14)
file
：
Boot file name, null terminated string, "generic" name or null in DHCPDISCOVER, fully
qualified directory-path name in DHCPOFFER.
15)
options
：
Optional parameters field. See the options documents (RFC 2132) for a list of
defined options. We will introduce some familiar options in the next section.
DHCP Option
This section defines a generalized use of the 'options' field for giving information useful to a
wide class of machines, operating systems and configurations. Sites with a single DHCP server
that is shared among heterogeneous clients may choose to define other, site-specific formats
for the use of the 'options' field. Figure 10-9 gives the format of options field.
Figure 10-9 DHCP Option
All options begin with a Code octet, which uniquely identifies the option followed by the length
octet. The value of the length octet does not include the Code and Length octets. The common
options are illustrated as below.
1)
option 1
：
Subnet Mask option. The subnet mask option is option1 which identifies the
assigned IP address with network, and its length is 4 octets.
2)
option 3
：
Router option. The router option is option 3 which specifies an IP address for
routers on the client's subnet.
3)
option 6
：
DNS option. The DNS option is option 6, and it assigns the IP address of domain
name server to the client which allows the client can use the web service in the internet.
4)
option 12
：
Host Name option. The option12 is used to specify the name of the client, which
may be requested by the DHCP server for authentication.
5)
option 50
：
Requested IP Address option. The option 50 is used in a DHCP-REQUEST
message to allow the client to request the particular IP address.
6)
option 51
：
Lease Time option. In DHCP-OFFER and DHCP-ACK message, the DHCP server
uses this option to specify the lease time in which the clients can use the IP address
legally.
7)
option 53
：
Message Type option. This option is used to convey the type of the DHCP
message. Legal values for this option show in Table 10-1:
Value Message Type
1
DHCP-DISCOVER
2
DHCP-OFFER
3
DHCP-REQUEST
4
DHCP-DECLINE
5
DHCP-ACK
6
DHCP-NAK
7
DHCP-RELEASE
8
DHCP-INFORM
Table 10-1 Option 53
173
