Hello Alan You're true and your explanations are very usefull. The IAD has no problem because it did what I programmed. But there are not solution to this source-nat problem. The problem will always appear on the DHCP renewal and I can't do any thing on this IAD. I will used the DHCP request sended at 87,5% of the lease time for workaround this. Thanks you for your explanations and your help. Thomas

...

When the renewal DHCP packets are send using unicast, my client try to reach FreeRadius. he send DHCP request using his IP address with source port 68 and the freeradius IP Address with destinatio port 67. My IAD source pat the flow using a dynamic random port.

That is not how DHCP works. DHCP clients use a source port of 68. DHCP relays use a source port of 67. Your IAD (whatever that is) is broken. Replace it with something that does DHCP properly.

...

I don't understand why Freeradius will not reply to the initial source port ?

Because DHCP is insane. The replies do *not* cause the src/dst ports to be swapped, as with every other UDP protocol.

...

...

3) This packet trace is not evidently the first requests, but a renewal? You're true, i see the problem only on the renewal (When 50% of the lease time have been reached).

Then look at the packet traces to see how the first request is different from the renewal.

Alan DeKok.