rump.dhcpclient —
simple dhcp client for rump kernels
The rump.dhcpclient utility is a very simple DHCP client
  which can be used to apply networking configuration on one interface in a rump
  kernel. Unlike full DHCP clients, rump.dhcpclient does
  not store leases or renew expired leases. The reason for this is the typical
  transient nature of a rump kernel. Additionally,
  rump.dhcpclient does not save DNS resolver
  information.
After having successfully configured networking,
    rump.dhcpclient prints out the networking
    configuration and lease time and exits.
Since rump.dhcpclient uses
    bpf(4) to send and receive raw
    network packets, the server must include support for bpf and vfs (for
    opening /dev/bpf). Otherwise, the following
    diagnostic message is printed:
rump.dhcpclient: bpf: Function not implemented
 
There is no easy way to release a lease.