Dynamic Host Configuration Protocol

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Dynamic Host Configuration Protocol (DHCP) is a client-server networking protocol. Most organizations use DHCP. A DHCP server provides configuration parameters specific to the DHCP client host requesting, generally, information required by the host to participate on the Internet network. DHCP also provides a mechanism for allocation of IP addresses to hosts.

DHCP appeared as a standard protocol in October 1993. RFC 2131 (http://www.faqs.org/rfcs/rfc2131.html) provides the latest (March 1997) DHCP definition.

The latest work on a protocol describing DHCPv6, DHCP in a IPv6 environment, was published in July 2003 as RFC 3315 (http://www.faqs.org/rfcs/rfcs3315.html)

Contents

IP Address Allocation

The DHCP protocol provides three methods of IP address allocation:

  • manual allocation, where the allocation is based on a table with MAC address - IP address pairs manually filled by the server admin. Only requesting clients with a MAC address listed in this table get the IP address according to the table.
  • automatic allocation, where a free IP address of a range given by the admin is permanently assigned to a requesting client.
  • dynamic allocation, the only method which provides dynamic reuse of IP addresses. A network administrator assigns a range of IP addresses to DHCP, and each client computer on the LAN has its TCP/IP software configured to request an IP address from the DHCP server when that client computer's network interface card starts up. The request-and-grant process uses a lease concept with a controllable time period. This eases the network installation procedure on the client computer side considerably.

Some DHCP server implementations can update the DNS name associated with the client hosts to reflect the new IP address by way of the DNS update protocol which was established with RFC 2136 (http://www.faqs.org/rfcs/rfc2136.html).

Client Configuration Parameters

A DHCP server can provide optional configurations to the client. DHCP Options are defined in RFC 2132 (http://www.ietf.org/rfc/rfc2132.txt)

List of configurable options:

Implementations

Microsoft introduced DHCP on their NT server with Windows NT version 3.5 in late 1994. Despite being called a new feature from Microsoft, it was not invented by them.

The Internet Software Consortium published DHCP software distributions for Unix variants with version 1.0.0 of the ISC DHCP Server released on December 6, 1997 and a more RFC compliant version 2.0 on June 22, 1999. The software is available at http://www.isc.org/sw/dhcp/

Other major implementations included Cisco with a DHCP server made available in Cisco IOS 12.0 in February 1999 and Sun who added DHCP support in the July 2001 release of Solaris 8.

A free DHCP server for the Microsoft Windows platform is available at http://tftpd32.jounin.net/

Usage

DHCP is used by most cable internet in the U.S. to allocate IP addresses. DSL providers in the US rarely use DHCP, using PPPoE instead. In addition, several routers provide DHCP support for networks of up to 255 computers, for assigning private IP addresses.

Protocol Anatomy

DHCP uses the same two IANA assigned ports for BOOTP: 67/udp for the server side, and 68/udp for the client side.

DHCP Discover

The client broadcasts on the local physical subnet to find available servers. The local router can be configured to forward DHCP packets to a DHCP server on a different subnet. This client implementation creates a UDP packet with the broadcast destination of 255.255.255.255 and also requests its last known IP address of 192.168.1.100 although this is not necessary and may be ignored by the server.

DHCP Offer

The server determines the configuration based on the client's hardware address that is specified in the CHADDR field. Here the server, 192.168.1.1, specifies the IP address in the YIADDR field.

DHCP Request

The client selects a configuration out of the DHCP Offer packets it received. Again, this client requests the 192.168.1.100 address that the server specified.

DHCP Acknowledge

The server acknowledges the request and broadcasts that on the local subnet. The client is expected to configure its network interface with the supplied options.

DHCPDISCOVER
UDP Src=0.0.0.0 Dest=255.255.255.255
OPHTYPEHLENHOPS
0x010x010x060x00
XID
0x3903F326
SECSFLAGS
0x00000x0000
CIADDR
0x00000000
YIADDR
0x00000000
SIADDR
0x00000000
GIADDR
0x00000000
CHADDR
0x00053C04
0x8D590000
0x00000000
0x00000000
192 octets of 0's. BOOTP legacy
DHCP option 53: DHCP Discover
DHCP option 50: 192.168.1.100 requested
DHCPOFFER
UDP Src=192.168.1.1 Dest=255.255.255.255
OPHTYPEHLENHOPS
0x020x010x060x00
XID
0x3903F326
SECSFLAGS
0x00000x0000
CIADDR
0x00000000
YIADDR
0xC0A80164
SIADDR
0x00000000
GIADDR
0x00000000
CHADDR
0x00053C04
0x8D590000
0x00000000
0x00000000
192 octets of 0's. BOOTP legacy
DHCP option 53: DHCP Offer
DHCP option 1: 255.255.255.0 subnet mask
DHCP option 3: 192.168.1.1 router
DHCP option 51: 1 day IP lease time
DHCP option 54: 192.168.1.1 DHCP server
DHCPREQUEST
UDP Src=0.0.0.0 Dest=255.255.255.255
OPHTYPEHLENHOPS
0x010x010x060x00
XID
0x3903F326
SECSFLAGS
0x00000x0000
CIADDR
0x00000000
YIADDR
0x00000000
SIADDR
0x00000000
GIADDR
0x00000000
CHADDR
0x00053C04
0x8D590000
0x00000000
0x00000000
192 octets of 0's. BOOTP legacy
DHCP option 53: DHCP Request
DHCP option 50: 192.168.1.100 requested
DHCPACK
UDP Src=192.168.1.1 Dest=255.255.255.255
OPHTYPEHLENHOPS
0x020x010x060x00
XID
0x3903F326
SECSFLAGS
0x00000x0000
CIADDR
0x00000000
YIADDR
0xC0A80164
SIADDR
0x00000000
GIADDR
0x00000000
CHADDR
0x00053C04
0x8D590000
0x00000000
0x00000000
192 octets of 0's. BOOTP legacy
DHCP option 53: DHCP ACK
DHCP option 1: 255.255.255.0 subnet mask
DHCP option 3: 192.168.1.1 router
DHCP option 51: 1 day IP lease time
DHCP option 54: 192.168.1.1 DHCP server

See also

External links

  • DHCP Server Security (http://www.windowsecurity.com/articles/DHCP-Security-Part1.html) - This article looks at the different types of threats faced by DHCP servers and countermeasures for mitigating these threats.cs:DHCP

da:DHCP de:Dynamic Host Configuration Protocol es:DHCP fr:Dynamic Host Configuration Protocol it:DHCP he:DHCP nl:Dynamic Host Configuration Protocol ja:Dynamic Host Configuration Protocol pl:DHCP ru:DHCP fi:DHCP sv:DHCP zh:DHCP

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