"DHCP" redirects here. This article is about the networking protocol. For other uses, see DHCP (disambiguation).
Dynamic Host Configuration Protocol (DHCP) is a protocol used by networked devices (clients) to obtain various parameters necessary for the clients to operate in an Internet Protocol (IP) network. By using this protocol, system administration workload greatly decreases, and devices can be added to the network with minimal or no manual configurations.
Dynamic Host Configuration Protocol is a way to administer network parameter assignment at a single DHCP server, or a group of such servers arranged in a fault-tolerant manner. Even in a network which has a few machines, Dynamic Host Configuration Protocol is useful because a machine can be added to the local network with little effort.
Even for servers whose addresses rarely change, DHCP is recommended for setting their addresses, so if the servers need to be readdressed (RFC2071), the changes need to be made in as few places as possible. For devices, such as routers and firewalls, that should not use DHCP, it can be useful to put Trivial File Transfer Protocol (TFTP) or SSH servers on the same machine that runs DHCP, again to centralize administration.
DHCP is also useful for directly assigning addresses to servers and desktop machines, and, through a Point-to-Point Protocol (PPP) proxy, for dialup and broadband on-demand hosts, as well as for residential Network address translation (NAT) gateways and routers. DHCP is usually not appropriate for infrastructure such as non-edge routers and DNS servers.
[edit] History
DHCP emerged as a standard protocol in October 1993 as defined in RFC 1531, succeeding the BOOTP protocol. The next RFC was 2131, released in 1997. The current DHCP definition can be found in RFC 2131, while a proposed standard for DHCP over IPv6 (DHCPv6) can be found in RFC 3315.
[edit] Basic Protocol Operation
The Dynamic Host Configuration Protocol (DHCP) automates the assignment of IP addresses, subnet masks, default gateway, and other IP parameters. [1]
When a DHCP-configured client (be it a computer or any other network aware device) connects to a network, the DHCP client sends a broadcast query requesting necessary information from a DHCP server. The DHCP server manages a pool of IP addresses and information about client configuration parameters such as the default gateway, the domain name, the DNS servers, other servers such as time servers, and so forth. Upon receipt of a valid request the server will assign the computer an IP address, a lease (the length of time for which the allocation is valid), and other TCP/IP configuration parameters, such as the subnet mask and the default gateway. The query is typically initiated immediately after booting and must be completed before the client can initiate IP-based communication with other hosts.
DHCP provides three modes for allocating IP addresses. The best-known mode is dynamic, in which the client is provided a "lease" on an IP address for a period of time. Depending on the stability of the network, this could range from hours (a wireless network at an airport) to months (for desktops in a wired lab). At any time before the lease expires, the DHCP client can request renewal of the lease on the current IP address. A properly-functioning client will use the renewal mechanism to maintain the same IP address throughout its connection to a single network, otherwise it may risk losing its lease while still connected, thus disrupting network connectivity while it renegotiates with the server for its original or a new IP address.
The two other modes for allocation of IP addresses are automatic (also known as DHCP Reservation), in which the address is permanently assigned to a client, and manual, in which the address is selected by the client (manually by the user or any other means) and the DHCP protocol messages are used to inform the server that the address has been allocated.
The automatic and manual methods are generally used when finer-grained control over IP address is required (typical of tight firewall setups), although typically a firewall will allow access to the range of IP addresses that can be dynamically allocated by the DHCP server.
[edit] Security
Due to its standardization before Internet security became an issue, the basic DHCP protocol does not include any security provisions, potentially exposing it to two types of attacks:[2]
Unauthorized DHCP Servers: as you can not specify the server you want, an unauthorized server can respond to client requests, sending the client network configuration values that are beneficial to a hijacker. As an example, a hacker can configure the DHCP server to configure clients to a DNS server which has been poisoned.
Unauthorized DHCP Clients: By masquerading as a legitimate client, an unauthorized client can gain access to network configuration and an IP address on a network it should otherwise not be allowed to utilize. Also, by flooding the DHCP server with requests for IP addresses, it is possible for an attacker to exhaust the pool of available IP addresses, disrupting normal network activity (a denial of service attack).
To combat these threats RFC 3118 ("Authentication for DHCP Messages") introduced authentication information into DHCP messages allowing clients and servers to reject information from invalid sources. Although support for this protocol is widespread, a large number of clients and servers still do not fully support authentication, thus forcing servers to support clients that do not support this feature. As a result, other security measures are usually implemented around the DHCP server (such as IPsec) to ensure that only authenticated clients and servers are granted access to the network.
Wherever possible, DHCP-assigned addresses should be dynamically linked to a secure DNS server, to allow troubleshooting by name rather than by a potentially unknown address. Effective DHCP-DNS linkage requires having a file of either MAC addresses or local names that will be sent to DNS that uniquely identifies physical hosts, IP addresses, and other parameters such as the default gateway, subnet mask, and IP addresses of DNS servers from a DHCP server. The DHCP server ensures that all IP addresses are unique, i.e., no IP address is assigned to a second client while the first client's assignment is valid (its lease has not expired). Thus IP address pool management is done by the server and not by a network administrator.
[edit] IP address allocation
Depending on implementation, the DHCP server has three methods of allocating IP-addresses (WARNING--the terminology below contradicts the terminology above in #Basic_Protocol_Operation):
dynamic allocation: A network administrator assigns a range of IP addresses to DHCP, and each client computer on the LAN has its IP software configured to request an IP address from the DHCP server during network initialization. The request-and-grant process uses a lease concept with a controllable time period, allowing the DHCP server to reclaim (and then reallocate) IP addresses that are not renewed (dynamic re-use of IP addresses).
automatic allocation: The DHCP server permanently assigns a free IP address to a requesting client from the range defined by the administrator.
manual allocation: The DHCP server allocates an IP address based on a table with MAC address - IP address pairs manually filled in by the server administrator. Only requesting clients with a MAC address listed in this table will be allocated an IP address.
Some DHCP server software can manage hosts by more than one of the above methods. For example, the known hosts on the network can be assigned an IP address based on their MAC address (manual allocation) whereas "guest" computers (such as laptops via WiFi) are allocated a temporary address out of a pool compatible with the network to which they're attached (dynamic allocation).
What is DHCP?
DHCP stands for "Dynamic Host Configuration Protocol".
What is DHCP's purpose?
DHCP's purpose is to enable individual computers on an IP network to extract their configurations from a server (the 'DHCP server') or servers, in particular, servers that have no exact information about the individual computers until they request the information. The overall purpose of this is to reduce the work necessary to administer a large IP network. The most significant piece of information distributed in this manner is the IP address.
Can DHCP work with Appletalk or IPX?
No, it is too tied to IP. Furthermore, they don't need it since they have always had automated mechanisms for assigning their own network addresses.
Who Created It? How Was It Created?
DHCP was created by the Dynamic Host Configuration Working Group of the Internet Engineering Task Force (IETF; a volunteer organization which defines protocols for use on the Internet). As such, it's definition is recorded in an Internet RFC and the Internet Activities Board (IAB) is asserting its status as to Internet Standardization. As of this writing (June 1998), DHCP is an Internet Draft Standard Protocol and is Elective. BOOTP is an Internet Draft Standard Protocol and is Recommended. For more information on Internet standardization, see RFC2300 (May 1998)
How is it different than BOOTP or RARP?
DHCP is based on BOOTP and maintains some backward compatibility. The main difference is that BOOTP was designed for manual pre-configuration of the host information in a server database, while DHCP allows for dynamic allocation of network addresses and configurations to newly attached hosts. Additionally, DHCP allows for recovery and reallocation of network addresses through a leasing mechanism.
RARP is a protocol used by Sun and other vendors that allows a computer to find out its own IP number, which is one of the protocol parameters typically passed to the client system by DHCP or BOOTP. RARP doesn't support other parameters and using it, a server can only serve a single LAN. DHCP and BOOTP are designed so they can be routed.
How is it different than VLANs?
