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Update UDP checksum manual computation doc

This commit is contained in:
gpotter2 2018-01-12 20:01:43 +01:00
parent b45097271c
commit ea779e37b2
1 changed files with 5 additions and 11 deletions
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16
doc/scapy/functions.rst
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@ -19,22 +19,16 @@ UDP checksum manually. The following steps must be performed:
from scapy.all import * from scapy.all import *
def pseudo_header(ip_src, ip_dst, ip_proto, length):
"""
Return a pseudo header according to RFC768
"""
# Prepare the binary representation of the pseudo header
return struct.pack("!4s4sHH", inet_aton(ip_src), inet_aton(ip_dst), ip_proto, length)
# Get the UDP checksum computed by Scapy # Get the UDP checksum computed by Scapy
packet = IP(dst="10.11.12.13", src="10.11.12.14")/UDP()/DNS() packet = IP(dst="10.11.12.13", src="10.11.12.14")/UDP()/DNS()
packet_raw = raw(packet) packet = IP(raw(packet)) # Build packet (automatically done when sending)
checksum_scapy = IP(packet_raw)[UDP].chksum checksum_scapy = packet[UDP].chksum
# Set the UDP checksum to 0 and compute the checksum 'manually' # Set the UDP checksum to 0 and compute the checksum 'manually'
packet = IP(dst="10.11.12.13", src="10.11.12.14")/UDP(chksum=0)/DNS() packet = IP(dst="10.11.12.13", src="10.11.12.14")/UDP(chksum=0)/DNS()
packet_raw = raw(packet) packet_raw = raw(packet)
udp_raw = packet_raw[20:] udp_raw = packet_raw[20:]
phdr = pseudo_header(packet.src, packet.dst, socket.IPPROTO_UDP, len(udp_raw)) # in4_chksum is used to automatically build a pseudo-header
chksum = in4_chksum(socket.IPPROTO_UDP, packet[IP], udp_raw) # For more infos, call "help(in4_chksum)"
assert(checksum_scapy == checksum(phdr + udp_raw)) assert(checksum_scapy == chksum)