#!/usr/bin/env python3
"""svos-whois-auth: Caddy forward_auth target for the SVOS front door on nh3-svos (eshpfi configs/svos-front/).

Caddy calls GET /auth with X-SVOS-Peer = the TCP peer of the accepted connection ({remote_host}:{remote_port},
taken from the socket by Caddy; never from a client header). We ask the local tailscaled (LocalAPI over its unix
socket) who that peer is and answer:
  200 + Tailscale-User-Login: <LoginName>  only if WhoIs resolves the peer AND the login is in the allow-list
  403                                      on anything else: no/garbled peer, WhoIs error, unknown peer, login not
                                           allowed, empty allow-list (fail closed)
The allow-list is /etc/svos-front/allowed-logins: one exact LoginName per line, '#' comments. Re-read per request.
Stdlib only; listens on 127.0.0.1 only.
"""
import http.client
import http.server
import ipaddress
import json
import socket
import sys
import urllib.parse

LISTEN = ("127.0.0.1", 8405)
TAILSCALED_SOCK = "/var/run/tailscale/tailscaled.sock"
ALLOW_FILE = "/etc/svos-front/allowed-logins"
TIMEOUT_S = 3.0


class _UnixHTTPConnection(http.client.HTTPConnection):
    def __init__(self, path):
        super().__init__("local-tailscaled.sock", timeout=TIMEOUT_S)
        self._path = path

    def connect(self):
        s = socket.socket(socket.AF_UNIX, socket.SOCK_STREAM)
        s.settimeout(TIMEOUT_S)
        s.connect(self._path)
        self.sock = s


def allowed_logins():
    try:
        with open(ALLOW_FILE, encoding="utf-8") as f:
            return {ln.strip() for ln in f if ln.strip() and not ln.lstrip().startswith("#")}
    except OSError:
        return set()


def parse_peer(value):
    """'100.64.0.4:51234' or '[fd7a::4]:51234' -> canonical 'ip:port' string, else None."""
    if not value:
        return None
    try:
        if value.startswith("["):
            host, _, port = value[1:].partition("]:")
        else:
            host, _, port = value.rpartition(":")
        ip = ipaddress.ip_address(host)
        port_n = int(port)
        if not 0 < port_n < 65536:
            return None
    except ValueError:
        return None
    return f"[{ip}]:{port_n}" if ip.version == 6 else f"{ip}:{port_n}"


def whois(peer):
    conn = _UnixHTTPConnection(TAILSCALED_SOCK)
    try:
        conn.request("GET", "/localapi/v0/whois?addr=" + urllib.parse.quote(peer, safe=""),
                     headers={"Sec-Tailscale": "localapi"})
        resp = conn.getresponse()
        body = resp.read()
        if resp.status != 200:
            return None
        data = json.loads(body)
        return (data.get("UserProfile") or {}).get("LoginName"), (data.get("Node") or {}).get("Name")
    finally:
        conn.close()


class Handler(http.server.BaseHTTPRequestHandler):
    server_version = "svos-whois-auth"

    def _deny(self, why):
        self.log_message("DENY %s", why)
        self.send_response(403)
        self.send_header("Content-Length", "0")
        self.end_headers()

    def do_GET(self):  # noqa: N802 (stdlib naming)
        if self.client_address[0] != "127.0.0.1":
            return self._deny("non-loopback caller")
        peer = parse_peer(self.headers.get("X-SVOS-Peer", ""))
        if peer is None:
            return self._deny("missing or malformed X-SVOS-Peer")
        try:
            found = whois(peer)
        except Exception as e:  # tailscaled down, timeout, bad JSON: fail closed
            return self._deny(f"whois error for {peer}: {e.__class__.__name__}")
        if not found or not found[0]:
            return self._deny(f"whois: no identity for {peer}")
        login, node = found
        if login not in allowed_logins():
            return self._deny(f"login {login!r} (node {node}) not in {ALLOW_FILE}")
        self.log_message("ALLOW %s login=%r node=%s", peer, login, node)
        self.send_response(200)
        self.send_header("Tailscale-User-Login", str(login))
        self.send_header("Content-Length", "0")
        self.end_headers()

    def log_message(self, format, *args):  # noqa: A002 (stdlib signature)
        sys.stderr.write((format % args) + "\n")


if __name__ == "__main__":
    http.server.ThreadingHTTPServer(LISTEN, Handler).serve_forever()