Mac OSX server command equivalent for dhclient?

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http://serverfault.com – Is there an MacOS command that makes a dhcp request, and renews the old lease, drops it for a new one, or usefully reports errors or lack of response from a dhcp server? This would both help fix networking on the machine after problems on the network without rebooting and would also be useful to diagnose wider networking problems from a mac. I can not find any command equivalent of dhclient though obviously some component must be serving this purpose. The question is, is that component exposed to a command line interface? I am biased to the command line for these features and may have ov (HowTos)