RFC 2131 DHCP

Dynamic Host Configuration Protocol

Definitions  DHCP Client – an Internet host using DHCP to obtain Configuration parameters  DHCP Server – an Internet host that returns configurations parameters to DHCP clients  BOOTP relay agent – an Internet host or router that passes DHCP messages between DHCP clients and DHCP servers  Binding – a collection of configuration parameters, including at least an IP address

Abstract The Dynamic Host Configuration Protocol provides a framework for passing configuration information to hosts on a TCP/IP network. DHCP is based on the Bootstrap Protocol, adding the capability of automatic allocation of reusable network addresses and additional configuration options. DHCP captures the behavior of BOOTP relay agents and DHCP participants can interoperate with BOOTP participants

DHCP Design Goals Clients should require no manual configuration Networks should require no manual configuration for individual clients DHCP should not require a server on each subnet A DHCP client must be prepared to receive multiple responses to a request for configuration parameters DHCP must coexist with statically configured hosts

Design goals specific to the transmission of network layer parameters  Guarantee that any specific network address will not be in use by more than one DHCP client at a time  Retain DHCP client configuration across DHCP client reboot  Retain DHCP client configuration across server reboots  Allow automated assignment of configuration parameters to new clients  Support fixed of permanent allocation of configuration parameters to specific clients

WARNING! A host should not act as a DHCP server unless explicitly configured to do so by a system administrator.

DHCP supports three mechanisms for IP address allocation.  Automatic Allocation – DHCP assigns a permanent IP address to a client  Manual Allocation – a clients IP address is assigned by the network administrator, and DHCP is used to simply convey the assigned address to the client  Dynamic Allocation – DHCP assigns and IP address to a client for a limited period of time

DHCP Packet

DHCP message type (option 53)  DHCPDISCOVER  DHCPOFFER  DHCPREQUEST  DHCPDECLINE  DHCPACK  DHCPNAK  DHCPRELEASE

DHCP consists of two components: Storage of network parameters for network clients A mechanism for allocation of network addresses to hosts.

Provide persistent storage of network parameters The first service provided by DHCP is to provide persistent storage of network parameters for network clients.

Key value pairs The DHCP service stores a key-value entry for each client where the key is some unique identifier and the value contains the configuration parameters for the client. Example key. IP-subnet-number, hardware address

Dynamic allocation of network addresses The second service provided by DHCP is the allocation of temporary or permanent network addresses to clients.