Dynamic Host Configuation Protocol (DHCP) Message Options
Dynamic Host Configuration Protocol (DHCP) message's options section carries additional configuration information between Dynamic Host Configuration Protocol (DHCP) client and the Dynamic Host Configuration Protocol (DHCP) server. The first four bytes of the field carry the "magic cookie" value 99, 130, 83, 99 in decimals (63, 82, 53, 63 in hexadecimals) to identify the information as vendor-independent option fields. Dynamic Host Configuration Protocol (DHCP) message's options contains an option code, option length, and option data. The option code identifies Dynamic Host Configuration Protocol (DHCP) option, option length is the length of the option data field (in bytes) and the option data is the information required.
Some of the important DHCP options are listed below.
The Dynamic Host Configuration Protocol (DHCP) message type Option
A Dynamic Host Configuration Protocol (DHCP) client and server exchanges different messages during the lease process. The DHCP message type option includes the DHCP message type option code (53), the length of the data field (1), and the message type (single byte) can be one of the following values.
Message Type indicates the DHCP message and can contain any of the following values.
1 = Dynamic Host Configuration Protocol (DHCP) Discover message (DHCPDiscover).
2 = Dynamic Host Configuration Protocol (DHCP) Offer message (DHCPOffer).
3 = Dynamic Host Configuration Protocol (DHCP) Request message (DHCPRequest).
4 = Dynamic Host Configuration Protocol (DHCP) Decline message (DHCPDecline).
5 = Dynamic Host Configuration Protocol (DHCP) Acknowledgment message (DHCPAck).
6 = Dynamic Host Configuration Protocol (DHCP) Negative Acknowledgment message (DHCPNak).
7 = Dynamic Host Configuration Protocol (DHCP) Release message (DHCPRelease).
8 = Dynamic Host Configuration Protocol (DHCP) Informational message (DHCPInform)
The subnet mask Option
The subnet mask option carries the subnet mask that the client should use. The client’s IPv4 address is in the ciaddr field in the fixed format section of the DHCP message, but the subnet mask is carried as an option. The subnet mask option has an option code of 1, a length of 4, and 32-bit mask.
The DNS Server Option
The Domain Name System is used for name resolution. A human friendly device name is resolved to an IP address using DNS. The DNS server option carries a list of addresses of DNS servers that the client can use. The DNS server option code is 6, the length (the number of DNS Servers listed in the option multiplied by four), and the IP addresses of the DNS servers.
The router Option
If the client needs to communicate to another network segment, it needs to know the default gateway (router's interface address facing the LAN). The router option has an option code of 3, a variable length (the number of routers listed in the option multiplied by four), and the IP addresses of the routers.
The requested IP address Option
When Dynamic Host Configuration Protocol (DHCP) clients send a DHCPDiscover message to find a Dynamic Host Configuration Protocol (DHCP) server, it can request a specific IP address be assigned by the DHCP server. The requested IP address Option is used for requesting a specific IP address from a Dynamic Host Configuration Protocol (DHCP) server.
The DNS Domain Name Option
The DNS Domain Name Option specifies the domain name the client should use.
The end Option
The end option is used to indicate the end of the options in the options section of a DHCP message. The end option has a fixed length. There is no length field and data field. The end option is a is a single byte set to the value 255.
In this lesson, we have learned some important Dynamic Host Configuration Protocol (DHCP) message options and the Dynamic Host Configuration Protocol (DHCP) message option formats. Click "Next" to continue.
