Windows 7 / Networking

Configure servers

It seldom happens that a server is ready to perform all the tasks you have planned for it immediately after installation. Typically some postinstallation configuration is required, and further configuration changes might become necessary after the server is in service.

This objective covers how to:

Create and configure scopes
Configure a DHCP reservation
Configure DHCP options
Configure client and server for PXE boot
Configure DHCP relay agent
Authorize DHCP server

Understanding DHCP

DHCP is a service that automatically configures the IP address and other TCP/IP settings on network computers by assigning addresses from a pool (called a scope) and reclaiming them when they are no longer in use.

Aside from being a time-consuming chore, manually configuring TCP/IP clients can result in typographical errors that cause addressing conflicts that interrupt network communications. DHCP prevents these errors and provides many other advantages, including automatic assignment of new addresses when computers are moved from one subnet to another and automatic reclamation of addresses that are no longer in use.

DHCP consists of three components, as follows:

A DHCP server application, which responds to client requests for TCP/IP configuration settings
A DHCP client, which issues requests to servers and applies the TCP/IP configuration settings it receives to the local computer
A DHCP communications protocol, which defines the formats and sequences of the messages exchanged by DHCP clients and servers

All of the Microsoft Windows operating systems include DHCP client capabilities, and all of the server operating systems (including Windows Server 2012) include the Microsoft DHCP Server.

The DHCP standards define three different IP address allocation methods, which are as follows:

Dynamic allocation: The DHCP server assigns an IP address to a client computer from a scope, for a specified length of time. Each client must periodically renew the lease to continue using the address. If the client allows the lease to expire, the address is returned to the scope for reassignment to another client.
Automatic allocation: The DHCP server permanently assigns an IP address to a client computer from a scope. Once the DHCP server assigns the address to the client, the only way to change it is to manually reconfigure the computer.
Manual allocation: The DHCP server permanently assigns a specific IP address to a specific computer on the network. In the Windows Server 2012 DHCP server, manually allocated addresses are called reservations.

In addition to IP addresses, DHCP also can provide clients with values for the other parameters needed to configure a TCP/IP client, including a subnet mask, default gateway, and DNS server addresses. The object is to eliminate the need for any manual TCP/IP configuration on a client system. For example, the Microsoft DHCP server includes more than 50 configuration parameters, which it can deliver along with the IP address, even though Windows clients can only use a subset of those parameters.

DHCP communications use eight different types of messages, all of which use the same basic packet format. DHCP traffic is carried within standard UDP/IP datagrams, using port 67 at the server and port 68 at the client.

DHCP options

The DHCP options field is a catch-all area designed to carry the various parameters (other than the IP address) used to configure the client system's TCP/IP stack. Because you can configure a DHCP server to deliver many options to clients, defining separate fields for each one would be impractical.

THE DHCP MESSAGE TYPE OPTION

The DHCP Message Type option identifies the overall function of the DHCP message and is required in all DHCP packets. The DHCP communication protocol defines eight different message types, as follows:

DHCPDISCOVER: Used by clients to request configuration parameters from a DHCP server
DHCPOFFER: Used by servers to offer IP addresses to requesting clients
DHCPREQUEST: Used by clients to accept or renew an IP address assignment
DHCPDECLINE: Used by clients to reject an offered IP address
DHCPACK: Used by servers to acknowledge a client's acceptance of an offered IP address
DHCPNAK: Used by servers to reject a client's acceptance of an offered IP address
DHCPRELEASE: Used by clients to terminate an IP address lease
DHCPINFORM: Used by clients to obtain additional TCP/IP configuration parameters from a server

BOOTP VENDOR INFORMATION EXTENSIONS

These options include many of the basic TCP/IP configuration parameters used by most client systems, such as the following:

Subnet Mask: Specifies which bits of the IP address identify the host system and which bits identify the network where the host system resides
Router: Specifies the IP address of the router (or default gateway) on the local network segment the client should use to transmit to systems on other network segments
Domain Name Server: Specifies the IP addresses of the servers the client will use for DNS name resolution
Host Name: Specifies the DNS host name the client system will use
Domain Name: Specifies the name of the DNS domain on which the system will reside

DHCP EXTENSIONS

These options are used to provide parameters that govern the DHCP lease negotiation and renewal processes.

Requested IP Address: Used by the client to request a particular IP address from the server
IP Address Lease Time: Specifies the duration of a dynamically allocated IP address lease
Server Identifier: Specifies the IP address of the server involved in a DHCP transaction; used by the client to address unicasts to the server
Parameter Request List: Used by the client to send a list of requested configuration options (identified by their code numbers) to the server
Message: Used to carry an error message from the server to the client in a DHCPNAK message
Renewal (T1) time value: Specifies the time period that must elapse before an IP address lease enters the renewing state
Rebinding (T2) time value: Specifies the time period that must elapse before an IP address lease enters the rebinding state