DHCP3-Server giving IP addresses depending on MAC

Nutnot · 04-11-2011, 06:52 PM

Hello, I have dhcp3-server (isc-dhcp-server) installed on my Debian and now I got a question about how it's giving the IP addresses to new devices.

For example: I connected my laptop and dhcp server gave me 192.168.1.5 address. Will it always give me the same ip address when I connect my laptop or it will eventually change after some time (week-month)??
If it's not changing it, then I am wrong about this.

BUT...
If I am somehow correct and if it will change in a week and give me another random (like 192.168.1.8) IP even I won't change my laptop network adapter, is it possible to configure dhcp server to always give same IP address depending on what MAC it is?

To make it clear, I want that when I connect new device (new laptop/pc) dhcp server would give it random IP but same time it would note the MAC address and never change the IP on that MAC.

I know about MAC filters, but setting filters is when you know MAC address since beginning and want to assign IP for it, but in my situation I don't know the MAC address.

I hope I made it clear, waiting for some help on this.

pljvaldez · 04-11-2011, 07:39 PM

You just have to edit the file /etc/dhcp3/dhcpd.conf (I believe that's the file, but I'm not in front of my Debian box right now). There's a section in the config file that talks about static routing based on MAC address. You should set that up for any machines you want to always have the same address.

sibe · 04-11-2011, 08:31 PM

Hi Nutnot,

basically, leased IPs change overtime, but as pljvaldez said, there is a way to statically bind an IP address to a particular client, using its MAC address.

First, you need to figure out the client's MAC address. Everytime a DHCP server leases an IP to a client, it saves the event in a file called dhcpd.leases (in my Red Hat box, it's under /var/lib/dhcpd directory). It looks like this :

Code:

lease 10.12.12.100 {
  starts 6 2011/01/15 14:09:59;
  ends 0 2011/01/16 14:09:59;
  tstp 0 2011/01/16 14:09:59;
  cltt 6 2011/01/15 14:09:59;
  binding state free;
  hardware ethernet 00:0c:29:f9:f0:f5;
  uid "\001\000\014)\371\360\365";
}

Take the MAC address and put it to your DHCP server's dhcpd.conf file, under this directive :

Code:

host some-host-name { hardware ethernet 00:0c:29:f9:f0:f5; fixed-address 10.12.12.100; }

Reload/restart your dhcp service, and now every time your client requests an IP, the server will assign it with a fixed address of 10.12.12.100.

Don't forget to exclude the fixed IP from the dynamic range option to avoid any potential conflict with another host.

----
sibe

Nutnot · 04-11-2011, 09:10 PM

Well I know the ways you just said how to solve this manually by looking at client's MAC address and assigning IP to it, but is it possible to make all this job done automatically by dhcp server?

I mean maybe when dhcp server leases and IP it could make it permanent IP for that particular MAC address?

sibe · 04-11-2011, 09:29 PM

Please apologize if I didn't get the question correctly, but AFAIK there's no way to achieve a permanent lease automatically. Unless, well, you put a very very high TTL on the default and maximum lease variable