32. How long should a lease be?
I’ve asked sites about this and have heard answers ranging from 15 minutes to a year. Most administrators will say it depends upon your goals, your site’s usage patterns, and service arrangements for your DHCP server.
A very relevant factor is that the client starts trying to renew the lease when it is halfway through: thus, for example, with a 4 day lease, the client which has lost access to its DHCP server has 2 days from when it first tries to renew the lease until the lease expires and the client must stop using the network. During a 2-day outage, new users cannot get new leases, but no lease will expire for any computer turned on at the time that the outage commences.
Another factor is that the longer the lease the longer time it takes for client configuration changes controlled by DHCP to propogate.
Some relevant questions in deciding on a lease time:
Do you have more users than addresses?
If so, you want to keep the lease time short so people don’t end up sitting on leases. Naturally, there are degrees. In this situation, I’ve heard examples cited of 15 minutes, 2 hours, and 2 days. Naturally, if you know you will have 20 users using 10 addresses in within a day, a 2 day lease is not practical.
Are you supporting mobile users?
If so, you may be in the situation of having more users than addresses on some particular IP number range. See above.
Do you have a typical or minimum amount of time that you are trying to support?
If your typical user is on for an hour at minimum, that suggest a hour lease at minimum.
How many clients do you have and how fast are the communications lines over which the DHCP packets will be run?
The shorter the lease, the higher the server and network load. In general, a lease of at least 2 hours is long enough that the load of even thousands of clients is negligible. For shorter leases, there may be a point beyond which you will want to watch the load. Note that if you have a communication line down for a long enough time for the leases to expire, you might see an unusually high load it returns. If the lease-time is at least double the communication line outage, this is avoided.
How long would it take to bring back up the DHCP server, and to what extent can your users live without it?
If the lease time is at least double the server outage, then running clients who already have leases will not lose them. If you have a good idea of your longest likely server outage, you can avoid such problems. For example, if your server-coverage is likely to recover the server within three hours at any time that clients are using their addresses, then a six hour lease will handle such an outage. If you might have a server go down on Friday right after work and may need all Monday’s work-day to fix it, then your maximum outage time is 3 days and a 6-day lease will handle it.
Do you have users who want to tell other users about their IP number?
If your users are setting up their own web servers and telling people how to get to them either by telling people the IP number or through a permanent DNS entry, then they are looking for an IP number that won’t be changing. While some sites would manually allocate any address that people expected to remain stable, other sites want to use DHCP’s ability to automate distribution of relatively permanent addresses. The relevant time is the maximum amount of time that you wish to allow the user to keep their machine turned off yet keep their address. For example, in a university, if students might have their computers turned off for as long as three weeks between semesters, and you wish them to keep their IP address, then a lease of six weeks or longer would suffice.
Some examples of lease-times that sites have used & their rationals:
15 minutes
To keep the maximum number of addresses free for distribution in cases where there will be more users than addresses.
6 hours
Long enough to allow the DHCP server to be fixed, e.g. 3 hours.
12 hours
If you need to take back an address, then you know that it will only take one night for the users’ lease to expire.
3 days
This is apparently Microsoft’s default, thus many sites use it.
6 days
Long enough that a weekend server outage that gets fixed on Monday will not result in leases terminating.
4 months
Long enough that students can keep their IP address over the summer hiatus. I believe this rational is workable if the summer hiatus is no more than 2 months.
One year
If a user has not used their address in six months, then they are likely to be gone. Allows administrator to recover those addresses after someone has moved on.