KVM port forward to guest (iptables) - only partial internet access? - iptables

Question

I am quite new to KVM and read a lot of threads here, but neither one was helpful in my case.
I am having a guest on a KVM host and want to forward port 80 from the host to port 80 in the guest. I did this with iptables:
Guest_name=theguest
Guest_ipaddr=192.168.122.43
Host_port=( '80' )
Guest_port=( '80' )
length=$(( ${#Host_port[#]} - 1 ))
if [ "${1}" = "${Guest_name}" ]; then
if [ "${2}" = "stopped" -o "${2}" = "reconnect" ]; then
for i in `seq 0 $length`; do
iptables -t nat -D PREROUTING -p tcp --dport ${Host_port[$i]} -j DNAT --to ${Guest_ipaddr}:${Guest_port[$i]}
iptables -D FORWARD -d ${Guest_ipaddr}/32 -p tcp -m state --state NEW -m tcp --dport ${Guest_port[$i]} -j ACCEPT
done
fi
if [ "${2}" = "start" -o "${2}" = "reconnect" ]; then
for i in `seq 0 $length`; do
iptables -t nat -A PREROUTING -p tcp --dport ${Host_port[$i]} -j DNAT --to ${Guest_ipaddr}:${Guest_port[$i]}
iptables -I FORWARD -d ${Guest_ipaddr}/32 -p tcp -m state --state NEW -m tcp --dport ${Guest_port[$i]} -j ACCEPT
done
fi
fi
Overall this works, but not always. http request to the apache are working, so the rules seem to be correct. If I do a wget request to a http address, nothing happens. If I do a wget request to a ftp address (with --user=theusername --password=thepassword), it works.
The biggest problem: apt-get update is not working. All it does is:
0% [Verbindung mit de.archive.ubuntu.com (141.30.13.20)] [Verbindung mit security.ubuntu.com (91.189.91.13)]
Only Ctrl-C helps at this stage.
The network in the KVM xml file is configured like this:
<interface type='network'>
<source network="default"/>
<mac address='themacaddress'/>
<model type='virtio'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>
</interface>
Can anybody help? Thanks a lot in advance!