[Dnsmasq-discuss] two dnsmasq servers providing dhcp for one domain

[Simon Kelley]

Anand Kameswaran wrote:
> Maybe this is a lot more straightforward than I am thinking, but I'm not 
> sure how I should configure DHCP, once I start using DHCP helper.
> 
> Let me try and explain the setup.  Each embedded device has 2 physical 
> NIC cards.  Eth0 and Eth1.  Eth0 is used for public communincations and 
> the VPN tunnels are setup between the two devices eth0 interfaces.  Eth1 
> had multiple virtual adapters to handle the subnetting, ie. 
> eth1:1...eth1:n  depending on the number of required subnets.
> 
> Now when configuring DNSMasq I had run into an oddity.  The machines 
> absolutely should not provide DHCP over eth0 as that would impact the 
> public network.  Initially I configured dnsmasq to explicitly listen on 
> all eth1.x interfaces, but it didn't work.  However, when I configured 
> dnsmasq to only exclude eth0 everything worked fine.
> 
> Here is the crux of the problem using dhcp_helper.  So device 1 is 
> providing DHCP/DNS.  I need to assign a different subnet for nodes on 
> device 2 than for device 1, so that routing can occur over the tunnels.  
> Obviously each side of the vpn tunnels needs unique subnets.  That is 
> one problem, although I know dnsmasq can serve up addresses in multiple 
> ranges.  The real problem I suspect is that requests from nodes on 
> device two, sent over broadcast will probably be received physically on 
> eth0.  ALthough their is in fact a "tunnel" device as well.  (btw using 
> GRE tunnels for the VPN - not encrypted as I am only concerned about 
> network segregation not security) 
> 
> Now I'm not really a good networking guy - so forgive me if my 
> terminology is slightly off, but hopefully my problem is clear.  And I 
> am not really to clear on how to configure dnsmasq once I have DHCP 
> helper working.  I need those requests that come over the DHCP_Helper to 
> get a different subnet.  Any suggestions?
> 
> Cheers,
> AK

You're worrying too much. It all happens automagically. Set up 
dhcp-helper to listen for broadcasts on the correct interface and bounce 
them to dnsmasq. When dhcp-helper does this, it puts the address of the 
interface on which the broadcast was received into the packet, in the 
giaddr field. When dnsmasq gets a packet with giaddr set, it uses that 
address to decide which subnet to allocate an address on, not the 
interface that dnsmasq received the packet on.

So:

dnsmasq, just list all the subnets you will need as dhcp-ranges.

dhcp-helper, select the interfaces  to listen for broadcasts using -i 
<interface> and/or -e <interface> and tell dhcp-helper where to send the 
packets to with -s <ip-addr of server running dnsmasq>

If the dnsmasq server is multi-homed you can use any of its addresses, 
as long as they are routable from the relay. The address of any 
interface that dhcp-helper is listening on must be routable from the 
host running dnsmasq.

HTH

Simon.