When a
computer which has obtained an
IP address via
DHCP shuts down, the DHCP
standard requires that it transmit a "
DHCPRELEASE" message to the DHCP server. This informs the server that the address is no longer in use, and that it may safely be assigned to another
host.
Microsoft Windows does not do this. When a Windows computer shuts down, it tells the DHCP server nothing at all. As a result, the address that had been assigned to that computer does not become available for re-use until its lease has timed out. Until that time, the address cannot be assigned, and is effectively wasted.
There are two major unpleasant consequences of this bug:
- If you have a large network of Windows boxen, but only a certain fraction of them are on at any given time, the network will consume rather more addresses than it really needs. Given that address space is often at a premium -- which is part of why many organizations use DHCP in the first place -- this is suboptimal.
- If you frequently dual-boot between a Windows operating system and another, such as GNU/Linux, BeOS, or *BSD, your computer can effectively tie up two IP addresses most of the time. In addition, if your DHCP server has been set to assign a fixed IP address to your MAC address, it may be unable to do so -- when you boot into your non-Windows operating system, Windows has still held onto the address, thus banning the DHCP server from assigning it again -- even to the same machine!
A
workaround for this bug is to decrease the
lease time on your DHCP server to a period of a few
minutes. This guarantees that once a Windows system has shut down, it will not be very long before its address can be reclaimed. However, this has the disadvantage of radically increasing the amount of
bandwidth overhead caused by DHCP, and increasing the chances of
random lossage.
On Windows 98 and Windows Me, the bug can, in fact, be fixed. To fix it, you must create a Registry key and modify another:
Key: HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\VxD\DHCP
Value name: ReleaseLeaseOnShutdown
Type: DWORD
Value data: 0x00000001 (1)
Key: HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\Shutdown
Value name: FastReboot
Type: STRING
Value data: 0
The reasons for which Microsoft has released a DHCP client which is broken by default may fairly be made the subject of conspiracy theory.
(The author of this writeup is not a Windows user; he picked up this information from a local source of Windows tech support in the process of debugging some strange addressing behavior of dual-booting Win98/Red Hat systems.)