anti
f715ac6bcd
In-decky-netns NFQUEUE rewriter (window/option-order/IP-ID) and raw-socket synthesizer for nmap probes Linux drops but the target OS answers (T2/T3), driven by os_fingerprint.OS_MANGLE. Packet-shaping logic is pure and unit-tested offline; scapy/netfilterqueue import lazily in the runtime loops. Entry: python -m decnet.cloak (run by the base container; CAP_NET_ADMIN).
132 lines
4.5 KiB
Python
132 lines
4.5 KiB
Python
# SPDX-License-Identifier: AGPL-3.0-or-later
|
|
"""
|
|
Egress SYN-ACK mangler — rewrites the TCP/IP option shape sysctl can't reach.
|
|
|
|
Split so the packet-shaping logic is pure and unit-testable without scapy, root,
|
|
or a live NFQUEUE:
|
|
|
|
- build_synack_options() / next_ipid() : pure, tested offline.
|
|
- _rewrite() : mutates a scapy packet (lazy import).
|
|
- run() : the NFQUEUE loop (needs CAP_NET_ADMIN).
|
|
|
|
scapy/netfilterqueue are imported lazily inside the runtime functions, mirroring
|
|
decnet/prober/tcpfp.py, so importing this module is cheap and side-effect-free.
|
|
"""
|
|
from __future__ import annotations
|
|
|
|
import os
|
|
import signal
|
|
import subprocess # nosec B404 — fixed-arg iptables, no shell
|
|
import sys
|
|
from typing import Any
|
|
|
|
from decnet.logging import get_logger
|
|
from decnet.os_fingerprint import MangleProfile, get_os_mangle
|
|
|
|
log = get_logger("cloak.mangler")
|
|
|
|
_QUEUE = 0
|
|
# Queue every egress packet carrying SYN (covers SYN-ACK incl. ECN/CWR variants);
|
|
# --queue-bypass means a dead handler never blackholes the decky.
|
|
_RULE = [
|
|
"OUTPUT", "-p", "tcp", "--tcp-flags", "SYN", "SYN",
|
|
"-j", "NFQUEUE", "--queue-num", str(_QUEUE), "--queue-bypass",
|
|
]
|
|
|
|
|
|
def next_ipid(prev: int, mode: str) -> int:
|
|
"""Next IP-ID for *mode*: 'incr' (TI=I), 'random' (TI=RD), 'keep' (unchanged).
|
|
|
|
'keep' returns -1 as a sentinel meaning "do not touch the kernel's value".
|
|
"""
|
|
if mode == "incr":
|
|
return (prev + 1) & 0xFFFF
|
|
if mode == "random":
|
|
# Not for security — only to read as randomized to nmap (TI=RD).
|
|
return int.from_bytes(os.urandom(2), "big") or 1
|
|
return -1
|
|
|
|
|
|
def build_synack_options(
|
|
orig_options: list[tuple[str, Any]], profile: MangleProfile
|
|
) -> list[tuple[str, Any]]:
|
|
"""Build the SYN-ACK TCP option list for *profile*, preserving the kernel's
|
|
live Timestamp value (so nmap's SEQ.TS increment-rate test still passes).
|
|
|
|
*orig_options* is a scapy-style ``[(name, value), ...]`` list.
|
|
"""
|
|
ts = next((v for n, v in orig_options if n == "Timestamp"), None)
|
|
out: list[tuple[str, Any]] = []
|
|
for code in profile.option_order:
|
|
if code == "MSS":
|
|
out.append(("MSS", profile.mss))
|
|
elif code == "WScale":
|
|
out.append(("WScale", profile.wscale))
|
|
elif code == "SAckOK":
|
|
out.append(("SAckOK", b""))
|
|
elif code == "NOP":
|
|
out.append(("NOP", None))
|
|
elif code == "TS":
|
|
if ts is not None: # only if sysctl kept timestamps on
|
|
out.append(("Timestamp", ts))
|
|
return out
|
|
|
|
|
|
def _is_synack(flags: int) -> bool:
|
|
return bool(flags & 0x02) and bool(flags & 0x10) # SYN & ACK
|
|
|
|
|
|
def _iptables(action: str) -> None:
|
|
subprocess.run(["iptables", action, *_RULE], check=True) # nosec B603 B607
|
|
|
|
|
|
def run(nmap_os: str) -> int:
|
|
"""Install the NFQUEUE rule and rewrite egress SYN-ACK for *nmap_os*."""
|
|
profile = get_os_mangle(nmap_os)
|
|
if profile is None:
|
|
log.info("cloak.mangler: no profile for %r — nothing to do", nmap_os)
|
|
return 0
|
|
|
|
from netfilterqueue import NetfilterQueue # type: ignore
|
|
from scapy.all import IP, TCP # type: ignore
|
|
|
|
ipid = [0x0400]
|
|
|
|
def _rewrite(pkt: Any) -> None:
|
|
try:
|
|
p = IP(pkt.get_payload())
|
|
if p.haslayer(TCP) and _is_synack(int(p[TCP].flags)):
|
|
p[TCP].window = profile.window
|
|
p[TCP].options = build_synack_options(p[TCP].options, profile)
|
|
nid = next_ipid(ipid[0], profile.ipid)
|
|
if nid >= 0:
|
|
ipid[0] = nid
|
|
p[IP].id = nid
|
|
# options length changed → dataofs MUST be recomputed, else the
|
|
# kernel emits a malformed segment that breaks real connections.
|
|
del p[IP].chksum, p[TCP].chksum, p[IP].len, p[TCP].dataofs
|
|
pkt.set_payload(bytes(p))
|
|
except Exception: # nosec B110 — never drop a packet on a rewrite bug
|
|
log.exception("cloak.mangler: rewrite failed; passing packet through")
|
|
pkt.accept()
|
|
|
|
_iptables("-A")
|
|
nfq = NetfilterQueue()
|
|
nfq.bind(_QUEUE, _rewrite)
|
|
|
|
def _cleanup(*_: Any) -> None:
|
|
try:
|
|
_iptables("-D")
|
|
finally:
|
|
sys.exit(0)
|
|
|
|
signal.signal(signal.SIGTERM, _cleanup)
|
|
signal.signal(signal.SIGINT, _cleanup)
|
|
log.info("cloak.mangler: rewriting SYN-ACK -> %s (window=%#x ipid=%s)",
|
|
nmap_os, profile.window, profile.ipid)
|
|
try:
|
|
nfq.run()
|
|
finally:
|
|
_iptables("-D")
|
|
return 0
|