Add DHCP relay agent, with Option 82 support

[troglobit]

Description

In many network setups it can be convenient to implement easy-replacement of end devices using DHCP Option 82. This is a feature where the DHCP request of a client is intercepted, by a DHCP relay agent on the switch it's connected to, to attach another DHCP option which includes information about the switch itself and the port the client is connected to. The relay agent then forwards the request to the DHCP server (one or many if redundancy is required), which in turn, based on policy, can either hand out a DHCP pool address or a fixed address reserved for the port on that switch.

Essentially, end devices would no longer just be devices with an IP address, but rather devices with dedicated roles in a train car, or on a factory floor. Replacing a broken end device with a new one shall yield the same IP address.

Tasks

• Investigate state of the Westermo dhcp-helper package
• Integrate the Westermo dhcp-helper package
• YANG support for setting up Option 82 ("IP per port")