1 #!/bin/env python3
2 # SPDX-License-Identifier: GPL-2.0
3
4 """
5 This script helps generate fragmented UDP packets.
6
7 While it is technically possible to dynamically generate
8 fragmented packets in C, it is much harder to read and write
9 said code. `scapy` is relatively industry standard and really
10 easy to read / write.
11
12 So we choose to write this script that generates a valid C
13 header. Rerun script and commit generated file after any
14 modifications.
15 """
16
17 import argparse
18 import os
19
20 from scapy.all import *
21
22
23 # These constants must stay in sync with `ip_check_defrag.c`
24 VETH1_ADDR = "172.16.1.200"
25 VETH0_ADDR6 = "fc00::100"
26 VETH1_ADDR6 = "fc00::200"
27 CLIENT_PORT = 48878
28 SERVER_PORT = 48879
29 MAGIC_MESSAGE = "THIS IS THE ORIGINAL MESSAGE, PLEASE REASSEMBLE ME"
30
31
32 def 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
43 def 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
56 def print_trailer(f):
57 f.write("\n")
58 f.write("#endif /* _IP_CHECK_DEFRAG_FRAGS_H */\n")
59
60
61 def 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
86 if __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)
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