the code to send ip fragment was copied from scapy, verified by tcpdump it indeed send two packet instead of one. However, GFW still can decode the content despite we are using overlapped ip fragmentation. But, 8.8.8.8 dropped the fragmented ip packet, for security reason.

overlapped_ip_fragmentation.py

from netfilterqueue import NetfilterQueue
import traceback
import subprocess
import signal
import dpkt
import socket

raw_socket = socket.socket(socket.AF_PACKET, socket.SOCK_RAW, socket.htons(3))
raw_socket.setsockopt(socket.SOL_SOCKET, socket.SO_SNDBUF, 2**30)

def split_ip_packet_to_overlapped_fragments(nfqueue_element):
    try:
        fragment1 = dpkt.ip.IP(nfqueue_element.get_payload())
        fragment1.off = 1 << 13 # Set More Fragment Flag
        fragment1.data = str(fragment1.data)[:16] + 3 * '0'
        fragment1.sum = 0
        fragment1.len = len(fragment1)
        fragment2 = dpkt.ip.IP(nfqueue_element.get_payload())
        fragment2.off = 2 # Offset is 2 * 8 bytes
        fragment2.data = str(fragment2.data)[16:]
        fragment2.sum = 0
        fragment2.len = len(fragment2)
        raw_socket.sendto(str(fragment1), ('pppoe-wan', 2048))
        raw_socket.sendto(str(fragment2), ('pppoe-wan', 2048))
        nfqueue_element.drop()
    except:
        traceback.print_exc()
        nfqueue_element.accept()

nfqueue = NetfilterQueue()
nfqueue.bind(0, split_ip_packet_to_overlapped_fragments)

def clean_up(*args):
    subprocess.call('iptables -D OUTPUT -m owner --uid-owner stowaway -j QUEUE', shell=True)

signal.signal(signal.SIGINT, clean_up)

try:
    subprocess.call('iptables -I OUTPUT -m owner --uid-owner stowaway -j QUEUE', shell=True)
    print('running..')
    nfqueue.run()
except KeyboardInterrupt:
    print('bye')