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DHCP Relay

Posted on 2013-01-20
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Last Modified: 2013-01-29
Hi All,

AS a experiment, I'm trying to setup dhcp relay inside my LAN. I have a Cable modem in bridge mode connected to my Cisco RVS4000 which is connected to a Netgear Router.

Cable Modem--RVS4000. NAT 192.168.40.X Subnet--Netgear, NAT, 192.168.23.X Subnet.

I have Server 2008 behind the Netgear handling DHCP. My goal is to have Server 2008 handle DHCP services for the Cisco network. I setup cisco to relay to the internal IP address of DHCP server 192.168.23.4 but its not working,

I know double-nat is never recommended but is that why relay is failing?
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Question by:bd14pb
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by:Jason Watkins
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Hi,

Are you allowing ports 67, 68 through the NAT, on the cisco end.
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by:Darr247
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The helper address should be the Netgear (Netgear, NAT, 192.168.23.X), not the DHCP server (192.168.23.4) itself.
http://www.cisco.com/en/US/docs/ios/12_4t/ip_addr/configuration/guide/htdhcpre.html

There's nothing inherently wrong with double NAT, by the way... it's actually a little more secure.
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by:bd14pb
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I changed the DHCP helper address in Cisco box to the address of the netgear on the 192.168.23.X subnet. Still not relaying.
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by:Darr247
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Turn off the firewall in the netgear and see if it works... if so, then you'll need to make a hole in its firewall to allow BOOTP broadcasts/replies on UDP ports 68/67 through.

You didn't make it to the netgear's outside (WAN) address did you?  (which I assume would be in the 192.168.40.0/24 network.)
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by:bd14pb
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I have firewall turned off on netgear. No its set at the netgears address on the 192.168.23.X subnet.
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by:Darr247
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Put the Cisco back to using the server IP (192.168.23.4) as the DHCP relay IP (previous link was not for the RVS4000).

In the Netgear's WAN setup, change the NAT filtering to Open.

If there is no NAT filtering option in the WAN setup, what is the specific model of the netgear?
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by:bd14pb
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You mean switch Cisco to the IP of the DHCP server on the 192.168.23.X network. or the IP of the Netgears interface on the LAN side, in that case 192.168.23.4?

Netgear is a WNR2000

No filtering option
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by:Darr247
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You originally said
>>> I have Server 2008 behind the Netgear handling DHCP.
>>> My goal is to have Server 2008 handle DHCP services for
>>> the Cisco network. I setup cisco to relay to the internal IP
>>> address of DHCP server 192.168.23.4 but its not working,
So I assumed 192.168.23.4 was the Server 2008's IP address.

The DHCP relay setting in the Cisco should be the IP address of the Server 2008 interface that's connected to the netgear's LAN side.

> No filtering option
That's odd, because it shows up in the WNR2000 manual.WNR2000 - WAN Setup - NAT FilteringWNR2000 - WAN Setup - NAT Filtering Configuration
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by:bd14pb
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Changed IP of DHCP relay on CIsco to IP of DHCP server on 192.168.23.X subnet and disabled filtering on netgear. Still no success.

I ran wireshark from a computer on the Cisco network and all I see are DHCP broadcast messages. No machine is responding to those broadcasts.
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by:Darr247
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Next step is to do the same on the LAN side of the Netgear to see if those BOOTP broadcast requests are making it through the WAN<->LAN bridge of the Netgear, then.
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by:bd14pb
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it doesnt seem like the cisco is relaying anything?! This is very frustrating. What could possibly be wrong? All firewalls turned off
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by:Darr247
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I don't understand why you deduce the cisco isn't relaying them.

I'm assuming you're using the bootp Display Filter so you see only the DHCP requests.

Here's a quick way to check. Unplug the network cable coming from the computer (the one with wireshark running on it) from the cisco's LAN and connect it to the cisco's WAN port. Give the network adapter an unused static IP in the 192.168.23.0 network, then trigger whatever you have asking for an address on the LAN side of the cisco to start asking again... if you see the DHCP requests appearing out of the WAN port, they're not being blocked by the cisco.
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by:vivigatt
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ip helper (dhcp relay) must be set to the DHCP server IP address, so that it forwards DHCP requests (DHCP DISCOVER) packets to the DHCP server
On the DHCP server itself, you must create a DHCP scope for the subnet that the forwarded DHCP DISCOVER packets come from.
Use Wireshark on the DHCP server itself, check if you see incoming DHCP DISCOVER packets from the other (relayed) subnets. You can filter on UDP 67 and 68. Check if your DHCP server sends DHCP replies corresponding to the forwarded DHCP packets.
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by:bd14pb
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Nothing is working guys. I dont see the DHCP requests getting to the server. DO I need to add a static route entry for 192.168.23.X network in the Cisco?
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Darr247 earned 350 total points
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If the cisco has a 192.168.23.xxx /24 address on its WAN interface (you should probably set that Static in the WAN setup, if you haven't already) it doesn't need a static route... that's where NAT will send everything (including the DHCP server's 192.168.23.xxx IP address that you've entered in the boxes below the DHCP Relay radio button) by default.

Did you set the the wireshark computer's network adapter to a 192.168.23.xxx address and plug its cat5e cable into the cisco's WAN port to see if the DHCP broadcasts are appearing there?  Because I just tried it on a WRVS4400N and the broadcast packets do appear there when the DHCP Relay's network matches the network of the WAN port.
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by:vivigatt
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Are your packets correctly routed from end to end ? I am talking about standard unicast packets.
Use pings and tarceroute (tracert), if need be.
Note that after having been processed by "ip helper", the DHCP broadcast packets are forwarded as unicast packets, directly to the DHCP server.
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by:Darr247
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> the DHCP broadcast packets are forwarded as unicast
> packets, directly to the DHCP server.

Maybe they are on Cisco's managed layer2 switches and 2 port (1 WAN, 1 LAN) routers using the 'ip helper' (I'd have to drive 10 miles and power up my test stand to check that), but they were not in the packets I saw coming out of the WAN port of the WRVS4400N router (which is essentially a RVS4000 router with a wireless section) using its DHCP Relay function. The relay agent and next server fields were empty, like soWireshark - DHCP framewith no destination (other than 255.255.255.255) specified.
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by:vivigatt
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I don't know about WRVS4400N or RVS4000, but if there is a "DHCP relay" or "ip helper" command or menu, the router is supposed to forward DHCP DISCOVER packets (sent to broadcast) as unicast packets, sent to the DHCP server address.
When this is so, you are perfectly right, the Relay Agent IP address must be set to the IP address of the router that did forward the packets.
(The Next Server IP address is actually the TFTP server that a diskless node would get its bootp/dhcp network boot program from, if a boot file name was set in "boot file name" field. I mention this just because you referred to next-server, but next server is not relevant to your issue as far as I can tell)
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by:bd14pb
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Hi All,

Thanks for all the responses. I have been away with family emergency. I will be chekcing the DHCP relay tonight. Thanks
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Author Closing Comment

by:bd14pb
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It looks like its working now. Thanks for help!
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