1 files changed, 92 insertions, 0 deletions
diff --git a/debian/tests/mitm.py b/debian/tests/mitm.py
new file mode 100644
index 0000000..cd2cd75
--- /dev/null
+++ b/debian/tests/mitm.py
@@ -0,0 +1,92 @@
+"""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}")