The Dynamic Host Configuration Protocol (DHCP) is a network protocol used to configure devices that are connected to a network (known as hosts) so they can communicate on that network using the Internet Protocol (IP). It involves clients and a server operating in a client-server model.
One common example of its use is in a typical personal home local area network (LAN). In this case, the server is a router while the clients are hosts (eg. personal computers, smart phones, printers, etc.). The router receives the configuration information through a modem from an internet service provider, which also operates DHCP servers with this router as one of the clients. The clients request configuration settings using the DHCP protocol such as an IP address, a default route and one or more DNS server addresses. Once the client implements these settings, the host is able to communicate on that internet.
The DHCP server maintains a database of available IP addresses and configuration information. When the server receives a request from a client, the DHCP server determines the network to which the DHCP client is connected, and then allocates an IP address or prefix that is appropriate for the client, and sends configuration information appropriate for that client. DHCP servers typically grant IP addresses to clients only for a limited interval. DHCP clients are responsible for renewing their IP address before that interval has expired, and must stop using the address once the interval has expired, if they have not been able to renew it.
DHCP is used for IPv4 and IPv6. While both versions serve the same purpose, the details of the protocol for IPv4 and IPv6 are sufficiently different that they may be considered separate protocols.
Hosts that do not use DHCP for address configuration may still use it to obtain other configuration information. Alternatively, IPv6 hosts may use stateless address autoconfiguration. IPv4 hosts may use link-local addressing to achieve limited local connectivity.
Nontechnical overview
DHCP allows computers (clients) to be assigned settings from a server in a client-server model. DHCP is ubiquitous in modern networks and is used in home networks as well as larger campus networks.DHCP is often used together with network address translation (NAT). Network address translation separates public (external) and private (internal) IP addresses. In home networks, the ISP server may assign a globally unique external IP address to a home router or modem and this IP address is used in internet communications. The router will then assign internal IP addresses to the clients connected to it, allowing the clients to broadcast only the external IP address. This improves security by limiting access to devices and also helps to conserve IPv4 addresses. To directly access a device from outside the network, port forwarding may be used.
Technical overview
Dynamic Host Configuration Protocol automates network-parameter assignment to network devices from one or more DHCP servers. Even in small networks, DHCP is useful because it makes it easy to add new machines to the network.When a DHCP-configured client (a computer or any other network-aware device) connects to a network, the DHCP client sends a broadcast query requesting necessary information to a DHCP server. The DHCP server manages a pool of IP addresses and information about client configuration parameters such as default gateway, domain name, the name servers, other servers such as time servers, and so forth. On receiving a valid request, the server assigns the computer an IP address, a lease (length of time the allocation is valid), and other IP configuration parameters, such as the subnet mask and the default gateway. The query is typically initiated immediately after booting, and must complete before the client can initiate IP-based communication with other hosts. Upon disconnecting, the IP address is returned to the pool for use by another computer. This way, many other computers can use the same IP address within minutes of each other.
Because the DHCP protocol must work correctly even before DHCP clients have been configured, the DHCP server and DHCP client usually must be connected to the same network link. In larger networks, this is not practical. On such networks, each network link contains one or more DHCP relay agents. These DHCP relay agents receive messages from DHCP clients and forward them to DHCP servers. DHCP servers send responses back to the relay agent, and the relay agent then sends these responses to the DHCP client on the local network link.
Depending on implementation, the DHCP server may have three methods of allocating IP-addresses:
- dynamic allocation: A network administrator assigns a range of IP addresses to DHCP, and each client computer on the LAN is 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.
- automatic allocation: The DHCP server permanently assigns a free IP address to a requesting client from the range defined by the administrator. This is like dynamic allocation, but the DHCP server keeps a table of past IP address assignments, so that it can preferentially assign to a client the same IP address that the client previously had.
- static allocation: The DHCP server allocates an IP address based on a table with MAC address/IP address pairs, which are manually filled in (perhaps by a network administrator). Only clients with a MAC address listed in this table will be allocated an IP address. This feature, which is not supported by all DHCP servers, is variously called Static DHCP Assignment by DD-WRT, fixed-address by the dhcpd documentation, Address Reservation by Netgear, DHCP reservation or Static DHCP by Cisco and Linksys, and IP reservation or MAC/IP binding by various other router manufacturers.
Technical details
DHCP uses the same two ports assigned by IANA for BOOTP: destination UDP port 67 for sending data to the server, and UDP port 68 for data to the client. DHCP communications are connectionless in nature.DHCP operations fall into four basic phases: IP discovery, IP lease offer, IP request, and IP lease acknowledgment. These points are often abbreviated as DORA (Discovery, Offer, Request, Acknowledgment).
DHCP clients and servers on the same subnet communicate via UDP broadcasts, initially. If the client and server are on different subnets, a DHCP Helper or DHCP Relay Agent may be used. Clients requesting renewal of an existing lease may communicate directly via UDP unicast, since the client already has an established IP address at that point.
DHCP discovery
The client broadcasts messages on the physical subnet to discover available DHCP servers. Network administrators can configure a local router to forward DHCP packets to a DHCP server which resides on a different subnet. This client implementation creates a User Datagram Protocol (UDP) packet with the broadcast destination of 255.255.255.255 or the specific subnet broadcast address.A DHCP client can also request its last-known IP address. If the client remains connected to a network for which this IP is valid, the server may grant the request. Otherwise, it depends whether the server is set up as authoritative or not. An authoritative server will deny the request, making the client ask for a new IP address immediately. A non-authoritative server simply ignores the request, leading to an implementation-dependent timeout for the client to give up on the request and ask for a new IP address.
DHCP offer
When a DHCP server receives an IP lease request from a client, it reserves an IP address for the client and extends an IP lease offer by sending a DHCPOFFER message to the client. This message contains the client's MAC address, the IP address that the server is offering, the subnet mask, the lease duration, and the IP address of the DHCP server making the offer.The server determines the configuration based on the client's hardware address as specified in the CHADDR (Client Hardware Address) field. Here the server, 192.168.1.1, specifies the client's IP address in the YIADDR (Your IP Address) field.
DHCP request
In response to the DHCP offer, the client replies with a DHCP request, unicast to the server, requesting the offered address. A client can receive DHCP offers from multiple servers, but it will accept only one DHCP offer. Based on the Transaction ID field in the request, servers are informed whose offer the client has accepted. When other DHCP servers receive this message, they withdraw any offers that they might have made to the client and return the offered address to the pool of available addresses. In some cases DHCP request message is broadcast, instead of being unicast to a particular DHCP server, because the DHCP client has still not received an IP address. Also, this way one message can let all other DHCP servers know that another server will be supplying the IP address without missing any of the servers with a series of unicast messages.DHCP acknowledgement
When the DHCP server receives the DHCPREQUEST message from the client, the configuration process enters its final phase. The acknowledgement phase involves sending a DHCPACK packet to the client. This packet includes the lease duration and any other configuration information that the client might have requested. At this point, the IP configuration process is completed.The protocol expects the DHCP client to configure its network interface with the negotiated parameters.
After the client obtains an IP address, the client may use the Address Resolution Protocol (ARP) to prevent IP conflicts caused by overlapping address pools of DHCP servers.
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