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Multiple DHCP servers on the network behavior

Posted on 2009-07-13
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Last Modified: 2012-05-07
Hello,

I have 4 Linksys WRT310N on my network. One is a OpenVPN server and the others OpenVPN clients. 2 of the clients have DHCP enabled. I also have software OpenVPN clients. The problem is some network clients are using remote gateways instead of the local one. For example in Miami the router DHCP server is 192.168.1.1. A client connected to that router is being assigned a IP by the Houston router, 192.168.1.2, when it should be assigned by the Miami router, to which it is connected directly. How to prevent such behavior?

Thank you!
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Question by:starusa
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Expert Comment

by:Qlemo
ID: 24840104
Have a look whether you have DHCP relay activated on the Linksys clients. As DHCP broadcasts cannot cross a routing connection, the Linksys at each end seems to be do something to allow for it. Or are you running OpenVPN in bridged mode (vs. routing, with own transfer networks)?
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Expert Comment

by:SteveJ
ID: 24866257
I think Qlemo may be onto your problem . . . there's no reason why your local linksys should ignore a DHCP request and pass it to a remote.

Good luck,
SteveJ
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Author Comment

by:starusa
ID: 24904622
Hey guys,
I'm not very into networking. I'm using tap0, so I guess that's bridging. DHCP Relay is the same as DHPC Forwarding? What about changing the router mode from Gateway to Router on the clients?
Thank you.
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Expert Comment

by:Qlemo
ID: 24905181
If the OpenVPN clients get IPs of the server network, it is bridged. If OpenVPN uses a separate IP address range, it is routed.

About Linksys router mode - I do not know whether that could influence the DHCP behaviour, but it might.

And DHC relay = DHCP forwarding.
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Author Comment

by:starusa
ID: 24905452
Well, in that case I'm using bridged mode. The router which is the vpn server is configured as 192.168.3.1 and the clients have the same 192.168.3.x. Also, DHCP forwarding is not enabled for any router.
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Accepted Solution

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Qlemo earned 2000 total points
ID: 24924142
I'm guessing only. I assume the DHCP requests are forwarded using OpenVPN because of the bridge mode. But that should not be the case, as the routers themselves are acting as DHCP servers.

Well, as a workaround you could use DHCP ClassID, set up on client by
ipconfig /setclassid "Local Area Connection" MyNewClassId
, and name the Scope of each site's DHCP server accordingly. A client without a ClassID is getting an arbitrary DHCP address, though.
This requires you use DHCP servers capable of Scopes / ClassIDs, of course ...

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