Different DNS server response for dhcpdiscover than for dhcpinform

[Michael Robbert]

We are currently running ISC DHCP 3.1.3b1 and have been running the same basic config for many years now, but recently we've had some clients that appear to behave differently and I think that I've tracked it down to what appears to be a known bug in the way the server responds to DHCPINFORM packets. There have been list discussions about this in the past and a workaround for one users situation was posted, but our situation is a little different and I'd like to know if this bug is supposed to be fixed or if anybody can suggest a different config that will work for us.
Our setup is that we send most of our clients to our two main DNS servers, but on a few subnets if the client MAC address is not registered with us (unknown to DHCP) we want to send them to a registration portal page so we give them a different DNS server that will direct them to our "captive portal". The problem is that we've found some clients appear to be occasionally sending DHCPINFORM packets at some point and are getting the public DNS servers so they are not getting redirected. Here is what I think are the relevant parts of our config:

option domain-name "Mines.EDU";
option domain-name-servers magma.Mines.EDU, ns1.Mines.EDU;
default-lease-time 7200;
max-lease-time 21600;
option broadcast-address 255.255.255.255;

authoritative;

#Main Campus subnet
subnet 138.67.0.0 netmask 255.255.192.0 {
        option broadcast-address 138.67.63.255;
        option subnet-mask 255.255.192.0;
        #known clients
        pool {
                failover peer "magma-massive";
                deny dynamic bootp clients;
                range 138.67.20.10 138.67.20.250;
                range 138.67.11.10 138.67.11.250;
                option routers 138.67.1.1;
                deny unknown clients;
        }
        #unknown clients
        pool {
                failover peer "magma-massive";
                deny dynamic bootp clients;
                range 138.67.13.10 138.67.13.240;
                range 138.67.10.10 138.67.10.240;
                option domain-name-servers slag.Mines.EDU;
                option routers 138.67.1.63;
                allow unknown clients;
                deny known clients;
        }
}

...
Many other subnets
...
include "/home/dhcpd/dhcp.registered";

Thanks in advance for any help!

Mike Robbert