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:

• The client's current address as recorded in the client's current binding, ELSE

• 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

• The address requested in the 'Requested IP Address' option, if that address is valid and not already allocated, ELSE

• 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:

• 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

• 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

• 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

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:

• The client's network address, as determined by the rules given earlier in this section,

• The expiration time for the client's lease, as determined by the rules given earlier in this section,

• 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,

  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.

• Any parameters from the existing binding that differ from the Host Requirements Document defaults,

• 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,

• 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,

• 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.