proxyagent.py

# Copyright 2019 The Matrix.org Foundation C.I.C.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
import base64
import logging
import re
from typing import Optional, Tuple
from urllib.request import getproxies_environment, proxy_bypass_environment

import attr
from zope.interface import implementer

from twisted.internet import defer
from twisted.internet.endpoints import HostnameEndpoint, wrapClientTLS
from twisted.internet.interfaces import IReactorCore
from twisted.python.failure import Failure
from twisted.web.client import URI, BrowserLikePolicyForHTTPS, _AgentBase
from twisted.web.error import SchemeNotSupported
from twisted.web.http_headers import Headers
from twisted.web.iweb import IAgent, IPolicyForHTTPS

from synapse.http.connectproxyclient import HTTPConnectProxyEndpoint

logger = logging.getLogger(__name__)

_VALID_URI = re.compile(br"\A[\x21-\x7e]+\Z")


@attr.s
class ProxyCredentials:
    username_password = attr.ib(type=bytes)

    def as_proxy_authorization_value(self) -> bytes:
        """
        Return the value for a Proxy-Authorization header (i.e. 'Basic abdef==').

        Returns:
            A transformation of the authentication string the encoded value for
            a Proxy-Authorization header.
        """
        # Encode as base64 and prepend the authorization type
        return b"Basic " + base64.encodebytes(self.username_password)


@implementer(IAgent)
class ProxyAgent(_AgentBase):
    """An Agent implementation which will use an HTTP proxy if one was requested

    Args:
        reactor: twisted reactor to place outgoing
            connections.

        proxy_reactor: twisted reactor to use for connections to the proxy server
                       reactor might have some blacklisting applied (i.e. for DNS queries),
                       but we need unblocked access to the proxy.

        contextFactory (IPolicyForHTTPS): A factory for TLS contexts, to control the
            verification parameters of OpenSSL.  The default is to use a
            `BrowserLikePolicyForHTTPS`, so unless you have special
            requirements you can leave this as-is.

        connectTimeout (Optional[float]): The amount of time that this Agent will wait
            for the peer to accept a connection, in seconds. If 'None',
            HostnameEndpoint's default (30s) will be used.

            This is used for connections to both proxies and destination servers.

        bindAddress (bytes): The local address for client sockets to bind to.

        pool (HTTPConnectionPool|None): connection pool to be used. If None, a
            non-persistent pool instance will be created.

        use_proxy (bool): Whether proxy settings should be discovered and used
            from conventional environment variables.
            This currently supports http:// and https:// proxies.
            A hostname without scheme is assumed to be http.

    Raises: ValueError if given a proxy with a scheme we don't support.
    """

    def __init__(
        self,
        reactor,
        proxy_reactor=None,
        contextFactory: Optional[IPolicyForHTTPS] = None,
        connectTimeout=None,
        bindAddress=None,
        pool=None,
        use_proxy=False,
    ):
        contextFactory = contextFactory or BrowserLikePolicyForHTTPS()

        _AgentBase.__init__(self, reactor, pool)

        if proxy_reactor is None:
            self.proxy_reactor = reactor
        else:
            self.proxy_reactor = proxy_reactor

        self._endpoint_kwargs = {}
        if connectTimeout is not None:
            self._endpoint_kwargs["timeout"] = connectTimeout
        if bindAddress is not None:
            self._endpoint_kwargs["bindAddress"] = bindAddress

        http_proxy = None
        https_proxy = None
        no_proxy = None
        if use_proxy:
            proxies = getproxies_environment()
            http_proxy = proxies["http"].encode() if "http" in proxies else None
            https_proxy = proxies["https"].encode() if "https" in proxies else None
            no_proxy = proxies["no"] if "no" in proxies else None

        # Parse credentials from https proxy connection string if present
        self.https_proxy_creds, https_proxy = parse_username_password(https_proxy)

        self.http_proxy_endpoint = _http_proxy_endpoint(
            http_proxy, self.proxy_reactor, contextFactory, **self._endpoint_kwargs
        )

        self.https_proxy_endpoint = _http_proxy_endpoint(
            https_proxy, self.proxy_reactor, contextFactory, **self._endpoint_kwargs
        )

        self.no_proxy = no_proxy

        self._policy_for_https = contextFactory
        self._reactor = reactor

    def request(self, method, uri, headers=None, bodyProducer=None):
        """
        Issue a request to the server indicated by the given uri.

        Supports `http` and `https` schemes.

        An existing connection from the connection pool may be used or a new one may be
        created.

        See also: twisted.web.iweb.IAgent.request

        Args:
            method (bytes): The request method to use, such as `GET`, `POST`, etc

            uri (bytes): The location of the resource to request.

            headers (Headers|None): Extra headers to send with the request

            bodyProducer (IBodyProducer|None): An object which can generate bytes to
                make up the body of this request (for example, the properly encoded
                contents of a file for a file upload). Or, None if the request is to
                have no body.

        Returns:
            Deferred[IResponse]: completes when the header of the response has
                 been received (regardless of the response status code).

                 Can fail with:
                    SchemeNotSupported: if the uri is not http or https

                    twisted.internet.error.TimeoutError if the server we are connecting
                        to (proxy or destination) does not accept a connection before
                        connectTimeout.

                    ... other things too.
        """
        uri = uri.strip()
        if not _VALID_URI.match(uri):
            raise ValueError("Invalid URI {!r}".format(uri))

        parsed_uri = URI.fromBytes(uri)
        pool_key = (parsed_uri.scheme, parsed_uri.host, parsed_uri.port)
        request_path = parsed_uri.originForm

        should_skip_proxy = False
        if self.no_proxy is not None:
            should_skip_proxy = proxy_bypass_environment(
                parsed_uri.host.decode(),
                proxies={"no": self.no_proxy},
            )

        if (
            parsed_uri.scheme == b"http"
            and self.http_proxy_endpoint
            and not should_skip_proxy
        ):
            # Cache *all* connections under the same key, since we are only
            # connecting to a single destination, the proxy:
            pool_key = ("http-proxy", self.http_proxy_endpoint)
            endpoint = self.http_proxy_endpoint
            request_path = uri
        elif (
            parsed_uri.scheme == b"https"
            and self.https_proxy_endpoint
            and not should_skip_proxy
        ):
            connect_headers = Headers()

            # Determine whether we need to set Proxy-Authorization headers
            if self.https_proxy_creds:
                # Set a Proxy-Authorization header
                connect_headers.addRawHeader(
                    b"Proxy-Authorization",
                    self.https_proxy_creds.as_proxy_authorization_value(),
                )

            endpoint = HTTPConnectProxyEndpoint(
                self.proxy_reactor,
                self.https_proxy_endpoint,
                parsed_uri.host,
                parsed_uri.port,
                headers=connect_headers,
            )
        else:
            # not using a proxy
            endpoint = HostnameEndpoint(
                self._reactor, parsed_uri.host, parsed_uri.port, **self._endpoint_kwargs
            )

        logger.debug("Requesting %s via %s", uri, endpoint)

        if parsed_uri.scheme == b"https":
            tls_connection_creator = self._policy_for_https.creatorForNetloc(
                parsed_uri.host, parsed_uri.port
            )
            endpoint = wrapClientTLS(tls_connection_creator, endpoint)
        elif parsed_uri.scheme == b"http":
            pass
        else:
            return defer.fail(
                Failure(
                    SchemeNotSupported("Unsupported scheme: %r" % (parsed_uri.scheme,))
                )
            )

        return self._requestWithEndpoint(
            pool_key, endpoint, method, parsed_uri, headers, bodyProducer, request_path
        )


def _http_proxy_endpoint(
    proxy: Optional[bytes],
    reactor: IReactorCore,
    tls_options_factory: IPolicyForHTTPS,
    **kwargs,
):
    """Parses an http proxy setting and returns an endpoint for the proxy

    Args:
        proxy: the proxy setting in the form: [scheme://][<username>:<password>@]<host>[:<port>]
            This currently supports http:// and https:// proxies.
            A hostname without scheme is assumed to be http.

        reactor: reactor to be used to connect to the proxy

        tls_options_factory: the TLS options to use when connecting through a https proxy
        kwargs: other args to be passed to HostnameEndpoint

    Returns:
        interfaces.IStreamClientEndpoint|None: endpoint to use to connect to the proxy,
            or None

    Raises: ValueError if given a proxy with a scheme we don't support.
    """
    if proxy is None:
        return None

    # Note: we can't use urlsplit/urlparse as that is completely broken for things without a scheme://
    scheme, host, port = parse_proxy(proxy)

    if scheme not in (b"http", b"https"):
        raise ValueError("Proxy scheme '{}' not supported".format(scheme.decode()))

    proxy_endpoint = HostnameEndpoint(reactor, host, port, **kwargs)

    if scheme == b"https":
        tls_options = tls_options_factory.creatorForNetloc(host, port)
        proxy_endpoint = wrapClientTLS(tls_options, proxy_endpoint)

    return proxy_endpoint


def parse_username_password(proxy: bytes) -> Tuple[Optional[ProxyCredentials], bytes]:
    """
    Parses the username and password from a proxy declaration e.g
    username:password@hostname:port.

    Args:
        proxy: The proxy connection string.

    Returns
        An instance of ProxyCredentials and the proxy connection string with any credentials
        stripped, i.e u:p@host:port -> host:port. If no credentials were found, the
        ProxyCredentials instance is replaced with None.
    """
    if proxy and b"@" in proxy:
        # We use rsplit here as the password could contain an @ character
        credentials, proxy_without_credentials = proxy.rsplit(b"@", 1)
        return ProxyCredentials(credentials), proxy_without_credentials

    return None, proxy


def parse_proxy(
    proxy: bytes, default_scheme: bytes = b"http", default_port: int = 1080
) -> Tuple[bytes, bytes, int]:
    """
    Parse the scheme, hostname and port from a proxy connection byte string.

    Args:
        proxy: The proxy connection string. Must be in the form '[scheme://]host[:port]'.
        default_scheme: The default scheme to return if one is not found in `proxy`. Defaults to http
        default_port: The default port to return if one is not found in `proxy`. Defaults to 1080

    Returns:
        A tuple containing the scheme, hostname and port.
    """
    # First check if we have a scheme present
    if b"://" in proxy:
        scheme, host = proxy.split(b"://", 1)
    else:
        scheme, host = default_scheme, proxy
    # Now check the leftover part for a port
    if b":" in host:
        new_host, port = host.rsplit(b":", 1)
        try:
            port = int(port)
            return scheme, new_host, port
        except ValueError:
            # the thing after the : wasn't a valid port; presumably this is an
            # IPv6 address.
            # TODO: this doesn't work when the last part of the IP is also just a number.
            #  We probably need to require ipv6's being wrapped in square brackets: [2001:db8:0:0:1::1]
            pass
    return scheme, host, default_port