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port forwarding

I have a small LAN and want to allow access to an internal(no real ip, sorry 
;-)) web server to Internet clients

The gateway is using iptables v1.2.11(debian sarge)

I have activated port forwarding, so that Internet traffic targeted at my 
gateway's port 80 is forwarded to the internal web server and works just fine.

(To make things a bit clear: 
the Internet accesses the gateway through $INET_IP, 
the LAN accesses the gateway through $LAN_IP, 
the IP of the internal web server is $HTTPD_IP)

What I also want, however, is to allow other LAN hosts have access to the same 
web server using the FQDN of the gateway(say http://mysite.dyndns.org). To do 
that correctly I must also SNAT LAN packets destined to $INET_IP of the 

iptables -t nat -A POSTROUTING -p TCP -i $LAN_IFACE -d $HTTPD_IP --dport 80 -j 
SNAT --to-source $LAN_IP

Which gives me an error: 
	iptables v1.2.11: Can't use -i with POSTROUTING

Finally, I want to be able to contact the web server from the gateway through 
the FQDN as well. So I need to DNAT locally generated packets so that they 
are sent to the proper host:

iptables -t nat -A OUTPUT -p TCP -d $INET_IP --dport 80 -j DNAT 
--to-destination $HTTPD_IP:80

Which also gives me an error:
	iptables v1.2.7a: Invalid argument

Firstly, I would really apreciate it if someone could tell me what is wrong 
with rules 2 and 3? 

Secondly, is there a way to achieve this functionality?

p.s. These rules are taken from Oscar Andreasson's extensive discussion of 
DNAT target in his "Iptables Tutorial 1.2.0", section 11.3 

Thanks a lot in advance, 

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