From owner-freebsd-net@FreeBSD.ORG  Thu Jul 12 20:41:46 2012
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Date: Thu, 12 Jul 2012 13:41:39 -0700
From: Yuri <yuri@rawbw.com>
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Subject: System doesn't detect unplugged network cable and doesn't set
 interface up properly with DHCP
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I have the simplest possible DHCP setup: ifconfig_re0="DHCP" in 
/etc/rc.conf.

When the system boots, it gets connected fine.

Now,  I disconnect my laptop and connect it to another network.
When cable is disconnected, IP address of this interface stays the same, 
old one is not removed.
When I plug it into another network, the same IP address stays. New IP 
doesn't get set. This is bad.
So I have to manually do 'ifconfig re0 down && remove <OLD-IP> && 
ifconfig re0 up'.

I believe, once interface is set as "DHCP", all those things should 
happen automatically. dhclient should drop the old IP when cable is 
unplugged, and should set it up anew when cable is plugged back.

Is my system misconfigured in some way, or this is the way how it works 
in FreeBSD?

Yuri