When using DHCP on your external interface you will need to edit two files. First edit /etc/wfe/conf/netzkartendata there is an entry for the external interface. It will look someting like eth1#external#x.x.x.x#255.255.255.0 put your dhcp address in place of the x's. Second if you have defined a name for the external interface to use in your rules you will need to edit /etc/wfe/conf/netdata. An entry in this file looks like X#external#x.x.x.x#255.255.255.0#V again replace x's with your ip and external with the name you have given your external interface.
