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checksum() example added in doc #691

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checksum() example added in doc

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Guillaume Valadon
Guillaume Valadon committed Jun 27, 2017
commit 10210cbe4ba2b1173c8eb3511cad1e296c696edd
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***********************
Calling Scapy functions
***********************

This section provides some examples that show how to benefit from Scapy
functions in your own code.

UDP checksum
============

The following example explains howto use the checksum() function to compute and
UDP checksum manually. The following steps must be performed:

1. compute the UDP pseudo header as described in RFC768
2. build an UDP packet with Scapy with p[UDP].chksum=0
3. call checksum() with the pseudo header and the UDP packet

::

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
packet = IP(dst="10.11.12.13", src="10.11.12.14")/UDP()/DNS()
packet_raw = str(packet)
checksum_scapy = IP(packet_raw)[UDP].chksum

# 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_raw = str(packet)
udp_raw = packet_raw[20:]
phdr = pseudo_header(packet.src, packet.dst, socket.IPPROTO_UDP, len(udp_raw))

assert(checksum_scapy == checksum(phdr + udp_raw))
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@@ -21,6 +21,7 @@ This document is under a `Creative Commons Attribution - Non-Commercial
advanced_usage
extending
build_dissect
functions

troubleshooting
development
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