webmentions_ssg/url_security.py


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import ipaddress
from urllib.parse import urlsplit

import dns.exception
import dns.resolver


class AddressResolutionError(Exception):
    pass


class NonPublicAddressError(Exception):
    pass


def ensure_public_url(url: str) -> None:
    hostname = urlsplit(url).hostname

    if hostname is None:
        raise NonPublicAddressError("URL has no hostname")

    try:
        answers = dns.resolver.resolve_name(hostname)
    except dns.exception.DNSException as exc:
        raise AddressResolutionError(
            f"Could not resolve hostname {hostname!r}"
        ) from exc

    addresses = {ipaddress.ip_address(address) for address in answers.addresses()}

    if not addresses:
        raise AddressResolutionError(f"Hostname {hostname!r} did not resolve")

    if any(not address.is_global for address in addresses):
        raise NonPublicAddressError(
            f"Hostname {hostname!r} resolves to a non-public address"
        )