How to calculate a packet checksum without sending it?

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

del packet.chksumpacket = 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.

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)]>>>

Add this patch to scapy/packet.py:

+    def checksum_silent(self):+        """+        Internal method that recalcs checksum without the annoying prints+        **AFTER old checksums are deleted.**+        """++        for f in self.fields_desc:+            if isinstance(f, ConditionalField) and not f._evalcond(self):+                continue+            fvalue = self.getfieldval(f.name)+            if isinstance(fvalue, Packet) or (f.islist and f.holds_packets and type(fvalue) is list):+                fvalue_gen = SetGen(fvalue,_iterpacket=0)+                for fvalue in fvalue_gen:+                    fvalue.checksum_silent()+        if self.payload:+            self.payload.checksum_silent()

Then instead of calling pkt.show2(), just call this functionpkt.checksum_silent(). (Remember to first do del pkt[IP].chksum and del pkt[UDP].chksum, etc.) as shown in the previous answer.

This function should be faster and be silent. (There may be additional things to trim as well; I hacked this code together and only tested to make sure it was silent with correct checksum.)