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net_checksums.py
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net_checksums.py
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# Copyright (C) 2015-2018 by Yuri Victorovich. All rights reserved.
# This code is licensed under BSD license.
## This is the module that recomputes IP and UDP packet checksums
import socket
import struct
import array
def is_big_endian():
return struct.pack("H",1) == "\x00\x01"
# Checksums are specified in rfc#768
if is_big_endian():
def checksum(pkt):
adj=False
if len(pkt) % 2 == 1:
pkt += bytearray([0])
adj=True
s = sum(array.array("H", pkt))
s = (s >> 16) + (s & 0xffff)
s += s >> 16
s = ~s
if adj:
pkt[len(pkt)-1:] = b''
return s & 0xffff
else:
def checksum(pkt):
adj=False
if len(pkt) % 2 == 1:
pkt += bytearray([0])
adj=True
s = sum(array.array("H", pkt))
s = (s >> 16) + (s & 0xffff)
s += s >> 16
s = ~s
if adj:
pkt[len(pkt)-1:] = b''
return (((s>>8)&0xff)|s<<8) & 0xffff
def packet_new_udp_headers_for_cksum(pkt):
header = bytearray(struct.pack("!4s4sBBH",
pkt[12:16],
pkt[16:20],
0,
socket.IPPROTO_UDP,
len(pkt)-20))
return header+pkt[20:]
def checksum_calc_udp_packet(pkt):
pkt[26:28] = bytearray(struct.pack("H", 0))
pkt[26:28] = bytearray(struct.pack("H", socket.htons(checksum(packet_new_udp_headers_for_cksum(pkt)))))