ISC DHCP server offers fixed IP addresses to ANY device!

Daniel Manser doenle85 at gmail.com
Fri Dec 19 15:47:21 UTC 2008


Dear List

We are currently having an issue with our ISC DHCP server offering
reserved IP addresses to virtually any device, not only to the MAC
address defined with a "fixed-address" statement.

I'm currently trying to set up an ISC DHCP server for our cable modem
infrastructure. I compiled the latest DHCPD source code (version 4.0.0)
on a fresh Debian 4 box.

Assuming that the IP address XX.169.176.4 is reserved for
00:11:22:33:44:55 (yes, it's a fake MAC and yes, I'm not very
creative), the DHCP server offers this address to ANY device. Here's an
extract of my log file:

Dec 19 15:41:06 dhcpdtest dhcpd: DHCPDISCOVER from 00:a0:12:24:74:a0
via XX.169.176.1
Dec 19 15:41:06 dhcpdtest dhcpd: DHCPOFFER on XX.169.176.4 to
00:a0:12:24:74:a0 via XX.169.176.1
Dec 19 15:41:06 dhcpdtest dhcpd: DHCPREQUEST for XX.169.176.4 to
00:a0:12:24:74:a0 via XX.169.176.1
Dec 19 15:41:06 dhcpdtest dhcpd: DHCPACK on XX.169.176.4 to
00:a0:12:24:74:a0 via XX.169.176.1

I have no idea why DHCPD offers XX.169.176.4. Please note that there
are no existing leases in the dhcpd.leases file.

Below my configuration files. For a better understanding, a short
explaination of my configuration:

Cable modems connect to a DHCP relay and get an IP address out of a
172.72.0.0 range. A device connected to the modem (a computer, or
router) requests a public IP address (fictitous ranges XX.169.176.4 to
XX.169.176.6 and XX.169.176.10 to XX.169.176.14). The address
XX.169.176.4 is reserved for "donaldduck" and should _not_ be offered
to any other device.

dhcpdtest:~# cat /etc/dhcpd.conf
server-identifier XX.196.149.52;
authorative;

include "/etc/dhcpd.d/networks.conf";

subnet 192.168.99.0 netmask 255.255.255.0 { }

subnet 172.27.0.0 netmask 255.255.255.0
{
	option subnet-mask 255.255.255.0;
	range 172.27.0.10 172.27.0.20;
	option routers 172.27.0.1;
	option time-offset 36000;
	default-lease-time 36000;
	max-lease-time 2592000;
	option tftp-server-name "XX.169.149.52";
	filename "THG540-10M-10M_d.cfg";
	include "/etc/dhcpd.d/modems.conf";
}

dhcpdtest:~# cat /etc/dhcpd.d/networks.conf
shared-network OURNETWORKS {
  option domain-name-servers XX.169.149.10;
  default-lease-time 36000;
  subnet XX.169.176.0 netmask 255.255.255.248 {
    option routers XX.169.176.1;
    range XX.169.176.4 XX.169.176.6;
    host donaldduck {
      hardware ethernet 00:11:22:33:44:55;
      fixed-address XX.169.176.4;
      }
    }
  subnet XX.169.176.8 netmask 255.255.255.248 {
    option routers XX.169.176.9;
    range XX.169.176.10 XX.169.176.14;
    }
}

dhcpdtest:~# cat /etc/dhcpd.d/modems.conf
host testmodem {
  hardware ethernet 00:1C:EA:B6:82:27;
  filename "light-d11-epc2203-upd-p1.cm.txt";
  fixed-address 172.27.0.99;
}

Any help/response on this issue is highly appreciated.

Daniel



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