setup.sh

# Sets up a tun/tap device on Linux
sudo ip tuntap add name tun0 mode tun user $USER
sudo ip link set tun0 up
sudo ip addr add 192.0.2.1 peer 192.0.2.2 dev tun0

sudo iptables -t nat -A POSTROUTING -s 192.0.2.2 -j MASQUERADE
sudo iptables -A FORWARD -i tun0 -s 192.0.2.2 -j ACCEPT
sudo iptables -A FORWARD -o tun0 -d 192.0.2.2 -j ACCEPT

syn.py

# Sends a SYN packet to `example.com`
import struct
from fcntl import ioctl


def openTun(tunName):
    tun = open("/dev/net/tun", "r+b", buffering=0)
    LINUX_IFF_TUN = 0x0001
    LINUX_IFF_NO_PI = 0x1000
    LINUX_TUNSETIFF = 0x400454CA
    flags = LINUX_IFF_TUN | LINUX_IFF_NO_PI
    ifs = struct.pack("16sH22s", tunName, flags, b"")
    ioctl(tun, LINUX_TUNSETIFF, ifs)
    return tun


syn = b'E\x00\x00,\x00\x01\x00\x00@\x06\x00\xc4\xc0\x00\x02\x02"\xc2\x95Cx\x0c\x00P\xf4p\x98\x8b\x00\x00\x00\x00`\x02\xff\xff\x18\xc6\x00\x00\x02\x04\x05\xb4'
tun = openTun(b"tun0")
tun.write(syn)
reply = tun.read(1024)
print(repr(reply))