Google Network Working Group S. Obsoletes: R. Please refer to the current edition of the "Internet Official Protocol Standards" STD 1 for the standardization state and status of this protocol. Distribution of this memo is unlimited. The data items themselves are also called "options. Future options will be specified in separate RFCs.
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Droms Request for Comments: Bucknell University Obsoletes: March Category: Standards Track Dynamic Host Configuration Protocol Status of this memo This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements.
Please refer to the current edition of the "Internet Official Protocol Standards" STD 1 for the standardization state and status of this protocol.
Distribution of this memo is unlimited. Table of Contents 1. Protocol Summary. The Client-Server Protocol. Specification of the DHCP client-server protocol. Security Considerations. Host Configuration Parameters. Format of a DHCP message. Timeline diagram of messages exchanged between DHCP client and servers when allocating a new network address.
Timeline diagram of messages exchanged between DHCP client and servers when reusing a previously allocated network address. State-transition diagram for DHCP clients. Description of fields in a DHCP message. DHCP messages. Fields and options used by DHCP servers. Client messages from various states. Fields and options used by DHCP clients. DHCP consists of two components: a protocol for delivering host-specific configuration parameters from a DHCP server to a host and a mechanism for allocation of network addresses to hosts.
DHCP is built on a client-server model, where designated DHCP server hosts allocate network addresses and deliver configuration parameters to dynamically configured hosts. Throughout the remainder of this document, the term "server" refers to a host providing initialization parameters through DHCP, and the term "client" refers to a host requesting initialization parameters from a DHCP server.
A host should not act as a DHCP server unless explicitly configured to do so by a system administrator. The diversity of hardware and protocol implementations in the Internet would preclude reliable operation if random hosts were allowed to respond to DHCP requests.
For example, IP requires the setting of many parameters within the protocol implementation software. Because IP can be used on many dissimilar kinds of network hardware, values for those parameters cannot be guessed or assumed to have correct defaults. IP hosts may not always be able to defend their network addresses, so that such a distributed address allocation scheme cannot be guaranteed to avoid allocation of duplicate network addresses.
In "dynamic allocation", DHCP assigns an IP address to a client for a limited period of time or until the client explicitly relinquishes the address.
A particular network will use one or more of these mechanisms, depending on the policies of the network administrator. Dynamic allocation is the only one of the three mechanisms that allows automatic reuse of an address that is no longer needed by the client to which it was assigned.
Thus, dynamic allocation is particularly useful for assigning an address to a client that will be connected to the network only temporarily or for sharing a limited pool of IP addresses among a group of clients that do not need permanent IP addresses. Dynamic allocation may also be a good choice for assigning an IP address to a new client being permanently connected to a network where IP addresses are sufficiently scarce that it is important to reclaim them when old clients are retired.
Manual allocation allows DHCP to be used to eliminate the error-prone process of manually configuring hosts with IP addresses in environments where for whatever reasons it is desirable to manage IP address assignment outside of the DHCP mechanisms.
The minimum lease time restriction has been removed. Finally, many editorial changes have been made to clarify the text as a result of experience gained in DHCP interoperability tests.
Hosts can locate routers through the ICMP router discovery mechanism [ 8 ]. BOOTP is a transport mechanism for a collection of configuration information. BOOTP is also extensible, and official extensions [ 17 ] have been defined for several configuration parameters.
Some Sun networks also use DRARP and an auto-installation mechanism to automate the configuration of new hosts in an existing network. In other related work, the path minimum transmission unit MTU discovery algorithm can determine the MTU of an arbitrary internet path [ 14 ]. The Address Resolution Protocol ARP has been proposed as a transport protocol for resource location and selection [ 6 ]. Finally, the Host Requirements RFCs [ 3 , 4 ] mention specific requirements for host reconfiguration and suggest a scenario for initial configuration of diskless hosts.
A client and server may negotiate for the transmission of only those parameters required by the client or specific to a particular subnet. DHCP allows but does not require the configuration of client parameters not directly related to the IP protocol. DHCP is not intended for use in configuring routers. One vendor may choose to include the item because a particular marketplace requires it or because it enhances the product, for example; another vendor may omit the same item.
Bindings are managed by DHCP servers. DHCP must allow local system administrators control over configuration parameters where desired; e. Each client should be able to discover appropriate local configuration parameters without user intervention and incorporate those parameters into its own configuration. Under normal circumstances, the network manager should not have to enter any per-client configuration parameters.
Some installations may include multiple, overlapping DHCP servers to enhance reliability and increase performance. The following list gives design goals specific to the transmission of the network layer parameters. A DHCP client should, whenever possible, be assigned the same configuration parameters e. There are some new, optional transactions that optimize the interaction between DHCP clients and servers that are described in sections 3 and 4.
The numbers in parentheses indicate the size of each field in octets. The names for the fields given in the figure will be used throughout this document to refer to the fields in DHCP messages.
First, DHCP defines mechanisms through which clients can be assigned a network address for a finite lease, allowing for serial reassignment of network addresses to different clients. Second, DHCP provides the mechanism for a client to acquire all of the IP configuration parameters that it needs in order to operate.
DHCP introduces a small change in terminology intended to clarify the meaning of one of the fields. Similarly, the tagged data items that were used inside the BOOTP "vendor extensions" field, which were formerly referred to as "vendor extensions," are now termed simply "options. See the options documents for a list of defined options. The semantics of this flag are discussed in section 4. The remaining bits of the flags field are reserved for future use. They MUST be set to zero by clients and ignored by servers and relay agents.
The model of DHCP persistent storage is that the DHCP service stores a key-value entry for each client, where the key is some unique identifier for example, an IP subnet number and a unique identifier within the subnet and the value contains the configuration parameters for the client. For example, the key might be the pair IP-subnet-number, hardware- address note that the "hardware-address" should be typed by the Droms Standards Track [Page 11] RFC Dynamic Host Configuration Protocol March type of hardware to accommodate possible duplication of hardware addresses resulting from bit-ordering problems in a mixed-media, bridged network allowing for serial or concurrent reuse of a hardware address on different subnets, and for hardware addresses that may not be globally unique.
Alternately, the key might be the pair IP-subnet-number, hostname , allowing the server to assign parameters intelligently to a DHCP client that has been moved to a different subnet or has changed hardware addresses perhaps because the network interface failed and was replaced. A client can query the DHCP service to retrieve its configuration parameters.
The client interface to the configuration parameters repository consists of protocol messages to request configuration parameters and responses from the server carrying the configuration parameters. The basic mechanism for the dynamic allocation of network addresses is simple: a client requests the use of an address for some period of time. The allocation mechanism the collection of DHCP servers guarantees not to reallocate that address within the requested time and attempts to return the same network address each time the client requests an address.
In this document, the period over which a network address is allocated to a client is referred to as a "lease" [ 11 ]. The client may extend its lease with subsequent requests. The client may issue a message to release the address back to the server when the client no longer needs the address. The client may ask for a permanent assignment by asking for an infinite lease. Even when assigning "permanent" addresses, a server may choose to give out lengthy but non-infinite leases to allow detection of the fact that the client has been retired.
In some environments it will be necessary to reassign network addresses due to exhaustion of available addresses. In such environments, the allocation mechanism will reuse addresses whose lease has expired. The server should use whatever information is available in the configuration information repository to choose an address to reuse. For example, the server may choose the least recently assigned address.
Several options have been defined so far. This option defines the "type" of the DHCP message. Additional options may be allowed, required, or not allowed, depending on the DHCP message type. The timeline diagram in figure 3 shows the timing relationships in a typical client-server interaction. If the client already knows its address, some steps may be omitted; this abbreviated interaction is described in section 3.
Servers need not reserve the offered network address, although the protocol will work more efficiently if the server avoids allocating the offered network address to another client.
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. The client may choose to wait for multiple responses. If the client detects that the address is already in use e. The client SHOULD wait a minimum of ten seconds before restarting the configuration process to avoid excessive network traffic in case of looping.
The client should choose to retransmit the DHCPREQUEST enough times to give adequate probability of contacting the server without causing the client and the user of that client to wait overly long before giving up; e. The timeline diagram in figure 4 shows the timing relationships in a typical client-server interaction for a client reusing a previously allocated network address.
The client performs a final check on the parameters as in section 3. At this point, the client is configured. It must instead request a new address by restarting the configuration process, this time using the non-abbreviated procedure described in section 3. Note that in this case, where the client retains its network address locally, the client will not normally relinquish its lease during a graceful shutdown.
Only in the case where the client explicitly needs to relinquish its lease, e. Throughout the protocol, times are to be represented in units of seconds. The time value of 0xffffffff is reserved to represent "infinity". Representing relative times in units of seconds in an unsigned 32 bit word gives a range of relative times from 0 to approximately years, which is sufficient for the relative times to be measured using DHCP.
Cisco Prime Network Registrar 10.0 DHCP User Guide
Droms Request for Comments: Bucknell University Obsoletes: March Category: Standards Track Dynamic Host Configuration Protocol Status of this memo This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" STD 1 for the standardization state and status of this protocol. Distribution of this memo is unlimited. Table of Contents 1.
Dynamic Host Configuration (dhc)
Please refer to the current edition of the "Internet Official Protocol Standards" STD 1 for the standardization state and status of this protocol. Distribution of this memo is unlimited. All Rights Reserved. Abstract Newer high-speed public Internet access technologies call for a high-speed modem to have a local area network LAN attachment to one or more customer premise hosts. However, a number of security and scaling problems arise with such "public" DHCP use.
Dynamic Host Configuration Protocol
Please refer to the current edition of the "Internet Official Protocol Standards" STD 1 for the standardization state and status of this protocol. Distribution of this memo is unlimited. All Rights Reserved. Table of Contents 1. Motivational Example
Causes the subsequent fields to align on word boundaries. End 1 octet End of valid information in the vendor field. Subsequent octets should be filled with the Pad options. The offset is expressed as a twos-complement bit integer. A positive offset indicates a location east of the zero meridian and a negative offset indicates a location west of the zero meridian. Router 3 4 octets minimum; multiples of 4 List of IP addresses for routers on the client subnet. Routers should be in order of preference.