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51-587: While some parts of BOOTP have been effectively superseded by the Dynamic Host Configuration Protocol (DHCP), which adds the feature of leases, parts of BOOTP are used to provide service to the DHCP protocol. Some DHCP servers also provide the legacy BOOTP functionality. When a network-connected computer boots up, its IP stack broadcasts BOOTP network messages requesting an IP address assignment. A BOOTP configuration server replies to

102-482: A BOOTP client is started, it has no IP address, so it broadcasts a message containing its MAC address onto the network. This message is called a “BOOTP request”, and it is picked up by the BOOTP server, which replies to the client with the following information that the client needs: When the client receives this information from the BOOTP server, it configures and initializes its TCP/IP protocol stack, and then connects to

153-483: A DHCP client. The information is a variable-length string of characters or octets which has a meaning specified by the vendor of the DHCP client. One method by which a DHCP client can communicate to the server that it is using a certain type of hardware or firmware is to set a value in its DHCP requests called the Vendor Class Identifier (VCI) (Option 60). The value to which this option is set gives

204-482: A DHCP server. In Unix-like systems this client-level refinement typically takes place according to the values in the configuration file /etc/dhclient.conf . Options are octet strings of varying length. This is called Type–length–value encoding. The first octet is the option code, the second octet is the number of following octets and the remaining octets are code dependent. For example, the DHCP message-type option for an offer would appear as 0x35, 0x01, 0x02, where 0x35

255-466: 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. When the server is reusing an IP address from its pool, it may first check (using ping ) to see if it

306-412: A denial-of-service attack, preventing the client from gaining access to network connectivity, or as a man-in-the-middle attack . Because the DHCP server provides the DHCP client with server IP addresses, such as the IP address of one or more DNS servers, an attacker can convince a DHCP client to do its DNS lookups through its own DNS server, and can therefore provide its own answers to DNS queries from

357-463: A limited geographical area. The networking equipments ( switches , routers ) and transmission media ( optical fiber , copper plant, Cat5 cabling etc.) are almost entirely owned by the campus tenant / owner: an enterprise, university, government etc. A campus area network is larger than a local area network but smaller than a metropolitan area network (MAN) or wide area network (WAN). College or university campus area networks often interconnect

408-416: A network device, capable of routing between the client's subnet and the subnet of the DHCP server. The DHCP client broadcasts on the local link; the relay agent receives the broadcast and transmits it to one or more DHCP servers using unicast . The IP addresses of the DHCP servers are manually configured in the relay agent. The relay agent stores its own IP address, from the interface on which it has received

459-533: A request to the DHCP server to release the DHCP information and the client deactivates its IP address. As client devices usually do not know when users may unplug them from the network, the protocol does not mandate the sending of DHCP Release . A DHCP server can provide optional configuration parameters to the client. RFC 2132 describes the available DHCP options defined by Internet Assigned Numbers Authority (IANA) - DHCP and BOOTP PARAMETERS. A DHCP client can select, manipulate and overwrite parameters provided by

510-409: A request. If the client and server are in different Broadcast Domains , 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. Additionally, there is a BROADCAST flag (1 bit in 2 byte flags field, where all other bits are reserved and so are set to 0)

561-405: A server: The client moves through DHCP states depending on how the server responds to the messages that the client sends. The DHCP ensures reliability in several ways: periodic renewal, rebinding, and failover. DHCP clients are allocated leases that last for some period of time. Clients begin to attempt to renew their leases once half the lease interval has expired. They do this by sending

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612-409: A single DHCP server may service the entire network when aided by DHCP relay agents located on the interconnecting routers. Such agents relay messages between DHCP clients and DHCP servers located on different subnets. Depending on implementation, the DHCP server may have three methods of allocating IP addresses: DHCP services are used for Internet Protocol version 4 (IPv4) and IPv6 . The details of

663-460: A unicast DHCPREQUEST message to the DHCP server that granted the original lease. If that server is down or unreachable, it will fail to respond to the DHCPREQUEST . However, in that case the client repeats the DHCPREQUEST from time to time, so if the DHCP server comes back up or becomes reachable again, the DHCP client will succeed in contacting it and renew the lease. If the DHCP server

714-469: A unique value, traditionally a 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 DHCP server may also take notice of the hardware-level MAC address (as specified in the CHADDR field). This field must be used to identify the client, if no Client ID is provided in the DHCP packet. The DHCP server determines

765-434: A university campus network, a corporate campus network serves to connect buildings. Examples of such are the networks at Googleplex and Microsoft 's campus. Campus networks are normally interconnected with high speed Ethernet links operating over optical fiber such as gigabit Ethernet and 10 Gigabit Ethernet . The range of CAN is 1 km to 5 km. If two buildings have the same domain and they are connected with

816-599: A variety of buildings, including administrative buildings, academic buildings, laboratories, university libraries, or student centers, residence halls , gymnasiums , and other outlying structures, like conference centers , technology centers, and training institutes. Early examples include the Stanford University Network at Stanford University , Project Athena at MIT , and the Andrew Project at Carnegie Mellon University . Much like

867-645: A wide range of extra configuration parameters to IP clients, including platform-specific parameters. Four years later, the DHCPINFORM message type (used for WPAD ) and other small changes were added. This definition, from 1997, remains the core of the standard for IPv4 networks. DHCPv6 was initially defined in 2003. After updates by many subsequent RFCs, its definition was replaced in 2018, where prefix delegation and stateless address autoconfiguration were now merged. Internet Protocol (IP) defines how devices communicate within and across local networks on

918-456: Is 6 octets long. The CHADDR is set to the MAC address used by the client. Some options are set as well. When a DHCP server receives a DHCPDISCOVER message from a client, which is an IP address lease request, the DHCP server reserves an IP address for the client and makes a lease offer by sending a DHCPOFFER message to the client. This message may contain the client's Client ID (Option 61, containing

969-543: Is being managed, DHCP clients communicate directly with DHCP servers. However, DHCP servers can also provide IP addresses for multiple subnets. In this case, a DHCP client that has not yet acquired an IP address cannot communicate directly with a DHCP server not on the same subnet, as the client's broadcast can only be received on its own subnet. In order to allow DHCP clients on subnets not directly served by DHCP servers to communicate with DHCP servers, DHCP relay agents can be installed on these subnets. A DHCP relay agent runs on

1020-461: Is code 53 for "DHCP message type", 0x01 means one octet follows and 0x02 is the value of "offer". The following tables list the available DHCP options. This table lists the DHCP message types, documented in RFC 2132, RFC 3203, RFC 4388, RFC 6926 and RFC 7724. These codes are the value in the DHCP extension 53, shown in the table above. An option exists to identify the vendor and functionality of

1071-406: Is free to be used. If this probe finds another computer using that address, the client should broadcast a DHCPDECLINE to the DHCP server(s). A DHCP client may request more information than the server sent with the original DHCPOFFER. The client may also request repeat data for a particular application. For example, browsers use DHCP Inform to obtain web proxy settings via WPAD . The client sends

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1122-410: Is not taken already. This may happen if a host is configured manually with an IP address that lies within the DHCP scope. Before claiming an IP address, the client should probe the newly received address (e.g. with ARP ), in order to find if there is another host present in the network with the proposed IP address. If there is no reply, this address does not conflict with that of another host, so it

1173-466: Is unreachable for an extended period of time, the DHCP client will attempt to rebind, by broadcasting its DHCPREQUEST rather than unicasting it. Because it is broadcast , the DHCPREQUEST message will reach all available DHCP servers. If some other DHCP server is able to renew the lease, it will do so at this time. In order for rebinding to work, when the client successfully contacts a backup DHCP server, that server must have accurate information about

1224-618: The Bootstrap Protocol (BOOTP) defined in September 1985. This introduced the concept of a relay agent, which allowed the forwarding of BOOTP packets across networks, allowing one central BOOTP server to serve hosts on many IP subnets. DHCP was first defined in October 1993. It is based on BOOTP, but can dynamically allocate IP addresses from a pool and reclaim them when they are no longer in use. It can also be used to deliver

1275-526: The server identification option in the DHCPREQUEST message, indicating the server whose offer the client has selected. When other DHCP servers receive this message, they withdraw any offers that they have made to the client and return their offered IP address to the pool of available addresses. When the DHCP server receives the DHCPREQUEST message from the client, the configuration process enters its final phase. The acknowledgement phase involves sending

1326-405: The DHCP protocol is commonly called DHCPv6 . The Reverse Address Resolution Protocol (RARP) was defined in 1984 for the configuration of simple devices, such as diskless workstations , with a suitable IP address. Acting in the data link layer , it made implementation difficult on many server platforms. It required that a server be present on each individual network link. RARP was superseded by

1377-517: The DHCP server a hint about any required extra information that this client needs in a DHCP response. Some types of set-top boxes set the VCI to inform the DHCP server about the hardware type and functionality of the device. An Aruba campus wireless access point , for example, supplies value 'ArubaAP' as option 60 in its DHCPDISCOVER message. The DHCP server can then augment its DHCPOFFER with an IP address of an Aruba wireless controller in option 43, so

1428-466: The Internet. A DHCP server can manage IP settings for devices on its local network, e.g., by assigning IP addresses to those devices automatically and dynamically. DHCP operates based on the client–server model . When a computer or other device connects to a network, the DHCP client software sends a DHCP broadcast query requesting the necessary information. Any DHCP server on the network may service

1479-401: The access point knows where to register itself. Setting a VCI by the client allows a DHCP server to differentiate between client machines and process the requests from them appropriately. The relay agent information option (option 82) specifies container for attaching sub-options to DHCP requests transmitted between a DHCP relay and a DHCP server. In small networks, where only one IP subnet

1530-520: The client can use to indicate in which way (broadcast or unicast) it can receive the DHCPOFFER: 0x8000 for broadcast, 0x0000 for unicast. Usually, the DHCPOFFER is sent through unicast. For those hosts which cannot accept unicast packets before IP addresses are configured, this flag can be used to work around this issue. The DHCP client broadcasts a DHCPDISCOVER message on the network subnet using

1581-467: The client may set the broadcast bit in the FLAGS field when sending a DHCPDISCOVER packet. The relay agent will use the 255.255.255.255 broadcast IP address (and the clients MAC address) to inform the client of the server's DHCPOFFER. The communication between the relay agent and the DHCP server typically uses both a source and destination UDP port of 67. A DHCP client can receive these messages from

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1632-502: The client's binding. Maintaining accurate binding information between two servers is a complicated problem; if both servers are able to update the same lease database, there must be a mechanism to avoid conflicts between updates on the independent servers. A proposal for implementing fault-tolerant DHCP servers was submitted to the Internet Engineering Task Force, but never formalized. If rebinding fails,

1683-600: The client's broadcast, in the GIADDR field of the DHCP packet. The DHCP server uses the GIADDR-value to determine the subnet, and subsequently the corresponding address pool, from which to allocate an IP address. When the DHCP server replies to the client, it sends the reply to the GIADDR-address, again using unicast. The relay agent then retransmits the response on the local network, using unicast (in most cases) to

1734-435: The client. This in turn allows the attacker to redirect network traffic through itself, allowing it to eavesdrop on connections between the client and network servers it contacts, or to simply replace those network servers with its own. Campus network A campus network , campus area network , corporate area network or CAN is a computer network made up of an interconnection of local area networks (LANs) within

1785-554: The configuration based on the client's hardware address as specified in the CHADDR (client hardware address) field. In the following example the server ( 192.168.1.1 ) specifies the client's IP address in the YIADDR (your IP address) field. In response to the DHCP offer, the client replies with a DHCPREQUEST message, broadcast to the server, requesting the offered address. A client can receive DHCP offers from multiple servers, but it will accept only one DHCP offer. The client must send

1836-472: The destination address 255.255.255.255 (limited broadcast) or the specific subnet broadcast address (directed broadcast). A DHCP client may also request an IP address in the DHCPDISCOVER, which the server may take into account when selecting an address to offer. For example, if HTYPE is set to 1, to specify that the medium used is Ethernet , HLEN is set to 6 because an Ethernet address (MAC address)

1887-481: The entire network and for the time period for which the allocation ( lease ) is valid. A DHCP client typically queries this information immediately after booting , and periodically thereafter before the expiration of the information. When a DHCP client refreshes an assignment, it initially requests the same parameter values, but the DHCP server may assign a new address based on the assignment policies set by administrators. On large networks that consist of multiple links,

1938-419: The lease will eventually expire. When the lease expires, the client must stop using the IP address granted to it in its lease. At that time it will restart the DHCP process from the beginning by broadcasting a DHCPDISCOVER message. Since its lease has expired, it will accept any IP address offered to it. Once it has a new IP address (presumably from a different DHCP server) it will once again be able to use

1989-532: The local network using standard IP routing, so that one central BOOTP server could serve hosts on many subnets. An increasing set of BOOTP vendor information extensions was defined to supply BOOTP clients of relevant information about the network, like default gateway , name server IP address , the domain name , etcetera. With the advent of the Dynamic Host Configuration Protocol , the BOOTP vendor information extensions were incorporated as DHCP option fields, to allow DHCP servers to also serve BOOTP clients. When

2040-505: The network location of their boot image , in addition to the IP address assignment. Enterprises used it to roll out a pre-configured client (e.g., Windows ) installation to newly installed PCs. Initially requiring the use of a boot floppy disk to establish the initial network connection, manufacturers of network interfaces later embedded the protocol in the firmware of interface cards as well as system boards with on-board network interfaces, thus allowing direct network booting. The BOOTP

2091-404: The network using a client–server architecture. The technology eliminates the need for individually configuring network devices manually, and consists of two network components, a centrally installed network DHCP server and client instances of the protocol stack on each computer or device. When connected to the network, and periodically thereafter, a client requests a set of parameters from

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2142-426: The network. However, since its IP address has changed, any ongoing connections will be broken. The basic methodology of DHCP was developed for networks based on Internet Protocol version 4 (IPv4). Since the development and deployment of IPv6 networks, DHCP has also been used for assigning parameters in such networks, despite the inherent features of IPv6 for stateless address autoconfiguration . The IPv6 version of

2193-462: The newly reserved IP address, in an Ethernet frame directed to the client's MAC address. The client should accept the packet as its own, even when that IP address is not yet set on the interface. Directly after processing the packet, the client sets the IP address on its interface and is ready for regular IP communication, directly thereafter. If the client's implementation of the IP stack does not accept unicast packets when it has no IP address yet,

2244-593: The protocol for IPv4 and IPv6 differ sufficiently that they may be considered separate protocols. For the IPv6 operation, devices may alternatively use stateless address autoconfiguration . IPv6 hosts may also use link-local addressing to achieve operations restricted to the local network link. The DHCP employs a connectionless service model, using the User Datagram Protocol (UDP). It is implemented with two UDP port numbers for its operations which are

2295-456: The protocol is designated as DHCPv6 . The base DHCP does not include any mechanism for authentication. Because of this, it is vulnerable to a variety of attacks. These attacks fall into three main categories: Because the client has no way to validate the identity of a DHCP server, unauthorized DHCP servers (commonly called " rogue DHCP ") can be operated on networks, providing incorrect information to DHCP clients. This can serve either as

2346-459: The request by assigning an IP address from a pool of addresses, which is preconfigured by an administrator. BOOTP is implemented using the User Datagram Protocol (UDP) for transport. Port number 67 is used by the server for receiving client requests, and port number 68 is used by the client for receiving server responses. BOOTP operates only on IPv4 networks. Historically, BOOTP has also been used for Unix-like diskless workstations to obtain

2397-404: The request. The DHCP server manages a pool of IP addresses and information about client configuration parameters such as default gateway , domain name , the name servers , and time servers . On receiving a DHCP request, the DHCP server may respond with specific information for each client, as previously configured by an administrator, or with a specific address and any other information valid for

2448-409: The same as for the bootstrap protocol ( BOOTP ). The server listens on UDP port number 67, and the client listens on UDP port number 68. DHCP operations fall into four phases: server discovery, IP lease offer, IP lease request, and IP lease acknowledgement. These stages are often abbreviated as DORA for discovery, offer, request, and acknowledgement. The DHCP operation begins with clients broadcasting

2499-628: The server on which the boot image is shared. The client loads the boot image and uses this information to load and start its operating system. The Dynamic Host Configuration Protocol (DHCP) was developed as an extension of BOOTP. BOOTP is defined in Requests for Comments (RFC) 951 and 1084. Dynamic Host Configuration Protocol The Dynamic Host Configuration Protocol ( DHCP ) is a network management protocol used on Internet Protocol (IP) networks for automatically assigning IP addresses and other communication parameters to devices connected to

2550-579: The server using DHCP. DHCP can be implemented on networks ranging in size from residential networks to large campus networks and regional ISP networks. Many routers and residential gateways have DHCP server capability. Most residential network routers receive a unique IP address within the ISP network. Within a local network, a DHCP server assigns a local IP address to each device. DHCP services exist for networks running Internet Protocol version 4 (IPv4), as well as version 6 ( IPv6 ). The IPv6 version of

2601-582: Was first defined in September 1985 as a replacement for the Reverse Address Resolution Protocol (RARP), published in June 1984. The primary motivation for replacing RARP with BOOTP is that RARP was a link layer protocol. This made implementation difficult on many server platforms, and required that a server be present on each individual IP subnet . BOOTP introduced the innovation of relay agents, which forwarded BOOTP packets from

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