#!/usr/bin/env python3

import socket
import sys

SRS_DOMAIN = "srs.warenform.de"
SOCKET_HOST = "127.0.0.1"
SOCKET_PORT = 10003
TIMEOUT = 5
MAX_RESPONSE = 4096


def read_netstring(sock):
    length_bytes = bytearray()

    while True:
        char = sock.recv(1)
        if not char:
            raise ValueError("Incomplete netstring length")
        if char == b":":
            break
        if char < b"0" or char > b"9" or len(length_bytes) >= 8:
            raise ValueError("Invalid netstring length")
        length_bytes.extend(char)

    if not length_bytes:
        raise ValueError("Empty netstring length")

    length = int(length_bytes)

    if length > MAX_RESPONSE:
        raise ValueError("Netstring response too large")

    data = bytearray()

    while len(data) < length + 1:
        chunk = sock.recv(length + 1 - len(data))
        if not chunk:
            raise ValueError("Incomplete netstring response")
        data.extend(chunk)

    if data[-1:] != b",":
        raise ValueError("Invalid netstring terminator")

    return bytes(data[:-1]).decode("utf-8")


def verify_srs(address):
    request = f"reverse {address}".encode("utf-8")
    packet = str(len(request)).encode("ascii") + b":" + request + b","

    with socket.create_connection(
        (SOCKET_HOST, SOCKET_PORT), timeout=TIMEOUT
    ) as sock:
        sock.settimeout(TIMEOUT)
        sock.sendall(packet)
        response = read_netstring(sock)

    if response.startswith("OK "):
        original = response[3:]
        if not original or "@" not in original:
            raise ValueError("Invalid reverse result")
        return "OK"

    if response.startswith("NOTFOUND "):
        return "REJECT 550 5.1.1 Invalid SRS address"

    raise ValueError(f"Unexpected socketmap response: {response}")


def check_recipient(recipient):
    if "@" not in recipient:
        return "DUNNO"

    localpart, domain = recipient.rsplit("@", 1)

    if domain.lower() != SRS_DOMAIN:
        return "DUNNO"

    if not localpart.upper().startswith(("SRS0=", "SRS1=")):
        return "REJECT 550 5.1.1 Invalid SRS address"

    try:
        return verify_srs(recipient)
    except (OSError, UnicodeError, ValueError) as exc:
        print(f"SRS verification error: {exc}", file=sys.stderr)
        return "DEFER 451 4.3.0 SRS verification unavailable"


def main():
    while True:
        attributes = {}

        for line in sys.stdin:
            line = line.rstrip("\r\n")

            if not line:
                break

            if "=" in line:
                key, value = line.split("=", 1)
                attributes[key] = value
        else:
            return

        recipient = attributes.get("recipient", "")
        action = check_recipient(recipient)

        print(f"action={action}\n", flush=True)


if __name__ == "__main__":
    main()