I have a network with 100 hosts, I want DHCPD to simple ignore 10 of them. How do I do that using software?

you can:
1. block the mac address of the 10 with an iptables rule like:

iptables -A INPUT -m mac --mac-source 00:aa:cc:bb:nn:dd -j DROP

2. in your DHCP server configuration (assuming to be /etc/dhcpd.conf), you can
define the mac address to ip definition, and don't define an IP range to be offerred:

host OKPC01 {
hardware ethernet 00:16:3e:0b:f2:29;
fixed-address 192.168.222.30;
}

Thanks.

Option 2 is best for me because I have two dhcp/LTSP servers and some hosts need to get IP settings from one server and some other hosts from the other server. Why you ask. Because it is a thin client network so every thin client has to get ip settings from the same dhcp/LTSP everytime they boot. Otherwise crazy things might happen.

But in this network I also have lots of normal desktop clients that also need IP settings from a dhcp server.

1 - Can I use this...

...and also especify a range for normal clients?

2 - If 1 is possible. I'd specify this range in one of the DHCP servers. What happens if a normal desktop client asks for Ip settings from a DHCP server and it's rejected. Would the desktop client try again to find another DHCP server in the network or would it just bail?

For the record, using iptables won't work if the daemon uses raw sockets (like dhcpd does).

So iptables wasn't really an option for this in the first place.