Chapter 1 - Introduction to NetX Duo DHCP server

In NetX Duo, the application's IP address is one of the supplied parameters to the nx_ip_create service call. Supplying the IP address poses no problem if the IP address is known to the application, either statically or through user configuration. However, there are some instances where the application doesn't know or care what its IP address is. In such situations, a zero IP address should be supplied to the nx_ip_create function and the application establishes communication with its DHCP server to dynamically request and obtain an IP address.

Dynamic IP Address Assignment

The basic service used to obtain a dynamic IP address from the network is the Reverse Address Resolution Protocol (RARP). This protocol is similar to ARP, except it is designed to obtain an IP address for itself instead of finding the MAC address for another network node. The low-level RARP message is broadcast on the local network and it is the responsibility of a server on the network to respond with an RARP response, which contains a dynamically allocated IP address.

Although RARP provides a service for dynamic allocation of IP addresses, it has several shortcomings. The most glaring deficiency is that RARP only provides dynamic allocation of the IP address. In most situations, more information is necessary in order for a device to properly participate on a network. In addition to an IP address, most devices need the network mask and the gateway IP address. The IP address of a DNS server and other network information may also be needed. RARP does not have the ability to provide this information.

RARP Alternatives

In order to overcome the deficiencies of RARP, researchers developed a more comprehensive IP address allocation mechanism called the Bootstrap Protocol (BOOTP). This protocol has the ability to dynamically allocate an IP address and also provide additional important network information. However, BOOTP has the drawback of being designed for static network configurations. It does not allow for quick or automated address assignment.

This is where the Dynamic Host Configuration Protocol (DHCP) is extremely useful. DHCP is designed to extend the basic functionality of BOOTP to include completely automated IP server allocation and completely dynamic IP address allocation through "leasing" an IP address to a client for a specified period of time. DHCP can also be configured to allocate IP addresses in a static manner like BOOTP.

DHCP Messages

Although DHCP greatly enhances the functionality of BOOTP, DHCP uses the same message format as BOOTP and supports the same vendor options as BOOTP. In order to perform its function, DHCP introduces seven new DHCP-specific options, as follows:

  • DISCOVER (1) (sent by DHCP Client)
  • OFFER (2) (sent by DHCP Server)
  • REQUEST (3) (sent by DHCP Client)
  • DECLINE (4) (sent by DHCP Client)
  • ACK (5) (sent by DHCP Server)
  • NACK (6) (sent by DHCP Server)
  • RELEASE (7) (sent by DHCP Client)
  • INFORM (8) (sent by DHCP Client)
  • FORCERENEW (9) (sent by DHCP Client)

DHCP Communication

The DHCP Server utilizes the UDP protocol to receive DHCP Client requests and transmit responses. Prior to having an IP address, UDP messages carrying the DHCP information are sent and received by utilizing the IP broadcast address of 255.255.255.255. However, if the Client knows the address of the DHCP Server it may send DHCP messages using unicast messages.

DHCP Server State Machine

The DHCP Server is implemented as a two-step state machine processed by an internal DHCP thread that is created during nx_dhcp_server_create processing. The main states of DHCP Server are 1) receiving a DISCOVER message from a DHCP client and 2) receiving a REQUEST message.

Below are the corresponding DHCP Client states:

  • NX_DHCP_STATE_BOOT Starting with a previous IP address
  • NX_DHCP_STATE_INIT Starting with no previous IP address value
  • NX_DHCP_STATE_SELECTING Waiting for a response from any DHCP server
  • NX_DHCP_STATE_REQUESTING DHCP Server identified, IP address request sent
  • NX_DHCP_STATE_BOUND DHCP IP Address lease established
  • NX_DHCP_STATE_RENEWING DHCP IP Address lease renewal time elapsed, renewal requested
  • NX_DHCP_STATE_REBINDING DHCP IP Address lease rebind time elapsed, renewal requested
  • NX_DHCP_STATE_FORCERENEW DHCP IP Address lease established, force renewal by server or by application
  • NX_DHCP_STATE_FAILED No server found or no response from server received

DHCP Additional Parameters

The NetX Duo DHCP Server has a default list of option parameters which is set in the configurable option NX_DHCP_DEFAULT_SERVER_OPTION_LIST in nxd_dhcp_server.h to supply DHCP Clients with common/critical network configuration parameters e.g. router or gateway address and DNS server for the DHCP Client.

DHCP RFCs

NetX Duo DHCP Server is compliant with RFC2132, RFC2131, and related RFCs.