Multi-homed DHCP server

kcarsey · 07-02-2014, 01:25 PM

Hello all, first post here and I am having some issues setting up a dhcp server at my workplace.

We have what I would consider a large network spanning 15+ large buildings, over 150 switches and over 1000 network devices plus a large wireless network.

I am trying to configure a Centos 6 box to server as a dhcp server on our network. We have layer 3 distrubition and core switches, and multiple vlans per building. I am wanting to put a dhcp server in the datacenter, on the datacenter vlan/subnet. I am getting some error messages in /var/log/messages and I am not sure how I need my local network set up on the server. I do not want to trunk every vlan on campus (100+ vlans) to this server. I was hoping to have the server have one interface IP, and use IP helper across the network to forward dhcprequest packets to the sever.

Error message

Code:

Jul  2 14:09:12 dhcp1 dhcpd: No subnet declaration for eth0 (10.72.228.40).
Jul  2 14:09:12 dhcp1 dhcpd: ** Ignoring requests on eth0.  If this is not what
Jul  2 14:09:12 dhcp1 dhcpd:    you want, please write a subnet declaration
Jul  2 14:09:12 dhcp1 dhcpd:    in your dhcpd.conf file for the network segment
Jul  2 14:09:12 dhcp1 dhcpd:    to which interface eth0 is attached. **
Jul  2 14:09:12 dhcp1 dhcpd:
Jul  2 14:09:12 dhcp1 dhcpd:
Jul  2 14:09:12 dhcp1 dhcpd: Not configured to listen on any interfaces!
Jul  2 14:09:12 dhcp1 dhcpd:
Jul  2 14:09:12 dhcp1 dhcpd: This version of ISC DHCP is based on the release available
Jul  2 14:09:12 dhcp1 dhcpd: on ftp.isc.org.  Features have been added and other changes
Jul  2 14:09:12 dhcp1 dhcpd: have been made to the base software release in order to make
Jul  2 14:09:12 dhcp1 dhcpd: it work better with this distribution.
Jul  2 14:09:12 dhcp1 dhcpd:
Jul  2 14:09:12 dhcp1 dhcpd: Please report for this software via the CentOS Bugs Database:
Jul  2 14:09:12 dhcp1 dhcpd:     http://bugs.centos.org/
Jul  2 14:09:12 dhcp1 dhcpd:
Jul  2 14:09:12 dhcp1 dhcpd: exiting.

Datacenter network is 10.72.228.0/22

dhcpd config with three test scopes

Code:

# DHCP Server Configuration file.
#   see /usr/share/doc/dhcp*/dhcpd.conf.sample
#   see 'man 5 dhcpd.conf'
#
option domain-name "xxxxxxxxx.edu";
option domain-name-servers xxx.xxx.xxx.xxx,xxx.xxx.xxx.xxx;
authorative;

#JDK-WIFI
#VLAN 475
#subnet 10.73.128.0 netmask 255.255.252.0
#       range 10.73.128.50 10.72.131.250;
#       option routers 10.73.128.1;
#       option broadcast-address 10.73.131.255;
#       default-lease-time 300;
#       max-lease-time 300;
#

#JDK-WIFI-MANAGE
#VLAN 476
subnet 10.73.132.0 netmask 255.255.255.0 {
        range 10.73.132.50 10.73.132.250;
        option routers 10.73.132.1;
        option broadcast-address 10.73.132.255;
        default-lease-time 300;
        max-lease-time 300;
}

#JDK-WIRED
#VLAN 477
subnet 10.73.136.0 netmask 255.255.252.0 {
        range 10.73.136.50 10.73.139.250;
        option routers 10.73.136.1;
        option broadcast-address 10.73.139.255;
        default-lease-time 300;
        max-lease-time 300;
}

Also current interface config

Code:

DEVICE="eth0"
BOOTPROTO="STATIC"
IPADDR=10.72.228.40
NETMASK=255.255.252.0
HWADDR="00:25:90:11:50:D6"
IPV6INIT="yes"
IPV6_AUTOCONF="yes"
NM_CONTROLLED="yes"
ONBOOT="yes"
TYPE="Ethernet"
UUID="37234b68-d7a5-4a43-8278-0f9dc270e982"

I have done some searching on this forum, and around the internet and have picked up on some things, but mostly questions and answers about serving a couple scopes. I plan on having around 80-100 dhcp scopes in this server when complete.

Thanks!

kirukan · 07-02-2014, 09:30 PM

Have you gone through this link
https://access.redhat.com/documentat...CP_Server.html

kcarsey · 07-02-2014, 09:40 PM

I have gone through a very similar writing except on the Centos help pages.

My question is, if I create virtual interfaces in the system for each subnet range, will those scopes still respond and hand out addresses even though the virtual adapter is not on the same subnet as the physical nic adapter? I was hoping to just set each L3 switch that is routing near the edge of the network to use IP helper to point to 10.72.228.40 and 10.72.228.45 for a second dhcp server for load balancing/redundancy. also how should I name them in networking-scripts. I am not a first time linux guy, just never done anything at all related to a dhcp server. Currently we are temporary using our layer 3 core switch as a dhcp server, and it is less than optimal.

The way it was previously done, is that for every vlan there was a network interface in /etc/sysconfig/networking-scripts such as ifcfg-eth0 then ifcfg-eth0.20 ifcfg-eth0.30 for vlan 20,30 etc. I am trying to move away from this, so that I do not have to trunk all my network's vlans into the datacenter network just for one or two dhcp servers. It places a lot of broadcast load on my switches, and also if I have a storm somewhere else in the network, it could affect the switches in the datacenter negatively.

Also, maybe I just need to declare one subnet in the same IP range as the network adapter on the server and then all the rest of the scopes will respond also on that single interface? I dont need to serve dhcp in my datacenter but if this is a sort of a workaround to get it to work then I am ok with that.

Thank you for the reply, I am sort of pulling my hair out at this point.

kirukan · 07-02-2014, 10:11 PM

I think configure dhcp relay agent in your router is the best option.

kcarsey · 07-03-2014, 07:02 AM

I agree that is going to be the best way to get this to work. Still unclear on what sort of setup I need on my server to allow it to operate as intended.

kcarsey · 07-03-2014, 11:41 AM

Got this to work finally. All I needed to do was define a scope in the same subnet as the interface IP address of the server which is our data center network. I had originally not intended to serve dhcp in this network because it is all servers, but this workaround seems to have worked. I can pull proper dhcp ip addresses from a network that is 3 hops away in our LAN.

Thanks!