class MPacketPreamble(Packet):
# IEEE 802.3br Figure 99-3
name = "MPacket Preamble"
fields_desc = [
StrFixedLenField("preamble", b"", length=8),
FCSField("fcs", 0, fmt="!I")
]
class Dot15d4FCS(Dot15d4):
'''
This class is a drop-in replacement for the Dot15d4 class above, except
it expects a FCS/checksum in the input, and produces one in the output.
This provides the user flexibility, as many 802.15.4 interfaces will have an AUTO_CRC setting # noqa: E501
that will validate the FCS/CRC in firmware, and add it automatically when transmitting. # noqa: E501
'''
name = "802.15.4 - FCS"
match_subclass = True
fields_desc = Dot15d4.fields_desc + [FCSField("fcs", None, fmt="<H")]
def compute_fcs(self, data):
# Do a CRC-CCITT Kermit 16bit on the data given
# Returns a CRC that is the FCS for the frame
# Implemented using pseudocode from: June 1986, Kermit Protocol Manual
# See also:
# http://regregex.bbcmicro.net/crc-catalogue.htm#crc.cat.kermit
crc = 0
for i in range(0, len(data)):
c = orb(data[i])
q = (crc ^ c) & 15 # Do low-order 4 bits
crc = (crc // 16) ^ (q * 4225)
q = (crc ^ (c // 16)) & 15 # And high 4 bits
crc = (crc // 16) ^ (q * 4225)
return struct.pack('<H', crc) # return as bytes in little endian order
def post_build(self, p, pay):
# construct the packet with the FCS at the end
p = Dot15d4.post_build(self, p, pay)
if self.fcs is None:
p = p[:-2]
p = p + self.compute_fcs(p)
return p
class Dot11FCS(Dot11):
name = "802.11-FCS"
match_subclass = True
fields_desc = Dot11.fields_desc + [FCSField("fcs", None, fmt="<I")]
def compute_fcs(self, s):
return struct.pack("!I", crc32(s) & 0xffffffff)[::-1]
def post_build(self, p, pay):
p += pay
if self.fcs is None:
p = p[:-4] + self.compute_fcs(p)
return p
class BTH(Packet):
name = "BTH"
fields_desc = [
ByteEnumField("opcode", 0, _bth_opcodes),
BitField("solicited", 0, 1),
BitField("migreq", 0, 1),
BitField("padcount", 0, 2),
BitField("version", 0, 4),
XShortField("pkey", 0xffff),
BitField("fecn", 0, 1),
BitField("becn", 0, 1),
BitField("resv6", 0, 6),
BitField("dqpn", 0, 24),
BitField("ackreq", 0, 1),
BitField("resv7", 0, 7),
BitField("psn", 0, 24),
FCSField("icrc", None, fmt="!I")]
@staticmethod
def pack_icrc(icrc):
return struct.pack("!I", icrc & 0xffffffff)[::-1]
def compute_icrc(self, p):
udp = self.underlayer
if udp is None or not isinstance(udp, UDP):
warning("Expecting UDP underlayer to compute checksum. Got %s.",
udp and udp.name)
return self.pack_icrc(0)
ip = udp.underlayer
if isinstance(ip, IP):
# pseudo-LRH / IP / UDP / BTH / payload
pshdr = Raw(b'\xff' * 8) / ip.copy()
pshdr.chksum = 0xffff
pshdr.ttl = 0xff
pshdr.tos = 0xff
pshdr[UDP].chksum = 0xffff
pshdr[BTH].fecn = 1
pshdr[BTH].becn = 1
pshdr[BTH].resv6 = 0xff
bth = pshdr[BTH].self_build()
payload = raw(pshdr[BTH].payload)
# add ICRC placeholder just to get the right IP.totlen and
# UDP.length
icrc_placeholder = b'\xff\xff\xff\xff'
pshdr[UDP].payload = Raw(bth + payload + icrc_placeholder)
icrc = crc32(raw(pshdr)[:-4]) & 0xffffffff
return self.pack_icrc(icrc)
else:
# TODO support IPv6
warning("The underlayer protocol %s is not supported.",
ip and ip.name)
return self.pack_icrc(0)
# RoCE packets end with ICRC - a 32-bit CRC of the packet payload and
# pseudo-header. Add the ICRC header if it is missing and calculate its
# value.
def post_build(self, p, pay):
p += pay
if self.icrc is None:
p = p[:-4] + self.compute_icrc(p)
return p
