Preparing Your Network for the DHCP Service (Task Map)

Before you set up your network to use DHCP, you must collect information to help you make decisions for configuring one or more servers. Use the following task map to identify the tasks for preparing your network for DHCP.

Task	Description	For Instructions
Map your network topology.	Determine and locate the services that are available on the network.	Mapping Your Network Topology
Determine the number of DHCP servers you need.	Use the expected number of DHCP clients as a basis for determining the number of DHCP servers you need.	Determining the Number of DHCP Servers
Update system files and netmasks table.	Reflect the network topology accurately.	Updating System Files and Netmask Tables

Mapping Your Network Topology

If you have not already done so, you should map the physical structure of your network. Indicate the location of routers and clients, and the location of servers that provide network services. This map of your network topology can help you determine which server to use for the DHCP service. The map can also help you determine the configuration information that the DHCP server can provide to clients.

See Chapter 2, Planning an IPv4 Addressing Scheme (Tasks for more information about planning your network.

The DHCP configuration process can gather some network information from the server's system and network files. Updating System Files and Netmask Tables discusses these files. However, you might want to give clients other service information, which you must enter into the server's macros. As you examine your network topology, record the IP addresses of any servers you want your clients to know about. The following servers, for example, might provide services on your network. The DHCP configuration does not discover these servers.

• Time server

• Log server

• Print server

• Install server

• Boot server

• Web proxy server

• Swap server

• X Window font server

• Trivial File Transfer Protocol (TFTP) server

Network Topology to Avoid

In some IP network environments, several local area networks (LANs) share the same network hardware media. The networks may use multiple network hardware interfaces or multiple logical interfaces. DHCP does not work well in this kind of shared media network. When multiple LANs run across the same physical network, a DHCP client's request arrives on all network hardware interfaces. This effect makes the client appear to be attached to all of the IP networks simultaneously.

DHCP must be able to determine the address of a client's network in order to assign an appropriate IP address to the client. If more than one network is present on the hardware media, the server cannot determine the client's network. The server cannot assign an IP address without knowing the network number.

You can use DHCP on only one of the networks. If one network does not suit your DHCP needs, you must reconfigure the networks. You should consider the following suggestions:

• Use a variable length subnet mask (VLSM) on your subnets to make better use of the IP address space you have. You may not need to run multiple networks on the same physical network. See the netmasks(4) man page for information about implementing variable length subnetting. For more detailed information about Classless Inter-Domain Routing (CIDR) and VLSM, see http://www.ietf.org/rfc/rfc1519.txt.

• Configure the ports on your switches to assign devices to different physical LANs. This technique preserves the mapping of one LAN to one IP network, required for Solaris DHCP. See the documentation for the switch for information about port configuration.

Determining the Number of DHCP Servers

The data store option that you choose has a direct effect on the number of servers you must have to support your DHCP clients. The following table shows the maximum number of DHCP and BOOTP clients that can be supported by one DHCP server for each data store.

Table 13-1 Estimated Maximum Number of Clients Supported by One DHCP Server

Data Store Type	Maximum Number of Clients Supported
Text files	10,000
NIS+	40,000
Binary files	100,000

This maximum number is a general guideline, not an absolute number. A DHCP server's client capacity depends greatly on the number of transactions per second that the server must process. Lease times and usage patterns have a significant impact on the transaction rate. For example, suppose leases are set to 12 hours and users turn their systems off at night. If many users turn on their systems at the same time in the morning, the server must handle transaction peaks as many clients request leases simultaneously. The DHCP server can support fewer clients in such an environment. The DHCP server can support more clients in an environment with longer leases, or an environment that consists of constantly connected devices such as cable modems.

The section Choosing the DHCP Data Store compares the types of data stores.

Updating System Files and Netmask Tables

During DHCP configuration, the DHCP tools scan various system files on your server for information that can be used to configure the server.

You must be sure the information in the system files is current before you run DHCP Manager or dhcpconfig to configure your server. If you notice errors after you configure the server, use DHCP Manager or dhtadm to modify the macros on the server.

The following table lists some of the information gathered during DHCP server configuration, and the sources for the information. Be sure this information is set correctly on the server before you configure DHCP on the server. If you make changes to the system files after you configure the server, you should reconfigure the service to reflect these changes.

Table 13-2 Information Used for DHCP Configuration

Information	Source	Comments
Time zone	System date, time zone settings	The date and time zone are initially set during Solaris installation. You can change the date by using the date command. You can change the time zone by editing the /etc/default/init file to set the TZ environment variable. See the TIMEZONE(4) man page for more information.
DNS parameters	/etc/resolv.conf	The DHCP server uses the /etc/resolv.conf file to obtain DNS parameters such as the DNS domain name and DNS server addresses. See System Administration Guide: Naming and Directory Services (DNS, NIS, and LDAP) or the resolv.conf(4) man page for more information about resolv.conf.
NIS or NIS+ parameters	System domain name, nsswitch.conf, NIS or NIS+	The DHCP server uses the domainname command to obtain the domain name of the server system. The nsswitch.conf file tells the server where to look for domain-based information. If the server system is an NIS or NIS+ client, the DHCP server performs a query to get NIS or NIS+ server IP addresses. See the nsswitch.conf(4) man page for more information.
Default router	System routing tables, user prompt	The DHCP server searches the network routing tables to find the default router for clients that are attached to the local network. For clients not on the same network, the DHCP server must prompt you for the information.
Subnet mask	Network interface, netmasks table	The DHCP server looks to its own network interfaces to determine the netmask and broadcast address for local clients. If the request was forwarded by a relay agent, the server obtains the subnet mask in the netmasks table on the relay agent's network.
Broadcast address	Network interface, netmasks table	For the local network, the DHCP server obtains the broadcast address by querying the network interface. For remote networks, the server uses the BOOTP relay agent's IP address and the remote network's netmask to calculate the broadcast address for the network.