DHCP relay

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If you want your router to forward DHCP requests to an external DHCP server you can configure the system to act as a DHCP relay agent. The DHCP relay agent works with IPv4 and IPv6 addresses.

Prerequisites

All interfaces used for the DHCP relay must be configured. See Network address setup.

DHCP relay example

Service-dhcp relay1.png

In this example the interfaces used for the DHCP relay are eth1 and eth2. The router receives DHCP client requests on eth1 and relays them through eth2 to the DHCP server at 10.0.1.4.

Configuration

Enable DHCP relay for eth1 and eth2:

# set service dhcp-relay interface eth1
# set service dhcp-relay interface eth2

Set the IP address of the DHCP server:

# set service dhcp-relay server 10.0.1.4

The router should discard DHCP packages already containing relay agent information to ensure that only requests from DHCP clients are forwarded:

# set service dhcp-relay relay-options relay-agents-packets discard

Commit the changes and show the results:

# commit
# show service dhcp-relay
     interface eth1
     interface eth2
     server 10.0.1.4
     relay-options {
        relay-agents-packets discard
     }

The DHCP relay agent can be restarted with:

$ restart dhcp relay-agent 

DHCPv6 relay example

Service-dhcpv6 relay1.png

In this example DHCPv6 requests are received by the router on eth1 ('listening interface') and forwarded through eth2 ('upstream interface') to the external DHCPv6 server at 2001:db8:100::4.

Configuration

Set eth1 to be the listening interface for the DHCPv6 relay:

# set service dhcpv6-relay listen-interface eth1

Set eth2 to be the upstream interface and specify the IPv6 address of the DHCPv6 server:

# set service dhcpv6-relay upstream-interface eth2 address 2001:db8:100::4

Commit the changes and show results:

# commit
# show service dhcpv6-relay
     listen-interface eth1 {
     }
     upstream-interface eth2 {
        address 2001:db8:100::4
     }

Show the current status of the DHCPv6 relay agent:

# show dhcpv6 relay-agent status

The DHCPv6 relay agent can be restarted with:

# restart dhcpv6 relay-agent

Additional parameters

DHCP relay agent options

  • # set service dhcp-relay relay-options hop-count count
Set the maximum hop count before packets are discarded. Range 0...255, default 10.
  • # set service dhcp-relay relay-options max-size size
Set maximum size of DHCP packets including relay agent information. If a DHCP packet size surpasses this value it will be forwarded without appending relay agent information. Range 64...1400, default 576.
  • set service dhcp-relay relay-options port port
Set the port used to relay DHCP client messages. Range 1...65535, default 67. After setting a different port, requests are still accepted on port 67 but replies are forwarded to 255.255.255.255 port 0 instead of 68.
  • set service dhcp-relay relay-options relay-agents-packet policy
Four policies for reforwarding DHCP packets exist:
append: The relay agent is allowed to append its own relay information to a received DHCP packet, disregarding relay information already present in the packet.
discard: Received packets which already contain relay information will be discarded.
forward: All packets are forwarded, relay information already present will be ignored.
replace: Relay information already present in a packet is stripped and replaced with the router's own relay information set.

DHCPv6 relay agent options

  • # set service dhcpv6-relay listen-port port
Set listening port for DHCPv6 requests. Default: 547.
  • # set service dhcpv6-relay max-hop-count count
Set maximum hop count before packets are discarded. Default: 10.
  • set service dhcpv6-relay use-interface-id-option
If this is set the relay agent will insert the interface ID. This option is set automatically if more than one listening interfaces are in use.