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DHCP Dynamic Host Configuration Protocol By Curtis Wright III DHCP Definition Dynamic Host Configuration Protocol(DHCP) provides a framework for passing configuration information to hosts on a TCP/IP network. DHCP is based on the Bootstrap Protocol(BOOTP) adding the capability of automatic allocation of reusable network addresses and additional configuration options. DHCP captures the behavior of BOOTP relay agents. DHCP participants can interoperate with BOOTP participants. DHCP Origins DHCP is the result of the Dynamic Host Configuration (DHC) working group of the IETF addressing the issue they defined on their first meeting in April 1989 in Cocoa Beach, FL of the automated allocation of a network address and transmission of other parameters such as the subnet mast and a default router. DHCP RFC Sequence RFC 1531 which was published in October of 1993 followed by 1541 same and month followed by RFC 2131 in 1997. Ralph Droms was the original author of DHCP and it currently involved on its ongoing evolution. Predecessors to DHCP 1. 2. Reverse Address Resolution Protocol (RARP) Bootstrap Protocol (BOOTP) RARP Provides a mechanism for a host to determine its IP address when it is only aware of its MAC address. RARP can determine and configure a diskless workstation with an IP address. No method for determining and delivering other configuration data. RARP server can service only a single subnet because of its complete reliance on Ethernet broadcasts. RFC 903 BOOTP Developed as answer to RARP’s limitations. BOOTP allows hosts to be dynamically configured to use the TCP/IP protocol suite. BOOTP delivers configuration information automatically instead of each TCP/IP host on a network being manually configured. RFC 951 BOOTP BOOTP is a client/server process where the BOOTP client, during the boot phase, requests configuration information from a BOOTP server. After receiving a request from a BOOTP client, the server looks up the clients MAC address in its BOOTP configuration database and sends a reply containing IP configuration information. Client receives the reply and configures its TCP/IP stack. BOOTP client will also load a boot file if the BOOTP server supplies a path using the fully qualified filename. The magic cookie, a mechanism for a BOOTP server to supply vendor-specific operating system(OS) options to a BOOTP client. Options include DNS servers, WINS or NetBIOS name server, time servers, etc. BOOTP Even with these improvements BOOTP had serious drawbacks such as: 1. BOOTP database was a static text file. File had to be maintained by hand as changes were made to the network. 2. Unable to dynamically allocate and distribute IP addresses. Dynamic IP addressing was critical when IP address space was limited. BOOTP Conversation Two types of BOOTP messages: 1. BOOTREQUEST 2. BOOTREPLY BOOTREQUEST/BOOTREPLY Have the sane packet structure. BOOTREQUEST includes information that allows the BOOTP server to determine what configuration data it must supply the requesting host. BOOTREPLY includes the information that the client requested of the server. BOOTREQUEST Contains following information: 1. Source’s MAC address 2.Destination’s MAC address 3. Destination’s IP address 4.Source’s IP address 5. Destination server hostname 6. Boot filename 7. Vendor-specific data BOOTP/DHCP similarities DHCP has many of the same characteristics as BOOTP such as 1.Client/server operating model 2.Packet structure 3.UDP port numbers a. messages from a client to a server are sent to the ‘server’ port (67). b. Messages from a server to a client are sent to the ‘client’ port (68). BOOTP/DHCP Differences There are two primary differences between DHCP and BOOTP: 1. DHCP defines a mechanism through which clients can be assigned a network address for a finite lease, allowing for serial reassignment of network addresses to different clients. 2. DHCP provides the mechanism for a client to acquire all of the IP configuration parameters that it needs in order to operate. DHCP Message format DHCP Message Field Desciptions Fields op htype hlen hops xid secs flags ciaddr yiaddr siaddr giaddr chaddr sname Oct ets 1 1 1 1 4 2 2 4 4 4 4 16 Descriptions op code/message type Hardware address type hardware address length Client sets to zero, optionally used by relay agents when booting via relay agent Transaction ID Filled in by client, seconds elapsed since client began address acquisition or renewal process. flags Client IP address ‘your’ (client) IP address IP address of next server to use in bootstrap; returned in DHCPOFFER, DHCPACK by server. Relay agent IP address Client hardware address DHCP Allocation Mechanisms DHCP supports three mechanisms for IP allocation: 1.Automatic – a permanent IP address is assigned to a client. 2.Dynamic – an IP address is assigned for a limited amount of time. 3.Manual – a client’s IP address is assigned by network administrator, and DHCP conveys that address to the client. A network could use one or more of these mechanisms depending on the network administrator’s policies. Dynamic Allocation 1. Allows automatic reuse of an address that is no longer needed by the client to which it is assigned. 2. Useful for assigning an address to a client that will be connected to the network temporarily or for sharing a limited pool of IP addresses among a group of clients that do not need permanent IP addresses. 3. Good for assigning an IP address to a new client being permanently connected to a network having a limited number of IP addresses that it is necessary to reclaim them when old clients are retired. DHCP server and client messages From client From server DHCPDISCOVER DHCPOFFER DHCPREQUEST DHCPACK DHCPDECLINE DHCPNACK DHCPRELEASE DHCPINFORM DHCP messages DHCPDISCOVER- Client broadcast to locate available servers. DHCPOFFER- Server to client in response to DHCPDISCOVER with offer of configuration parameters. DHCPREQUEST – Client message to servers either (a) requesting offered parameters from one server and implicitly declining offers from all others, (b) confirming correctness of previously allocated address after, e.g., system reboot, or (c) extending the lease on a particular network address DHCPACK Server to client with configuration parameters, including committed network address. DHCP messages (contd) DHCPNACK – Server to client indicating client notion of network address is incorrect or client’s lease as expired. DHCPDECLINE – Client to server indicating network address is already in use. DHCPRELEASE – Client to server relinquishing network address and cancelling remaining lease DHCPINFORM – Client to server, asking only for local configuration parameters; client already has externally configured network address. DHCP client and server messages when allocating a new network address Server (Not Selected) Client Server(selected) Begins initialization DHCPDISCOVER DHCPDISCOVER Determines configuration Determines Configuration DHCPOFFER Collects Replies Selects configuration DHCPREQUEST DHCPREQUEST Commits configuration DHCPACK Initialization complete Graceful shutdown DHCPRELEASE Discards lease DHCP client and server messages for previously allocated network address Server Server Client Begins Initialization Locates Configuration Locates Configuration DHCPACK DHCPACK Initialization complete Constructing and sending DHCP messages DHCP clients and servers both construct DHCP messages by filling in fields in the fixed format section of the message and appending tagged data items in the variable length option area. Options area includes first a four-octet ‘magic cookie’, followed by the options which comes from BOOTP. The last option must ALWAYS be the ‘end’ option. Constructing and sending DHCP messages (contd) DHCP messages broadcast by a client prior to that client obtaining its IP address must have the source address field in the IP header set to 0. DHCP clients are responsible for all message retransmission. The client MUST adopt a retransmission strategy that incorporates a randomized exponential backoff algorithm to determine the delay between retransmissions. DHCP servers and BOOTP relay agents attempt to deliver DHCPOFFER, DHCPACK, and DHCPNACK messages directly to the client using unicast delivery. DHCP server administrative controls A DHCP server has a block of network addresses from with it can satisfy requests for new addresses. Each server maintains a database of allocated addresses and leases in local permanent storage. DHCP server are not required to respond to every DHCPDISCOVER and DHCPREQUEST message they receive. DHCP server needs to use some unique identifier to associate a client with its lease. DHCP clients are free to use any strategy in selecting a DHCP server among those from which the client receives a DHCPOFFER message. DHCP Server Behavior While not required for correct operation of DHCP, the sever SHOULD NOT use the selected network address before the client responds to the server’s DHCPOFFER message. Procedure for server choosing an expiration time for a lease: If the client has not requested a specific lease in the DHCPDISCOVER message and the client already has an assigned network address, the server returns the lease expiration time previously assigned to that address(note that the client must explicitly request a specific lease to extend the expiration time on a previously assigned address), ELSEIF the client has not requested a specific lease in the DHCPDISCOVER message and the client does not have an assigned network address, the server assigns a locally configured default lease time, ELSE If the client has requested a specific lease in the DHCPDISCOVER message(regardless of whether the client DHCP server behavior (contd) The sever must return to the client: 1. The client’s network address 2. The expiration time for the client’s lease 3. Parameters requested by the client based on specified rules. 4. The server MUST supply as many of the requested parameters as possible and MUST omit any parameters it cannot provide. 5.Any parameters from the existing binding that differ from the Host Requirements Document defaults, 6.Any parameters specific to this client 7.Any parameter specific to this client’s class; the parameters must be identified by an exact match between the client’s vendor class identifiers and the client’s classes identified in the server 8.Parameters with non-default values on the client’s subnet. State transition diagram for client INITREBOOT INIT REBOOTING DHCPOFFER/ Discard SELECTING DHCPOFFER/Collect Replies REQUESTING REBINDING DHCPACK/Record lease,set timers T1, T2 BOUND T1 expires Send DHCPREQUEST to Leasing server RENEWING DHCPACK/Record lease, set timers T1, T2 DHCPOFFER,DHCPACK,DHCPNAK Discard DHCPAK/ Halt Network Some DHCP drawbacks A malicious user could make trouble by putting up an unofficial DHCP server. The "broadcast flag": DHCP includes a way in which client implementations unable to receive a packet with a specific IP address can ask the server or relay agent to use the broadcast IP address in the replies (a "flag" set by the client in the requests An easy mistake to make in setting up a DHCP server is to fail to set all the necessary global parameters. Long leases can be disadvantageous in cases where you need to change a configuration parameter or withdraw an address from use. DHCP security DHCP which is built on UDP and IP and is used to make maintenance of remote and/or diskless hosts easier in its current form is quite insecure. IPv6(IPng) or IP Next Generation It has been foreseen due to the rapid growth of the Internet and its initial design that there will come a point where there will not be any available addresses to connect new hosts. IPv6 is one of several solutions that has been proposed to deal with this issue. IPv6 has a stateless autoconfiguration capability. Stateless autoconfiguration does not require manual host configuration, minimal router configuration, and no additional servers. A host is able generate its own addresses using a combination of locally available information and information advertised by routers. Future of DHCP 1. DHCP for IPv6 is the future for DHCP . 2. DHCPv6 complements IPv6 by providing a stateful autoconfiguration option to facilitate the automatic configuration of DHCP clients(RFC 3315). In stateful autoconfiguration, hosts obtain interface addresses and/or configuration information and parameters from a server. The server maintains a database of the addresses that have been to which hosts. 3.DHCP clients can get configuration options such as DNS server addresses. 4. DHCPv6 uses a combination of unicast and multicast messages instead of broadcast message for the majority of its conversation. References 1.www.dhcp.org 2.DHCP for Windows 2000 3.The DHCP handbook Exam Questions 1. 2. Provide timeline diagram of messages exchanged between DHCP client and servers when allocating a new network address. List the predecessors to DHCP with their advantages and disadvantages and explain why DCHP is better. Exam Question Answers Server (not selected) Client Server (selected) V V V | | | | Begins Initialization | | | | | -------------------------------/ | \ ---------------------------------------- | | / DHCPDISCOVER | DHCPDISCOVER \ | | | | Determines Configuration | Determines Configuration | | | | \----------------------| ----------------------------------/ | | DHCPOFFER \ | / DHCPOFFER | | \ | / | | Collects Replies | | \ | | | Selects Configuration | | | | | ___________________/ | \__________________________ | | / DHCPREQUEST | DHCPREQUEST \ | | | | | | Commits configuration | | | | | _________________________/ | | | / DHCPACK | | | | | Initialization complete | Exam Question Answers(Contd) Predecessors to DHCP are 1.RARP(Reverse Address Resolution Protocol) Benefits a. Provides a mechanism for a host to determine its IP address when it is only aware of its MAC address b. RARP can determine and configure a diskless workstation with an IP address. Disadvantages a. No method for determining and delivering other configuration data. b. RARP server can service only a single subnet because of its complete reliance on Ethernet broadcasts Exam Question Answers(Contd) 2. BOOTP(Bootstrap Protocol) Benefits a. BOOTP allows hosts to be dynamically configured to use the TCP/IP protocol suite b. BOOTP delivers configuration information automatically instead of each TCP/IP host on a network being manually configured Disadvantages a. BOOTP database was a static text file which was maintained by hand as network changes were made. b. Unable to dynamically allocate and distribute IP addresses. Exam Question Answers(Contd) DHCP is better because a. DHCP defines a mechanism through which clients can be assigned a network address for a finite lease, allowing for serial reassignment of network addresses to different clients. b. DHCP provides the mechanism for a client to acquire all of the IP configuration parameters that it needs in order to operate.