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"""Flask app which MITM's an archive to generate out-of-order apt lists.
Specifically, it prepends an additional Packages file stanza for a non-existent
lower version of apt: a fixed version of debootstrap will find the second
(correct) apt stanza and succeed; a broken version of debootstrap will find
only the first (non-existent) apt stanza and fail.
"""
import functools
import gzip
import hashlib
import os
import requests
from debian.deb822 import Packages
from flask import Flask, redirect
app = Flask(__name__)
ARCH = os.environ.get("FLASK_ARCH", "amd64")
DIST = os.environ.get("FLASK_DIST", "bookworm")
DISTRO = os.environ.get("FLASK_DISTRO", "debian")
MIRROR = os.environ.get("FLASK_MIRROR", "http://deb.debian.org")
if DISTRO == "debian":
hash_funcs = [hashlib.md5, hashlib.sha256]
else:
# Ubuntu includes SHA1 still
hash_funcs = [hashlib.md5, hashlib.sha1, hashlib.sha256]
def _munge_release_file(url: str) -> bytes:
"""Given a Release file URL, rewrite it for our modified Packages content."""
original = requests.get(MIRROR + "/" + url).content
packages_content = _packages_content(
f"{DISTRO}/dists/{DIST}/main/binary-{ARCH}/Packages"
)
size = bytes(str(len(packages_content)), "ascii")
sums = [
bytes(hash_func(packages_content).hexdigest(), "ascii")
for hash_func in hash_funcs
]
new_lines = []
filename = f"main/binary-{ARCH}/Packages".encode("ascii")
for line in original.splitlines():
if not line.endswith(filename):
new_lines.append(line)
continue
new_lines.append(b" ".join([b"", sums.pop(0), size, filename]))
return b"\n".join(new_lines)
@functools.lru_cache
def _packages_content(url: str) -> bytes:
"""Given a Packages URL, fetch it and prepend a broken apt stanza."""
resp = requests.get(MIRROR + "/" + url + ".gz")
upstream_content = gzip.decompress(resp.content)
# Find the first `apt` stanza
for stanza in Packages.iter_paragraphs(upstream_content):
if stanza["Package"] == "apt":
break
# Generate the broken stanza
new_version = stanza["Version"] + "~test"
stanza["Filename"] = stanza["Filename"].replace(stanza["Version"], new_version)
stanza["Version"] = new_version
# Prepend the stanza to the upstream content
return bytes(stanza) + b"\n" + upstream_content
@app.route("/<path:url>", methods=["GET", "POST"])
def root(url):
"""Handler for all requests."""
if (
url == f"{DISTRO}/dists/{DIST}/InRelease"
or "by-hash" in url
or "Packages.xz" in url
or "Packages.gz" in url
):
# 404 these URLs to force clients to fetch by path and without compression, to
# make MITM easier
return "", 404
if url == f"{DISTRO}/dists/{DIST}/Release":
# If Release is being fetched, return our modified version
return _munge_release_file(url)
if url == f"{DISTRO}/dists/{DIST}/main/binary-{ARCH}/Packages":
# If Packages is being fetched, return our modified version
return _packages_content(url)
# For anything we don't need to modify, redirect clients to upstream mirror
return redirect(f"{MIRROR}/{url}")
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