DHCP directed broadcast passing NAT

Unanswered Question
Aug 14th, 2008

can I use an IP helper-address to direct DHCP broadcasts to a DHCP on the other side of a NAT? I'm thinking not.

I have this problem too.
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tdrais Thu, 08/14/2008 - 16:53

Never tried this one but you can in general nat traffic originated by the router itself. It just gets a little tricky because router generated messages do not com in a NAT IN interface.

Lets assume we go thought the trouble to get the message to nat. The reason I suspect it won't work is because the way the helper is using fields inside the dhcp request itself to indicate which interface it came from.

So lets assume you have a interface that send a message to a helper and places in the packet as the source interface. Now the source in the ip header of the packet gets natted to but the one in the packet is left untouched and gets sent to the helper. The helper gets the packet looks inside and uses the to generate a ip and places it back in the packets. This is good thing because if it used it would have issues. Now he will attempt to send the packet back to the gateway using which of course doesn't work because it needs to return to

It been a while since I read the helper RFC but I am pretty sure it uses the internal address and not the address on the packets. This is not to say that someone might have a option on a DHCP server to get around this issue.


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