClientID (or if 
ClientID is not present, Client MAC address) for address assignment. 
There are some configs that can force different behavior (with some 
limitations). Regardless of address assignment, there are facilities 
for assigning different option based on admin specified criteria.

On the roadmap, but not implemented, is a config option to override 
the default behavior. That should allow address assignment based on 
arbitrary criteria (such as network switch port).

Simon Hobson

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