Linux setting network from static to dhcp client

if you want to setting eth0 to dhcp client , you can use this command:
sudo dhclient eth0

if you get the message: File exists, or something like that..

Basically what happens is that dhclient adds a route to the routing table.

It tries this while the route is already in the table.

Check

ip route

for a route which was added by the dhcp server.

For having the lease renewed do

sudo dhclient -r

if thats not enough you can remove all leases by removing the file and getting a new lease

sudo rm /var/lib/dhcp/dhclient.leases; sudo dhclient eth0

Depending on your exact setup this might be an issue with having to type your password twice, so watch out for that.