butchkn01
asked on
DHCP leases with no DHCP server
We recently temporarily powered down our primary domain controller which runs the DHCP service. This was part of a of a larger upgrade process. The next day, while this server was still down, all the client PCs were powered on and had valid IP address. Doing a IPConfig showed a valid lease obtained, the lease renewal date was correct future date and the DHCP server was the original DHCP server. I expect Active Directory or DNS is involved in the answer. All this with no DHCP. Clients are all Windows XP SP2 and SP3. The question is how and why did the clients retain their IP addresses?
Butch
Butch
Clients will retain their leases until the original lease period expires, the default is 8 days. presumably none of the clients leases expired and had to be renewed during the period that the server was down.
How long is the lease?
Once they hit 50 percent of their lease time they will try to renew their lease with their current DHCP server. At 85 percent they will try to contact any DHCP server.
I think it is 85%...it is around that number.
Thanks
Mike
Once they hit 50 percent of their lease time they will try to renew their lease with their current DHCP server. At 85 percent they will try to contact any DHCP server.
I think it is 85%...it is around that number.
Thanks
Mike
storkyIV is correct; once a machine get's it's IP lease, for however long it is set, the DHCP server isn't needed until the lease expires. Some of your leases could run out on your clients but they would still have access until the DHCP server was available. New machines that don't have IP addresses leased from your DHCP server would not be able to participate in your network unless an available static IP address was assigned.
this can possible because of cache credential of AD user in client system.
try to renew ip from client side.
start > run > cmd > ipconfig /release > ipconfig /renew.
it should not take ip while DHCP is down.
try to renew ip from client side.
start > run > cmd > ipconfig /release > ipconfig /renew.
it should not take ip while DHCP is down.
FYI @mkline71, its actually an nice round 87.5% - (who tought of that number ?)
If the machines can't renew the lease then they keep it anyway - until the lease has 100% expired - only then can the DHCP server re-issue the lease to another client.
If the machines can't renew the lease then they keep it anyway - until the lease has 100% expired - only then can the DHCP server re-issue the lease to another client.
87.5...excellent, that is an odd number, thanks kcts :)
I've just had a brainwave 87.5% - so assuming the default 8 day lease, thats at the end of day 7 , thats why this odd number must have been selected - 1 day to go...
ASKER
So, If a DHCP server is down and the client is powered off, the client PC will will use the same IP address when it is powered up until that lease has expired. If that is the case where is the IP address stored in the client or is it stored in the client.
It's kept in the DNS table.
Is DNS on a seperate server?
Is DNS on a seperate server?
ASKER
DNS is on a separate server.
DNS resolves addresses it knows about. The client's NIC gets the IP address which is actually a binary number that the NIC card holds in memory.
>>So, If a DHCP server is down and the client is powered off, the client PC will will use the same IP address when it is powered up until that lease has expired. If that is the case where is the IP address stored in the client or is it stored in the client. <<
Yes it is - its in HKLM\SYSTEM\CurrentControl
There may be several entries- at least one per interface.
Yes it is - its in HKLM\SYSTEM\CurrentControl
There may be several entries- at least one per interface.
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ASKER
What all this means to me is that if the DHCP server is down, the client is Windows XP, the lease has not expired and since Microsoft DHCP server shares information with DNS (which is on another server), the clients will continue to function on the network. The most important piece is the lease expiration time, keep it at a reasonable length to cover a DHCP outage. Would this conclusion be the same for non-Microsoft clients and servers?
DHCP is a universal protocol - it is functionally the same in all implimentations of DHCP
... if the lease is too long and you have a lot of movement of PCs (Laptops coming in and out of your network for example), all DHCP leases can get used as the DCHP server cannot re-issue an IP while the client still holds the lease, even though that client may have been removed from the network
DHCP leases of 72 hours are reasonable.
ASKER
Explanation helped locate the registry location
Basically once DHCP issues the IP address, the clients and any switches will keep memory of this until the lease expires.
Once the lease expires, the client would not be able to renew it's IP / lease until the DHCP server is powered back up.
I would check to see how long you have set the leases to be issued for.