Dhcp Options; Dhcp Options Overview; Introduction To Dhcp Options - 3Com MSR 50 Series Configuration Manual

568 C 32: DHCP O
HAPTER

DHCP Options

DHCP Options Overview

Introduction to DHCP
Options
VERVIEW
flags: The leftmost bit is defined as the BROADCAST (B) flag. This flag indicates
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whether the DHCP server sends a reply back by unicast or broadcast. 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.
ciaddr: Client IP address.
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yiaddr: 'your' (client) IP address, assigned by the server.
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siaddr: Server IP address, from which the clients obtained configuration
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parameters.
giaddr: The first relay agent IP address a request message traveled.
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chaddr: Client hardware address.
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sname: The server host name, from which the client obtained configuration
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parameters.
file: Bootfile name and routing information, defined by the server to the client.
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options: Optional parameters field that is variable in length, which includes the
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message type, lease, DNS IP address, WINS IP address and so forth.
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
the new features of 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.
Figure 164 shows the DHCP option format.
Figure 164 DHCP option format
0
Option type
Value (variable)
The common DHCP options are:
Option 6: DNS server option. It specifies the DNS server IP address to be
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assigned to the client.
Option 51: IP address lease option.
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Option 53: DHCP message type option. It identifies the type of the DHCP
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message.
Option 55: Parameter request list option. It is used by a DHCP client to request
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specified configuration parameters. The option contains values that correspond
to the parameters requested by the client.
7 15
Option length