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.
An option space is a collection of options. ISC (Internet Systems Consortium) DHCP has five predefined option spaces: dhcp, agent, server, nwip, and fqdn. The NIOS appliance supports only the predefined DHCP option space, which contains the industry standard options as well as additional options you can configure as needed:
You can also create option spaces to define new groups of options. For example, 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 vendor identifier set in option 60 and a list of requested vendor specific options (option 43). The DHCP server then responds with the list of replies for the various options encapsulated into option 43.
Note that custom options defined in the DHCP option space are included in the options section of the DHCP messages that DHCP servers and clients exchange. Custom options defined in a user-defined option space are always encapsulated in option 43 in DHCP messages.
You can apply options globally at the Grid level, or more specifically at the member, network, network container, range, host and roaming host levels.
A network inherits DHCP options from its parent object. You can override the inherited DHCP options configured at the Grid or Member level for the networks and shared networks. If a network has a parent network container and parent shared network and if you override the DHCP options on the shared network, then the network inherits the shared network values. If you override the DHCP options at the network container level, then the network inherits the network container values. Otherwise, the network continues to inherit DHCP options from its parent Grid or member. A shared network without a parent network container continues to inherit DHCP options from its parent Grid or member. The parent object is derived from the first network within the shared network. A network container inherits DHCP options from its parent and grandparent network containers. A network container does not inherit DHCP options defined at the Grid or member level.
To override an inherited value, click Override next to it and complete the appropriate fields. When you click Override, the appliance displays the value inherited from its parent object (if any). If you do not set any value at the higher level, the appliance displays the default DHCP options. The following table lists the default DHCP Options:
Table 26.1 Default DHCP Options
Name | Default Value | |
---|---|---|
IPV4 Common DHCP Options | Enable PXE Lease Time | Disabled |
Lease Time | 43200 | |
Routers | Empty List | |
Domain Name | Empty | |
DNS Servers | Empty List | |
Broadcast Address | 127.0.0.1 | |
Custom DHCP Options | "" | |
Lease Scavenging | Disabled | |
Authoritative (Advanced) | Disabled | |
Lease Deletion (Advanced) | Disabled | |
Ignore Optionlist (Advanced) | Disabled | |
IPv4 DDNS | Enable DDNS Updates | Disabled |
DDNS Domain Name | "" | |
DDNS Update TTL | 0 | |
DDNS Update Method | Interim | |
Generate Hostname | Disabled | |
Fixed Address Updates | Disabled | |
Option 81 Support | Disabled | |
Lease Renewal Update | Disabled | |
IPv4 Threshold Options | Enable DHCP Thresholds | Disabled |
High - Trigger | 95 | |
High - Reset | 85 | |
Low - Trigger | 0 | |
Low - Reset | 10 | |
Enable SNMP Warnings | Disabled | |
Enable Email Warnings | Disabled | |
Email Addresses | Empty List | |
IPv4 BOOTP/PXE | Enable PXE Lease Time | Disabled |
Lease Time (Value) | 0 | |
Deny-BOOTP-Requests | Disabled | |
Boot File | "" | |
Next Server | "" | |
Boot Server | "" | |
IPv6 DHCP Options | Valid Lifetime | 43200 |
Preferred Lifetime | 27000 | |
Domain Name | "" | |
DNS Servers | ||
Custom DHCP Options | ||
Lease Deletion | Enabled | |
Lease Scavenging | Disabled | |
IPv6 DDNS Options | Enable DDNS Updates | |
DDNS Domain Name | "" | |
DDNS Update TTL | 0 | |
DDNS Update Method | Interim | |
Generate Hostname | Disabled | |
FQDN Support
| Disabled | |
Lease Renewal Update | Disabled |
You can also create an option filter the appliance uses to filter address requests by the DHCP options of requesting hosts. The filter instructs the appliance to either grant or deny an address request if the requesting host matches the filter. For information, see Defining Option Filters.
The DHCP option configuration conforms to the following RFCs:
Each DHCP option is identified by a name and an option code number, and specifies a data type. The data type for some options is predefined. For example, in the DHCP option space, the data type for option 1: subnet-mask is an IP address. You cannot change the data type for this option. The data type for some options is user-defined and can be in one of the formats shown in Table 26.2.
Table 26.2 DHCP Option Data Types
Data type | Specifies |
---|---|
String | An ASCII text string (the same as the text data type) or a list of hexadecimal characters separated by colons Formatting to distinguish an ASCII text string from a hexadecimal string is important. For details, see the following section |
Boolean | A flag with a value of either true or false (or on or off ) |
IP address | A single IP address |
Array of IP addresses | A series of IP addresses, separated by commas You can optionally include a space after each comma |
Text | An ASCII text string |
8-, 16-, or 32-bit unsigned integer | A numeric range of the following possible values 8-bit unsigned integer: from 0 to 255 16-bit unsigned integer: from 0 to 65,535 32-bit unsigned integer: from 0 to 4,294,967,295 |
8-, 16-, or 32-bit signed integer | A numeric range of the following possible values 8-bit signed integer: from -128 to 127 16-bit signed integer: from -32,768 to 32,767 32-bit signed integer: from -2,147,483,648 to 2,147,483,647 |
Domain-list | A list of domain names, separated by spaces |
When defining a hexadecimal string for a DHCP option (such as option 43, vendor encapsulated options), use only hexadecimal characters (0-9, a-f, or A-F) without spaces and separated by colons. The accepted form for a hexadecimal string, as presented in a regular expression, is [0-9a-fA-F]{1,2}(:[0-9a-fA-F]{1,2})*
Two examples of correctly written hexadecimal strings:
A few examples of incorrectly written hexadecimal strings:
The DHCP module treats incorrectly written hexadecimal strings as simple text strings, not hexadecimal strings. If the string appears in quotes, it is a text string.
To use DHCP options, you can do the following:
You can define basic DHCP options that the DHCP server uses to provide configuration information to DHCP clients. The server includes these options in its DHCP messages.
To define DHCP options:
3. Save the configuration and click Restart if it appears at the top of the screen.
DHCP members support the DHCP option space by default. You can create additional option spaces to provide additional configuration or service information. Note that custom options defined in a user-defined option space are always encapsulated in option 43 in DHCP messages
To add a custom option space:
After you create an option space and add options to it, you can apply the options as described in Applying DHCP Options.
You can define custom options in the DHCP option space or in an option space that you configured, as follows:
Click the Add icon to add more options.
4. Save the configuration.
Some options may apply to all networks and some may apply to specific ranges and even hosts. When you apply an option, you select the object to which the option is applied, such as the Grid member, or network, and then specify a value for the option.
Use the following guidelines when specifying option values:
Here are some examples of option names and correctly formatted values:
Option name | Value | Comment |
---|---|---|
option 61 dhcp-client-identifier | MyPC | Double quotes are no longer needed for string type values |
dhcp-client-identifier | 43:4c:49:45:54:2d:46:4f:4f | Series of octets specified in hex, separated by colons for a Data-string type value |
netbios-name-servers | 10.1.1.5,10.1.1.10 | Multiple IP addresses separated by commas |
option-80 | ABC123 | Custom option number 80 set to the string ABC123. |
To apply DHCP options:
Member: From the Data Management tab, select the DHCP tab -> Members tab -> Members -> member check box, and then click the Edit icon.
Network: From the Data Management tab, select the DHCP tab -> Networks tab -> Networks -> network check box, and then click the Edit icon.
Network Container: From the Data Management tab, select the IPAM tab -> network_container check box, and then click the Edit icon.
DHCP Range: From the DataManagement tab, select the DHCP tab -> Networks tab -> Networks -> network -> addr_range check box, and then click the Edit icon.
Fixed Address: From the Data Management tab, select the DHCP tab -> Networks tab -> Networks -> network -> fixed_address check box, and then click the Edit icon.
Reservation: From the Data Management tab, select the DHCP tab -> Networks tab -> Networks -> network -> reservation check box, and then click the Edit icon.
Host Address: From the Data Management tab, select the DHCP tab tab -> Networks tab -> Networks -> network -> host_record check box, and then click the Edit icon. Select the host IP address, and then click the Edit icon.
Roaming Host: From the Data Management tab, select the DHCP tab tab -> Networks tab -> Roaming Hosts -> roaming_host check box, and then click the Edit icon.
2. In the DHCP Properties editor, select the IPv4 DHCP Options and complete the following:
3. Save the configuration and click Restart if it appears at the top of the screen.
In this example, you configure two custom options in the DHCP option space, and apply them to a DHCP range in the network 192.168.2.0/24.
Add the custom options to the DHCP options space:
Enter values for the newly defined custom options and apply them to a DHCP range:
4. Save the configuration and click Restart if it appears at the top of the screen.
The member then includes options 150 and 209 in its DHCP messages to clients that are allocated IP addresses from the DHCP range 192.168.2.10 - 100.
The appliance uses option 60 (vendor-class-identifier) to forward client requests to the DHCP server for services that the clients require. You can define option 60 match rules and filter on these rules. You can set these rules for the Grid and override for a member.
To define option 60 for the Grid or member:
3. Save the configuration and click Restart if it appears at the top of the screen.
A typical relationship between a DHCP client, relay agent, and the NIOS appliance on a network is as follows:
The situation is different for individual hosts connecting to the Internet through an ISP, usually over a circuit-switched data network.
To avoid broadcasting the DHCPOFFER over the network segment on which the host made the request, the relay agent sends the response directly to the host over the established circuit.
Option 82 assists the agent in forwarding address assignments across the proper circuit. When a relay agent receives a DHCPDISCOVER message, it can add one or two agent IDs in the DHCP option 82 suboption fields to the message.
The relay agent IDs are:
Note
For information about the relay agent option, refer to RFC3046, DHCP Relay Agent Information Option.
In addition to the relay agent IDs, NIOS also supports the Option 82 Link Selection and Server ID Override sub-options, which allow DHCPv4 to operate in a network architecture where direct communication between the DHCP server and DHCP client is undesirable or infeasible. You can configure these sub-options to direct DHCP traffic to go through the relay agent and have more control over your DHCP communications.
The Link Selection sub-option provides a mechanism to separate the subnet/link in which the DHCP client resides from the GIADDR (Gateway IP address). The GIADDR field in a DHCP message is populated by the relay agent and is typically used to inform the DHCP server about the subnet in which the DHCP client resides and to inform the DHCP server of the IP address to use to communicate with the relay agent. In situations where the GIADDR might not be the appropriate subnet from which IP addresses should be allocated, you can use the Link Selection sub-option to explicitly set the subnet from which IP addresses are allocated to the client.
The Server ID Override sub-option allows the relay agent to tell the DHCP server what IP address, instead of the server's address, must be used in the response. Generally, the response from the server contains the IP address of the DHCP server itself in the Server-ID option. You can use the Server ID Override sub-option to specify a new value for the server ID that is inserted in the reply packet by the DHCP server. Configuring the Server ID Override sub-option allows the relay agent to have the clients send all unicast messages to the relay agent instead of the DHCP server.
Note
If you want the Link Selection and Server ID Override sub-options to be included in the DHCP relayed messages, you must configure them on the DHCP relay agent. You cannot configure them on NIOS. For more information about these sub-options, refer to https://tools.ietf.org/html/rfc3527 and https://tools.ietf.org/html/rfc5107.
On the NIOS appliance, you can do the following with option 82:
When you define the circuit ID or remote ID of the relay agent as a host identifier, you can choose the logging format Grid Manager uses to display the IDs in the detailed lease information panel. For information about viewing lease information, see Viewing Detailed Lease Information.
To define logging format for the agent ID, circuit ID and remote ID, complete the following:
Note
You cannot override this Grid setting at the member level. Also, changing the logging format requires a DHCP service restart.
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