In this video from ITFreeTraining, I will look at what is a DHCP lease. When a DHCP
server allocates an IP Address to a device, the lease determines how long that device
can use that IP Address. This video looks at what is a lease and the important time
periods associated with that lease.
To start with, what is a DHCP lease? A DHCP lease is similar to a lease that you may take
out on a house. That is the lease is for a certain time. The lease can also be extended
before it ends. The lease may be ended early or renewed.
Just like when you take out a lease on a house, you don’t own the house, but you get the
right to use the house while the lease is in place. If the lease expires, you need to
find a new place to live. Let’s have a closer look at how a DHCP lease works.
Consider that you have a device on the network that needs an IP Address to communicate. In
this case a desktop computer. The device will communicate on the network and locate a DHCP
server. The DHCP server will allocate the device an IP Address and a lease time. Essentially
the device is free to use that IP Address for the lease time.
The lease time is configured on the DHCP server. On Windows, the default lease time is eight
days. On Linux the default lease time differs depending on the distribution.
The device can also request a certain lease time, but ultimately it is up to the server
to decide what lease time it wants to allocate to the device. Windows will always allocate
the lease time that is configured on the server. On Linux, the administrator can configure
a default value that will be used if the device does not request a certain time. If the device
does request a certain lease time, Linux supports a maximum value that it will allocate up to.
It is also possible to configure the lease time to unlimited; however, this is not recommended.
As we will see in later videos, renewing a lease also allows network configuration to
be changed. Setting the lease time to unlimited makes it harder for the administrator to make
changes to network configuration at a later date.
Let’s consider what happens when the client has been given a lease. The lease in this
example is set for eight days which is the default for Windows. For the first half of
the lease, which is four days in this example, the client will not attempt to renew the lease.
This is referred to as Timer 1 or T1. The administrator is free during this time period
to perform a manual renewal if they wish. However, the device will not attempt to renew
the lease during this period.
The next timer is Timer 2 or T2. This is 87.5% of the lease time or seven eighths. In this
example, this will be the time period of five to seven days. During this time, the device
will attempt to renew its lease. It does this by attempting to contact the original DHCP
server that it obtained the lease from.
Once the lease time becomes greater than T2, the device goes into what is referred to as
rebinding. Rebinding means that the device will attempt to obtain an IP Address from
any DHCP server. If the device is able to contact the original DHCP server, it will
be able to renew its IP Address. If it contacts a different DHCP server it will obtain a different
IP Address.
Now let’s consider an example. On this network, there are two DHCP servers and one desktop
computer. The desktop computer requires an IP Address so it requests one from a DHCP
server. In later videos I will go into this process in more detail.
The desktop computer now has a lease which allows it to use an IP Address on the network.
In this example, I will once again use an eight-day lease, so for the first four days
it uses the lease without trying to renew it.
On the fifth day, it will attempt to renew it. The desktop will keep doing this up to
the seventh day. If the desktop is able to contact the DHCP Server within these three
days, the lease will be renewed. When this occurs, the lease time goes back to zero.
The process now repeats. For the first half of the lease time, the desktop does not attempt
to renew its lease. For this example, let’s consider that the first DHCP server has failed.
The desktop will not be able to renew its lease. However, it will attempt to keep contacting
the DHCP server and will not attempt to contact any other DHCP servers.
On the last day, the desktop will attempt to contact any DHCP server on the network
to renew or obtain a new IP Address. In this case, the second DHCP server is contacted.
The second DHCP server cannot renew the IP Address; however, it can give the desktop
a new IP Address. The desktop is now able to communicate on the network using the new
IP Address. Since a new IP Address has been allocated to the desktop, the lease time starts
again from zero.
You can see the process keeps repeating itself. If the DHCP server is always available, the
device will generally always use the same IP Address. However, in some cases the administrator
may want to change the IP Addresses that are given to devices. When this occurs, it is
possible during the renewal phase that the device is given another IP Address. Having
the IP Address change can cause problems with open connections to the computer; let’s
have a look at how IPv6 handles that issue.
DHCP for IPv6 adds an additional time value. This is the valid lifetime value. In Windows,
the default value is 12 days. Let’s have a look at how this works.
For the first 50% of the lease time, the device won’t make an attempt to renew the lease.
Up to Timer 2, the device will attempt to renew the IP Address with the DHCP server.
In this example, let’s say the DHCP server is not available.
After Timer 2, the device will attempt to contact any DHCP server. In this example,
let’s say that the device was successful and was given a new IP Address. In IPv4, the
IP Address will change which can be disruptive to some applications. IPv6 prevents this from
happening by having the IP Address changed to a deprecated state. When in this state,
the IP Address can still be used, but its use is discouraged. New connections should
use the new IP Address.
Using this method, if an application sensitive to IP Address changes was currently transferring
data using that IP Address, the application could keep using the old IP Address until
it completed transferring its data. Thus preventing disruption to the application.
Before I end this video, I will perform a quick summary of the main points covered.
The lease time is a period during which the device can use an IP Address for. On Windows
the default is eight days. Non-Windows systems may also add the option of a device being
able to request a different lease time than the default.
Timer 1 or T1 is 50% of the lease time. Timer 2 or T2 is 87.5% of the lease time. A device
will attempt to renew its IP Address after T1. The device will only do this with the
DHCP server that it obtained the original lease from. It is also possible for the DHCP
server not to renew the lease. When this occurs, the device will obtain a new IP Address rather
than using the old one.
Generally, DHCP servers will always allow the device to continue using the same IP Address
unless the administrator makes changes to the DHCP server. For example, the administrator
removes the IP Address from the pool of allocable IP Addresses.
If the device is not able to renew its IP Address after Timer 2 has expired, the device
will attempt to rebind. In this state, the device will contact any DHCP server to obtain
a lease. This essentially means, that the IP Address of the device may change. DHCP
servers tend to be pretty reliable, so it should be rare that a device will need to
change its IP Address.
That’s it for DHCP leases. I hope you have found this video informative. For the other
free videos in our DHCP course and others, please see our web site or YouTube channel.
Until next time, I would like to thank you for watching.
References
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How DHCP Lease Renewal Works
When the DHCP lease reaches 50 percent of the
lease time, the client attempts to renew the lease.
This automatic process occurs in the background.
Computers might have the same IP address that
was assigned by the DHCP server for a long time if
they operate continually on a network without
being shut down.
To renew the IP address lease, the client
broadcasts a DHCPREQUEST message. The
server that leased the IP address originally sends
a DHCPACK message back to the client; this
message contains any new parameters that have
changed since the original lease was created.
If the DHCP client cannot contact the DHCP server, then the client waits until 87.5 percent
of the lease
time expires. If the renewal is unsuccessful, which means 100 percent of the lease time
has expired, then
the client computer attempts to contact the configured default gateway. If the gateway
does not respond,
the client assumes that it is on a new subnet and enters the Discovery phase, where it attempts
to obtain
an IP configuration from any DHCP server.
Client computers also attempt renewal during the startup process. This is because client
computers might
have been moved while they were offline; for example, a laptop computer might be plugged
into a new
subnet. If renewal is successful, the lease period is reset. The DHCP role on Windows
Server 2012 supports
a new feature, DHCP Server Failover protocol. This protocol enables synchronization of lease
information
between DHCP servers and increases DHCP service availability. If one DHCP server is not available,
the
other DHCP servers continue to service clients in the same subnet.