secs: Filled in by the client, the number of seconds elapsed since the client began address
acquisition or renewal process. Currently this field is reserved and set to 0.
flags: The leftmost bit is defined as the BROADCAST (B) flag. If this flag is set to 0, the DHCP
server sent a reply back by unicast; if this flag is set to 1, the DHCP server sent a reply back by
broadcast. The remaining bits of the flags field are reserved for future use.
ciaddr: Client IP address.
yiaddr: 'your' (client) IP address, assigned by the server.
siaddr: Server IP address, from which the clients obtained configuration parameters.
giaddr: IP address of the first relay agent a request message traveled.
chaddr: Client hardware address.
sname: Server host name, from which the client obtained configuration parameters.
file: Bootfile name and path information, defined by the server to the client.
options: Optional parameters field that is variable in length, which includes the message type,
lease, domain name server IP address, and WINS IP address.
DHCP Options Overview
The DHCP message adopts the same format as the Bootstrap Protocol (BOOTP) message for
compatibility, but differs from it in the option field, which identifies new features for DHCP.
DHCP uses the option field in DHCP messages to carry control information and network configuration
parameters, implementing dynamic address allocation and providing more network configuration
information for clients.
shows the DHCP option format.
Figure 1-4 DHCP option format
Introduction to DHCP Options
The common DHCP options are as follows:
Option 3: Router option. It specifies the gateway address to be assigned to the client.
Option 6: DNS server option. It specifies the DNS server IP address to be assigned to the client.
Option 51: IP address lease option.
Option 53: DHCP message type option. It identifies the type of the DHCP message.
Option 55: Parameter request list option. It is used by a DHCP client to request specified
configuration parameters. The option contains values that correspond to the parameters requested
by the client.
Option 66: TFTP server name option. It specifies a TFTP server to be assigned to the client.
Option 67: Bootfile name option. It specifies the bootfile name to be assigned to the client.
Option 150: TFTP server IP address option. It specifies the TFTP server IP address to be assigned
to the client.