DHCP Messages

Table F.1 shows the fields in a DHCP message.

Table F.1 DHCP Message Fields

Field Name	Octets	Description
op	1	Message op code/message type. 1 = BOOTREQUEST, 2 = BOOTREPLY
htype	1	Hardware address type.
hlen	1	Hardware address length.
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 messages and responses between a client and a server.
secs	2	Filled in by client; seconds elapsed since client began address acquisition or renewal process.
flags	2	Flags. To work around some clients that cannot accept IP unicast datagrams before the TCP/IP software is configured, DHCP uses the flags field. The leftmost bit is defined as the BROADCAST (B) flag. The remaining bits of the flags field are reserved for future use. They must be set to zero by clients and ignored by servers and relay agents.
ciaddr	4	Client IP address; only filled in if client is in BOUND, RENEW, or REBINDING state and can respond to ARP requests.
yiaddr	4	Client IP address.
siaddr	4	IP address of next server to use in bootstrap; returned in DHCPOffer, DHCPAck by server.
giaddr	4	Relay agent IP address; used in booting via a relay agent.
chaddr	16	Clients hardware address.
sname	64	Optional server host name, null terminated string.
file	128	File name, null terminated string; "generic" name or null in DHCPDiscover, fully qualified path name in DHCPOffer.
options	variable	Optional parameters field. For more information about DHCP options, see "DHCP Options" in this book.

Table F.2 shows the fields and options used by DHCP servers.

Note

In Tables F.2 and F.3, "MUST," "MUST NOT," "SHOULD," and "MAY" refer to whether or not that particular information must, must not, should, or may be included in the message.

Table F.2 Message Fields and Options

Field	DHCPOffer	DHCPAck	DHCPNak
op	BOOTREPLY	BOOTREPLY	BOOTREPLY
htype	Dependent on hardware type; see RFC 1700, Assigned Numbers , for more information.
hlen	Hardware address length in octets
hops	0	0	0
xid	xid from client DHCPDiscover message	xid from client DHCPRequest message	xid from client DHCPRequest message
secs	0	0	0
ciaddr	0	ciaddr from 0 DHCPRequest or 0	0
yiaddr	IP address offered to client	IP address assigned to client	0
siaddr	IP address of next bootstrap server	IP address of next bootstrap server	0
flags	flags from client DHCPDiscover	flags from client DHCPRequest	flags from client DHCPRequest
giaddr	giaddr from client DHCPDiscover	giaddr from client DHCPRequest	giaddr from client DHCPRequest
chaddr	chaddr from client DHCPDiscover	chaddr from client DHCPRequest	chaddr from client DHCPRequest
sname	Server host name or options	Server host name or options	(Unused)
file	Client file or options	Client file or options	(Unused)
options	Options	Options
Option	DHCPOffer	DHCPAck	DHCPNak
Requested IP address	MUST NOT	MUST NOT	MUST NOT
IP address lease time	MUST	MUST (DHCPRequest)	MUST NOT
		MUST NOT (DHCPInform)
Use file or sname field	MAY	MAY	MUST NOT
DHCP message type	DHCPOffer	DHCPAck	DHCPNak
Parameter request list	MUST NOT	MUST NOT	MUST NOT
Message	SHOULD	SHOULD	SHOULD
Client identifier	MUST NOT	MUST NOT	MAY
Vendor class identifier	MAY	MAY	MAY
Server identifier	MUST	MUST	MUST
Maximum message size	MUST NOT	MUST NOT	MUST NOT
All others	MAY	MAY	MUST NOT

Table F.3 shows the fields and options used by DHCP clients.

Table F.3 DHCP Client Message Fields and Options

Field	DHCPDiscover DHCPInform	DHCPRequest	DHCPDecline DHCPRelease
op	BOOTREQUEST	BOOTREQUEST	BOOTREQUEST
htype	Dependent on hardware type; see RFC 1700, Assigned Numbers , for more information.	Dependent on hardware type; see RFC 1700, Assigned Numbers , for more information.	Dependent on hardware type; see RFC 1700, Assigned Numbers , for more information.
hlen	Hardware address length in octets	Hardware address length in octets	Hardware address length in octets
hops	0	0	0
xid	Selected by client	xid from server DHCPOffer message	Selected by client
secs	0 or seconds since DHCP process started	0 or seconds since DHCP process started	0
flags	Set BROADCAST flag if client requires broadcast reply	Set BROADCAST flag if client requires broadcast reply	0
ciaddr	0 (DHCPDiscover) or network address (DHCPInform)	0 or network address (BOUND/RENEW/ REBIND)	0 (DHCPDecline) network address (DHCPRelease)
yiaddr	0	0	0
siaddr	0	0	0
giaddr	0	0	0
chaddr	hardware address	hardware address	hardware address
sname	Options, if indicated in sname/file option; otherwise unused	Options, if indicated in sname/file option; otherwise unused	(Unused)
file	Options, if indicated in sname/file option; otherwise unused	Options, if indicated in sname/file option; otherwise unused	(Unused)
options	Options	Options	(Unused)
Requested IP address	MAY (DISCOVER) MUST NOT (INFORM)	MUST (in SELECTING or INIT-REBOOT) MUST NOT (in BOUND or RENEWING)	MUST (DHCPDecline) MUST NOT (DHCPRelease)
IP address lease time	MAY (DISCOVER) MUST NOT (INFORM)	MAY	MUST NOT
Use file or sname field	MAY	MAY	MAY
DHCP message type	DHCPDiscover/ DHCPInform	DHCPRequest	DHCPDecline/ DHCPRelease
Client identifier	MAY	MAY	MAY
Vendor class identifier	MAY	MAY	MUST NOT
Server identifier	MUST NOT	MUST (after SELECTING, MUST NOT (after INIT-REBOOT, BOUND, RENEWING, or REBINDING)	MUST
Parameter request list	MAY	MAY	MUST NOT
Maximum message size	MAY	MAY	MUST NOT
Message	SHOULD NOT	SHOULD NOT	SHOULD
Site-specific	MAY	MAY	MUST NOT
All others	MAY	MAY	MUST NOT