DHCP Message Formats
DHCP Message Formats |
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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 |