hostname problem

Hi, I have a problem whit my hostname. When ik get my IP from my ISP via DHCP then will be chechged my hostname to cust. The DNS from my IP is cust.x.xxx.adsl.cistron.nl I see in my log that dhcp chancges my reslov.conf. Why ? I dont know how to fix this problem... I only know that mysql can`t start because i dont have a host name anymore. When i do hostname -f i see dont know host (or something) When i boot whit out a inernet contection there is no problem till i make a contection. Greets Wouter