1#!/bin/env python3 2# SPDX-License-Identifier: GPL-2.0 3 4""" 5This script helps generate fragmented UDP packets. 6 7While it is technically possible to dynamically generate 8fragmented packets in C, it is much harder to read and write 9said code. `scapy` is relatively industry standard and really 10easy to read / write. 11 12So we choose to write this script that generates a valid C 13header. Rerun script and commit generated file after any 14modifications. 15""" 16 17import argparse 18import os 19 20from scapy.all import * 21 22 23# These constants must stay in sync with `ip_check_defrag.c` 24VETH1_ADDR = "172.16.1.200" 25VETH0_ADDR6 = "fc00::100" 26VETH1_ADDR6 = "fc00::200" 27CLIENT_PORT = 48878 28SERVER_PORT = 48879 29MAGIC_MESSAGE = "THIS IS THE ORIGINAL MESSAGE, PLEASE REASSEMBLE ME" 30 31 32def print_header(f): 33 f.write("// SPDX-License-Identifier: GPL-2.0\n") 34 f.write("/* DO NOT EDIT -- this file is generated */\n") 35 f.write("\n") 36 f.write("#ifndef _IP_CHECK_DEFRAG_FRAGS_H\n") 37 f.write("#define _IP_CHECK_DEFRAG_FRAGS_H\n") 38 f.write("\n") 39 f.write("#include <stdint.h>\n") 40 f.write("\n") 41 42 43def print_frags(f, frags, v6): 44 for idx, frag in enumerate(frags): 45 # 10 bytes per line to keep width in check 46 chunks = [frag[i : i + 10] for i in range(0, len(frag), 10)] 47 chunks_fmted = [", ".join([str(hex(b)) for b in chunk]) for chunk in chunks] 48 suffix = "6" if v6 else "" 49 50 f.write(f"static uint8_t frag{suffix}_{idx}[] = {{\n") 51 for chunk in chunks_fmted: 52 f.write(f"\t{chunk},\n") 53 f.write(f"}};\n") 54 55 56def print_trailer(f): 57 f.write("\n") 58 f.write("#endif /* _IP_CHECK_DEFRAG_FRAGS_H */\n") 59 60 61def main(f): 62 # srcip of 0 is filled in by IP_HDRINCL 63 sip = "0.0.0.0" 64 sip6 = VETH0_ADDR6 65 dip = VETH1_ADDR 66 dip6 = VETH1_ADDR6 67 sport = CLIENT_PORT 68 dport = SERVER_PORT 69 payload = MAGIC_MESSAGE.encode() 70 71 # Disable UDPv4 checksums to keep code simpler 72 pkt = IP(src=sip,dst=dip) / UDP(sport=sport,dport=dport,chksum=0) / Raw(load=payload) 73 # UDPv6 requires a checksum 74 # Also pin the ipv6 fragment header ID, otherwise it's a random value 75 pkt6 = IPv6(src=sip6,dst=dip6) / IPv6ExtHdrFragment(id=0xBEEF) / UDP(sport=sport,dport=dport) / Raw(load=payload) 76 77 frags = [f.build() for f in pkt.fragment(24)] 78 frags6 = [f.build() for f in fragment6(pkt6, 72)] 79 80 print_header(f) 81 print_frags(f, frags, False) 82 print_frags(f, frags6, True) 83 print_trailer(f) 84 85 86if __name__ == "__main__": 87 dir = os.path.dirname(os.path.realpath(__file__)) 88 header = f"{dir}/ip_check_defrag_frags.h" 89 with open(header, "w") as f: 90 main(f) 91