DHCP Relay#

DHCP → DRDHCP Relays

The DHCP protocol is used to dynamically assign IP addresses to a host that asks for one. Because a requesting host has no assigned address yet, the initial DHCP Discover is sent from source IP 0.0.0.0 to the limited broadcast 255.255.255.255, which routers do not forward across subnet boundaries. This is a problem in cases where the DHCP server manages multiple subnets: it would need to be reachable in every broadcast domain, which is often not feasible.

Instead, we use a DHCP relay that intercepts the broadcast DHCP message and forwards it as a unicast packet to the DHCP server, recording its own IP address as the gateway address. This intermediary device then relays messages in both directions: DHCP client requests are forwarded as unicast packets to the server, and server responses are forwarded back to the client.

The DHCPRelay can be configured on two interface types, which are always tied to a virtual Router:

• An IRBInterface that connects to a BridgeDomain, which can have many sub-interfaces connected to it
• A RoutedInterface is essentially a broadcast domain on a single physical interface, although there may of course be many devices behind that interface

Sub-options#

DHCP relay options can be defined which add additional information to the DHCP request, such as the sub-interface on which the relay received the DHCP request.

For DHCPv4, the following options are supported:

• CircuitID
• RemoteID

For DHCPv6, the following options are supported:

• RemoteID
• InterfaceID
• ClientLinkLayerAddress

instance 2 (dhcp-router),
   transmitted DHCP Boot Request to 10.10.10.10 Port 67

   H/W Type: Ethernet(10Mb)  H/W Address Length: 6
   ciaddr: 192.168.0.119     yiaddr: 0.0.0.0
   siaddr: 0.0.0.0           giaddr: 192.168.0.14
   chaddr: aa:13:8c:4d:db:8d    xid: 0x54dea877

   DHCP options:
   [82] Relay agent information: len = 40
      [1] Circuit-id: dc1-leaf4|dhcp-router|irb0|0:0       <-- Added by the DHCP relay
      [2] Remote-id: (hex) aa 13 8c 4d db 8d               <-- Added by the DHCP relay
   [53] Message type: Request
   [57] Max msg size: 576
   [55] Param request list: len = 7
             1  Subnet mask
             3  Router
             6  Domain name server
            12  Host name
            15  Domain name
            28  Broadcast addr
            42  NTP server
   [60] Class id: udhcp 1.36.1
   [61] Client id: (hex) 01 aa 13 8c 4d db 8d
   [255] End

Dependencies#

The DHCPRelay resource has no dependencies. However, it is only configured on a particular node if at least one IRBInterface or RoutedInterface is present on that node.

Referenced resources#

IRBInterface#

If the DHCP relay is configured on IRBInterfaces (which connect a BridgeDomain to a Router), then those IRB interfaces should exist and be configured with at least one primary IP address. If an IRBInterface does not have a primary IP on a node, the DHCP relay will not be deployed on that interface.

RoutedInterface#

If the DHCP relay is configured on RoutedInterfaces (which connect an Interface to a Router), then those routed interfaces should exist and be configured with at least one primary IP address. If a RoutedInterface does not have a primary IP on a node, the DHCP relay will not be deployed on that interface.

Examples#

apiVersion: services.eda.nokia.com/v2
kind: DHCPRelay
metadata:
  name: my-dhcp-relay
  namespace: services
spec:
  irbInterfaceSelectors:
    - dhcp = enabled
  router: dhcp-router
  servers:
    - 10.10.10.10
  subOptions:
    - CircuitID
    - RemoteID

cat << 'EOF' | kubectl apply -f -
apiVersion: services.eda.nokia.com/v2
kind: DHCPRelay
metadata:
  name: my-dhcp-relay
  namespace: services
spec:
  irbInterfaceSelectors:
    - dhcp = enabled
  router: dhcp-router
  servers:
    - 10.10.10.10
  subOptions:
    - CircuitID
    - RemoteID
EOF

Custom Resource Definition#

To browse the Custom Resource Definition go to crd.eda.dev.