Dynamic Host Configuration Protocol

DHCP Message

Format of a DHCP message

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)                   |
+---------------------------------------------------------------+

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.

DHCP Option

DHCP Options and BOOTP Vendor Extensions RFC2132