DHCP
Headers
|
0 |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
9 |
10 |
11 |
12 |
13 |
14 |
15 |
16 |
17 |
18 |
19 |
20 |
21 |
22 |
23 |
24 |
25 |
26 |
27 |
28 |
29 |
30 |
31 |
|
Op (1 byte) |
htype (1 byte) |
hlen (1 byte) |
hops (1 byte) |
|
0 |
1 |
2 |
3 |
|
xid (4 bytes) |
|
4 |
5 |
6 |
7 |
|
secs (2 bytes) |
flags (2 bytes) |
|
8 |
9 |
10 |
11 |
|
ciaddr (4 bytes) |
|
12 |
13 |
14 |
15 |
|
yiaddr (4 bytes) |
|
16 |
17 |
18 |
19 |
|
siaddr (4 bytes) |
|
20 |
21 |
22 |
23 |
|
chaddr (16 bytes) |
|
24 |
- |
- |
39 |
|
sname (64 bytes) |
|
40 |
- |
- |
103 |
|
File (128 bytes) |
|
104 |
- |
- |
231 |
|
Options (variable length) |
|
232 |
- |
- |
- |
Notes:
Op the
message type (see DHCP message types table below)
Htype Hardware address type (see HTYPE codes table below)
Hlen Hardware address length
Hops set to zero by a client but can optionally be used
when booting via a relay agent
Xid Transaction ID, a random number choosen by the client,
used by the client and server so that no-one get confused
between messages.
Secs The number in seconds since the address was
allocated, the client fills this part
Flags Exactly what it says on the tin
Ciaddr The clients IP address, but only if they have one,
basically if the client is BOUND, RENEW or REBINDING.
Yiaddr Your (client) IP address.
Siaddr IP address of the next server to use in bootstrap.
Is returned in DHCPOFFER and DHCPACK messages by the DCHP
server.
Giaddr VPN server IP address used in booting via relay
agents
Chaddr Client Hardware address (should be unique
LOL)
Sname Optional server host name, should be a null
terminated string
File Boot file name, it should be a null terminated
string. For DHCPDISCOVER messages it should be a fully
generic name or null, for DHCPOFFER messages it should be a
fully qualified directory path name.
Options For optional parameters.
DHCP
Message Type (OP codes)
|
Message |
Description |
Code?? |
|
Discover |
A broadcast from a client to recognise server. |
|
|
Offer |
A response from a server with a proposal of
parameters. |
Op code 2 - BOOTREPLY |
|
Request |
A client broadcasts to a preferred server, declining
all others. |
Op code 1 - BOOTREQUEST |
|
Ack |
The server assigns an IP address. |
|
|
Nak |
The server rejects the clients request. |
|
|
Decline |
The client found a problem with the assigned
address. |
|
|
Release |
The client returns the assigned IP address before
the lease expires. |
|
Format
of the Flags field
|
0 |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
9 |
10 |
11 |
12 |
13 |
14 |
15 |
|
B |
Must Be Zerod |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
B
= Broadcast
The rest
of the byte must be zerod this is for future use.
Hardware
Address type codes HTYPE codes
|
Code |
Definition |
|
1 |
Ethernet (10Mb) |
|
2 |
Experimental Ethernet (3Mb) |
|
3 |
Amateur Radio AX.25 |
|
4 |
Proteon ProNET Token Ring |
|
5 |
Chaos |
|
6 |
IEEE 802 Networks |
|
7 |
ARCNET |
|
8 |
Hyperchannel |
|
9 |
Lanstar |
|
10 |
Autonet Short Address |
|
11 |
LocalTalk |
|
12 |
LocalNet (IBM PCNet or SYTEK LocalNET) |
|
13 |
Ultra link |
|
14 |
SMDS |
|
15 |
Frame Relay |
|
16 |
Asynchronous Transmission Mode (ATM) |
|
17 |
HDLC |
|
18 |
Fibre Channel |
|
19 |
Asynchronous Transmission Mode (ATM) |
|
20 |
Serial Line |
|
21 |
Asynchronous Transmission Mode (ATM) |
