TCP/IP and Data Communications Administration Guide

Lease Time Policy

DHCP provides a mechanism for dynamically allocating IP addresses. These IP addresses carry a lease period, which you can set to permanent or temporary. Your site policy must include decisions about the number of temporary and permanent IP addresses, and the lease period of temporary IP addresses.

To reap the most benefit from your DHCP service, it's best to allow DHCP to dynamically manage your IP address assignment for you. With DHCP, the client and the server negotiate a loan of an IP address configuration for a certain time period, known as a lease time. You can set a lease time policy based on server, network, client vendor class, or individual client IP address through the use of the LeaseTim and LeaseNeg symbols in certain macro definitions in your dhcptab.

The LeaseNeg and LeaseTim symbols allow you to set policy for your site. LeaseTim is a relative time, such as 24 hours, or 2 hours, or 10 hours. When a client is assigned an IP address (or renegotiates a lease on an IP address it is already assigned), the LeaseTim value is added to the absolute time the client received its DHCP reply. Absolute time is the time now, such as September 27, 1996. The LeaseTim value plus the absolute current time is stored in the client's dhcp_network record as an absolute future time when the client's lease on its IP address expires.

The LeaseTim symbol setting defines the maximum lease time interval you allow to clients. Typically, this value should remain relatively small, so that: clients and servers stay in synch; IP addresses that are not being used are reclaimed in the most timely fashion; and the renumbering of networks happens more smoothly.

However, the value must be large enough so that if your DHCP server becomes unavailable, your clients continue to function until the machine(s) running the DHCP service can be repaired. One to 3 days is an optimal LeaseTim policy. Select values that work well in your environment.

The LeaseNeg symbol determines whether or not a client can renegotiate its lease with the server before the lease expires. If this symbol is present, then the client can renegotiate its lease. LeaseNeg allows clients to operate on the network without lease-related interrupts of existing connections.

Omitting LeaseNeg is useful for environments where you have more machines than you have IP addresses, and thus want to enforce a time limit on the use of any IP address. Enforcing a time limit on machines in a Computer Science class lab is an example of this. Like LeaseTim, LeaseNeg can be used in a variety of different macros in your dhcptab. See dhcptab(4) and dhcp_network(4) for more information.

Machines that export services such as mail or web pages must retain their IP address. However, you'd still like to be able to detect when the IP address used by this node is no longer being used (perhaps when the machine is retired). You can achieve this by marking this node's dhcp_network record as being manually assigned (by you) and by setting the node's flag field to MANUAL. For example, if the hostname is gandalf and the network is, type pntadm - gandalf -f MANUAL

You can allocate IP addresses with permanent leases. However, you will not be able to use the DHCP service to automatically reclaim these IP addresses for you. Therefore, keep the number of permanent IP addresses to a minimum, manageable number.

The DHCP service uses this field to determine when dhcp_network record entries have expired and can be reclaimed. You can alter this value through the e option of the pntadm command. Through this command, you can set a client's lease expiration time to the past, although we recommend that you only set it into the future to avoid adversely affecting the client and the client's user(s).

Each time the DHCP service allocates a dynamic IP address or renegotiates a lease on an existing binding, this field in the dhcp_network table is updated.

The lease flag indicates the conditions under which the IP address can be assigned. The flag setting can be a combination of the following:

0 (Dynamic)

The IP address lease has an expiration time. When the lease expires it can be renewed, if that is indicated by the site policy. If the current client does not renew the lease, then the IP address can be assigned to another client. When the flag is set to 0, the client can request that the lease time be changed.

1 (Permanent)

The IP address lease is assigned permanently, and the client cannot change the lease time. However, the client using the IP address can release it. When it is released, it can be assigned to another client.

2 (Manual)

The IP address is assigned to a specific client machine. It cannot be released by the client. As long as the flag is set to 2, the IP address can be reassigned only if an administrator changes it manually.

4 (Unusable)

The IP address is unusable. You can set the flag to 4 to prevent an IP address from being assigned. The DHCP server marks an IP address as unusable if it attempts to locate the IP address and finds that it is already in use. Before it assigns an IP address, the DHCP server normally pings it to see if it is already in use for some reason. This setting is configurable in dhcpconfig.

The flag can also have a combination of settings. For example, if the flag is set to 3, it is a combination of 1 and 2; that is, it is both permanent and manual. The IP address has a permanent lease, and was assigned by the administrator.