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DHCP woes

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Last Modified: 2012-05-05
I would like to help a school who is having problems in a mixed Mac and Windows environment (mostly Mac workstations with a handful of Windows XP workstations).  One of the problems appears to be the proliferation of DHCP servers.  For example, they have a Mac server (running ancient Mac OS 8.6) that acts as the primary DHCP server.  They also have a DHCP server available on their Windows server, the DSL router, the SonicWALL firewall box, and at least two or three other wireless routers (both Linksys and Mac Airport) scattered around the school.  I have turned off DHCP on all devices except the Mac Server, but I'm still getting mixed results.  I wonder if I should choose a "higher order" device, like the SonicWALL box to act as the sole DHCP server?  And that way, when I eventually turn on VPN services, the SonicWALL will be able to handle those requests also?  Does anyone have any tips or insight into the best way to implement DHCP in such a mixed environment?
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Commented:
I like the idea of having more than one device or server acting as a DHCP server in case one fails or is down for maintenance.  However, does that cause a conflict or confusion for a client workstation if it sends out a DHCP request, and then receives more than one response?
No. As cleaner said, just do not overlap your DHCP ranges.

No having multiple devices/servers providing dhcp services is not an issue since:

 A workstation will broadcast a request for address assignment, the dhcp service will then provide a response to the request. The first response received by the workstation will be the one used by the workstation and the workstation will make use of that ip address for the duration of the lease time allowed by the dhcp server. Additionally the workstation will notify the dhcp server of its use of the address so as to keep the server from serving that IP address to another workstation.

The only issue with using multiple dhcp servers is that a single workstation may have more than one address tied to it until the lease expires for an address that is no longer in use. For example, if you were to boot  the workstation and it received an address from the MAC with a lease time of 86400 seconds (1 Day), and you were then to reboot the workstation prior to the lease expiration time, and have an additional address served to the machine by the PC server, the workstation would then have two addresses assigned to it  1. from the mac, and 2. another from the pc. This is not an issue as long as you have more addresses for use than you have workstations. In this scenario the MAC would keep the address in an used status until after the lease expiration time has expired. It will then place the ip address back into its unused range and will serve it again upon need

Regards,
tiggrdave
"The only issue with using multiple dhcp servers is that a single workstation may have more than one address tied to it until the lease expires for an address that is no longer in use. For example, if you were to boot  the workstation and it received an address from the MAC with a lease time of 86400 seconds (1 Day), and you were then to reboot the workstation prior to the lease expiration time, and have an additional address served to the machine by the PC server, the workstation would then have two addresses assigned to it  1. from the mac, and 2. another from the pc. This is not an issue as long as you have more addresses for use than you have workstations. In this scenario the MAC would keep the address in an used status until after the lease expiration time has expired. It will then place the ip address back into its unused range and will serve it again upon need"
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I don't believe this is the case (I'm happy to be proven wrong though)...

If you have an IP already from a DHCP server, when you reboot you will simply keep the same IP and only send a DHCPDiscover packet if the client doesn't have an IP, otherwise it will just keep the existing address until it needs to renew (at 50% of the lease) or then send another DHCPDiscover at 87.5% of the lease duration at which point any DHCP server alive can renew the request if possible or hand out a new lease.

Hello The Cleaner,


Actually, I believe you to be correct.

The issue I referenced is one that occurred this morning on a segment of our internal network. About 15 minutes ago we found that we had configuration issues on a server's  interface/route  set-up which caused  some very strange results while rebooting  several workstations.  

Sorry for any confusion this may have caused.

tiggrdave

Author

Commented:
Thanks to all for the help!

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