Introduction

Background

Typically in CORD, the orchestrator (XOS) would instantiate containers or VMs in compute nodes and inform the fabric (via REST-API) of the IP/MAC and location of the instance. Without the use of XOS, it was necessary to configure the end-hosts (eg. compute nodes) in ONOS via the NetworkConfigHostProvider. With the use of a DHCP server and the ONOS dhcprelay app, this limitation has been removed. It is no longer necessary to configure the fabric with this information, as ONOS can learn the location, mac address, vlan-id and DHCP assigned IP address via the dhcprelay app, during the DHCP v6 solicit/advertise/request/reply process.

In this release, we add the following new capabilities:

It is no longer necessary to configure mac address of the DHCP server in the relay app config. ONOS will automatically discover the MAC and VLAN associated with the DHCP server IP address.

DHCP v6 relay app is vlan-aware. The server and client could be any mix of tagged or untagged hosts in same or different vlans.

DHCP server needs not be physically connected to the fabric. It could be located "behind" an external gateway router.

Configuration

The dhcprelay app makes one major assumption. It assumes that DHCP server is connected in the data-plane, i.e. DHCP server is either connected to fabric in one of reachable subnets or it is reachable by a gateway router that itself is connected to fabric in dataplane. We treat these cases separately.

 "of:0000000000000205/24" : {
      "interfaces" : [ {
        "name" : "dhcp-server-intf",
        "ips" : [ "10.0.2.254/24", "2001:db8:1::254/64" ],
        "vlan-tagged" : [ 20 ]
      } ]
    }

A second part of the configuration for DHCP relay app requires a json configuration under the key apps:

"apps" : {
      "org.onosproject.dhcp-relay" : {
         "default" : {
              "dhcpServerConnectPoint": "of:0000000000000205/24",
              "serverIps": ["10.0.2.253", "2001:db8:2::2"]
         }
      }
    }

Note that dhcprelay app is configured with location of the DHCP server (switchport to which it is connected to the fabric). It is also configured with DHCP server IP's, but it is no longer necessary to configure MAC address of the server. ONOS will automatically learn the MAC and VLAN corresponding to the server IP.

Configuring a DHCP Server reachable via external router

In this case, it is actually the external router that is directly connected to the fabric. This external router is already configured in the ports section of network-config (for vRouter functionality). For example, if the external router is connected to switch of:205 on port 1

"of:0000000000000205/1" : {
      "interfaces" : [ {
        "ips" : [ "192.168.101.2/30", "2000::c0a8:6402/120" ],
        "mac" : "a2:9b:32:9d:7f:b3",
        "name" : "internet-router"
      } ]
    }

As before the “ips” and “mac” configured on port 1, actually correspond to the addresses configured in Quagga.

The app config in this case, includes an additional field necessary to inform the dhcp-relay app of the gatewayIP through which the DHCP server can be reached.

"apps" : {
      "org.onosproject.dhcp-relay" : {
           "default" : {
                "dhcpServerConnectPoint": "of:0000000000000205/1",
                "serverIps": ["10.0.2.253", "2001:db8:2::2"],
                "gatewayIps": ["192.168.101.1", "1000::100:1"]
           }
       }
    }

Note that the dhcpserverConnectPoint should now be the switchport to which the external router is connected to the fabric.

Configure DHCP server

Install DHCP server

Modern DHCP servers should support relayed DHCP request. However, the way to configure them are probably different case to case. Here we use isc-dhcp-server on Ubuntu as an example.
To install the DHCP server, simply run:

sudo apt-get install isc-dhcp-server

Configure DHCP server

 Two configuration files are required by DHCP server.

First, we need to specify which network interface the DHCP server should listen on. To do that, we need to modify /etc/default/isc-dhcp-server and change the following line.

INTERFACES="eth1"

Next, we need to specify the subnet we want to lease. To do that, we need to modify /etc/dhcp/dhcpd.conf and add the following lines.
Note that the subnet of eth1 needs to be included. Otherwise, the DHCP server will not listen to the interface even though we have specified that in /etc/default/isc-dhcp-server.

subnet6 2001:db8:1::/64 {
        # Range for clients
        range6 2001:db8:1::129 2001:db8:1::250;

        # Range for clients requesting a temporary address
        range6 2001:db8:1::/64 temporary;
}
# A subnet that matches the interface IP address is required by isc-dhcp-server
subnet6 2001:db8:2::/64 {
        # Range for clients
        range6 2001:db8:2::129 2001:db8:2::254;

        # Range for clients requesting a temporary address
        range6 2001:db8:2::/64 temporary;
      
        # Prefix range for delegation to sub-routers
        prefix6 2001:db8:1:: 2001:db8:10:: /56;

}

Finally, restart the DHCP server.

sudo service isc-dhcp-server restart 

Testing

The host should be able to obtain an IP address from the pool we specified (2001:db8:1::129-250). Try to run dhclient and see if the host can get an IP address and IP prefix.

sudo dhclient -6 -N eth1       # for obtaining ip address
sudo dhclient -6 -P -N  eth1   # for obtaining ip address and prefix together

sudo dhclient -6 -r eth1       # for releasing ip address
sudo dhclient -6 -P -r  eth1   # for releasing prefix 

If something goes wrong, check /var/log/syslog for DHCP server log and run tcpdump on DHCP server to see if the DHCP packets from the host reach the server correctly.