Relay agents, NAT, and offers to giaddr

Bruce Hudson Bruce.Hudson at Dal.Ca
Fri Sep 15 16:35:18 UTC 2006

>    The router usually has a better idea about network topology than 
> other devices, which is a good reason for making it the relay agent.
>> No, it still will NOT work. The GIAddr will still be a non-routable 
>> rfc1918 address.
>    That's really an implementation choice.  NAT boxes already keep state 
> for UDP queries and responses, and re-write packet contents.  There's no 
> reason the NAT box can't do NAT for DHCP relay, too.  In that case, 
> giaddr could be its external IP.

    If the NAT box puts its external address in the giaddr field, the DHCP
server is going to assign an external address to the client and send this
back to the NAT box. The NAT box will then need to determine a unique
internal address to translate this to; and maintain this mapping in some
type of persistent storage so that the client's lease and the server's
lease information stay synchronized. You will also need to allow for a
unique internal IP per client; something NAT is usually used to avoid.

    It seems easier just to put a DHCP server on the internal network, or
build it into the NAT box.
Bruce A. Hudson				| Bruce.Hudson at Dal.CA
UCIS, Networks and Systems		|
Dalhousie University			|
Halifax, Nova Scotia, Canada		| (902) 494-3405

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