[gentoo-server] iptables NAT and service move from host to host

[Christian Parpart]

Hi,

we're about to move some services from an old host to a newer one,
however, we want to keep up the old host for a week after the migration, so, 
that everyone may choose their own good time to switch their IP's in their 
local configs.

that is, I want to all IP packets incoming at $old_ip to be redirected to 
$new_ip via iptables.

unfortunately, in my little test, that didn't work '(

Executing the following on the $old_ip's host.

$ old_ip=public.ip.1
$ new_ip=public.ip.2

$ iptables -t nat -A PREROUTING  -p tcp --dport 81 \
	-d $old_ip -i eth0 -j DNAT --to-destination $new_ip:80
$ iptables -t nat -A POSTROUTING -p tcp --sport 80 -\
	s $new_ip -o eth0 -j SNAT --to-source $old_ip:81

now, a $(telnet $old_ip:81) shall be equivalent to $(telnet $new_ip:80).
but it seems that no packet is reaching the $new_ip's host.

So has anybody a nice hint for me where I ran into what pitfall?

Thanks in advance,
Christian Parpart.