redirect doesn't seem to work?

[Ben Russo <ben@umialumni.com>]

Have a UNIX server.  User wants to write programs to receive SNMPTraps.
I *thought* I could redirect incomming traffic to port 162 to a high
port, (like 4162) where the user could have his programs receive the
messages with no special privelages?

Anyway, a completely normal box, no special firewalling rules....
The only rule I create is:

iptables -t nat -A POSTROUTING -p udp -m udp --dport 162 -j REDIRECT
--to-ports 4162

Now, the user starts his program that binds to udp port 4162 ( I can see
it with "netstat -nap" ) but he doesn't see any packets coming in.
I run tcpdump -n | grep snmptrap and I see packets coming from the
network to the server on udp port 162, and the server sending back icmp
unreachable packets back to the sources??

Why doesn't the redirect work?

-Ben.