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I know there are mixed feelings about running multiple DHCP servers on one LAN, but I'm wondering about the best way to set their ranges.

In my case, I have a router which performs DHCP and also my Ubuntu Server which I use for FOG ghosting (among other things of course). Since FOG packages itself with a DHCP server, I figured it best to use it and help things run smoother for FOG. I'd probably rather run just one DHCP server so that I could look over all the leases in one place.

Anyways, is it better to distribute the ip ranges among the DHCP servers? or is it okay to just overlap them. or is there even a difference.

Also, would it be possible (and advantageous) to have my wireless router service only the wireless requests, while having the Ubuntu Server Box service the Ethernet ones?

Thanks,

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3 Answers 3

up vote 2 down vote accepted

A DHCP server will service all the requests it receives. If the wireless Ethernet network and the wired Ethernet network are the same broadcast domain then you don't want to be running two DHCP servers in that single broadcast domain (in the vast majority of cases). If they're separate broadcast domains (implying that they're different IP subnets, too) you can run a DHCP server on a host in each broadcast domain, or if the router separating the subnets supports DHCP relaying you could run a single DHCP server and receive relayed requests from the clients on the "far" subnet.

I suspect you've got a single broadcast domain, since you're talking about "overlapping" the address scopes. That being the case you really should have only one DHCP server servicing that entire broadcast domain. (There are situations where you could have more than one in a broadcast domain-- typically in a failover scenario-- but that's beyond the scope of your question.)

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I guess you answered the part about the wireless and ethernet being serviced by different DHCP servers. Both wireless and ethernet are all on the same broadcast domain, so I guess they can't have their own DCHP server. but there is a bit I'm still unclear about... you said "you don't want to be running two DHCP servers in that single broadcast domain" .. is there a reason for this? I'd imagine its because they can conflict with each other~ OK, But if I had to run two DHCP servers, should they have different ranges? –  neildaemond Jan 6 '12 at 8:18
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I don't buy the idea of "split scopes" in the same broadcast domain unless the DHCP server software has a method of coordinating leases "behind the scenes". ISC dhcpd, for example, has failover functionality that intelligently handles failover. Two random DHCP servers with "split scopes", though, are just going to create a situation in which your leases are served randomly by the DHCP server that answers first. That's too non-deterministic for me. –  Evan Anderson Jan 6 '12 at 11:09
    
I agree Evan, non-deterministic is baaad. Good argument for not having multiple DHCP servers in the same broadcast domain, thanks. –  neildaemond Jan 6 '12 at 13:00

As Evan mentioned you don't want two DHCP servers on the same broadcast domain. Which DHCP server responds to a client's DHCPREQUEST/DHCPRENEW is whichever one gets the DHCPREQUEST first. Neither DHCP server is going to be aware of the other's lease database (which clients have which addresses) so you're almost guaranteed to get IP address conflicts and just like Highlander there can only be one! (Things won't work right if you have the same IP address assigned to more than one station).

The exception to this is doing DHCP Split Scope. The idea is that half of your available addresses goes to one DHCP server and the other half to other. This way you can guarantee that you won't end up with a the same address assigned to multiple clients AND you get some redundancy in your DHCP services.



Also, would it be possible (and advantageous) to have my wireless router service only the wireless requests, while having the Ubuntu Server Box service the Ethernet ones?

If you had more than one broadcast domain this can be nice. You can subnet your addressing space and give one subnet to the wireless clients and another to your wired clients. This makes routing, filtering, and monitoring them easier.

EDIT: I just looked at your other question. What you want is either a separate network (either physical or VLANed) for your FOG clients with one subnet and associated DHCP services and another network for your employee machines (with their own services).

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Thanks kce, double thanks for checking out my other question (where I ended up putting everything all on the same network with one router at the gateway and un-managed switches to everything else. I'm a bit of a noob when it comes to subnets, so I kept it simple). "subnet your addressing space and give one subnet to the wireless clients and another to your wired clients" -> will my wireless and wired clients have access to the same network resources if they are on different subnets? –  neildaemond Jan 6 '12 at 10:01
    
If you route them the way, then yes. :D –  kce Jan 6 '12 at 17:27
    
oh man, I still have a lot to learn about subnets and routing O_o –  neildaemond Jan 7 '12 at 2:07
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Take a look at the fantastic question: How does subnetting work? –  kce Jan 7 '12 at 22:20

What about setting the router in the Access Point mode? DHCP responses will be passed from the Ubuntu Server to the wireless clients directly.

Basically every wireless router supports the AP mode, just disable DHCP server on the router, enable WiFi radio and plug in the router to the network using one of the LAN ports.

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Great idea!! done. –  neildaemond Jan 6 '12 at 12:22

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