iptables confusion.

[Ken D'Ambrosio]

Every time I think I'm getting to the point where I might understand IP 
Tables, I do something that proves that, no, I really don't.  Today's 
confusion:  I want to set up a virtual NIC to do port forwarding.  But 
first, I wanted to get the port forward part of the equation straight.  
So I wound up executing these commands:

iptables -t nat -A PREROUTING -p tcp --dport 8774 -j DNAT --to 
172.23.242.39:8774
iptables -A FORWARD -d 172.23.242.39 -p tcp --dport 8774 -j ACCEPT
iptables -t nat -A POSTROUTING -j MASQUERADE

Worked great.  I then did an "ifconfig eth0:1 172.23.9.139 netmask 
255.255.255.0" to see if I could telnet to port 8774 on it.  I could.  
So then I did "iptables --flush", and it did.  When I type "iptables 
--list", I now get:

Chain INPUT (policy ACCEPT)
target     prot opt source               destination

Chain FORWARD (policy ACCEPT)
target     prot opt source               destination

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination

Terrific.  Pretty much what I expected.  Telnetting to port 8774 on eth0 
fails, as expected... but telnetting to port 8774 on the virtual works 
great.  I even fired up Firefox to make sure, and youbetchya, it's 
interacting with the remote server.

Why?

-Ken