BOOTP/DHCP message format

BOOTP/DHCP message format

 Here, I will explain the message format of DHCP. Since DHCP is upward compatible with BOOTP (Bootstrap), the structure of the messages has remained largely unchanged.

First, I will explain the BOOTP message format.

 BOOTP and DHCP messages use almost the same format structure. Only the final field is different. BOOTP calls this optional field the "vendor specific area" and limits it to 64 bytes.

For DHCP, this field is of variable length and is called "options" and allows DHCP option information to be conveyed.

The figure below shows the BOOTP message format.

DHCP message format

DHCP is an extension of BOOTP, and the message format etc. are almost the same.

The diagram below shows the DHCP message format.

DHCP basic frame

The basic frame of a DHCP message is shown below.

The contents of each field in the DHCP message are as follows.

●Major items of DHCP message

fieldNameexplanation
op
(1 byte)
OpCodeIdentifies whether the packet is a request or a response.
htype
(1 byte)
hardware typeSpecifies the type of hardware address.
hlen
(1 byte)
hardware lengthSpecifies the hardware address length.
hops
(bytes)
hopSet to 0 by the client and incremented each time a request goes through the router.
xid
(4 bytes)
transaction IDA random number chosen by the client. All DHCP messages have the same xid.
secs
(2 bytes)
secondsSets the number of seconds that have passed since the DHCP process was started.
flags
(2 bytes)
flagSpecifies whether the message is broadcast or unicast.
ciaddr
(4 bytes)
client
IP address
Only used when the client knows its own IP address, like the Bound, Renew, and Rebinding states.
yiaddr
(4 bytes)
Destination
IP address
If the client IP address is 0.0.0.0, the DHCP server sets this field to the offered client IP address.
siaddr
(4 bytes)
Server
IP address
If the client knows the IP address of the DHCP server, set this field to the IP address of the DHCP server. Otherwise, it is used in DHCPOFFER, DHCPACK from the DHCP server.
giaddr
(4 bytes)
router
IP address
Gateway IP address, set by DHCP/BOOTP relay agent.
chaddr
(16 bytes)
client
MAC address
DHCP client MAC address.
name
(64 bytes)
server nameOptional server host name.
file
(bytes)
boot file nameBootfile name.
options
(variable length)
Optional parametersOptional parameters provided by the DHCP server.

 Huge fields like sname and file don't make much sense in DHCP. This field is provided to defend its position as an extension of BOOTP.