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 exchange 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 of these 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 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 probing of newly allocated addresses. The server transmits the DHCPOFFER message to the client, using the BOOTP relay agent if necessary.
DHCP messages
| 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 has 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
Timeline diagram
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
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 client's binding 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 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.
If the client neither receives a DHCPACK or a DHCPNAK message, the client times out and retransmits the DHCPREQUEST message, per 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.
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 yet 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 |
| | |
| /|\ |
| _________ __/ | \__________ |
| /DHCPREQUEST | 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
If the client's request is invalid (e.g., the client has moved to a new subnet), servers SHOULD respond to the client with a DHCPNAK message. 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 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.
If the client neither receives a DHCPACK or a DHCPNAK message, the client times out and retransmits the DHCPREQUEST message, per 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 MAY choose to use the previously allocated network address and configuration parameters for the remainder of the unexpired lease. This corresponds to moving to the 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 forever). 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 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 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 option 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 requested options 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 be ignored by servers, and multiple servers may not return the same values for some options. The 'requested IP address' option is to be filled in only in a DHCPREQUEST message when the client is verifying previously allocated, cached network parameters. 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.