DHCP Offer in both Unicast and Broadcast for Vista Broadcast Workaround

[Arthur Winters]

All,

You may know that Vista forces the entire DHCP process to be in 
broadcasts.  The issue I am having is that I am trying to use Cisco 'arp 
authorized' which only allows ARP entries to be added from a valid 
source (the DHCP server).  The problem is the arp authorized commands 
look at the unicast version of the DHCP Offer packet to get the MAC 
address and IP address of the valid client and adds it to the ARP table. 

The issue is, if the entire process is done in broadcasts, the router 
does not know who a valid client is.  The client will still get a valid 
IP address, its just the router will not allow them access.  We could 
set a registry flag in Vista to not use broadcasts, but we prefer not to 
do this as we will have many users we need to do this for, and will 
become a nightmare.

The next option that came up was to some how modify the source code of 
ISC-DHCP to send both a unicast offer and a broadcast offer if the 
broadcast flag is set in the discover packet.  There for Cisco will get 
the packet it is looking for and Vista will get its packet it is looking 
for.

I know it is a Vista issue, not a DHCP or Cisco issue, but we need to 
find a work around that does not involve having the users modify their 
software for it to work properly.

Thanks,

Arthur