ALIF: Understanding DHCP Option 125

Option 125 is a vendor-specific DHCP Server feature that is required if you are using AIM’s multi-subnet mode. It provides additional information to the Infinity Receivers and Transmitters when requesting an IP Address from the DHCP Server. The option gives the endpoints the IP Address of the Primary and any other Backup or Satellite AIM Servers that reside on the network. It is also used to setup Satellite AIM’s. To simplify the process you can download a Windows DHCP Option 125 application, if you require.

The information is provided in a hex code which can be formatted slightly differently depending on the DHCP Server, but will typically look like:-

0000621f0a01080a0014050a001406

The Hex code is broken down to the following elements:

The Adder Enterprise and Sub-option 1 elements of the hex code are fixed and do not change, however the IP Address and Length A & B hex values will change depending on your own Infinity installation.

The first IP address in the list must always be the Primary AIM Server, followed by the Backup then Satellite AIM’s.

There are a number of online websites that help you convert an IP Address into a hex number. Alternatively, you could use a Calculator with a Hex and Dec function which you may find is built into your computers operating system.

An IP Address hex code can be calculated by converting each of the four decimal octet’s individually into hex then combining them.

10.0.20.5 = 0a001405

The Length A and B values will change depending on the number of IP Addresses that are included.

Each hex IP Address is 4-bytes in length, for example:

The Length B value is the total byte count of the hex IP Addresses. In the example below, there are 2x hex IP addresses. Each hex IP address is 4-bytes, therefore Length B is 8-bytes in total which equates to 08 in hex.

Length A includes Sub-option 1 (1-byte) and the Length B value (1-byte) plus the total of Length B (8-bytes). This makes the total 10-bytes which equates to 0a in hex.

Example

In the following example an additional IP address will be added to the existing DHCP option 125 hex code shown previously. It will be a Satellite AIM whose IP address is 10.0.30.1 which is on a different subnet.

Step 1: Convert IP Address 10.0.30.1 into hex, which is 0a001e01. Add it to the end of the hex code.

0000621f..01..0a0014050a0014060a001e01

Step 2: Calculate Length B. Since we now have 3x IP addresses and each hex IP address is 4-bytes, the total length is 12-bytes which equates to 0c in hex.

0000621f..010c0a0014050a0014060a001e01

Step 3: Calculate Length A. This is the total of Length B plus the byte count for Sub-option 1 and the Length B value. The Sub-option 1 and Length B value are 1-byte each, the Length B total is 12-bytes. This makes the total of Length A to be 14-bytes which equate to 0e in hex.

0000621f0e010c0a0014050a0014060a001e01

Formatting Hex Code

The formatting of the option can vary between DHCP servers. You will need to check the manual for the specific server. For example:-

No delimiter

0000621f0a01080a0014050a001406

Dots/Periods

0000.621f.0a01.080a.0014.050a.0014.06

Colons

00:00:62:1f:0a:01:08:0a:00:14:05:0a:00:14:06

Example DHCP Configurations

Example: ISC DHCPd, Adder, AIM servers: 10.0.20.5, 10.0.20.6

Example: Cisco IOS DHCPd, Adder, AIM servers: 10.0.20.5, 10.0.20.6

Example: HP 2920 Switch. Adder AIM Servers: 10.0.10.10, 10.0.10.11

Example: Setting up DHCP Option 125 on Microsoft Windows Server 2012
Note: This assumes that you have setup the IP address scope to issue a range of IP addresses.

• Open the DHCP server configuration console (found within administration tools).
• Expand the server tree in question on the left hand side.
• Right click the IPv4 object in the left hand tree Select "Set Predefined Options".
• With the Option Class set to "DHCP Standard Options"
• Click Add
• Enter a Name, for example, Adder AIMs
• Set the Data type to Binary
• Set the Code to 125
• Enter a Description (optional)
• Press OK to close the dialog window.
• Open the DHCP Scope that you have created to issue IP addresses.
• In the Scope Options section, right mouse click and choose Configure Options.
• On the General tab, scroll down the list and find option 125.
• Tick the checkbox for option 125
• In the data entry section, you need to enter the hex number in the binary section.
  
  
  
• Click OK to close the dialog window.
• For our setup to take immediate effect a restart the DHCP service was required.

Checking an Endpoint

When a transmitter or receiver has successfully acquired a DHCP IP address along with Option 125, it will automatically assume that it is under the AIM Managers control and you should see the IP address of the AIM Manager(s) listed on the About page.

For example, the screenshot below has been taken from ALIF2020. In the AIM section that you will see that the AIM servers are 10.0.20.5 and 10.0.20.6 respectively.