Problem to redirect udp packet with ipfw fwd

[sebtest007]

Hello,
I would like implement an ipfw fwd udp packet in my firewall. To fwd forward all DNS requests to my internal DNS server.

I try that with this line configuration:

### Forwarding dns connection from unauth client  
ipfw add 60001 fwd 127.0.0.1,53 udp from 10.101.0.0/20 to any 53

But It doesn't work. When I try to resolve nslookup [url=http://www.google.com]http://www.google.com[/url] on my client , I receive this message on my server:

Nov 25 12:40:20 GW-HOTEL1 named[1363]: client 10.101.0.100#61313 (http://www.google.com): error sending response: address not available
Nov 25 12:40:22 GW-HOTEL1 named[1363]: client 10.101.0.100#61314 (http://www.google.com): error sending response: address not available
Nov 25 12:40:25 GW-HOTEL1 named[1363]: client 10.101.0.100#61315 (http://www.google.com): error sending response: address not available
Nov 25 12:40:27 GW-HOTEL1 named[1363]: client 10.101.0.100#61316 (http://www.google.com): error sending response: address not available

However when I use ipfw fwd with tcp it works fine.

### Forwarding http connection from unauth client  
ipfw add 60002 fwd 127.0.0.1,80 tcp from 10.101.0.0/20 to any 80

Any ideas?

Thanks in advance for your help.

Regards
Sebastien