Hi,

After I used TAP device by -net nic -net tap,ifname=mytap and I tried to use
user mode network by -net nic -net user, a Windows XP guest doesn't get
IP address from a built-in DHCP server.

It is fixed by an attached patch.

DHCPRELEASE and DHCPNACK are introduced.

DHCPRELEASE code is borrowed from VirtualBox. Windows 2000/XP tries to call
DHCPREQUEST and get old IP address when it boots. I made a code to reply
DHCPNACK to the request. Then the Win2k/XP called DHCPDISCOVER and try to
get a new IP address.

I tested Windows 98SE/2000/XP, Knoppix 3.8, Morphix, Fedora Core 3 and
RedHat 7.2 guest.
There is not problem except RH7.2. It can get IP address but it is
10.0.2.16. dhcpcd in RH7.2 tries to call DHCPDISCOVER two times. So it
consumes two entries in the built-in DHCP server. It seems that it is a bug
of dhcpcd in RH7.2.

Regards,
Kazu