Simplest method? Manually kill dhclient. Then manually start dhclient on that interface. Voila! Done.
The very first time you connect via DHCP, the handshake looks like so:
client: DHCPDISCOVER broadcasting for a DHCP server
server: DHCPOFFER reply with an IP
client: DHCPREQUEST reply with the offered IP
server: DHCPACK marking that IP as used by the client
When you stop/start dhclient, the following occurs:
client: DHCPDISCOVER
server: DHCPOFFER with an IP
client: DHCPREQUEST with the last IP it had
server: DHCPACK, marking that IP as in use by the client
If that IP is no longer available (ie, assigned to another client), then the server sends a DHCPNACK (Not ACKnowledged). Then the offer/request happens again with another IP.
There's really no such thing as "DHCP renew" option. It's just the client requesting the last IP that it had, and the server either accepting it or sending another IP.
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