Question

I’m writing my first program in Python.
I’m using a third-party kit for interacting with network packets (Scapy).

When I use this method:

p=sr1(IP(dst="google.com")/TCP())

the result is:

<IP  version=4L ihl=5L tos=0x0 len=44 id=18972 flags= frag=0L ttl=44
proto=tcp chksum=0x25bd src=173.194.39.134 dst=192.168.137.2
options=[] |<TCP  sport=www dport=ftp_data seq=1295300429 ack=1
dataofs=6L reserved=0L flags=SA window=14300 chksum=0x3c7e urgptr=0
options=[('MSS', 1430)] |<Padding  load='\x00\x00' |>>>

I can access to chksum of IP simply by p.chksum, but how can I access to chksum of TCP?

Answer 1

As a general guideline, you should use

>>> p[TCP].chksum

However this only works for received packets because the checksum is calculated on transmit. You can retrieve the checksum for an untransmitted packet with the show2() method.