DHCP: Message format

Message Format

As mentioned earlier, the format of the DHCP messages is based on the format of BOOTP messages in order to keep compatible with BOOTP relay agents and BOOTP clients.

Here is the DHCP message format. The numbers in parentheses indicate the size of each field in octets.

Description of fields in a DHCP message

FIELD	OCTETS	DESCRIPTION
op	1	Message op code / message type. 1 = BOOTREQUEST, 2 = BOOTREPLY
htype	1	Hardware address type (e.g., '1' = 10Mb Ethernet)
hlen	1	Hardware address length (e.g. '6' for 10Mb Ethernet)
hops	1	Client sets to zero, optionally used by relay agents when booting via a relay agent.
xid	4	Transaction ID. A random number chosen by the client, used by the client and server to associate the request message with its response.
secs	2	Seconds passed since client began the request process. Filled in by client.
flags	2	Flags
ciaddr	4	Client IP address. Filled in by client if it knows its IP address (from previous requests or from manual configurations) and can respond to ARP requests.
yiaddr	4	'your' (client) IP address. Server's response to client.
siaddr	4	Server IP address. Address of sending server or of the next server to use in the next bootstrap process step.
giaddr	4	Relay agent IP address, used in booting via a relay agent.
chaddr	16	Client hardware address.
sname	64	Optional server host name. Null terminated string.
file	128	Boot file name. Null terminated string; "generic" name or null in request, fully qualified directory-path name in reply.
options	variable	Field to hold the optional parameters. (See next section).

The options field in a DHCP message

Apart from the small amount of the fields imported from the BOOTP frame format there came a need for a lot more fields, some of them changing from one message to another, others from one subnet to another etc.

That need came first after defining the BOOTP protocol which led to the BOOTP extension mechanism, where the last field in the frame format, the 'vendor extensions' filed was of variable length and could contain the extra information. DHCP improved this mechanism, changing the field name to 'options' and adding more options.

One way to categorize those options would be to split them into two groups:

Configuration parameters.
Message control information.

All options begin with a tag octet, which uniquely identifies the option. The next octet is the option length specifier, its value does not include the two octets specifying the tag and length. The length octet is followed by length octets of data.

All these options/vendor extensions are defined in RFC 2132, where they are split to the following groups:

RFC 1497 Vendor Extensions.
IP Layer Parameters per Host.
IP Layer Parameters per Interface.
Link Layer Parameters per Interface.
TCP Parameters.
Application and Service Parameters.
DHCP Extensions.

Some important DHCP options:

Message Type (a DHCP control).: Specifies the type of the DHCP message in order to be more specific than the originally BOOTP field 'op'. Different message types are used at different stages of the client/server interaction.
Appears in every DHCP message (therefore the options field is never empty).
Renewal Time Value (a DHCP control).: Specifies the time interval from address assignment until the client attempts to contact the server that originally issued the client's network address before the lease expire.
Parameter Request List (a DHCP control).: A list of valid DHCP option codes. Used by a DHCP client to request values for specified configuration parameters.
Subnet Mask (a Configuration parameter): Specifies the client's subnet mask.
DNS Option (a Configuration parameter): Specifies a list of DNS name servers available to the client.

Quiz

The Options field purpose is to:

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