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
butchkn01Asked:
Who is Participating?

[Product update] Infrastructure Analysis Tool is now available with Business Accounts.Learn More

x
I wear a lot of hats...

"The solutions and answers provided on Experts Exchange have been extremely helpful to me over the last few years. I wear a lot of hats - Developer, Database Administrator, Help Desk, etc., so I know a lot of things but not a lot about one thing. Experts Exchange gives me answers from people who do know a lot about one thing, in a easy to use platform." -Todd S.

storkyIVCommented:
Hello,

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.
0
Brian PiercePhotographerCommented:
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.
0
Mike KlineCommented:
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

0
Determine the Perfect Price for Your IT Services

Do you wonder if your IT business is truly profitable or if you should raise your prices? Learn how to calculate your overhead burden with our free interactive tool and use it to determine the right price for your IT services. Download your free eBook now!

gemartiCommented:
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.

0
xtreminatorDIYerCommented:
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.
0
Brian PiercePhotographerCommented:
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.
0
Mike KlineCommented:
87.5...excellent,  that is an odd number,  thanks kcts :)
0
Brian PiercePhotographerCommented:
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...
0
butchkn01Author Commented:
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.
0
storkyIVCommented:
It's kept in the DNS table.
Is DNS on a seperate server?
0
butchkn01Author Commented:
DNS is on a separate server.
0
gemartiCommented:
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.
0
Brian PiercePhotographerCommented:
>>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\CurrentControlSet\Services\TCPIP\Parameters\Interfaces\
There may be several entries- at least one per interface.
0
Brian PiercePhotographerCommented:
I forgot the pic
IP.JPG
0

Experts Exchange Solution brought to you by

Your issues matter to us.

Facing a tech roadblock? Get the help and guidance you need from experienced professionals who care. Ask your question anytime, anywhere, with no hassle.

Start your 7-day free trial
butchkn01Author Commented:
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?
0
Brian PiercePhotographerCommented:
DHCP is a universal protocol - it is functionally the same in all implimentations of DHCP
0
Brian PiercePhotographerCommented:
... 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
0
gemartiCommented:
DHCP leases of 72 hours are reasonable.
0
butchkn01Author Commented:
Explanation helped locate the registry location
0
It's more than this solution.Get answers and train to solve all your tech problems - anytime, anywhere.Try it for free Edge Out The Competitionfor your dream job with proven skills and certifications.Get started today Stand Outas the employee with proven skills.Start learning today for free Move Your Career Forwardwith certification training in the latest technologies.Start your trial today
Active Directory

From novice to tech pro — start learning today.