Dhcp Version 4

The DHCP version 4 process is responsible for IPv4 address configuration for cable modems, MTA-s and CPE devices. IPv4 addressing still necesery for all devices, because most of the Internet uses only IPv4 nowadays. Actual dhcpd.conf can be checked under "/ip6m_dhcp/dhcpd" url of a working IP6M instance.

The configuration file has the following sections:

header, global options
dhcp classes
subnets
hosts

Global options

option option-122 code 122 = string;
next-server 192.168.89.10;
omapi-port 7911;
log-facility local7;
log(info, concat("LEASE LOG|", pick-first-value(binary-to-ascii(10, 8, ".", leased-address), ""), "|", binary-to-ascii(16, 8, ":", option agent.remote-id), "|", binary-to-ascii(16, 8, ":", option dhcp-client-identifier)));
option option-120 code 120 = unsigned integer 32;
option option-120 0;
option log-servers 192.168.89.10;
option tftp-server-name "192.168.89.10";
option space packetcable;
option packetcable.primary-dhcp-server code 1 = ip-address;
option packetcable.secondary-dhcp-server code 2 = ip-address;
option packetcable.provisioning-server code 3 = { boolean, ip-address };
option packetcable.as-request-timers code 4 = { unsigned integer 32, unsigned integer 32, unsigned integer 32 };
option packetcable.ap-request-timers code 5 = { unsigned integer 32, unsigned integer 32, unsigned integer 32 };
option packetcable.kerberos-realm-name code 6 = text;
option packetcable.kerberos-use-tgt code 7 = boolean;
option packetcable.provisioning-timer code 8 = unsigned integer 8;
option packetcable.new-provisioning-server code 15 = { boolean, ip-address };
option packetcable-encapsulation code 122 = encapsulate packetcable;
option option-66 code 66 = string;
option packetcable.primary-dhcp-server 192.168.89.10;
option packetcable.secondary-dhcp-server 192.168.89.10;
option packetcable.provisioning-server true 192.168.89.10;
option packetcable.kerberos-realm-name "\005BASIC\0011\000";
option packetcable.kerberos-use-tgt false;
option time-servers 192.168.89.10;
option time-offset 0;
option domain-name "ip6m.net";
use-host-decl-names on;

Most of these lines define packetcable.* options (option-122 dhcp option), which is required for docsis voip mta configuration.

192.168.89.10 is a sample IPv4 address of the IP6M server.

DHCP Classes

Dhcp classes needed when ip address assignment cannot be static based on the client's mac address, because it doesn't known, but it has to be done based on other information, for example the dhcp forwarding agent that sent the dhcp request to the server. This is the typical situation when the server has to assign IP address to an unknown mac address behind a known cable modem.

An example of such a class is:

class "CMTS-1" {
  match if (
    (option agent.remote-id = 44:32:C8:21:9:E7) and (option dhcp-client-identifier != 1:44:32:C8:21:09:E8) and (option dhcp-client-identifier != ff:c8:21:09:e8:0:3:0:1:44:32:c8:21:9:e8) or
    (option agent.remote-id = 44:32:C8:41:8C:67) and (option dhcp-client-identifier != 1:44:32:C8:41:8C:68) and (option dhcp-client-identifier != ff:c8:41:8c:68:0:3:0:1:44:32:c8:41:8c:68) or
    (option agent.remote-id = 0:1E:6B:E6:B4:A5) and (option dhcp-client-identifier != 1:00:1E:6B:E6:B4:A6) and (option dhcp-client-identifier != ff:6b:e6:b4:a6:0:3:0:1:0:1e:6b:e6:b4:a6) or
    (option agent.remote-id = C:EE:E6:E5:35:2C) and (option dhcp-client-identifier != 1:0C:EE:E6:E5:35:2D) and (option dhcp-client-identifier != ff:e6:e5:35:2d:0:3:0:1:c:ee:e6:e5:35:2d) or
    (option agent.remote-id = 90:6E:BB:54:F0:D0) or
    (option agent.remote-id = 0:11:E6:F2:2C:94) and (option dhcp-client-identifier != 1:00:11:E6:F2:2C:95) and (option dhcp-client-identifier != ff:e6:f2:2c:95:0:3:0:1:0:11:e6:f2:2c:95)   );
}

Let's analyze the first row:

(option agent.remote-id = 44:32:C8:21:9:E7) and (option dhcp-client-identifier != 1:44:32:C8:21:09:E8) and (option dhcp-client-identifier != ff:c8:21:09:e8:0:3:0:1:44:32:c8:21:9:e8)

A dhcp request is matched if its forwarding agent is "44:32:C8:21:9:E7", and its dhcp client id (until Docsis 2.0 mac address) is not (!=) one of the other two identifiers. In this example the other two identifier is the ID of the voip MTA in the cable modem, so only other mac addresses will match in this class. The MTA will get it's IP address in an other class or static assignment.

Subnets

A typical subnet declaration with dynamic ip assigment to CPE devices based on the class above is the following:

subnet 10.2.0.0 netmask 255.255.0.0 {
  option subnet-mask 255.255.0.0;
  option broadcast-address 10.2.255.255;
  option routers 10.2.0.1;
  option time-servers 192.168.89.10;
  option time-offset 7200;
  option domain-name-servers 8.8.8.8, 8.8.4.4;
  max-lease-time 7200;
  default-lease-time 7200;
  server-identifier 192.168.89.10;
  pool {
    allow members of "CMTS-1";
    range 10.2.0.100 10.2.255.254;
  }
}

Of course, every class and other networks need to have their own subnet declaration. A minimal subnet declaration like this:

subnet 192.168.89.0 netmask 255.255.255.0 {
}

is needed for the server local network, even if we don't want to server any clients in this subnet.

Hosts

Docsis cable modems, MTA-s have their own host records as follows:

host modem-2 {
	hardware ethernet 44:32:c8:21:09:e7;
	fixed-address 10.1.0.3;
	filename "cm/cm-2.cfg";
}
host modem-3 {
	hardware ethernet 44:32:c8:41:8c:67;
	fixed-address 10.1.0.4;
	filename "cm/cm-3.cfg";
}
host modem-4 {
	hardware ethernet 00:1e:6b:e6:b4:a5;
	fixed-address 10.1.0.5;
	filename "cm/cm-4.cfg";
}

These records fairly simple, the request that matches the ethernet mac address gets the ip address defined in the "fixed-address" field and a "filename" option that is required by cable modems, where they find their configuration file to download by TFTP. (See next section)

A complete, working dhcp.conf file is generated by IP6M if you install the "ip6m_testdata" module, or add your own configuration.

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