URL previewing support

README.rst

@@ -104,7 +104,7 @@ Installing prerequisites on Ubuntu or Debian::
 
     sudo apt-get install build-essential python2.7-dev libffi-dev \
                          python-pip python-setuptools sqlite3 \
-                         libssl-dev python-virtualenv libjpeg-dev
+                         libssl-dev python-virtualenv libjpeg-dev libxslt1-dev
 
 Installing prerequisites on ArchLinux::
 
@@ -557,6 +557,23 @@ as the primary means of identity and E2E encryption is not complete. As such,
 we are running a single identity server (https://matrix.org) at the current
 time.
 
+
+URL Previews
+============
+
+Synapse 0.15.0 introduces an experimental new API for previewing URLs at
+/_matrix/media/r0/preview_url.  This is disabled by default.  To turn it on
+you must enable the `url_preview_enabled: True` config parameter and explicitly
+specify the IP ranges that Synapse is not allowed to spider for previewing in
+the `url_preview_ip_range_blacklist` configuration parameter.  This is critical
+from a security perspective to stop arbitrary Matrix users spidering 'internal'
+URLs on your network.  At the very least we recommend that your loopback and
+RFC1918 IP addresses are blacklisted.
+
+This also requires the optional lxml and netaddr python dependencies to be
+installed.
+
+
 Password reset
 ==============
 

UPGRADE.rst

@@ -30,6 +30,14 @@ running:
     python synapse/python_dependencies.py | xargs -n1 pip install
 
 
+Upgrading to v0.15.0
+====================
+
+If you want to use the new URL previewing API (/_matrix/media/r0/preview_url)
+then you have to explicitly enable it in the config and update your dependencies
+dependencies.  See README.rst for details.
+
+
 Upgrading to v0.11.0
 ====================
 

docs/url_previews.rst

@@ -0,0 +1,74 @@
+URL Previews
+============
+
+Design notes on a URL previewing service for Matrix:
+
+Options are:
+
+ 1. Have an AS which listens for URLs, downloads them, and inserts an event that describes their metadata.
+   * Pros:
+     * Decouples the implementation entirely from Synapse.
+     * Uses existing Matrix events & content repo to store the metadata.
+   * Cons:
+     * Which AS should provide this service for a room, and why should you trust it?
+     * Doesn't work well with E2E; you'd have to cut the AS into every room
+     * the AS would end up subscribing to every room anyway.
+
+ 2. Have a generic preview API (nothing to do with Matrix) that provides a previewing service:
+   * Pros:
+     * Simple and flexible; can be used by any clients at any point
+   * Cons:
+     * If each HS provides one of these independently, all the HSes in a room may needlessly DoS the target URI
+     * We need somewhere to store the URL metadata rather than just using Matrix itself
+     * We can't piggyback on matrix to distribute the metadata between HSes.
+
+ 3. Make the synapse of the sending user responsible for spidering the URL and inserting an event asynchronously which describes the metadata.
+   * Pros:
+     * Works transparently for all clients
+     * Piggy-backs nicely on using Matrix for distributing the metadata.
+     * No confusion as to which AS
+   * Cons:
+     * Doesn't work with E2E
+     * We might want to decouple the implementation of the spider from the HS, given spider behaviour can be quite complicated and evolve much more rapidly than the HS.  It's more like a bot than a core part of the server.
+
+ 4. Make the sending client use the preview API and insert the event itself when successful.
+   * Pros:
+      * Works well with E2E
+      * No custom server functionality
+      * Lets the client customise the preview that they send (like on FB)
+   * Cons:
+      * Entirely specific to the sending client, whereas it'd be nice if /any/ URL was correctly previewed if clients support it.
+
+ 5. Have the option of specifying a shared (centralised) previewing service used by a room, to avoid all the different HSes in the room DoSing the target.
+
+Best solution is probably a combination of both 2 and 4.
+ * Sending clients do their best to create and send a preview at the point of sending the message, perhaps delaying the message until the preview is computed?  (This also lets the user validate the preview before sending)
+ * Receiving clients have the option of going and creating their own preview if one doesn't arrive soon enough (or if the original sender didn't create one)
+
+This is a bit magical though in that the preview could come from two entirely different sources - the sending HS or your local one.  However, this can always be exposed to users: "Generate your own URL previews if none are available?"
+
+This is tantamount also to senders calculating their own thumbnails for sending in advance of the main content - we are trusting the sender not to lie about the content in the thumbnail.  Whereas currently thumbnails are calculated by the receiving homeserver to avoid this attack.
+
+However, this kind of phishing attack does exist whether we let senders pick their thumbnails or not, in that a malicious sender can send normal text messages around the attachment claiming it to be legitimate.  We could rely on (future) reputation/abuse management to punish users who phish (be it with bogus metadata or bogus descriptions).   Bogus metadata is particularly bad though, especially if it's avoidable.
+
+As a first cut, let's do #2 and have the receiver hit the API to calculate its own previews (as it does currently for image thumbnails).  We can then extend/optimise this to option 4 as a special extra if needed.
+
+API
+---
+
+GET /_matrix/media/r0/preview_url?url=http://wherever.com
+200 OK
+{
+    "og:type"        : "article"
+    "og:url"         : "https://twitter.com/matrixdotorg/status/684074366691356672"
+    "og:title"       : "Matrix on Twitter"
+    "og:image"       : "https://pbs.twimg.com/profile_images/500400952029888512/yI0qtFi7_400x400.png"
+    "og:description" : "“Synapse 0.12 is out! Lots of polishing, performance & bugfixes: /sync API, /r0 prefix, fulltext search, 3PID invites https://t.co/5alhXLLEGP”"
+    "og:site_name"   : "Twitter"
+}
+
+* Downloads the URL
+  * If HTML, just stores it in RAM and parses it for OG meta tags
+    * Download any media OG meta tags to the media repo, and refer to them in the OG via mxc:// URIs.
+  * If a media filetype we know we can thumbnail: store it on disk, and hand it to the thumbnailer. Generate OG meta tags from the thumbnailer contents.
+  * Otherwise, don't bother downloading further.

synapse/config/repository.py

@@ -16,14 +16,16 @@
 from ._base import Config
 from collections import namedtuple
 
+import sys
+
 ThumbnailRequirement = namedtuple(
     "ThumbnailRequirement", ["width", "height", "method", "media_type"]
 )
 
 
 def parse_thumbnail_requirements(thumbnail_sizes):
     """ Takes a list of dictionaries with "width", "height", and "method" keys
-    and creates a map from image media types to the thumbnail size, thumnailing
+    and creates a map from image media types to the thumbnail size, thumbnailing
     method, and thumbnail media type to precalculate
 
     Args:
@@ -53,12 +55,25 @@ class ContentRepositoryConfig(Config):
     def read_config(self, config):
         self.max_upload_size = self.parse_size(config["max_upload_size"])
         self.max_image_pixels = self.parse_size(config["max_image_pixels"])
+        self.max_spider_size = self.parse_size(config["max_spider_size"])
         self.media_store_path = self.ensure_directory(config["media_store_path"])
         self.uploads_path = self.ensure_directory(config["uploads_path"])
         self.dynamic_thumbnails = config["dynamic_thumbnails"]
         self.thumbnail_requirements = parse_thumbnail_requirements(
             config["thumbnail_sizes"]
         )
+        self.url_preview_enabled = config["url_preview_enabled"]
+        if self.url_preview_enabled:
+            try:
+                from netaddr import IPSet
+                if "url_preview_ip_range_blacklist" in config:
+                    self.url_preview_ip_range_blacklist = IPSet(
+                        config["url_preview_ip_range_blacklist"]
+                    )
+                if "url_preview_url_blacklist" in config:
+                    self.url_preview_url_blacklist = config["url_preview_url_blacklist"]
+            except ImportError:
+                sys.stderr.write("\nmissing netaddr dep - disabling preview_url API\n")
 
     def default_config(self, **kwargs):
         media_store = self.default_path("media_store")
@@ -80,7 +95,7 @@ def default_config(self, **kwargs):
         # the resolution requested by the client. If true then whenever
         # a new resolution is requested by the client the server will
         # generate a new thumbnail. If false the server will pick a thumbnail
-        # from a precalcualted list.
+        # from a precalculated list.
         dynamic_thumbnails: false
 
         # List of thumbnail to precalculate when an image is uploaded.
@@ -100,4 +115,62 @@ def default_config(self, **kwargs):
         - width: 800
           height: 600
           method: scale
+
+        # Is the preview URL API enabled?  If enabled, you *must* specify
+        # an explicit url_preview_ip_range_blacklist of IPs that the spider is
+        # denied from accessing.
+        url_preview_enabled: False
+
+        # List of IP address CIDR ranges that the URL preview spider is denied
+        # from accessing.  There are no defaults: you must explicitly
+        # specify a list for URL previewing to work.  You should specify any
+        # internal services in your network that you do not want synapse to try
+        # to connect to, otherwise anyone in any Matrix room could cause your
+        # synapse to issue arbitrary GET requests to your internal services,
+        # causing serious security issues.
+        #
+        # url_preview_ip_range_blacklist:
+        # - '127.0.0.0/8'
+        # - '10.0.0.0/8'
+        # - '172.16.0.0/12'
+        # - '192.168.0.0/16'
+
+        # Optional list of URL matches that the URL preview spider is
+        # denied from accessing.  You should use url_preview_ip_range_blacklist
+        # in preference to this, otherwise someone could define a public DNS
+        # entry that points to a private IP address and circumvent the blacklist.
+        # This is more useful if you know there is an entire shape of URL that
+        # you know that will never want synapse to try to spider.
+        #
+        # Each list entry is a dictionary of url component attributes as returned
+        # by urlparse.urlsplit as applied to the absolute form of the URL.  See
+        # https://docs.python.org/2/library/urlparse.html#urlparse.urlsplit
+        # The values of the dictionary are treated as an filename match pattern
+        # applied to that component of URLs, unless they start with a ^ in which
+        # case they are treated as a regular expression match.  If all the
+        # specified component matches for a given list item succeed, the URL is
+        # blacklisted.
+        #
+        # url_preview_url_blacklist:
+        # # blacklist any URL with a username in its URI
+        # - username: '*'
+        #
+        # # blacklist all *.google.com URLs
+        # - netloc: 'google.com'
+        # - netloc: '*.google.com'
+        #
+        # # blacklist all plain HTTP URLs
+        # - scheme: 'http'
+        #
+        # # blacklist http(s)://www.acme.com/foo
+        # - netloc: 'www.acme.com'
+        #   path: '/foo'
+        #
+        # # blacklist any URL with a literal IPv4 address
+        # - netloc: '^[0-9]+\.[0-9]+\.[0-9]+\.[0-9]+$'
+
+        # The largest allowed URL preview spidering size in bytes
+        max_spider_size: "10M"
+
+
         """ % locals()