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Server 2012 R2, Cisco RV082

This is for a very small network system where the DC's have to be shutdown periodically for a day or so. 2 DC's exist & are also acting as DHCP servers. It's a temporary physical constraint that is being worked on for a resolution. In the meantime it is desired to be able to plug into the network and gain internet access when the DHCP servers are not operating -- without having to go into the router and reconfigure it each time.

Is it possible to engage the router's DHCP for a small set of addresses for those periods when the DC's are offline and keep that from interfering with the DC DHCP servers when they are online? What I'm looking for is a technique or setup that will ensure the router doesn't serve IP's when the DC's are online and filling that role.

I am asking this question because in my study of ADDS/DNS/DHCP operation I thought I read where there are significant benefits to have all of this done by the servers and not have a router's DHCP operation interfere. For that reason I turned off the router DHCP, but if the DC's are offline, I need the router's DHCP. I want to know if and how I can turn the router DHCP back on without impacting server ADDS/DNS/DHCP operation when the DC's are online.

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  • If you can keep the router powered on then why can't you keep the servers powered on? – joeqwerty Aug 14 '15 at 20:32
  • Fair question: It's a physical security issue. Plans are underway to improve security, but until then, when there is a risk of loss, things are turned off from time to time. If the router was lost, no big deal; server loss, much bigger deal. – Alan Aug 19 '15 at 21:46
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It's not all that difficult. When the servers are being turned off enable DHCP on the router. Disable DHCP on the router when the servers are being turned on.

Any DHCP server on the network can service clients. A client will use whichever DHCP server it gets a response from first. So in your scenario, you'll need to enable DHCP on the router when you turn the servers off and disable it on the router when you turn the servers on.

On a side note: I think you're going to find this solution/situation problematic. A DHCP client that isn't actively in the rebinding phase is going to continue using it's original DHCP lease configuration, including the DNS servers assigned as part of that lease... those being the servers that are turned off. You'll need to perform an ipconfig/release and ipconfig/renew on the DHCP clients in order to get DNS name resolution working as they'll then need to use the router for DNS and will need the router to assign itself as the DNS server with the DHCP lease from the router.

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  • Got it; didn't think of that. Will look to see this problem in action next time I'm shutting down the DCs. Thx! Good motivation to get the security situation resolved! – Alan Aug 20 '15 at 21:13
  • Glad to help... – joeqwerty Aug 20 '15 at 23:33
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For small site you can simply remove the DHCP's role from the server and leave it to the router for ever, I seen that often.

You setup the DHCP on the router to give the domain dns suffix and to give only the domain dns. Server on or at off, the DHCP will never fail.

The problem you will face is more for the DNS. Usually you give only the DC's IP as a DNS's server, but when the computer will fail to resolve any IP via the DC (because it's off) you will need an external's DNS. The computer switch to use the second DNS and will not restart to use the first DNS if the second DNS never fail. A manual operation to ask your user to restart their computer will be necessary to make them see the domain correctly again. You could give your user a small script to set their dns too... like illustrated there

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  • The problem with this route is that if a client is given a DNS server that can't communicate with any of the DCs, it will be unable to communicate with any domain-based resources. – I say Reinstate Monica Aug 17 '15 at 1:50
  • @Twisty read the op question.. its exactly the problem I state, as both DC are powered down... The op will have to give external DNS as I stated.. even if its not a good idea at all – yagmoth555 Aug 17 '15 at 2:50
  • My feedback is meant to be constructive, not negative. Your answer had a sense of permanence to it vs. a temporary work around. If left this way permanently, my observation stands. If you mean this to be implemented only as long as the DCs are offline, then a clarifying edit would garner an upvote from me. – I say Reinstate Monica Aug 17 '15 at 2:57
  • @Twisty good point, but I told the easy way, as he want to close the DC anyhow, I kinda stand behind joeqwerty comment now, as why he close the dc but not the router, that make no sense. He could even run a virtual dc on a secretary computer to back the dns. My answer is only good if the OP could do a site to site vpn to be able to reach another dns, or the virtual dc to have two working dns gave by the router – yagmoth555 Aug 17 '15 at 3:09
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    @Twisty I will edit tomorrow, as I see its not wrote ok. The DHCP can be run on the router and give only the domain dns and suffix. I seen that in a lot of small office that add a domain later on and forget to take the dhcp role on the server, but that work, even if the dc are on. That point is not stated correctly in my answer. The point I wanted the OP to answer me, is how is dns work when all dc are off. That make confusion in my post – yagmoth555 Aug 17 '15 at 3:25
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Yeah, just ensure that the router sets DHCP in non-authoritative mode, and te DC are in authoritative.

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  • I'll check that out on my router. thx – Alan Aug 14 '15 at 18:50
  • The RV082 is an entry class router, the online manual does not show such option. In non-authoritative the DHCP does not offer new bail, so that mean a manual operation is necessary anyway for @Alan – yagmoth555 Aug 14 '15 at 20:22
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    It's not all that difficult. When the servers are being turned off enable DHCP on the router. Disable DHCP on the router when the servers are being turned on. All of this talk of authoritative, non-authoritative is nonsense. – joeqwerty Aug 14 '15 at 20:31
  • @joeqwerty. Want to make sure I understand you correctly, basically you're saying I really will have to go into the router setups to turn DHCP on and off. In other words, there's no real way for me to tell DHCP in the router to act in a 'secondary role', in other words, let the server DC's handle everything, unless they don't. I didn't recall any way to do this, but I thought I would ask in case I missed something. – Alan Aug 19 '15 at 21:42
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    DHCP doesn't work like that. Any DHCP server on the network can service clients. A client will use whichever DHCP server it gets a response from first. So in your scenario, you'll need to enable DHCP on the router when you turn the servers off and disable it on the router when you turn the servers on. – joeqwerty Aug 19 '15 at 22:09

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