System Administration Guide, Volume 3

Making Decisions for IP Address Management

This section discusses the decisions you need to make when configuring IP addresses to be managed by DHCP. The topics parallel the dialogs of DHCP Manager's Address Wizard, but can also be used to make decisions if you use the dhcpconfig utility.

As part of the DHCP service setup, you determine several aspects of the IP addresses that the server is to manage. If your network needs more than one DHCP server, you must decide how to divide responsibility for the addresses so you can assign some to each server. Before you begin configuring your server you must decide on the following:

Number and Ranges of IP Addresses

During the initial server configuration, DHCP Manager allows you to add one block, or range, of IP addresses under DHCP management by specifying the total number of addresses and the first address in the block. DHCP Manager creates a list of contiguous addresses from this information. If you have several blocks of noncontiguous addresses, you can add the others by running DHCP Manager's Address Wizard again, after the initial configuration.

Before configuring your IP addresses, know how many addresses are in the initial block of addresses you want to add and the IP address of the first address in the range.

Client Host Name Generation

The dynamic nature of DHCP means that an IP address is not permanently associated with the host name of the system that is using it. The Solaris DHCP server can generate a client name to associate with each IP address, if you select this option. The generated client names are mapped to IP addresses in /etc/hosts or the NIS/NIS+ hosts tables. The client names use a prefix, or root name, plus a dash and a number assigned by the server. For example, if the root name is charlie, the client names will be charlie-1, charlie-2, charlie-3, and so on.

By default, generated client names begin with the name of the DHCP server that manages them. This is useful in environments having more than one DHCP server because you can quickly see in the DHCP network tables which clients any given DHCP server manages. However, you can change the root name to any name you choose.

Before configuring your IP addresses, decide if you want the server to generate client names, and if so, what root name to use for the names.

Note -

The client names are not automatically added to the DNS domain, thus the client names are not known outside your name service (NIS/NIS+) domain. However, you can load them into DNS manually. See "Administering DNS" in Solaris Naming Administration Guide and the in.named(1M) manual page for more information about DNS.

Default Client Configuration Macros

In Solaris DHCP, a macro is a collection of network configuration options and their assigned values. Macros are used to determine what network configuration information to send to a DHCP client.

During the initial configuration of the DHCP server, several macros are created, using information gathered from system files and from prompting the system administrator:

The network address macro is automatically processed for all clients located on that network. The locale macro is included in the server macro, so it is processed when the server macro is processed.

While configuring IP addresses for the first network, you must select a client configuration macro to be used for all DHCP clients using the addresses you are configuring. By default, the server macro is selected because it is contains information needed by all clients using this server. Clients receive the options contained in the network address macro before those in the server macro. See "Order of Macro Processing" for more information about macro processing order.

Dynamic and Permanent Lease Type

The lease type determines whether the lease policy (lease time and negotiation) is used for the addresses you are configuring. During initial server configuration, DHCP Manager allows you to select either dynamic or permanent leases for the addresses you are adding. The dhcpconfig utility allows only dynamic leases.

When an address has a dynamic lease, the DHCP server can manage the address by allocating it to a client, extending the lease time, detecting when it is no longer in use, and reclaiming it. When an address has a permanent lease, the DHCP server can only allocate it to a client, after which the client owns the address until the client explicitly releases it. When the address is released, the server can assign it to another client. The address is not subject to the lease policy as long as it is configured with a permanent lease type.

When you configure a range of IP addresses, the lease type you select applies to all the addresses in the range. To get the most benefit from DHCP, you should use dynamic leases for most of the addresses. You can later modify individual addresses to make them permanent if necessary, but the total number of permanent leases should be kept to a minimum.

Reserved Addresses and Lease Type

Addresses can be reserved by manually assigning them to particular clients. A reserved address can have a permanent or dynamic lease associated with it. When a reserved address is assigned a permanent lease, the address can only be allocated to the client that is bound to the address, the DHCP server cannot allocate the address to another client, and the address cannot be reclaimed by the DHCP server.

If a reserved address is assigned a dynamic lease, the address can only be allocated to the client that is bound to the address, but the client must track lease time and negotiate for a lease extension as if the address were not reserved. This allows you to track when the client is using the address by looking at the network table.

You cannot create reserved addresses for all the IP addresses during the initial configuration because they are intended to be used sparingly for individual addresses.