furtka/furtka/https.py

"""Local-CA HTTPS helpers for the `tls internal` setup.

Caddy generates the local root CA lazily on first start and keeps it under
$XDG_DATA_HOME/caddy/pki/authorities/local/ — our packaged caddy.service
sets `XDG_DATA_HOME=/var/lib`, so on the target that resolves to
/var/lib/caddy/pki/authorities/local/. The private key stays 0600 /
caddy-owned; we only ever read the public root.crt next to it.

HTTPS is **opt-in** since 26.15-alpha. Default Caddyfile has no `:443`
site block, so `tls internal` never triggers cert issuance. The
/settings toggle drops a snippet file into /etc/caddy/furtka-https.d/
that adds the hostname+tls-internal block (plus the redirect snippet
inside /etc/caddy/furtka.d/ for HTTP→HTTPS). Disabling the toggle
removes both snippets and reloads — Caddy falls back to HTTP-only.

Why opt-in: fresh-install boxes used to always serve a self-signed
cert on :443. Any browser that had ever trusted a previous Furtka
box's local CA rejected the new cert with an unbypassable
SEC_ERROR_BAD_SIGNATURE — Firefox in particular has no "Advanced →
Accept" for that case. Making HTTPS explicit means fresh installs
never hit that trap; users who want HTTPS download the rootCA.crt
first and then click the toggle.

This module exposes:
- status(): CA fingerprint + current toggle state
- set_force_https(enabled): write/remove BOTH snippets atomically,
  reload Caddy, roll back on failure.
"""

import base64
import hashlib
import re
import subprocess
from pathlib import Path

CA_CERT_PATH = Path("/var/lib/caddy/pki/authorities/local/root.crt")
SNIPPET_DIR = Path("/etc/caddy/furtka.d")
REDIRECT_SNIPPET = SNIPPET_DIR / "redirect.caddyfile"
REDIRECT_CONTENT = "redir https://{host}{uri} permanent\n"
HTTPS_SNIPPET_DIR = Path("/etc/caddy/furtka-https.d")
HTTPS_SNIPPET = HTTPS_SNIPPET_DIR / "https.caddyfile"
HOSTNAME_FILE = Path("/etc/hostname")

_PEM_RE = re.compile(
    r"-----BEGIN CERTIFICATE-----\s*(.+?)\s*-----END CERTIFICATE-----",
    re.DOTALL,
)


class HttpsError(Exception):
    """Recoverable failure from set_force_https — the caller should 5xx."""


def _read_hostname(hostname_file: Path = HOSTNAME_FILE) -> str:
    """Return the box's hostname, stripped. Falls back to 'furtka' so a
    missing /etc/hostname doesn't produce an empty site block that Caddy
    would reject at parse time."""
    try:
        value = hostname_file.read_text().strip()
    except (FileNotFoundError, PermissionError, OSError):
        return "furtka"
    return value or "furtka"


def _https_snippet_content(hostname: str) -> str:
    """Caddy site block the HTTPS toggle installs at opt-in.

    Serves <hostname>.local and <hostname> on :443 with Caddy's
    `tls internal` (local CA auto-issuance), and imports the shared
    furtka_routes snippet so the :443 listener exposes the same
    routes as :80. Must be written at top-level (not inside another
    site block) — that's why the Caddyfile imports furtka-https.d at
    top-level rather than inside :80.
    """
    return f"{hostname}.local, {hostname} {{\n\ttls internal\n\timport furtka_routes\n}}\n"


def _ca_fingerprint(ca_path: Path) -> str | None:
    try:
        pem = ca_path.read_text()
    except (FileNotFoundError, PermissionError, IsADirectoryError):
        return None
    match = _PEM_RE.search(pem)
    if not match:
        return None
    try:
        der = base64.b64decode("".join(match.group(1).split()))
    except (ValueError, base64.binascii.Error):
        return None
    return hashlib.sha256(der).hexdigest().upper()


def _format_fingerprint(hex_upper: str) -> str:
    return ":".join(hex_upper[i : i + 2] for i in range(0, len(hex_upper), 2))


def status(
    ca_path: Path = CA_CERT_PATH,
    https_snippet: Path = HTTPS_SNIPPET,
) -> dict:
    """force_https is True iff the HTTPS listener snippet exists.

    Before 26.15-alpha this checked the redirect snippet instead — but
    the redirect alone without a :443 listener wouldn't actually serve
    HTTPS, so the listener snippet is the authoritative "HTTPS is on"
    signal.
    """
    fp = _ca_fingerprint(ca_path)
    return {
        "ca_available": fp is not None,
        "fingerprint_sha256": _format_fingerprint(fp) if fp else None,
        "force_https": https_snippet.is_file(),
        "ca_download_url": "/rootCA.crt",
    }


def _default_reload() -> None:
    subprocess.run(
        ["systemctl", "reload", "caddy"],
        check=True,
        capture_output=True,
        text=True,
    )


def set_force_https(
    enabled: bool,
    snippet_dir: Path = SNIPPET_DIR,
    snippet: Path = REDIRECT_SNIPPET,
    https_snippet_dir: Path = HTTPS_SNIPPET_DIR,
    https_snippet: Path = HTTPS_SNIPPET,
    hostname_file: Path = HOSTNAME_FILE,
    reload_caddy=_default_reload,
) -> bool:
    """Toggle HTTPS by writing or removing two snippets atomically:

    1. The top-level HTTPS hostname+tls-internal block (enables :443
       listener + Caddy's `tls internal` cert issuance)
    2. The :80-scoped redirect snippet (forces HTTP → HTTPS)

    Reload Caddy after the snippet swap. On reload failure both
    snippets are reverted to their pre-call state so a bad config
    can't leave Caddy wedged.
    """
    snippet_dir.mkdir(mode=0o755, parents=True, exist_ok=True)
    https_snippet_dir.mkdir(mode=0o755, parents=True, exist_ok=True)

    had_redirect = snippet.is_file()
    previous_redirect = snippet.read_text() if had_redirect else None
    had_https = https_snippet.is_file()
    previous_https = https_snippet.read_text() if had_https else None

    if enabled:
        snippet.write_text(REDIRECT_CONTENT)
        https_snippet.write_text(_https_snippet_content(_read_hostname(hostname_file)))
    else:
        if had_redirect:
            snippet.unlink()
        if had_https:
            https_snippet.unlink()

    try:
        reload_caddy()
    except subprocess.CalledProcessError as e:
        _revert(snippet, previous_redirect)
        _revert(https_snippet, previous_https)
        msg = (e.stderr or e.stdout or "").strip() or f"exit {e.returncode}"
        raise HttpsError(f"caddy reload failed: {msg}") from e
    except FileNotFoundError as e:
        _revert(snippet, previous_redirect)
        _revert(https_snippet, previous_https)
        raise HttpsError(f"systemctl not available: {e}") from e
    return enabled


def _revert(snippet: Path, previous: str | None) -> None:
    if previous is None:
        try:
            snippet.unlink()
        except FileNotFoundError:
            pass
    else:
        snippet.write_text(previous)