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)