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