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Configuring DHCP Options

Owned by Sri Raghu
Last updated: Sept 06, 2024
5 min read

DHCP options provide specific configuration and service information to DHCP clients. These options appear as variable-length fields at the end of the DHCP messages that DHCP servers and clients exchange. For example, DHCP option 3 is used to list the available routers in the network of the client, and option 6 is used to list the available DNS servers.

To configure DHCP options, complete the following:

  1. In the Infoblox Portal, click Manage > IPAM/DHCP > Global DHCP Configuration.

  2. On the Global DHCP Configuration page, click DHCP Options and go to IPv4 DHCP OPTION CODES. Click Add to add an IPv4 DHCP option code or an option group to the list. Or click Remove to delete the entry. Then complete the following:

    • Type: Select Option or Option Group based on what you want to add to the list.

    • Space: Choose a value from the drop-down list. You can choose the Space only when Option is selected. The list displays either options or option groups that are configured based on the Type you choose.

    • Name: Choose a value from the drop-down list. The list displays names of the options or option groups based on the Space you choose.

    • Option Type: Displays the option type. For example, it can be an 8-bit integer, 16-bit integer, 32-bit integer, unsigned 8-bit integer, unsigned 16-bit integer, unsigned 32-bit integer, Boolean, FQDN, Hexadecimal, IPv4 Address, or Text. 

    • Value: Enter the value that you want to use for the selected DHCP option. The Value field is only available if Type is selected as Option. For option types other than hexadecimal strings, this field has a limit of 255 bytes. In the message sent on the wire, hexadecimal strings longer than 255 bytes will be broken up into multiple instances; it is up to the client to re-assemble these strings into a single option.

  3. To add more options or option spaces to the filter, click Add.
  4. Configure the Header Options. To obtain the configuration, some devices use one or more of the following fields in the DHCPv4 packet: next-server, server-NIOS-X Server name, and boot-file-name. Configure the following:
    • Server Name: Specify the server's NIOS-X Server name, which can be up to 64 bytes long.
    • File Name: Specify the configuration file's name, which can be up to 128 bytes long.
    • Server Address:  Specify the IPv4 address.
  5. Configure the Vendor Encapsulated Option Space. There are two vendor encapsulated option spaces available, option 43 and option 125. 
    • Option Space (43): You can specify the hexadecimal value while creating the option space or create an encapsulated option space.  
      • Hexadecimal in an Option Space: For Type, select Option. For Space, select the option space you have created. For more information on option spaces, see Creating Option Spaces. Create an option space with Type as hexadecimal. If you choose the Option Type as hexadecimal, specify the value as aa:bb:cc, aa-bb-cc, or aabbcc.
      • Vendor Encapsulated Option Space: Click Select DHCP Option Space. Choose an option space from the list and click Select. The selected option spaces will include only option space 43. You can create additional option spaces to define vendor-specific options, which are encapsulated in option 43. When a DHCP client requests vendor-specific options, it makes a request using the list of requested vendor-specific options (option 43). 
    • Option Space (125): Click Select DHCP Option Space. Choose an option space from the list, and click Select. The option space is replaced with the appropriate vendor ID. The name of the option space must follow the convention vendor-27167, where 27167 is to be replaced with the appropriate vendor ID. You can configure the Vendor Specific Option option space at the global level in the Global DHCP Configuration, DHCP Config Profiles, and IP Space. You can also use Vendor Specific Option (option code 43) for specific objects such as Hardware Filter and Option Filter. For the Vendor Specific Option (option code 43), change the value of option code TYPE from No Value to Hexadecimal. For more information, see Configuring Option Spaces. 

  6. Click DHCP Options and go to IPv6 DHCP OPTION CODES. Click Add to add an IPv6 DHCP option code or an option group to the list. Or click Remove to delete the entry. Then complete the following:

    • Type: Select Option or Option Group based on what you want to add to the list.

    • Space: Choose a value from the drop-down list. You can choose the Space only when Option is selected. The list displays either options or option groups that are configured based on the Type you choose.

    • Name: Choose a value from the drop-down list. The list displays names of the options or option groups based on the Space you choose.

    • Option Type: Displays the option type. For example, it can be an 8-bit integer, 16-bit integer, 32-bit integer, unsigned 8-bit integer, unsigned 16-bit integer, unsigned 32-bit integer, Boolean, FQDN, Hexadecimal, IPv6 Address, or Text. 

    • Value: Enter the value that you want to use for the selected DHCP option. The Value field is only available if Type is selected as Option. Universal DDI will not add a trailing dot to FQDN formatted options. If the client receiving the option requires it to have a trailing dot, configure it on the Infoblox Portal as part of the option's value. 

  7. Click Save & Close to save the details or click Cancel to exit.

Guidelines for DHCP Router offset

If you choose the Option Type as IPv4 address, IPv6 address or arrays of those, you can use + and - operators for assigning offsets. For these option types provide a valid IP address or specify an offset value between -9 to -1 or +1 to +9. Make sure to specify the + or - sign before the integer. Zero is not allowed.  ​This feature is useful when an option, such as the routers DHCP option (option 3), follows a predictable pattern based on the associated subnet (such as the first or last usable IP address in the subnet). The offset variable can be used for options that support IPv4, IPv6, and an array of IPs and is not restricted to routers (option code 3).

You can configure the DHCP router offset as per the following guidelines:

  • Subnet and lower (i.e. range) would require an IP address.
  • Anything “above” a subnet (address block, IP space, config profile, global) would require a +/- offset.

For example, if you use +3 at the global level for option code 3 and you have a subnet 10.0.0.0/24 then the computed value that will be shown in the Infoblox Portal will be 10.0.0.3. You can configure the offset at the global level and override with an IP address for objects such as subnet (and lower).