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from secrets import SystemRandom
from typing import Any, Optional, Union
from pyroute2.dhcp.fsm import State
random = SystemRandom()
def random_xid_prefix() -> int:
'''A (max) 32 bit random int with its lowest nibble set to 0.
These last 4 bits are used by the client to store its state, in the form
of the associated `State` value, when sending a DHCP message.
'''
return random.randint(0x00000010, 0xFFFFFFF0)
class Xid:
'''Transaction IDs used to identify responses to DHCP requests.
We use the last nibble to store the state the message was sent in.
The most significant bits store a random value we use to match requests
and responses, since the RFC specifies that the server must send back
the same value when answering. (see RFC section 4.1)
'''
def __init__(self, value: Optional[Union[int, 'Xid']] = None):
if isinstance(value, int):
int_value = value
elif isinstance(value, Xid):
int_value = int(value)
elif value is None:
int_value = random_xid_prefix()
else:
raise TypeError(f'{value!r} is not an xid')
assert int_value < 0xFFFFFFFF # we have 32 bits
self._value = int_value
@property
def random_part(self) -> int:
'''The random part of the transaction id.'''
return self._value & 0xFFFFFFF0
@property
def request_state(self) -> Optional[State]:
'''The state in which the request was sent.
Since servers answer with the same transaction ID as the request,
we can use this to know what client state does a response answer to.
'''
try:
return State(self._value & 0xF)
except ValueError:
return None
def for_state(self, state: State) -> 'Xid':
'''A new Xid built from the random part + the state.'''
return Xid(self.random_part | state)
def __index__(self) -> int:
'''Allows xids to be used as int.'''
return self._value
def matches(self, received_xid: 'Xid'):
'''Loose match, whether the random part of both XIDs match.
This can be used to check if a message is indeed in response
to a request we sent.
'''
return self.random_part == received_xid.random_part
def __eq__(self, other: Any) -> bool:
'''Xids compare to other xids or ints.'''
if isinstance(other, Xid):
return self._value == other._value
elif isinstance(other, int):
return self._value == other
return False
def __str__(self) -> str:
return hex(self._value)
def __repr__(self) -> str:
return f"Xid({self})"