How to calculate a packet checksum without sending it?

Question

I'm using scapy, and I want to create a packet and calculate its' checksum without sending it. Is there a way to do it?

Thanks.

Answer 1

I've also tried to avoid show2() because it print the packet. I've found in the source a better solution:

del packet.chksum packet = packet.__class__(bytes(packet)) 

This code regenerate the packet with the correct checksum without any print and actually is what show2() run in the background before printing.

Answer 2

You need to delete the .chksum value from the packet after you create it; then call .show2()

>>> from scapy.layers.inet import IP >>> from scapy.layers.inet import ICMP >>> from scapy.layers.inet import TCP >>> target = "10.9.8.7" >>> ttl = 64 >>> id = 32711 >>> sport = 2927 >>> dport = 80 >>> pak = IP(dst=target, src = "100.99.98.97", ttl=ttl, flags="DF", id=id, len=1200, chksum = 0)/TCP(flags="S", sport=sport, dport=int(dport), options=[('Timestamp',(0,0))], chksum = 0) >>> del pak[IP].chksum >>> del pak[TCP].chksum >>> pak.show2() ###[ IP ]###   version   = 4L   ihl       = 5L   tos       = 0x0   len       = 1200   id        = 32711   flags     = DF   frag      = 0L   ttl       = 64   proto     = tcp   chksum    = 0x9afd   src       = 100.99.98.97   dst       = 10.9.8.7   \options   \ ###[ TCP ]###      sport     = 2927      dport     = www      seq       = 0      ack       = 0      dataofs   = 8L      reserved  = 0L      flags     = S      window    = 8192      chksum    = 0x2c0e      urgptr    = 0      options   = [('Timestamp', (0, 0)), ('EOL', None)] >>>