stateful UDP with unknown source port on INPUT?

["noa levy" <noalevy@hotmail.com>]

Hi,
I am working on a machine that needs to send SNMP informs, and receive ACKs 
for these informs. I am using iptables as my firewall, and I need to find a 
way to open a pinhole for the ACK to get back to me. My problem is that SNMP 
informs are sent to a predefined UDP port (162) but the ACKs are returned 
from a random port.
The following rules exist to allow established UDP sessions:
iptables -A INPUT-p udp -m state ?state ESTABLISHED -j ACCEPT
iptables -A OUTPUT -p udp -m state ?state NEW,ESTABLISHED -j ACCEPT

My problem is that, since the source port of the ACK is a random one, the 
ACKs are dropped. Is there a way to allow all incoming UDP packets with an 
unknown source port and an established dest port?
Any help will be greatly appreciated,
Thanks,
Noa

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