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diff --git a/doc/rfc/rfc2131.txt b/doc/rfc/rfc2131.txt new file mode 100644 index 0000000..f45d9b8 --- /dev/null +++ b/doc/rfc/rfc2131.txt @@ -0,0 +1,2523 @@ + + + + + + +Network Working Group R. Droms +Request for Comments: 2131 Bucknell University +Obsoletes: 1541 March 1997 +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. + +Abstract + + The Dynamic Host Configuration Protocol (DHCP) provides a framework + for passing configuration information to hosts on a TCPIP network. + DHCP is based on the Bootstrap Protocol (BOOTP) [7], adding the + capability of automatic allocation of reusable network addresses and + additional configuration options [19]. DHCP captures the behavior of + BOOTP relay agents [7, 21], and DHCP participants can interoperate + with BOOTP participants [9]. + +Table of Contents + + 1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . 2 + 1.1 Changes to RFC1541. . . . . . . . . . . . . . . . . . . . . . 3 + 1.2 Related Work. . . . . . . . . . . . . . . . . . . . . . . . . 4 + 1.3 Problem definition and issues . . . . . . . . . . . . . . . . 4 + 1.4 Requirements. . . . . . . . . . . . . . . . . . . . . . . . . 5 + 1.5 Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6 + 1.6 Design goals. . . . . . . . . . . . . . . . . . . . . . . . . 6 + 2. Protocol Summary. . . . . . . . . . . . . . . . . . . . . . . 8 + 2.1 Configuration parameters repository . . . . . . . . . . . . . 11 + 2.2 Dynamic allocation of network addresses . . . . . . . . . . . 12 + 3. The Client-Server Protocol. . . . . . . . . . . . . . . . . . 13 + 3.1 Client-server interaction - allocating a network address. . . 13 + 3.2 Client-server interaction - reusing a previously allocated + network address . . . . . . . . . . . . . . . . . . . . . . . 17 + 3.3 Interpretation and representation of time values. . . . . . . 20 + 3.4 Obtaining parameters with externally configured network + address . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 + 3.5 Client parameters in DHCP . . . . . . . . . . . . . . . . . . 21 + 3.6 Use of DHCP in clients with multiple interfaces . . . . . . . 22 + 3.7 When clients should use DHCP. . . . . . . . . . . . . . . . . 22 + 4. Specification of the DHCP client-server protocol. . . . . . . 22 + + + +Droms Standards Track [Page 1] + +RFC 2131 Dynamic Host Configuration Protocol March 1997 + + + 4.1 Constructing and sending DHCP messages. . . . . . . . . . . . 22 + 4.2 DHCP server administrative controls . . . . . . . . . . . . . 25 + 4.3 DHCP server behavior. . . . . . . . . . . . . . . . . . . . . 26 + 4.4 DHCP client behavior. . . . . . . . . . . . . . . . . . . . . 34 + 5. Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . .42 + 6. References . . . . . . . . . . . . . . . . . . . . . . . . . .42 + 7. Security Considerations. . . . . . . . . . . . . . . . . . . .43 + 8. Author's Address . . . . . . . . . . . . . . . . . . . . . . .44 + A. Host Configuration Parameters . . . . . . . . . . . . . . . .45 +List of Figures + 1. Format of a DHCP message . . . . . . . . . . . . . . . . . . . 9 + 2. Format of the 'flags' field. . . . . . . . . . . . . . . . . . 11 + 3. Timeline diagram of messages exchanged between DHCP client and + servers when allocating a new network address. . . . . . . . . 15 + 4. Timeline diagram of messages exchanged between DHCP client and + servers when reusing a previously allocated network address. . 18 + 5. State-transition diagram for DHCP clients. . . . . . . . . . . 34 +List of Tables + 1. Description of fields in a DHCP message. . . . . . . . . . . . 10 + 2. DHCP messages. . . . . . . . . . . . . . . . . . . . . . . . . 14 + 3. Fields and options used by DHCP servers. . . . . . . . . . . . 28 + 4. Client messages from various states. . . . . . . . . . . . . . 33 + 5. Fields and options used by DHCP clients. . . . . . . . . . . . 37 + +1. Introduction + + The Dynamic Host Configuration Protocol (DHCP) provides configuration + parameters to Internet hosts. 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. Also, + distributed address allocation schemes depend on a polling/defense + + + +Droms Standards Track [Page 2] + +RFC 2131 Dynamic Host Configuration Protocol March 1997 + + + mechanism for discovery of addresses that are already in use. 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. + + DHCP supports three mechanisms for IP address allocation. In + "automatic allocation", DHCP assigns a permanent IP address to a + client. 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). In "manual allocation", a client's IP + address is assigned by the network administrator, and DHCP is used + simply to convey the assigned address to the client. 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 format of DHCP messages is based on the format of BOOTP messages, + to capture the BOOTP relay agent behavior described as part of the + BOOTP specification [7, 21] and to allow interoperability of existing + BOOTP clients with DHCP servers. Using BOOTP relay agents eliminates + the necessity of having a DHCP server on each physical network + segment. + +1.1 Changes to RFC 1541 + + This document updates the DHCP protocol specification that appears in + RFC1541. A new DHCP message type, DHCPINFORM, has been added; see + section 3.4, 4.3 and 4.4 for details. The classing mechanism for + identifying DHCP clients to DHCP servers has been extended to include + "vendor" classes as defined in sections 4.2 and 4.3. 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. + + + + +Droms Standards Track [Page 3] + +RFC 2131 Dynamic Host Configuration Protocol March 1997 + + +1.2 Related Work + + There are several Internet protocols and related mechanisms that + address some parts of the dynamic host configuration problem. The + Reverse Address Resolution Protocol (RARP) [10] (through the + extensions defined in the Dynamic RARP (DRARP) [5]) explicitly + addresses the problem of network address discovery, and includes an + automatic IP address assignment mechanism. The Trivial File Transfer + Protocol (TFTP) [20] provides for transport of a boot image from a + boot server. The Internet Control Message Protocol (ICMP) [16] + provides for informing hosts of additional routers via "ICMP + redirect" messages. ICMP also can provide subnet mask information + through the "ICMP mask request" message and other information through + the (obsolete) "ICMP information request" message. 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. Morgan has + proposed extensions to BOOTP for dynamic IP address assignment [15]. + The Network Information Protocol (NIP), used by the Athena project at + MIT, is a distributed mechanism for dynamic IP address assignment + [19]. The Resource Location Protocol RLP [1] provides for location + of higher level services. Sun Microsystems diskless workstations use + a boot procedure that employs RARP, TFTP and an RPC mechanism called + "bootparams" to deliver configuration information and operating + system code to diskless hosts. (Sun Microsystems, Sun Workstation + and SunOS are trademarks of Sun Microsystems, Inc.) 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. + +1.3 Problem definition and issues + + DHCP is designed to supply DHCP clients with the configuration + parameters defined in the Host Requirements RFCs. After obtaining + parameters via DHCP, a DHCP client should be able to exchange packets + with any other host in the Internet. The TCP/IP stack parameters + supplied by DHCP are listed in Appendix A. + + + + + +Droms Standards Track [Page 4] + +RFC 2131 Dynamic Host Configuration Protocol March 1997 + + + Not all of these parameters are required for a newly initialized + client. 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 also does + not address registration of newly configured clients with the Domain + Name System (DNS) [12, 13]. + + DHCP is not intended for use in configuring routers. + +1.4 Requirements + + Throughout this document, the words that are used to define the + significance of particular requirements are capitalized. These words + are: + + o "MUST" + + This word or the adjective "REQUIRED" means that the + item is an absolute requirement of this specification. + + o "MUST NOT" + + This phrase means that the item is an absolute prohibition + of this specification. + + o "SHOULD" + + This word or the adjective "RECOMMENDED" means that there + may exist valid reasons in particular circumstances to ignore + this item, but the full implications should be understood and + the case carefully weighed before choosing a different course. + + o "SHOULD NOT" + + This phrase means that there may exist valid reasons in + particular circumstances when the listed behavior is acceptable + or even useful, but the full implications should be understood + and the case carefully weighed before implementing any behavior + described with this label. + + + + + + + + + +Droms Standards Track [Page 5] + +RFC 2131 Dynamic Host Configuration Protocol March 1997 + + + o "MAY" + + This word or the adjective "OPTIONAL" means that this item is + truly optional. 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. + +1.5 Terminology + + This document uses the following terms: + + o "DHCP client" + + A DHCP client is an Internet host using DHCP to obtain + configuration parameters such as a network address. + + o "DHCP server" + + A DHCP server is an Internet host that returns configuration + parameters to DHCP clients. + + o "BOOTP relay agent" + + A BOOTP relay agent or relay agent is an Internet host or router + that passes DHCP messages between DHCP clients and DHCP servers. + DHCP is designed to use the same relay agent behavior as specified + in the BOOTP protocol specification. + + o "binding" + + A binding is a collection of configuration parameters, including + at least an IP address, associated with or "bound to" a DHCP + client. Bindings are managed by DHCP servers. + +1.6 Design goals + + The following list gives general design goals for DHCP. + + o DHCP should be a mechanism rather than a policy. DHCP must + allow local system administrators control over configuration + parameters where desired; e.g., local system administrators + should be able to enforce local policies concerning allocation + and access to local resources where desired. + + + + + + + +Droms Standards Track [Page 6] + +RFC 2131 Dynamic Host Configuration Protocol March 1997 + + + o Clients should require no manual configuration. Each client + should be able to discover appropriate local configuration + parameters without user intervention and incorporate those + parameters into its own configuration. + + o Networks should require no manual configuration for individual + clients. Under normal circumstances, the network manager + should not have to enter any per-client configuration + parameters. + + o DHCP should not require a server on each subnet. To allow for + scale and economy, DHCP must work across routers or through the + intervention of BOOTP relay agents. + + o A DHCP client must be prepared to receive multiple responses + to a request for configuration parameters. Some installations + may include multiple, overlapping DHCP servers to enhance + reliability and increase performance. + + o DHCP must coexist with statically configured, non-participating + hosts and with existing network protocol implementations. + + o DHCP must interoperate with the BOOTP relay agent behavior as + described by RFC 951 and by RFC 1542 [21]. + + o DHCP must provide service to existing BOOTP clients. + + The following list gives design goals specific to the transmission of + the network layer parameters. DHCP must: + + o Guarantee that any specific network address will not be in + use by more than one DHCP client at a time, + + o Retain DHCP client configuration across DHCP client reboot. A + DHCP client should, whenever possible, be assigned the same + configuration parameters (e.g., network address) in response + to each request, + + o Retain DHCP client configuration across server reboots, and, + whenever possible, a DHCP client should be assigned the same + configuration parameters despite restarts of the DHCP mechanism, + + o Allow automated assignment of configuration parameters to new + clients to avoid hand configuration for new clients, + + o Support fixed or permanent allocation of configuration + parameters to specific clients. + + + + +Droms Standards Track [Page 7] + +RFC 2131 Dynamic Host Configuration Protocol March 1997 + + +2. Protocol Summary + + From the client's point of view, DHCP is an extension of the BOOTP + mechanism. This behavior allows existing BOOTP clients to + interoperate with DHCP servers without requiring any change to the + clients' initialization software. RFC 1542 [2] details the + interactions between BOOTP and DHCP clients and servers [9]. There + are some new, optional transactions that optimize the interaction + between DHCP clients and servers that are described in sections 3 and + 4. + + Figure 1 gives the format of a DHCP message and table 1 describes + each of the fields in the DHCP message. 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. + + There are two primary differences between DHCP and BOOTP. 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. What was the "vendor extensions" field + in BOOTP has been re-named the "options" field in DHCP. 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." + + + + + + + + + + + + + + + + + + + + + +Droms Standards Track [Page 8] + +RFC 2131 Dynamic Host Configuration Protocol March 1997 + + + 0 1 2 3 + 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | op (1) | htype (1) | hlen (1) | hops (1) | + +---------------+---------------+---------------+---------------+ + | xid (4) | + +-------------------------------+-------------------------------+ + | secs (2) | flags (2) | + +-------------------------------+-------------------------------+ + | ciaddr (4) | + +---------------------------------------------------------------+ + | yiaddr (4) | + +---------------------------------------------------------------+ + | siaddr (4) | + +---------------------------------------------------------------+ + | giaddr (4) | + +---------------------------------------------------------------+ + | | + | chaddr (16) | + | | + | | + +---------------------------------------------------------------+ + | | + | sname (64) | + +---------------------------------------------------------------+ + | | + | file (128) | + +---------------------------------------------------------------+ + | | + | options (variable) | + +---------------------------------------------------------------+ + + Figure 1: Format of a DHCP message + + DHCP defines a new 'client identifier' option that is used to pass an + explicit client identifier to a DHCP server. This change eliminates + the overloading of the 'chaddr' field in BOOTP messages, where + 'chaddr' is used both as a hardware address for transmission of BOOTP + reply messages and as a client identifier. The 'client identifier' + is an opaque key, not to be interpreted by the server; for example, + the 'client identifier' may contain a hardware address, identical to + the contents of the 'chaddr' field, or it may contain another type of + identifier, such as a DNS name. The 'client identifier' chosen by a + DHCP client MUST be unique to that client within the subnet to which + the client is attached. If the client uses a 'client identifier' in + one message, it MUST use that same identifier in all subsequent + messages, to ensure that all servers correctly identify the client. + + + + +Droms Standards Track [Page 9] + +RFC 2131 Dynamic Host Configuration Protocol March 1997 + + + DHCP clarifies the interpretation of the 'siaddr' field as the + address of the server to use in the next step of the client's + bootstrap process. A DHCP server may return its own address in the + 'siaddr' field, if the server is prepared to supply the next + bootstrap service (e.g., delivery of an operating system executable + image). A DHCP server always returns its own address in the 'server + identifier' option. + + FIELD OCTETS DESCRIPTION + ----- ------ ----------- + + op 1 Message op code / message type. + 1 = BOOTREQUEST, 2 = BOOTREPLY + htype 1 Hardware address type, see ARP section in "Assigned + Numbers" RFC; e.g., '1' = 10mb ethernet. + hlen 1 Hardware address length (e.g. '6' for 10mb + ethernet). + hops 1 Client sets to zero, optionally used by relay agents + when booting via a relay agent. + xid 4 Transaction ID, a random number chosen by the + client, used by the client and server to associate + messages and responses between a client and a + server. + secs 2 Filled in by client, seconds elapsed since client + began address acquisition or renewal process. + flags 2 Flags (see figure 2). + ciaddr 4 Client IP address; only filled in if client is in + BOUND, RENEW or REBINDING state and can respond + to ARP requests. + yiaddr 4 'your' (client) IP address. + siaddr 4 IP address of next server to use in bootstrap; + returned in DHCPOFFER, DHCPACK by server. + giaddr 4 Relay agent IP address, used in booting via a + relay agent. + chaddr 16 Client hardware address. + sname 64 Optional server host name, null terminated string. + file 128 Boot file name, null terminated string; "generic" + name or null in DHCPDISCOVER, fully qualified + directory-path name in DHCPOFFER. + options var Optional parameters field. See the options + documents for a list of defined options. + + Table 1: Description of fields in a DHCP message + + The 'options' field is now variable length. A DHCP client must be + prepared to receive DHCP messages with an 'options' field of at least + length 312 octets. This requirement implies that a DHCP client must + be prepared to receive a message of up to 576 octets, the minimum IP + + + +Droms Standards Track [Page 10] + +RFC 2131 Dynamic Host Configuration Protocol March 1997 + + + datagram size an IP host must be prepared to accept [3]. DHCP + clients may negotiate the use of larger DHCP messages through the + 'maximum DHCP message size' option. The options field may be further + extended into the 'file' and 'sname' fields. + + In the case of a client using DHCP for initial configuration (before + the client's TCP/IP software has been completely configured), DHCP + requires creative use of the client's TCP/IP software and liberal + interpretation of RFC 1122. The TCP/IP software SHOULD accept and + forward to the IP layer any IP packets delivered to the client's + hardware address before the IP address is configured; DHCP servers + and BOOTP relay agents may not be able to deliver DHCP messages to + clients that cannot accept hardware unicast datagrams before the + TCP/IP software is configured. + + To work around some clients that cannot accept IP unicast datagrams + before the TCP/IP software is configured as discussed in the previous + paragraph, DHCP uses the 'flags' field [21]. The leftmost bit is + defined as the BROADCAST (B) flag. The semantics of this flag are + discussed in section 4.1 of this document. 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. Figure 2 gives the + format of the 'flags' field. + + 1 1 1 1 1 1 + 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + |B| MBZ | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + + B: BROADCAST flag + + MBZ: MUST BE ZERO (reserved for future use) + + Figure 2: Format of the 'flags' field + +2.1 Configuration parameters repository + + The first service provided by DHCP is to provide persistent storage + of network parameters for network clients. 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 2131 Dynamic Host Configuration Protocol March 1997 + + + 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). The protocol defines + that the key will be (IP-subnet-number, hardware-address) unless the + client explicitly supplies an identifier using the 'client + identifier' option. 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. + +2.2 Dynamic allocation of network addresses + + The second service provided by DHCP is the allocation of temporary or + permanent network (IP) addresses to clients. 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. As a consistency check, the allocating + server SHOULD probe the reused address before allocating the address, + e.g., with an ICMP echo request, and the client SHOULD probe the + newly received address, e.g., with ARP. + + + + + +Droms Standards Track [Page 12] + +RFC 2131 Dynamic Host Configuration Protocol March 1997 + + +3. The Client-Server Protocol + + DHCP uses the BOOTP message format defined in RFC 951 and given in + table 1 and figure 1. The 'op' field of each DHCP message sent from + a client to a server contains BOOTREQUEST. BOOTREPLY is used in the + 'op' field of each DHCP message sent from a server to a client. + + The first four octets of the 'options' field of the DHCP message + contain the (decimal) values 99, 130, 83 and 99, respectively (this + is the same magic cookie as is defined in RFC 1497 [17]). The + remainder of the 'options' field consists of a list of tagged + parameters that are called "options". All of the "vendor extensions" + listed in RFC 1497 are also DHCP options. RFC 1533 gives the + complete set of options defined for use with DHCP. + + Several options have been defined so far. One particular option - + the "DHCP message type" option - must be included in every DHCP + message. This option defines the "type" of the DHCP message. + Additional options may be allowed, required, or not allowed, + depending on the DHCP message type. + + Throughout this document, DHCP messages that include a 'DHCP message + type' option will be referred to by the type of the message; e.g., a + DHCP message with 'DHCP message type' option type 1 will be referred + to as a "DHCPDISCOVER" message. + +3.1 Client-server interaction - allocating a network address + + The following summary of the protocol exchanges between clients and + servers refers to the DHCP messages described in table 2. 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 3.2. + + 1. The client broadcasts a DHCPDISCOVER message on its local physical + subnet. The DHCPDISCOVER message MAY include options that suggest + values for the network address and lease duration. BOOTP relay + agents may pass the message on to DHCP servers not on the same + physical subnet. + + 2. Each server may respond with a DHCPOFFER message that includes an + available network address in the 'yiaddr' field (and other + configuration parameters in DHCP options). 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. When allocating a new address, + servers SHOULD check that the offered network address is not + + + +Droms Standards Track [Page 13] + +RFC 2131 Dynamic Host Configuration Protocol March 1997 + + + already in use; e.g., the server may probe the offered address + with an ICMP Echo Request. Servers SHOULD be implemented so that + network administrators MAY choose to disable probes of newly + allocated addresses. The server transmits the DHCPOFFER message + to the client, using the BOOTP relay agent if necessary. + + Message Use + ------- --- + + 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. + + DHCPNAK - Server to client indicating client's notion of network + address is incorrect (e.g., client has moved to new + subnet) 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. + + Table 2: DHCP messages + + + + + + + + + + + + +Droms Standards Track [Page 14] + +RFC 2131 Dynamic Host Configuration Protocol March 1997 + + + Server Client Server + (not selected) (selected) + + v v v + | | | + | Begins initialization | + | | | + | _____________/|\____________ | + |/DHCPDISCOVER | DHCPDISCOVER \| + | | | + Determines | Determines + configuration | configuration + | | | + |\ | ____________/ | + | \________ | /DHCPOFFER | + | DHCPOFFER\ |/ | + | \ | | + | Collects replies | + | \| | + | Selects configuration | + | | | + | _____________/|\____________ | + |/ DHCPREQUEST | DHCPREQUEST\ | + | | | + | | Commits configuration + | | | + | | _____________/| + | |/ DHCPACK | + | | | + | Initialization complete | + | | | + . . . + . . . + | | | + | Graceful shutdown | + | | | + | |\ ____________ | + | | DHCPRELEASE \| + | | | + | | Discards lease + | | | + v v v + Figure 3: Timeline diagram of messages exchanged between DHCP + client and servers when allocating a new network address + + + + + + + +Droms Standards Track [Page 15] + +RFC 2131 Dynamic Host Configuration Protocol March 1997 + + + 3. The client receives one or more DHCPOFFER messages from one or more + servers. The client may choose to wait for multiple responses. + The client chooses one server from which to request configuration + parameters, based on the configuration parameters offered in the + DHCPOFFER messages. The client broadcasts a DHCPREQUEST message + that MUST include the 'server identifier' option to indicate which + server it has selected, and that MAY include other options + specifying desired configuration values. The 'requested IP + address' option MUST be set to the value of 'yiaddr' in the + DHCPOFFER message from the server. This DHCPREQUEST message is + broadcast and relayed through DHCP/BOOTP relay agents. To help + ensure that any BOOTP relay agents forward the DHCPREQUEST message + to the same set of DHCP servers that received the original + DHCPDISCOVER message, the DHCPREQUEST message MUST use the same + value in the DHCP message header's 'secs' field and be sent to the + same IP broadcast address as the original DHCPDISCOVER message. + The client times out and retransmits the DHCPDISCOVER message if + the client receives no DHCPOFFER messages. + + 4. The servers receive the DHCPREQUEST broadcast from the client. + Those servers not selected by the DHCPREQUEST message use the + message as notification that the client has declined that server's + offer. The server selected in the DHCPREQUEST message commits the + binding for the client to persistent storage and responds with a + DHCPACK message containing the configuration parameters for the + requesting client. The combination of 'client identifier' or + 'chaddr' and assigned network address constitute a unique + identifier for the client's lease and are used by both the client + and server to identify a lease referred to in any DHCP messages. + Any configuration parameters in the DHCPACK message SHOULD NOT + conflict with those in the earlier DHCPOFFER message to which the + client is responding. The server SHOULD NOT check the offered + network address at this point. The 'yiaddr' field in the DHCPACK + messages is filled in with the selected network address. + + If the selected server is unable to satisfy the DHCPREQUEST message + (e.g., the requested network address has been allocated), the + server SHOULD respond with a DHCPNAK message. + + A server MAY choose to mark addresses offered to clients in + DHCPOFFER messages as unavailable. The server SHOULD mark an + address offered to a client in a DHCPOFFER message as available if + the server receives no DHCPREQUEST message from that client. + + 5. The client receives the DHCPACK message with configuration + parameters. The client SHOULD perform a final check on the + parameters (e.g., ARP for allocated network address), and notes the + duration of the lease specified in the DHCPACK message. At this + + + +Droms Standards Track [Page 16] + +RFC 2131 Dynamic Host Configuration Protocol March 1997 + + + point, the client is configured. If the client detects that the + address is already in use (e.g., through the use of ARP), the + client MUST send a DHCPDECLINE message to the server and restarts + the configuration process. The client SHOULD wait a minimum of ten + seconds before restarting the configuration process to avoid + excessive network traffic in case of looping. + + If the client receives a DHCPNAK message, the client restarts the + configuration process. + + The client times out and retransmits the DHCPREQUEST message if the + client receives neither a DHCPACK or a DHCPNAK message. The client + retransmits the DHCPREQUEST according to the retransmission + algorithm in section 4.1. 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.g., a client + retransmitting as described in section 4.1 might retransmit the + DHCPREQUEST message four times, for a total delay of 60 seconds, + before restarting the initialization procedure. If the client + receives neither a DHCPACK or a DHCPNAK message after employing the + retransmission algorithm, the client reverts to INIT state and + restarts the initialization process. The client SHOULD notify the + user that the initialization process has failed and is restarting. + + 6. The client may choose to relinquish its lease on a network address + by sending a DHCPRELEASE message to the server. The client + identifies the lease to be released with its 'client identifier', + or 'chaddr' and network address in the DHCPRELEASE message. If the + client used a 'client identifier' when it obtained the lease, it + MUST use the same 'client identifier' in the DHCPRELEASE message. + +3.2 Client-server interaction - reusing a previously allocated network + address + + If a client remembers and wishes to reuse a previously allocated + network address, a client may choose to omit some of the steps + described in the previous section. 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. + + + + + + + + + + + +Droms Standards Track [Page 17] + +RFC 2131 Dynamic Host Configuration Protocol March 1997 + + + 1. The client broadcasts a DHCPREQUEST message on its local subnet. + The message includes the client's network address in the + 'requested IP address' option. As the client has not received its + network address, it MUST NOT fill in the 'ciaddr' field. BOOTP + relay agents pass the message on to DHCP servers not on the same + subnet. If the client used a 'client identifier' to obtain its + address, the client MUST use the same 'client identifier' in the + DHCPREQUEST message. + + 2. Servers with knowledge of the client's configuration parameters + respond with a DHCPACK message to the client. Servers SHOULD NOT + check that the client's network address is already in use; the + client may respond to ICMP Echo Request messages at this point. + + Server Client Server + + v v v + | | | + | Begins | + | initialization | + | | | + | /|\ | + | _________ __/ | \__________ | + | /DHCPREQU EST | DHCPREQUEST\ | + |/ | \| + | | | + Locates | Locates + configuration | configuration + | | | + |\ | /| + | \ | ___________/ | + | \ | / DHCPACK | + | \ _______ |/ | + | DHCPACK\ | | + | Initialization | + | complete | + | \| | + | | | + | (Subsequent | + | DHCPACKS | + | ignored) | + | | | + | | | + v v v + + Figure 4: Timeline diagram of messages exchanged between DHCP + client and servers when reusing a previously allocated + network address + + + +Droms Standards Track [Page 18] + +RFC 2131 Dynamic Host Configuration Protocol March 1997 + + + If the client's request is invalid (e.g., the client has moved + to a new subnet), servers SHOULD respond with a DHCPNAK message to + the client. Servers SHOULD NOT respond if their information is not + guaranteed to be accurate. For example, a server that identifies a + request for an expired binding that is owned by another server SHOULD + NOT respond with a DHCPNAK unless the servers are using an explicit + mechanism to maintain coherency among the servers. + + If 'giaddr' is 0x0 in the DHCPREQUEST message, the client is on + the same subnet as the server. The server MUST + broadcast the DHCPNAK message to the 0xffffffff broadcast address + because the client may not have a correct network address or subnet + mask, and the client may not be answering ARP requests. + Otherwise, the server MUST send the DHCPNAK message to the IP + address of the BOOTP relay agent, as recorded in 'giaddr'. The + relay agent will, in turn, forward the message directly to the + client's hardware address, so that the DHCPNAK can be delivered even + if the client has moved to a new network. + + 3. The client receives the DHCPACK message with configuration + parameters. The client performs a final check on the parameters + (as in section 3.1), and notes the duration of the lease specified + in the DHCPACK message. The specific lease is implicitly identified + by the 'client identifier' or 'chaddr' and the network address. At + this point, the client is configured. + + If the client detects that the IP address in the DHCPACK message + is already in use, the client MUST send a DHCPDECLINE message to the + server and restarts the configuration process by requesting a + new network address. This action corresponds to the client + moving to the INIT state in the DHCP state diagram, which is + described in section 4.4. + + If the client receives a DHCPNAK message, it cannot reuse its + remembered network address. It must instead request a new + address by restarting the configuration process, this time + using the (non-abbreviated) procedure described in section + 3.1. This action also corresponds to the client moving to + the INIT state in the DHCP state diagram. + + The client times out and retransmits the DHCPREQUEST message if + the client receives neither a DHCPACK nor a DHCPNAK message. The + client retransmits the DHCPREQUEST according to the retransmission + algorithm in section 4.1. 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.g., a client + retransmitting as described in section 4.1 might retransmit the + + + +Droms Standards Track [Page 19] + +RFC 2131 Dynamic Host Configuration Protocol March 1997 + + + DHCPREQUEST message four times, for a total delay of 60 seconds, + before restarting the initialization procedure. If the client + receives neither a DHCPACK or a DHCPNAK message after employing + the retransmission algorithm, the client MAY choose to use the + previously allocated network address and configuration parameters + for the remainder of the unexpired lease. This corresponds to + moving to BOUND state in the client state transition diagram shown + in figure 5. + + 4. The client may choose to relinquish its lease on a network + address by sending a DHCPRELEASE message to the server. The + client identifies the lease to be released with its + 'client identifier', or 'chaddr' and network address in the + DHCPRELEASE message. + + 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.g., the client + is about to be moved to a different subnet, will the client send + a DHCPRELEASE message. + +3.3 Interpretation and representation of time values + + A client acquires a lease for a network address for a fixed period of + time (which may be infinite). Throughout the protocol, times are to + be represented in units of seconds. The time value of 0xffffffff is + reserved to represent "infinity". + + As clients and servers may not have synchronized clocks, times are + represented in DHCP messages as relative times, to be interpreted + with respect to the client's local clock. Representing relative + times in units of seconds in an unsigned 32 bit word gives a range of + relative times from 0 to approximately 100 years, which is sufficient + for the relative times to be measured using DHCP. + + The algorithm for lease duration interpretation given in the previous + paragraph assumes that client and server clocks are stable relative + to each other. If there is drift between the two clocks, the server + may consider the lease expired before the client does. To + compensate, the server may return a shorter lease duration to the + client than the server commits to its local database of client + information. + +3.4 Obtaining parameters with externally configured network address + + If a client has obtained a network address through some other means + (e.g., manual configuration), it may use a DHCPINFORM request message + + + +Droms Standards Track [Page 20] + +RFC 2131 Dynamic Host Configuration Protocol March 1997 + + + to obtain other local configuration parameters. Servers receiving a + DHCPINFORM message construct a DHCPACK message with any local + configuration parameters appropriate for the client without: + allocating a new address, checking for an existing binding, filling + in 'yiaddr' or including lease time parameters. The servers SHOULD + unicast the DHCPACK reply to the address given in the 'ciaddr' field + of the DHCPINFORM message. + + The server SHOULD check the network address in a DHCPINFORM message + for consistency, but MUST NOT check for an existing lease. The + server forms a DHCPACK message containing the configuration + parameters for the requesting client and sends the DHCPACK message + directly to the client. + +3.5 Client parameters in DHCP + + Not all clients require initialization of all parameters listed in + Appendix A. Two techniques are used to reduce the number of + parameters transmitted from the server to the client. First, most of + the parameters have defaults defined in the Host Requirements RFCs; + if the client receives no parameters from the server that override + the defaults, a client uses those default values. Second, in its + initial DHCPDISCOVER or DHCPREQUEST message, a client may provide the + server with a list of specific parameters the client is interested + in. If the client includes a list of parameters in a DHCPDISCOVER + message, it MUST include that list in any subsequent DHCPREQUEST + messages. + + The client SHOULD include the 'maximum DHCP message size' option to + let the server know how large the server may make its DHCP messages. + The parameters returned to a client may still exceed the space + allocated to options in a DHCP message. In this case, two additional + options flags (which must appear in the 'options' field of the + message) indicate that the 'file' and 'sname' fields are to be used + for options. + + The client can inform the server which configuration parameters the + client is interested in by including the 'parameter request list' + option. The data portion of this option explicitly lists the options + requested by tag number. + + In addition, the client may suggest values for the network address + and lease time in the DHCPDISCOVER message. The client may include + the 'requested IP address' option to suggest that a particular IP + address be assigned, and may include the 'IP address lease time' + option to suggest the lease time it would like. Other options + representing "hints" at configuration parameters are allowed in a + DHCPDISCOVER or DHCPREQUEST message. However, additional options may + + + +Droms Standards Track [Page 21] + +RFC 2131 Dynamic Host Configuration Protocol March 1997 + + + be ignored by servers, and multiple servers may, therefore, not + return identical values for some options. The 'requested IP address' + option is to be filled in only in a DHCPREQUEST message when the + client is verifying network parameters obtained previously. The + client fills in the 'ciaddr' field only when correctly configured + with an IP address in BOUND, RENEWING or REBINDING state. + + If a server receives a DHCPREQUEST message with an invalid 'requested + IP address', the server SHOULD respond to the client with a DHCPNAK + message and may choose to report the problem to the system + administrator. The server may include an error message in the + 'message' option. + +3.6 Use of DHCP in clients with multiple interfaces + + A client with multiple network interfaces must use DHCP through each + interface independently to obtain configuration information + parameters for those separate interfaces. + +3.7 When clients should use DHCP + + A client SHOULD use DHCP to reacquire or verify its IP address and + network parameters whenever the local network parameters may have + changed; e.g., at system boot time or after a disconnection from the + local network, as the local network configuration may change without + the client's or user's knowledge. + + If a client has knowledge of a previous network address and is unable + to contact a local DHCP server, the client may continue to use the + previous network address until the lease for that address expires. + If the lease expires before the client can contact a DHCP server, the + client must immediately discontinue use of the previous network + address and may inform local users of the problem. + +4. Specification of the DHCP client-server protocol + + In this section, we assume that a DHCP server has a block of network + addresses from which it can satisfy requests for new addresses. Each + server also maintains a database of allocated addresses and leases in + local permanent storage. + +4.1 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. The options + area includes first a four-octet 'magic cookie' (which was described + in section 3), followed by the options. The last option must always + + + +Droms Standards Track [Page 22] + +RFC 2131 Dynamic Host Configuration Protocol March 1997 + + + be the 'end' option. + + DHCP uses UDP as its transport protocol. DHCP messages from a client + to a server are sent to the 'DHCP server' port (67), and DHCP + messages from a server to a client are sent to the 'DHCP client' port + (68). A server with multiple network address (e.g., a multi-homed + host) MAY use any of its network addresses in outgoing DHCP messages. + + The 'server identifier' field is used both to identify a DHCP server + in a DHCP message and as a destination address from clients to + servers. A server with multiple network addresses MUST be prepared + to to accept any of its network addresses as identifying that server + in a DHCP message. To accommodate potentially incomplete network + connectivity, a server MUST choose an address as a 'server + identifier' that, to the best of the server's knowledge, is reachable + from the client. For example, if the DHCP server and the DHCP client + are connected to the same subnet (i.e., the 'giaddr' field in the + message from the client is zero), the server SHOULD select the IP + address the server is using for communication on that subnet as the + 'server identifier'. If the server is using multiple IP addresses on + that subnet, any such address may be used. If the server has + received a message through a DHCP relay agent, the server SHOULD + choose an address from the interface on which the message was + recieved as the 'server identifier' (unless the server has other, + better information on which to make its choice). DHCP clients MUST + use the IP address provided in the 'server identifier' option for any + unicast requests to the DHCP server. + + 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. + + If the 'giaddr' field in a DHCP message from a client is non-zero, + the server sends any return messages to the 'DHCP server' port on the + BOOTP relay agent whose address appears in 'giaddr'. If the 'giaddr' + field is zero and the 'ciaddr' field is nonzero, then the server + unicasts DHCPOFFER and DHCPACK messages to the address in 'ciaddr'. + If 'giaddr' is zero and 'ciaddr' is zero, and the broadcast bit is + set, then the server broadcasts DHCPOFFER and DHCPACK messages to + 0xffffffff. If the broadcast bit is not set and 'giaddr' is zero and + 'ciaddr' is zero, then the server unicasts DHCPOFFER and DHCPACK + messages to the client's hardware address and 'yiaddr' address. In + all cases, when 'giaddr' is zero, the server broadcasts any DHCPNAK + messages to 0xffffffff. + + If the options in a DHCP message extend into the 'sname' and 'file' + fields, the 'option overload' option MUST appear in the 'options' + field, with value 1, 2 or 3, as specified in RFC 1533. If the + + + +Droms Standards Track [Page 23] + +RFC 2131 Dynamic Host Configuration Protocol March 1997 + + + 'option overload' option is present in the 'options' field, the + options in the 'options' field MUST be terminated by an 'end' option, + and MAY contain one or more 'pad' options to fill the options field. + The options in the 'sname' and 'file' fields (if in use as indicated + by the 'options overload' option) MUST begin with the first octet of + the field, MUST be terminated by an 'end' option, and MUST be + followed by 'pad' options to fill the remainder of the field. Any + individual option in the 'options', 'sname' and 'file' fields MUST be + entirely contained in that field. The options in the 'options' field + MUST be interpreted first, so that any 'option overload' options may + be interpreted. The 'file' field MUST be interpreted next (if the + 'option overload' option indicates that the 'file' field contains + DHCP options), followed by the 'sname' field. + + The values to be passed in an 'option' tag may be too long to fit in + the 255 octets available to a single option (e.g., a list of routers + in a 'router' option [21]). Options may appear only once, unless + otherwise specified in the options document. The client concatenates + the values of multiple instances of the same option into a single + parameter list for configuration. + + 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. The delay between retransmissions SHOULD be + chosen to allow sufficient time for replies from the server to be + delivered based on the characteristics of the internetwork between + the client and the server. For example, in a 10Mb/sec Ethernet + internetwork, the delay before the first retransmission SHOULD be 4 + seconds randomized by the value of a uniform random number chosen + from the range -1 to +1. Clients with clocks that provide resolution + granularity of less than one second may choose a non-integer + randomization value. The delay before the next retransmission SHOULD + be 8 seconds randomized by the value of a uniform number chosen from + the range -1 to +1. The retransmission delay SHOULD be doubled with + subsequent retransmissions up to a maximum of 64 seconds. The client + MAY provide an indication of retransmission attempts to the user as + an indication of the progress of the configuration process. + + The 'xid' field is used by the client to match incoming DHCP messages + with pending requests. A DHCP client MUST choose 'xid's in such a + way as to minimize the chance of using an 'xid' identical to one used + by another client. For example, a client may choose a different, + random initial 'xid' each time the client is rebooted, and + subsequently use sequential 'xid's until the next reboot. Selecting + a new 'xid' for each retransmission is an implementation decision. A + client may choose to reuse the same 'xid' or select a new 'xid' for + each retransmitted message. + + + +Droms Standards Track [Page 24] + +RFC 2131 Dynamic Host Configuration Protocol March 1997 + + + Normally, DHCP servers and BOOTP relay agents attempt to deliver + DHCPOFFER, DHCPACK and DHCPNAK messages directly to the client using + uicast delivery. The IP destination address (in the IP header) is + set to the DHCP 'yiaddr' address and the link-layer destination + address is set to the DHCP 'chaddr' address. Unfortunately, some + client implementations are unable to receive such unicast IP + datagrams until the implementation has been configured with a valid + IP address (leading to a deadlock in which the client's IP address + cannot be delivered until the client has been configured with an IP + address). + + A client that cannot receive unicast IP datagrams until its protocol + software has been configured with an IP address SHOULD set the + BROADCAST bit in the 'flags' field to 1 in any DHCPDISCOVER or + DHCPREQUEST messages that client sends. The BROADCAST bit will + provide a hint to the DHCP server and BOOTP relay agent to broadcast + any messages to the client on the client's subnet. A client that can + receive unicast IP datagrams before its protocol software has been + configured SHOULD clear the BROADCAST bit to 0. The BOOTP + clarifications document discusses the ramifications of the use of the + BROADCAST bit [21]. + + A server or relay agent sending or relaying a DHCP message directly + to a DHCP client (i.e., not to a relay agent specified in the + 'giaddr' field) SHOULD examine the BROADCAST bit in the 'flags' + field. If this bit is set to 1, the DHCP message SHOULD be sent as + an IP broadcast using an IP broadcast address (preferably 0xffffffff) + as the IP destination address and the link-layer broadcast address as + the link-layer destination address. If the BROADCAST bit is cleared + to 0, the message SHOULD be sent as an IP unicast to the IP address + specified in the 'yiaddr' field and the link-layer address specified + in the 'chaddr' field. If unicasting is not possible, the message + MAY be sent as an IP broadcast using an IP broadcast address + (preferably 0xffffffff) as the IP destination address and the link- + layer broadcast address as the link-layer destination address. + +4.2 DHCP server administrative controls + + DHCP servers are not required to respond to every DHCPDISCOVER and + DHCPREQUEST message they receive. For example, a network + administrator, to retain stringent control over the clients attached + to the network, may choose to configure DHCP servers to respond only + to clients that have been previously registered through some external + mechanism. The DHCP specification describes only the interactions + between clients and servers when the clients and servers choose to + interact; it is beyond the scope of the DHCP specification to + describe all of the administrative controls that system + administrators might want to use. Specific DHCP server + + + +Droms Standards Track [Page 25] + +RFC 2131 Dynamic Host Configuration Protocol March 1997 + + + implementations may incorporate any controls or policies desired by a + network administrator. + + In some environments, a DHCP server will have to consider the values + of the vendor class options included in DHCPDISCOVER or DHCPREQUEST + messages when determining the correct parameters for a particular + client. + + A DHCP server needs to use some unique identifier to associate a + client with its lease. The client MAY choose to explicitly provide + the identifier through the 'client identifier' option. If the client + supplies a 'client identifier', the client MUST use the same 'client + identifier' in all subsequent messages, and the server MUST use that + identifier to identify the client. If the client does not provide a + 'client identifier' option, the server MUST use the contents of the + 'chaddr' field to identify the client. It is crucial for a DHCP + client to use an identifier unique within the subnet to which the + client is attached in the 'client identifier' option. Use of + 'chaddr' as the client's unique identifier may cause unexpected + results, as that identifier may be associated with a hardware + interface that could be moved to a new client. Some sites may choose + to use a manufacturer's serial number as the 'client identifier', to + avoid unexpected changes in a clients network address due to transfer + of hardware interfaces among computers. Sites may also choose to use + a DNS name as the 'client identifier', causing address leases to be + associated with the DNS name rather than a specific hardware box. + + DHCP clients are free to use any strategy in selecting a DHCP server + among those from which the client receives a DHCPOFFER message. The + client implementation of DHCP SHOULD provide a mechanism for the user + to select directly the 'vendor class identifier' values. + +4.3 DHCP server behavior + + A DHCP server processes incoming DHCP messages from a client based on + the current state of the binding for that client. A DHCP server can + receive the following messages from a client: + + o DHCPDISCOVER + + o DHCPREQUEST + + o DHCPDECLINE + + o DHCPRELEASE + + o DHCPINFORM + + + + +Droms Standards Track [Page 26] + +RFC 2131 Dynamic Host Configuration Protocol March 1997 + + + Table 3 gives the use of the fields and options in a DHCP message by + a server. The remainder of this section describes the action of the + DHCP server for each possible incoming message. + +4.3.1 DHCPDISCOVER message + + When a server receives a DHCPDISCOVER message from a client, the + server chooses a network address for the requesting client. If no + address is available, the server may choose to report the problem to + the system administrator. If an address is available, the new address + SHOULD be chosen as follows: + + o The client's current address as recorded in the client's current + binding, ELSE + + o The client's previous address as recorded in the client's (now + expired or released) binding, if that address is in the server's + pool of available addresses and not already allocated, ELSE + + o The address requested in the 'Requested IP Address' option, if that + address is valid and not already allocated, ELSE + + o A new address allocated from the server's pool of available + addresses; the address is selected based on the subnet from which + the message was received (if 'giaddr' is 0) or on the address of + the relay agent that forwarded the message ('giaddr' when not 0). + + As described in section 4.2, a server MAY, for administrative + reasons, assign an address other than the one requested, or may + refuse to allocate an address to a particular client even though free + addresses are available. + + Note that, in some network architectures (e.g., internets with more + than one IP subnet assigned to a physical network segment), it may be + the case that the DHCP client should be assigned an address from a + different subnet than the address recorded in 'giaddr'. Thus, DHCP + does not require that the client be assigned as address from the + subnet in 'giaddr'. A server is free to choose some other subnet, + and it is beyond the scope of the DHCP specification to describe ways + in which the assigned IP address might be chosen. + + While not required for correct operation of DHCP, the server SHOULD + NOT reuse the selected network address before the client responds to + the server's DHCPOFFER message. The server may choose to record the + address as offered to the client. + + The server must also choose an expiration time for the lease, as + follows: + + + +Droms Standards Track [Page 27] + +RFC 2131 Dynamic Host Configuration Protocol March 1997 + + + o 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), ELSE + + o IF 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 + + o IF the client has requested a specific lease in the DHCPDISCOVER + message (regardless of whether the client has an assigned network + address), the server may choose either to return the requested + lease (if the lease is acceptable to local policy) or select + another lease. + +Field DHCPOFFER DHCPACK DHCPNAK +----- --------- ------- ------- +'op' BOOTREPLY BOOTREPLY BOOTREPLY +'htype' (From "Assigned Numbers" RFC) +'hlen' (Hardware address length in octets) +'hops' 0 0 0 +'xid' 'xid' from client 'xid' from client 'xid' from client + DHCPDISCOVER DHCPREQUEST DHCPREQUEST + message message message +'secs' 0 0 0 +'ciaddr' 0 'ciaddr' from 0 + DHCPREQUEST or 0 +'yiaddr' IP address offered IP address 0 + to client assigned to client +'siaddr' IP address of next IP address of next 0 + bootstrap server bootstrap server +'flags' 'flags' from 'flags' from 'flags' from + client DHCPDISCOVER client DHCPREQUEST client DHCPREQUEST + message message message +'giaddr' 'giaddr' from 'giaddr' from 'giaddr' from + client DHCPDISCOVER client DHCPREQUEST client DHCPREQUEST + message message message +'chaddr' 'chaddr' from 'chaddr' from 'chaddr' from + client DHCPDISCOVER client DHCPREQUEST client DHCPREQUEST + message message message +'sname' Server host name Server host name (unused) + or options or options +'file' Client boot file Client boot file (unused) + name or options name or options +'options' options options + + + +Droms Standards Track [Page 28] + +RFC 2131 Dynamic Host Configuration Protocol March 1997 + + +Option DHCPOFFER DHCPACK DHCPNAK +------ --------- ------- ------- +Requested IP address MUST NOT MUST NOT MUST NOT +IP address lease time MUST MUST (DHCPREQUEST) MUST NOT + MUST NOT (DHCPINFORM) +Use 'file'/'sname' fields MAY MAY MUST NOT +DHCP message type DHCPOFFER DHCPACK DHCPNAK +Parameter request list MUST NOT MUST NOT MUST NOT +Message SHOULD SHOULD SHOULD +Client identifier MUST NOT MUST NOT MAY +Vendor class identifier MAY MAY MAY +Server identifier MUST MUST MUST +Maximum message size MUST NOT MUST NOT MUST NOT +All others MAY MAY MUST NOT + + Table 3: Fields and options used by DHCP servers + + Once the network address and lease have been determined, the server + constructs a DHCPOFFER message with the offered configuration + parameters. It is important for all DHCP servers to return the same + parameters (with the possible exception of a newly allocated network + address) to ensure predictable client behavior regardless of which + server the client selects. The configuration parameters MUST be + selected by applying the following rules in the order given below. + The network administrator is responsible for configuring multiple + DHCP servers to ensure uniform responses from those servers. The + server MUST return to the client: + + o The client's network address, as determined by the rules given + earlier in this section, + + o The expiration time for the client's lease, as determined by the + rules given earlier in this section, + + o Parameters requested by the client, according to the following + rules: + + -- IF the server has been explicitly configured with a default + value for the parameter, the server MUST include that value + in an appropriate option in the 'option' field, ELSE + + -- IF the server recognizes the parameter as a parameter + defined in the Host Requirements Document, the server MUST + include the default value for that parameter as given in the + Host Requirements Document in an appropriate option in the + 'option' field, ELSE + + -- The server MUST NOT return a value for that parameter, + + + +Droms Standards Track [Page 29] + +RFC 2131 Dynamic Host Configuration Protocol March 1997 + + + The server MUST supply as many of the requested parameters as + possible and MUST omit any parameters it cannot provide. The + server MUST include each requested parameter only once unless + explicitly allowed in the DHCP Options and BOOTP Vendor + Extensions document. + + o Any parameters from the existing binding that differ from the Host + Requirements Document defaults, + + o Any parameters specific to this client (as identified by + the contents of 'chaddr' or 'client identifier' in the DHCPDISCOVER + or DHCPREQUEST message), e.g., as configured by the network + administrator, + + o Any parameters specific to this client's class (as identified + by the contents of the 'vendor class identifier' + option in the DHCPDISCOVER or DHCPREQUEST message), + e.g., as configured by the network administrator; 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, + + o Parameters with non-default values on the client's subnet. + + The server MAY choose to return the 'vendor class identifier' used to + determine the parameters in the DHCPOFFER message to assist the + client in selecting which DHCPOFFER to accept. The server inserts + the 'xid' field from the DHCPDISCOVER message into the 'xid' field of + the DHCPOFFER message and sends the DHCPOFFER message to the + requesting client. + +4.3.2 DHCPREQUEST message + + A DHCPREQUEST message may come from a client responding to a + DHCPOFFER message from a server, from a client verifying a previously + allocated IP address or from a client extending the lease on a + network address. If the DHCPREQUEST message contains a 'server + identifier' option, the message is in response to a DHCPOFFER + message. Otherwise, the message is a request to verify or extend an + existing lease. If the client uses a 'client identifier' in a + DHCPREQUEST message, it MUST use that same 'client identifier' in all + subsequent messages. If the client included a list of requested + parameters in a DHCPDISCOVER message, it MUST include that list in + all subsequent messages. + + + + + + + +Droms Standards Track [Page 30] + +RFC 2131 Dynamic Host Configuration Protocol March 1997 + + + Any configuration parameters in the DHCPACK message SHOULD NOT + conflict with those in the earlier DHCPOFFER message to which the + client is responding. The client SHOULD use the parameters in the + DHCPACK message for configuration. + + Clients send DHCPREQUEST messages as follows: + + o DHCPREQUEST generated during SELECTING state: + + Client inserts the address of the selected server in 'server + identifier', 'ciaddr' MUST be zero, 'requested IP address' MUST be + filled in with the yiaddr value from the chosen DHCPOFFER. + + Note that the client may choose to collect several DHCPOFFER + messages and select the "best" offer. The client indicates its + selection by identifying the offering server in the DHCPREQUEST + message. If the client receives no acceptable offers, the client + may choose to try another DHCPDISCOVER message. Therefore, the + servers may not receive a specific DHCPREQUEST from which they can + decide whether or not the client has accepted the offer. Because + the servers have not committed any network address assignments on + the basis of a DHCPOFFER, servers are free to reuse offered + network addresses in response to subsequent requests. As an + implementation detail, servers SHOULD NOT reuse offered addresses + and may use an implementation-specific timeout mechanism to decide + when to reuse an offered address. + + o DHCPREQUEST generated during INIT-REBOOT state: + + 'server identifier' MUST NOT be filled in, 'requested IP address' + option MUST be filled in with client's notion of its previously + assigned address. 'ciaddr' MUST be zero. The client is seeking to + verify a previously allocated, cached configuration. Server SHOULD + send a DHCPNAK message to the client if the 'requested IP address' + is incorrect, or is on the wrong network. + + Determining whether a client in the INIT-REBOOT state is on the + correct network is done by examining the contents of 'giaddr', the + 'requested IP address' option, and a database lookup. If the DHCP + server detects that the client is on the wrong net (i.e., the + result of applying the local subnet mask or remote subnet mask (if + 'giaddr' is not zero) to 'requested IP address' option value + doesn't match reality), then the server SHOULD send a DHCPNAK + message to the client. + + + + + + + +Droms Standards Track [Page 31] + +RFC 2131 Dynamic Host Configuration Protocol March 1997 + + + If the network is correct, then the DHCP server should check if + the client's notion of its IP address is correct. If not, then the + server SHOULD send a DHCPNAK message to the client. If the DHCP + server has no record of this client, then it MUST remain silent, + and MAY output a warning to the network administrator. This + behavior is necessary for peaceful coexistence of non- + communicating DHCP servers on the same wire. + + If 'giaddr' is 0x0 in the DHCPREQUEST message, the client is on + the same subnet as the server. The server MUST broadcast the + DHCPNAK message to the 0xffffffff broadcast address because the + client may not have a correct network address or subnet mask, and + the client may not be answering ARP requests. + + If 'giaddr' is set in the DHCPREQUEST message, the client is on a + different subnet. The server MUST set the broadcast bit in the + DHCPNAK, so that the relay agent will broadcast the DHCPNAK to the + client, because the client may not have a correct network address + or subnet mask, and the client may not be answering ARP requests. + + o DHCPREQUEST generated during RENEWING state: + + 'server identifier' MUST NOT be filled in, 'requested IP address' + option MUST NOT be filled in, 'ciaddr' MUST be filled in with + client's IP address. In this situation, the client is completely + configured, and is trying to extend its lease. This message will + be unicast, so no relay agents will be involved in its + transmission. Because 'giaddr' is therefore not filled in, the + DHCP server will trust the value in 'ciaddr', and use it when + replying to the client. + + A client MAY choose to renew or extend its lease prior to T1. The + server may choose not to extend the lease (as a policy decision by + the network administrator), but should return a DHCPACK message + regardless. + + o DHCPREQUEST generated during REBINDING state: + + 'server identifier' MUST NOT be filled in, 'requested IP address' + option MUST NOT be filled in, 'ciaddr' MUST be filled in with + client's IP address. In this situation, the client is completely + configured, and is trying to extend its lease. This message MUST + be broadcast to the 0xffffffff IP broadcast address. The DHCP + server SHOULD check 'ciaddr' for correctness before replying to + the DHCPREQUEST. + + + + + + +Droms Standards Track [Page 32] + +RFC 2131 Dynamic Host Configuration Protocol March 1997 + + + The DHCPREQUEST from a REBINDING client is intended to accommodate + sites that have multiple DHCP servers and a mechanism for + maintaining consistency among leases managed by multiple servers. + A DHCP server MAY extend a client's lease only if it has local + administrative authority to do so. + +4.3.3 DHCPDECLINE message + + If the server receives a DHCPDECLINE message, the client has + discovered through some other means that the suggested network + address is already in use. The server MUST mark the network address + as not available and SHOULD notify the local system administrator of + a possible configuration problem. + +4.3.4 DHCPRELEASE message + + Upon receipt of a DHCPRELEASE message, the server marks the network + address as not allocated. The server SHOULD retain a record of the + client's initialization parameters for possible reuse in response to + subsequent requests from the client. + +4.3.5 DHCPINFORM message + + The server responds to a DHCPINFORM message by sending a DHCPACK + message directly to the address given in the 'ciaddr' field of the + DHCPINFORM message. The server MUST NOT send a lease expiration time + to the client and SHOULD NOT fill in 'yiaddr'. The server includes + other parameters in the DHCPACK message as defined in section 4.3.1. + +4.3.6 Client messages + + Table 4 details the differences between messages from clients in + various states. + + --------------------------------------------------------------------- + | |INIT-REBOOT |SELECTING |RENEWING |REBINDING | + --------------------------------------------------------------------- + |broad/unicast |broadcast |broadcast |unicast |broadcast | + |server-ip |MUST NOT |MUST |MUST NOT |MUST NOT | + |requested-ip |MUST |MUST |MUST NOT |MUST NOT | + |ciaddr |zero |zero |IP address |IP address| + --------------------------------------------------------------------- + + Table 4: Client messages from different states + + + + + + + +Droms Standards Track [Page 33] + +RFC 2131 Dynamic Host Configuration Protocol March 1997 + + +4.4 DHCP client behavior + + Figure 5 gives a state-transition diagram for a DHCP client. A + client can receive the following messages from a server: + + o DHCPOFFER + + o DHCPACK + + o DHCPNAK + + The DHCPINFORM message is not shown in figure 5. A client simply + sends the DHCPINFORM and waits for DHCPACK messages. Once the client + has selected its parameters, it has completed the configuration + process. + + Table 5 gives the use of the fields and options in a DHCP message by + a client. The remainder of this section describes the action of the + DHCP client for each possible incoming message. The description in + the following section corresponds to the full configuration procedure + previously described in section 3.1, and the text in the subsequent + section corresponds to the abbreviated configuration procedure + described in section 3.2. + + + + + + + + + + + + + + + + + + + + + + + + + + + + +Droms Standards Track [Page 34] + +RFC 2131 Dynamic Host Configuration Protocol March 1997 + + + -------- ------- +| | +-------------------------->| |<-------------------+ +| INIT- | | +-------------------->| INIT | | +| REBOOT |DHCPNAK/ +---------->| |<---+ | +| |Restart| | ------- | | + -------- | DHCPNAK/ | | | + | Discard offer | -/Send DHCPDISCOVER | +-/Send DHCPREQUEST | | | + | | | DHCPACK v | | + ----------- | (not accept.)/ ----------- | | +| | | Send DHCPDECLINE | | | +| REBOOTING | | | | SELECTING |<----+ | +| | | / | | |DHCPOFFER/ | + ----------- | / ----------- | |Collect | + | | / | | | replies | +DHCPACK/ | / +----------------+ +-------+ | +Record lease, set| | v Select offer/ | +timers T1, T2 ------------ send DHCPREQUEST | | + | +----->| | DHCPNAK, Lease expired/ | + | | | REQUESTING | Halt network | + DHCPOFFER/ | | | | + Discard ------------ | | + | | | | ----------- | + | +--------+ DHCPACK/ | | | + | Record lease, set -----| REBINDING | | + | timers T1, T2 / | | | + | | DHCPACK/ ----------- | + | v Record lease, set ^ | + +----------------> ------- /timers T1,T2 | | + +----->| |<---+ | | + | | BOUND |<---+ | | + DHCPOFFER, DHCPACK, | | | T2 expires/ DHCPNAK/ + DHCPNAK/Discard ------- | Broadcast Halt network + | | | | DHCPREQUEST | + +-------+ | DHCPACK/ | | + T1 expires/ Record lease, set | | + Send DHCPREQUEST timers T1, T2 | | + to leasing server | | | + | ---------- | | + | | |------------+ | + +->| RENEWING | | + | |----------------------------+ + ---------- + Figure 5: State-transition diagram for DHCP clients + + + + + + + +Droms Standards Track [Page 35] + +RFC 2131 Dynamic Host Configuration Protocol March 1997 + + +4.4.1 Initialization and allocation of network address + + The client begins in INIT state and forms a DHCPDISCOVER message. + The client SHOULD wait a random time between one and ten seconds to + desynchronize the use of DHCP at startup. The client sets 'ciaddr' + to 0x00000000. The client MAY request specific parameters by + including the 'parameter request list' option. The client MAY + suggest a network address and/or lease time by including the + 'requested IP address' and 'IP address lease time' options. The + client MUST include its hardware address in the 'chaddr' field, if + necessary for delivery of DHCP reply messages. The client MAY + include a different unique identifier in the 'client identifier' + option, as discussed in section 4.2. If the client included a list + of requested parameters in a DHCPDISCOVER message, it MUST include + that list in all subsequent messages. + + The client generates and records a random transaction identifier and + inserts that identifier into the 'xid' field. The client records its + own local time for later use in computing the lease expiration. The + client then broadcasts the DHCPDISCOVER on the local hardware + broadcast address to the 0xffffffff IP broadcast address and 'DHCP + server' UDP port. + + If the 'xid' of an arriving DHCPOFFER message does not match the + 'xid' of the most recent DHCPDISCOVER message, the DHCPOFFER message + must be silently discarded. Any arriving DHCPACK messages must be + silently discarded. + + The client collects DHCPOFFER messages over a period of time, selects + one DHCPOFFER message from the (possibly many) incoming DHCPOFFER + messages (e.g., the first DHCPOFFER message or the DHCPOFFER message + from the previously used server) and extracts the server address from + the 'server identifier' option in the DHCPOFFER message. The time + over which the client collects messages and the mechanism used to + select one DHCPOFFER are implementation dependent. + + + + + + + + + + + + + + + + +Droms Standards Track [Page 36] + +RFC 2131 Dynamic Host Configuration Protocol March 1997 + + +Field DHCPDISCOVER DHCPREQUEST DHCPDECLINE, + DHCPINFORM DHCPRELEASE +----- ------------ ----------- ----------- +'op' BOOTREQUEST BOOTREQUEST BOOTREQUEST +'htype' (From "Assigned Numbers" RFC) +'hlen' (Hardware address length in octets) +'hops' 0 0 0 +'xid' selected by client 'xid' from server selected by + DHCPOFFER message client +'secs' 0 or seconds since 0 or seconds since 0 + DHCP process started DHCP process started +'flags' Set 'BROADCAST' Set 'BROADCAST' 0 + flag if client flag if client + requires broadcast requires broadcast + reply reply +'ciaddr' 0 (DHCPDISCOVER) 0 or client's 0 (DHCPDECLINE) + client's network address client's network + network address (BOUND/RENEW/REBIND) address + (DHCPINFORM) (DHCPRELEASE) +'yiaddr' 0 0 0 +'siaddr' 0 0 0 +'giaddr' 0 0 0 +'chaddr' client's hardware client's hardware client's hardware + address address address +'sname' options, if options, if (unused) + indicated in indicated in + 'sname/file' 'sname/file' + option; otherwise option; otherwise + unused unused +'file' options, if options, if (unused) + indicated in indicated in + 'sname/file' 'sname/file' + option; otherwise option; otherwise + unused unused +'options' options options (unused) + + + + + + + + + + + + + + + + +Droms Standards Track [Page 37] + +RFC 2131 Dynamic Host Configuration Protocol March 1997 + + +Option DHCPDISCOVER DHCPREQUEST DHCPDECLINE, + DHCPINFORM DHCPRELEASE +------ ------------ ----------- ----------- +Requested IP address MAY MUST (in MUST + (DISCOVER) SELECTING or (DHCPDECLINE), + MUST NOT INIT-REBOOT) MUST NOT + (INFORM) MUST NOT (in (DHCPRELEASE) + BOUND or + RENEWING) +IP address lease time MAY MAY MUST NOT + (DISCOVER) + MUST NOT + (INFORM) +Use 'file'/'sname' fields MAY MAY MAY +DHCP message type DHCPDISCOVER/ DHCPREQUEST DHCPDECLINE/ + DHCPINFORM DHCPRELEASE +Client identifier MAY MAY MAY +Vendor class identifier MAY MAY MUST NOT +Server identifier MUST NOT MUST (after MUST + SELECTING) + MUST NOT (after + INIT-REBOOT, + BOUND, RENEWING + or REBINDING) +Parameter request list MAY MAY MUST NOT +Maximum message size MAY MAY MUST NOT +Message SHOULD NOT SHOULD NOT SHOULD +Site-specific MAY MAY MUST NOT +All others MAY MAY MUST NOT + + Table 5: Fields and options used by DHCP clients + + If the parameters are acceptable, the client records the address of + the server that supplied the parameters from the 'server identifier' + field and sends that address in the 'server identifier' field of a + DHCPREQUEST broadcast message. Once the DHCPACK message from the + server arrives, the client is initialized and moves to BOUND state. + The DHCPREQUEST message contains the same 'xid' as the DHCPOFFER + message. The client records the lease expiration time as the sum of + the time at which the original request was sent and the duration of + the lease from the DHCPACK message. The client SHOULD perform a + check on the suggested address to ensure that the address is not + already in use. For example, if the client is on a network that + supports ARP, the client may issue an ARP request for the suggested + request. When broadcasting an ARP request for the suggested address, + the client must fill in its own hardware address as the sender's + hardware address, and 0 as the sender's IP address, to avoid + confusing ARP caches in other hosts on the same subnet. If the + + + +Droms Standards Track [Page 38] + +RFC 2131 Dynamic Host Configuration Protocol March 1997 + + + network address appears to be in use, the client MUST send a + DHCPDECLINE message to the server. The client SHOULD broadcast an ARP + reply to announce the client's new IP address and clear any outdated + ARP cache entries in hosts on the client's subnet. + +4.4.2 Initialization with known network address + + The client begins in INIT-REBOOT state and sends a DHCPREQUEST + message. The client MUST insert its known network address as a + 'requested IP address' option in the DHCPREQUEST message. The client + may request specific configuration parameters by including the + 'parameter request list' option. The client generates and records a + random transaction identifier and inserts that identifier into the + 'xid' field. The client records its own local time for later use in + computing the lease expiration. The client MUST NOT include a + 'server identifier' in the DHCPREQUEST message. The client then + broadcasts the DHCPREQUEST on the local hardware broadcast address to + the 'DHCP server' UDP port. + + Once a DHCPACK message with an 'xid' field matching that in the + client's DHCPREQUEST message arrives from any server, the client is + initialized and moves to BOUND state. The client records the lease + expiration time as the sum of the time at which the DHCPREQUEST + message was sent and the duration of the lease from the DHCPACK + message. + +4.4.3 Initialization with an externally assigned network address + + The client sends a DHCPINFORM message. The client may request + specific configuration parameters by including the 'parameter request + list' option. The client generates and records a random transaction + identifier and inserts that identifier into the 'xid' field. The + client places its own network address in the 'ciaddr' field. The + client SHOULD NOT request lease time parameters. + + The client then unicasts the DHCPINFORM to the DHCP server if it + knows the server's address, otherwise it broadcasts the message to + the limited (all 1s) broadcast address. DHCPINFORM messages MUST be + directed to the 'DHCP server' UDP port. + + Once a DHCPACK message with an 'xid' field matching that in the + client's DHCPINFORM message arrives from any server, the client is + initialized. + + If the client does not receive a DHCPACK within a reasonable period + of time (60 seconds or 4 tries if using timeout suggested in section + 4.1), then it SHOULD display a message informing the user of the + problem, and then SHOULD begin network processing using suitable + + + +Droms Standards Track [Page 39] + +RFC 2131 Dynamic Host Configuration Protocol March 1997 + + + defaults as per Appendix A. + +4.4.4 Use of broadcast and unicast + + The DHCP client broadcasts DHCPDISCOVER, DHCPREQUEST and DHCPINFORM + messages, unless the client knows the address of a DHCP server. The + client unicasts DHCPRELEASE messages to the server. Because the + client is declining the use of the IP address supplied by the server, + the client broadcasts DHCPDECLINE messages. + + When the DHCP client knows the address of a DHCP server, in either + INIT or REBOOTING state, the client may use that address in the + DHCPDISCOVER or DHCPREQUEST rather than the IP broadcast address. + The client may also use unicast to send DHCPINFORM messages to a + known DHCP server. If the client receives no response to DHCP + messages sent to the IP address of a known DHCP server, the DHCP + client reverts to using the IP broadcast address. + +4.4.5 Reacquisition and expiration + + The client maintains two times, T1 and T2, that specify the times at + which the client tries to extend its lease on its network address. + T1 is the time at which the client enters the RENEWING state and + attempts to contact the server that originally issued the client's + network address. T2 is the time at which the client enters the + REBINDING state and attempts to contact any server. T1 MUST be + earlier than T2, which, in turn, MUST be earlier than the time at + which the client's lease will expire. + + To avoid the need for synchronized clocks, T1 and T2 are expressed in + options as relative times [2]. + + At time T1 the client moves to RENEWING state and sends (via unicast) + a DHCPREQUEST message to the server to extend its lease. The client + sets the 'ciaddr' field in the DHCPREQUEST to its current network + address. The client records the local time at which the DHCPREQUEST + message is sent for computation of the lease expiration time. The + client MUST NOT include a 'server identifier' in the DHCPREQUEST + message. + + Any DHCPACK messages that arrive with an 'xid' that does not match + the 'xid' of the client's DHCPREQUEST message are silently discarded. + When the client receives a DHCPACK from the server, the client + computes the lease expiration time as the sum of the time at which + the client sent the DHCPREQUEST message and the duration of the lease + in the DHCPACK message. The client has successfully reacquired its + network address, returns to BOUND state and may continue network + processing. + + + +Droms Standards Track [Page 40] + +RFC 2131 Dynamic Host Configuration Protocol March 1997 + + + If no DHCPACK arrives before time T2, the client moves to REBINDING + state and sends (via broadcast) a DHCPREQUEST message to extend its + lease. The client sets the 'ciaddr' field in the DHCPREQUEST to its + current network address. The client MUST NOT include a 'server + identifier' in the DHCPREQUEST message. + + Times T1 and T2 are configurable by the server through options. T1 + defaults to (0.5 * duration_of_lease). T2 defaults to (0.875 * + duration_of_lease). Times T1 and T2 SHOULD be chosen with some + random "fuzz" around a fixed value, to avoid synchronization of + client reacquisition. + + A client MAY choose to renew or extend its lease prior to T1. The + server MAY choose to extend the client's lease according to policy + set by the network administrator. The server SHOULD return T1 and + T2, and their values SHOULD be adjusted from their original values to + take account of the time remaining on the lease. + + In both RENEWING and REBINDING states, if the client receives no + response to its DHCPREQUEST message, the client SHOULD wait one-half + of the remaining time until T2 (in RENEWING state) and one-half of + the remaining lease time (in REBINDING state), down to a minimum of + 60 seconds, before retransmitting the DHCPREQUEST message. + + If the lease expires before the client receives a DHCPACK, the client + moves to INIT state, MUST immediately stop any other network + processing and requests network initialization parameters as if the + client were uninitialized. If the client then receives a DHCPACK + allocating that client its previous network address, the client + SHOULD continue network processing. If the client is given a new + network address, it MUST NOT continue using the previous network + address and SHOULD notify the local users of the problem. + +4.4.6 DHCPRELEASE + + If the client no longer requires use of its assigned network address + (e.g., the client is gracefully shut down), the client sends a + DHCPRELEASE message to the server. Note that the correct operation + of DHCP does not depend on the transmission of DHCPRELEASE messages. + + + + + + + + + + + + +Droms Standards Track [Page 41] + +RFC 2131 Dynamic Host Configuration Protocol March 1997 + + +5. Acknowledgments + + The author thanks the many (and too numerous to mention!) members of + the DHC WG for their tireless and ongoing efforts in the development + of DHCP and this document. + + The efforts of J Allard, Mike Carney, Dave Lapp, Fred Lien and John + Mendonca in organizing DHCP interoperability testing sessions are + gratefully acknowledged. + + The development of this document was supported in part by grants from + the Corporation for National Research Initiatives (CNRI), Bucknell + University and Sun Microsystems. + +6. References + + [1] Acetta, M., "Resource Location Protocol", RFC 887, CMU, December + 1983. + + [2] Alexander, S., and R. Droms, "DHCP Options and BOOTP Vendor + Extensions", RFC 1533, Lachman Technology, Inc., Bucknell + University, October 1993. + + [3] Braden, R., Editor, "Requirements for Internet Hosts -- + Communication Layers", STD 3, RFC 1122, USC/Information Sciences + Institute, October 1989. + + [4] Braden, R., Editor, "Requirements for Internet Hosts -- + Application and Support, STD 3, RFC 1123, USC/Information + Sciences Institute, October 1989. + + [5] Brownell, D, "Dynamic Reverse Address Resolution Protocol + (DRARP)", Work in Progress. + + [6] Comer, D., and R. Droms, "Uniform Access to Internet Directory + Services", Proc. of ACM SIGCOMM '90 (Special issue of Computer + Communications Review), 20(4):50--59, 1990. + + [7] Croft, B., and J. Gilmore, "Bootstrap Protocol (BOOTP)", RFC 951, + Stanford and SUN Microsystems, September 1985. + + [8] Deering, S., "ICMP Router Discovery Messages", RFC 1256, Xerox + PARC, September 1991. + + [9] Droms, D., "Interoperation between DHCP and BOOTP", RFC 1534, + Bucknell University, October 1993. + + + + + +Droms Standards Track [Page 42] + +RFC 2131 Dynamic Host Configuration Protocol March 1997 + + + [10] Finlayson, R., Mann, T., Mogul, J., and M. Theimer, "A Reverse + Address Resolution Protocol", RFC 903, Stanford, June 1984. + + [11] Gray C., and D. Cheriton, "Leases: An Efficient Fault-Tolerant + Mechanism for Distributed File Cache Consistency", In Proc. of + the Twelfth ACM Symposium on Operating Systems Design, 1989. + + [12] Mockapetris, P., "Domain Names -- Concepts and Facilities", STD + 13, RFC 1034, USC/Information Sciences Institute, November 1987. + + [13] Mockapetris, P., "Domain Names -- Implementation and + Specification", STD 13, RFC 1035, USC/Information Sciences + Institute, November 1987. + + [14] Mogul J., and S. Deering, "Path MTU Discovery", RFC 1191, + November 1990. + + [15] Morgan, R., "Dynamic IP Address Assignment for Ethernet Attached + Hosts", Work in Progress. + + [16] Postel, J., "Internet Control Message Protocol", STD 5, RFC 792, + USC/Information Sciences Institute, September 1981. + + [17] Reynolds, J., "BOOTP Vendor Information Extensions", RFC 1497, + USC/Information Sciences Institute, August 1993. + + [18] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC 1700, + USC/Information Sciences Institute, October 1994. + + [19] Jeffrey Schiller and Mark Rosenstein. A Protocol for the Dynamic + Assignment of IP Addresses for use on an Ethernet. (Available + from the Athena Project, MIT), 1989. + + [20] Sollins, K., "The TFTP Protocol (Revision 2)", RFC 783, NIC, + June 1981. + + [21] Wimer, W., "Clarifications and Extensions for the Bootstrap + Protocol", RFC 1542, Carnegie Mellon University, October 1993. + +7. Security Considerations + + DHCP is built directly on UDP and IP which are as yet inherently + insecure. Furthermore, DHCP is generally intended to make + maintenance of remote and/or diskless hosts easier. While perhaps + not impossible, configuring such hosts with passwords or keys may be + difficult and inconvenient. Therefore, DHCP in its current form is + quite insecure. + + + + +Droms Standards Track [Page 43] + +RFC 2131 Dynamic Host Configuration Protocol March 1997 + + + Unauthorized DHCP servers may be easily set up. Such servers can + then send false and potentially disruptive information to clients + such as incorrect or duplicate IP addresses, incorrect routing + information (including spoof routers, etc.), incorrect domain + nameserver addresses (such as spoof nameservers), and so on. + Clearly, once this seed information is in place, an attacker can + further compromise affected systems. + + Malicious DHCP clients could masquerade as legitimate clients and + retrieve information intended for those legitimate clients. Where + dynamic allocation of resources is used, a malicious client could + claim all resources for itself, thereby denying resources to + legitimate clients. + +8. Author's Address + + Ralph Droms + Computer Science Department + 323 Dana Engineering + Bucknell University + Lewisburg, PA 17837 + + Phone: (717) 524-1145 + EMail: droms@bucknell.edu + + + + + + + + + + + + + + + + + + + + + + + + + + + +Droms Standards Track [Page 44] + +RFC 2131 Dynamic Host Configuration Protocol March 1997 + + +A. Host Configuration Parameters + + IP-layer_parameters,_per_host:_ + + Be a router on/off HRC 3.1 + Non-local source routing on/off HRC 3.3.5 + Policy filters for + non-local source routing (list) HRC 3.3.5 + Maximum reassembly size integer HRC 3.3.2 + Default TTL integer HRC 3.2.1.7 + PMTU aging timeout integer MTU 6.6 + MTU plateau table (list) MTU 7 + IP-layer_parameters,_per_interface:_ + IP address (address) HRC 3.3.1.6 + Subnet mask (address mask) HRC 3.3.1.6 + MTU integer HRC 3.3.3 + All-subnets-MTU on/off HRC 3.3.3 + Broadcast address flavor 0x00000000/0xffffffff HRC 3.3.6 + Perform mask discovery on/off HRC 3.2.2.9 + Be a mask supplier on/off HRC 3.2.2.9 + Perform router discovery on/off RD 5.1 + Router solicitation address (address) RD 5.1 + Default routers, list of: + router address (address) HRC 3.3.1.6 + preference level integer HRC 3.3.1.6 + Static routes, list of: + destination (host/subnet/net) HRC 3.3.1.2 + destination mask (address mask) HRC 3.3.1.2 + type-of-service integer HRC 3.3.1.2 + first-hop router (address) HRC 3.3.1.2 + ignore redirects on/off HRC 3.3.1.2 + PMTU integer MTU 6.6 + perform PMTU discovery on/off MTU 6.6 + + Link-layer_parameters,_per_interface:_ + Trailers on/off HRC 2.3.1 + ARP cache timeout integer HRC 2.3.2.1 + Ethernet encapsulation (RFC 894/RFC 1042) HRC 2.3.3 + + TCP_parameters,_per_host:_ + TTL integer HRC 4.2.2.19 + Keep-alive interval integer HRC 4.2.3.6 + Keep-alive data size 0/1 HRC 4.2.3.6 + +Key: + + MTU = Path MTU Discovery (RFC 1191, Proposed Standard) + RD = Router Discovery (RFC 1256, Proposed Standard) + + + +Droms Standards Track [Page 45] + |