Mac OSX server command equivalent for dhclient?
To renew the lease:
sudo ipconfig set <interface> DHCP
Usually interface is en0 or en1.
To see all the gory details, run sudo ipconfig setverbose 1 before you run the above, then tail /var/log/system.log. After you're done, remember to run sudo ipconfig setverbose 0 to turn it off again.