11.6. DHCP Operations

The DHCP server provided by dr-provision provides the basis for many of our more advanced bare-metal automation capabilities, including dynamic network boot control and UEFI secure boot support.

11.6.1. Startup Configuration

Whether the DHCP server is enabled and what ports it will listen on are controlled by the following startup options:

  1. –disable-dhcp to disable basic DHCP functionality.
  2. –disable-pxe to disable extended PXE/BINL support (mainly used by Windows)
  3. –dhcp-port to change the DHCP port from the default of 67 to something else.
  4. –binl-port to change the BINL port from the default of 4011 to something else.

If none of these options are present, then we default to DHCP and BINL being enabled on their standard ports. The DHCP server listens on all interfaces, and that behaviour cannot be modified at the present time.

11.6.2. Runtime Configuration

By default (without any Subnets or Reservations defined) the DHCP server will ignore any traffic it receives. To start responding to requests, Subnets and/or Reservations must be defined.

11.6.2.1. Subnets

A Subnet ( Subnet ) defines an address range that dr-provision attempt to handle DHCP traffic from and the IP addressing information that should be applied to any interfaces requesting configuration from within that range.

Whether any given packet maps into a Subnet depends on one of the following conditions being true:

  • The DHCP packet was not relayed to us, and there is an address on the interface it was received on that is within range of that Subnet.
  • The DHCP packet was relayed to us, and the GIADDR field in the packet is within the range of that Subnet.
  • The DHCP packet was relayed to us, and Option 82 Suboption 5 is set and within the range of the Subnet.

Additionally, if Option 82 Suboption 11 is set, we will format ServerID in the response packet as appropriate.

11.6.2.2. Reservations

A Reservation ( Reservation ) defines a one-to-one mapping between a specific network interface to be configured and the IP addressing information that should be applied to that interface.

A Reservation can define an IP address that is outside the bounds of any Subnet.

11.6.3. Runtime Behaviour

  • If there is no Subnet or Lease that covers an incoming DHCP packet, it will be ignored.
  • DHCP Options from Subnets and Reservations stack. If an incoming request could be handled by both a Reservation and a Subnet, then the DHCP options on the Reservation will take precedence over the ones from the Subnet.
  • If you set DHCP logging to Debug or Trace, the dr-provision log will contain all inbound and outbound DHCP traffic, along with messages about how any given packet was handled.
  • We rely on CHADDR for Lease and Subnet lookups. DUIDs are ignored, as they are often different between the firmware, IPXE/PXELINUX, Sledgehammer, and the final installed OS.
  • If the CHADDR in a DHCP packet corresponds to a recorded HardwareAddr for a Machine, the DHCP server will react intelligently to network boot requests.