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dhcp protocol: https://datatracker.ietf.org/doc/html/rfc2131
dhcp options : https://datatracker.ietf.org/doc/html/rfc2132
dhcp message:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| op (1) | htype (1) | hlen (1) | hops (1) |
+---------------+---------------+---------------+---------------+
| xid (4) |
+-------------------------------+-------------------------------+
| secs (2) | flags (2) |
+-------------------------------+-------------------------------+
| ciaddr (4) |
+---------------------------------------------------------------+
| yiaddr (4) |
+---------------------------------------------------------------+
| siaddr (4) |
+---------------------------------------------------------------+
| giaddr (4) |
+---------------------------------------------------------------+
| |
| chaddr (16) |
| |
| |
+---------------------------------------------------------------+
| |
| sname (64) |
+---------------------------------------------------------------+
| |
| file (128) |
+---------------------------------------------------------------+
| |
| options (variable) |
+---------------------------------------------------------------+
dhcp message flags field:
1 1 1 1 1 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|B| MBZ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
B: BROADCAST flag
MBZ: MUST BE ZERO (reserved for future use)
dhcp message field description:
FIELD OCTETS DESCRIPTION
----- ------ -----------
op 1 Message op code / message type.
1 = BOOTREQUEST, 2 = BOOTREPLY
htype 1 Hardware address type, see ARP section in "Assigned
Numbers" RFC; 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
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 ([0]: broadcast flag).
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 'your' (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 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 DHCPDISCOVER, fully qualified
directory-path name in DHCPOFFER.
options var Optional parameters field. See the options
documents for a list of defined options.
- A DHCP server always returns its own address in the 'server identifier'
option.
- Client must accept DHCP of at least 576 octets (312 octects of option)
- Time values (eg lease) are relative times in seconds and should be treated
relative wo each nodes clock.
- Services:
- Parameter repository (2.1), used for persistent storage of network
parameters.
- Dynamic nw address allocation (2.2)
- Protocol
- op == BOOTREQUEST client -> server message
- op == BOOTREPLY server -> client message
- options[0:4] -> magic cookie {0x63, 0x82, 0x53, 0x63}
- option (53 - DHCP Message Type) required in all dhcp messages
1. Client broadcasts DHCPDISCOVER (53).
2. Server respond with DHCPOFFER (53).
- yiaddr -> allocated address
- additional options
3. Client broadcasts DHCPREQUEST (53).
- must include 'server identifier' option
- 'requested ip address' option must be set to 'yiaddr' offerened in 2.
4. Server sends DHCPACK (53) to client.
- fill 'yiaddr' with allocated address
- add configuration parameters
5. Client may send DHCPDECLINE (53) if it detects addr already in use.
6. Client may send DHCPRELEASE (53).
- Constructing dhcp message
- variable options must end with 'end (255)' option
- client -> server udp port 67
- server -> client udp port 68
- @server (msg received by client)
- giaddr !=0 -> using BOOTP relay -> send resp to @giaddr:67
- giaddr == 0 && ciaddr != 0 -> send resp to @ciaddr:68
- giaddr == 0 && ciaddr == 0 && flags.b == 1 -> send resp to broadcast 0xffff_ffff
- giaddr == 0 && ciaddr == 0 && flags.b == 0 -> send resp to client hw addr + yiaddr
- Options:
- 'maximum DHCP message size'
- Client may negotiate lager dhcp messages.
- 'server identifier'
- A DHCP server always returns its own address in the option.
- 'client identifier'
- Client may pass explicit client identifier to server.
- 'DHCP message type' (REQUIRED)
- type of dhcp message
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