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You can use option filters to classify DHCP clients and decide which DHCP options each group of clients can receive. By default, regardless of the networks in which the DHCP clients reside and whether an option filter is applied to a DHCP range or range template, all DHCP clients that match the filter criteria receive the DHCP options and values you define in the filter. You can change this configuration so the appliance does not use the filter to classify DHCP clients. For information about how to configure this, see Defining Option Filters.
You can add DHCP options and the Hardware Operator option to an option filter. (For information about the Hardware Operator option, see DHCP Hardware Operator.) Depending on whether the options you add to the filter are also defined at the Grid, member, network, and DHCP range levels, and whether you add the filter to the Class Filter List or Logic Filter List, the appliance either appends them to the existing options or overwrites the option values before returning them to the matching clients. For more information about how the appliance returns DHCP options, see Adding Filters to the Logic Filter List.
The appliance can filter an address request by the options (such as root-server-ip-address or user-class) of the requesting host. Depending on how you apply an option filter, the appliance can grant or deny an address request if the requesting host matches the filter criteria. You can also create complex match rules that use the AND and OR logic to further define the filter criteria. When you select match rules in Grid Manager, you can preview the rules before committing them to the filter. Grid Manager provides an expression builder that automatically builds the rules after you define them. For information, see Defining Option Filters.
To define an option filter and apply it to an address range:

1. Define an option filter based on either the predefined or custom DHCP options. For information, see Defining Option Filters.
2. Apply the filter to a DHCP address range or range template in the Class Filter List or Logic Filter List. For information, see Applying Filters to DHCP Objects.

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After you add all the match rules, you can click Preview to view the rules that are written to the dhcpd configuration file or click Reset to remove the previously configured rules and start again. For information about how to use match rules, see Using Match Rules in Option Filters. bookmark2522

4. Click Next and complete the following to define which DHCP options to return to the matching client:

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The appliance generates the following rules in the dhcpd configuration file:
 

You can also define more complex rules using the AND and OR logic as follows:

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The NIOS appliance can filter DHCP address requests by user class filters. A user class indicates a category of user, application, or device of which the DHCP client is a member. User class identifiers are configured on DHCP clients and are sent during a DHCP address request operation. The client includes the user class identifier in DHCP option 77 when sending DHCPDISCOVER and DHCPREQUEST messages.
By using user class identifiers, a DHCP server can screen address requests and assign addresses from select address ranges based on the different user class identifiers it receives. For example, if you assign a user class filter named mobile to a range of addresses from 10.1.1.31–10.1.1.80, the appliance selects an address from that range if it receives an address request that includes the user class name mobile and there are still addresses available in that range. You might want mobile users to receive these addresses because you have given them shorter lease times than other, more stationary DHCP clients. See Figure 31.6.

If the NIOS appliance receives address requests with the user class mobile and there are no available addresses in address range 2 but there are available addresses in ranges 1 and 3, the appliance begins assigning addresses from address range 3 (because its addresses are higher than those in range 1). Then, if all addresses in range 3 are in use, the appliance begins assigning addresses from address range 1. If you want the appliance to assign addresses to mobile users (that is, those identified with the user class mobile) exclusively from address range 2, then you must apply user class filters for "mobile" to address ranges 1 and 3 that deny lease requests matching that user class.

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