Hi Oliverleure,
Take a look at this link http://manpages.ubuntu.com/manpages/intrepid/man5/udhcpd.conf.5.html it will show you the proper way and multiple options of configuring udhcpd. Also, take a look at this examples http://git.busybox.net/busybox/tree/examples/udhcp/udhcpd.conf . The error you are getting “ address already in use” might be because while setting the dns and dhcp addresses, you might have them configured to overlap each other. Make sure the dns addresses range is separate from the dhcp addresses.
Regards
Sergio