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Solving the CORS Streaming Problem A Practical Approach with M3U8 and Flask

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    Ashlin Darius Govindasamy
    Twitter

Solving the CORS Streaming Problem: A Practical Approach with M3U8 and Flask

Introduction

Streaming media content efficiently and securely is a cornerstone of modern web applications. However, developers often encounter challenges related to Cross-Origin Resource Sharing (CORS), especially when dealing with M3U8 playlists in HTTP Live Streaming (HLS). This article explores the CORS streaming problem and presents a practical solution using a Flask-based proxy server to seamlessly stream M3U8 content across different origins.

GitHub

https://github.com/adgsenpai/m3u8server

Understanding the CORS Streaming Problem

CORS is a security feature implemented in web browsers that restricts web pages from making requests to a different domain than the one that served the web page. While essential for protecting users, CORS can inadvertently block legitimate requests, such as streaming media from external servers.

M3U8 files are playlists used in HLS to list the media segments (typically .ts files) that a client needs to fetch to play streaming content. When these media segments are hosted on a different origin than the web application, CORS policies can prevent the client from accessing them, resulting in playback failures.

Common Scenarios Leading to CORS Issues

  1. Third-Party Hosting: Media segments are hosted on external servers that do not permit cross-origin requests.
  2. Dynamic Content Delivery: Media segments are served from dynamically generated URLs with varying origins.
  3. Content Security Policies: Strict CORS configurations on media servers restrict access from unauthorized origins.

The Flask-Based Proxy Solution

To overcome CORS restrictions without altering the original media servers, a proxy server can be employed. The proxy modifies M3U8 playlists and streams media segments through a controlled origin that is permitted by CORS policies.

How It Works

  1. Intercepting Playlist Requests: The client requests the M3U8 playlist through the proxy server.
  2. Modifying Playlist URLs: The proxy server fetches the original M3U8 file, parses it, and rewrites the URLs of the media segments to route through the proxy.
  3. Serving Modified Playlists: The client receives the modified M3U8 playlist, ensuring all subsequent media segment requests go through the proxy.
  4. Handling Media Segments: When the client requests a media segment, the proxy fetches it from the original server and serves it to the client, adhering to CORS policies.

Benefits of Using a Proxy

  • Bypasses CORS Restrictions: Routes all media requests through a single, controlled origin.
  • Enhances Performance: Implements caching strategies to reduce latency and server load.
  • Maintains Security: Keeps the original media servers untouched, preserving their security configurations.

Implementation with Flask

Flask, a lightweight Python web framework, is well-suited for creating a proxy server. Below is a simplified overview of the implementation steps.

Setting Up the Flask Application

  1. Install Dependencies:

    pip install Flask requests m3u8 cachetools Flask-CORS

  2. Create the Proxy Server:

    from flask import Flask, request, Response, abort, make_response
import requests
import m3u8
from urllib.parse import urljoin, urlencode
from cachetools import TTLCache
import threading
from flask_cors import CORS

app = Flask(__name__)
CORS(app, resources={r"/proxy": {"origins": "*"}})

PROXY_BASE_URL = "https://yourdomain.com/proxy"

# Caches for playlists and segments with a TTL of 5 minutes
playlist_cache = TTLCache(maxsize=1000, ttl=300)
segment_cache = TTLCache(maxsize=10000, ttl=300)
cache_lock = threading.Lock()

def is_m3u8(url):
    return url.endswith('.m3u8')

def get_filename(url):
    return url.split('/')[-1] or 'file'

@app.route('/proxy')
def proxy():
    target_url = request.args.get('url')
    if not target_url:
        abort(400, description="Missing 'url' parameter.")

    filename = get_filename(target_url)

    if is_m3u8(target_url):
        with cache_lock:
            cached_playlist = playlist_cache.get(target_url)
        if cached_playlist:
            resp = make_response(cached_playlist)
            resp.headers['Content-Type'] = 'application/vnd.apple.mpegurl'
            resp.headers['Content-Disposition'] = f'inline; filename="{filename}"'
            return resp

        try:
            response = requests.get(target_url, timeout=10)
            response.raise_for_status()
        except requests.RequestException as e:
            abort(502, description=f"Error fetching the URL: {e}")

        try:
            playlist = m3u8.loads(response.text)
        except Exception as e:
            abort(500, description=f"Error parsing M3U8 file: {e}")

        base_url = response.url

        for segment in playlist.segments:
            absolute_uri = urljoin(base_url, segment.uri)
            proxied_uri = f"{PROXY_BASE_URL}?{urlencode({'url': absolute_uri})}"
            segment.uri = proxied_uri

        if playlist.is_variant:
            for playlist_variant in playlist.playlists:
                original_uri = playlist_variant.uri
                absolute_uri = urljoin(base_url, original_uri)
                proxied_uri = f"{PROXY_BASE_URL}?{urlencode({'url': absolute_uri})}"
                playlist_variant.uri = proxied_uri

        modified_playlist = playlist.dumps()

        with cache_lock:
            playlist_cache[target_url] = modified_playlist

        resp = make_response(modified_playlist)
        resp.headers['Content-Type'] = 'application/vnd.apple.mpegurl'
        resp.headers['Content-Length'] = len(modified_playlist)
        resp.headers['Content-Disposition'] = f'inline; filename="{filename}"'
        return resp

    else:
        with cache_lock:
            cached_segment = segment_cache.get(target_url)
        if cached_segment:
            resp = Response(cached_segment, mimetype='video/MP2T')
            resp.headers['Content-Disposition'] = f'inline; filename="{filename}"'
            return resp

        try:
            response = requests.get(target_url, timeout=10)
            response.raise_for_status()
        except requests.RequestException as e:
            abort(502, description=f"Error fetching the URL: {e}")

        content_type = response.headers.get('Content-Type', '')
        mime_type = 'video/MP2T' if target_url.endswith('.ts') else (content_type or 'application/octet-stream')

        content = response.content

        with cache_lock:
            segment_cache[target_url] = content

        resp = Response(content, mimetype=mime_type)
        resp.headers['Content-Disposition'] = f'inline; filename="{filename}"'
        resp.headers['Content-Length'] = len(content)
        return resp

if __name__ == '__main__':
    app.run(host='0.0.0.0', port=8000, debug=True)

How the Proxy Server Works

  1. Handling Playlist Requests:

    • When the client requests an M3U8 playlist via the proxy, the server checks if the modified playlist is cached.
    • If cached, it serves the playlist directly from the cache.
    • If not, it fetches the original playlist, modifies the segment URLs to route through the proxy, caches the modified playlist, and serves it to the client.

  2. Handling Media Segment Requests:

    • When the client requests a media segment (e.g., .ts file) via the proxy, the server checks if the segment is cached.
    • If cached, it serves the segment directly from the cache.
    • If not, it fetches the segment from the original server, caches it, and serves it to the client.

  3. Caching Mechanism:

    • Playlist Cache (playlist_cache): Stores modified M3U8 playlists for 5 minutes to reduce repeated processing.
    • Segment Cache (segment_cache): Stores media segments for 5 minutes to minimize redundant network requests.

  4. Thread Safety:

    • A threading lock (cache_lock) ensures that cache access is thread-safe, preventing race conditions in a multi-threaded environment.

  5. CORS Handling:

    • The Flask-CORS extension is configured to allow cross-origin requests to the proxy endpoint, ensuring that the client can access the modified playlists and segments without CORS issues.

Practical Examples

Original M3U8 Playlist

#EXTM3U
#EXT-X-VERSION:3
#EXT-X-TARGETDURATION:10
#EXTINF:10.0,
segment1.ts
#EXTINF:10.0,
segment2.ts
#EXT-X-ENDLIST

Modified M3U8 Playlist via Proxy

#EXTM3U
#EXT-X-VERSION:3
#EXT-X-TARGETDURATION:10
#EXTINF:10.0,
https://yourdomain.com/proxy?url=https%3A%2F%2Foriginalserver.com%2Fsegment1.ts
#EXTINF:10.0,
https://yourdomain.com/proxy?url=https%3A%2F%2Foriginalserver.com%2Fsegment2.ts
#EXT-X-ENDLIST

Explanation:

  • The proxy modifies each segment URL to route through https://yourdomain.com/proxy, ensuring that all media segment requests comply with CORS policies.

Caching Example

  1. First Request:

    • Client requests the modified playlist via the proxy.
    • Proxy fetches and modifies the original playlist.
    • Modified playlist is cached.
    • Client receives the modified playlist.

  2. Subsequent Requests Within 5 Minutes:

    • Client requests the same playlist via the proxy.
    • Proxy serves the playlist from the cache.
    • Reduces latency and server load.

Conclusion

Addressing CORS restrictions in M3U8 streaming is crucial for ensuring seamless media playback across different origins. By implementing a Flask-based proxy server that modifies M3U8 playlists and streams media segments through a controlled origin, developers can effectively bypass CORS limitations without compromising performance or security. This practical approach, complemented by efficient caching mechanisms and thread-safe operations, offers a robust solution to the CORS streaming problem in modern web applications.

Appendix

Complete Source Code

from flask import Flask, request, Response, abort, make_response
import requests
import m3u8
from urllib.parse import urlparse, urljoin, urlencode
from cachetools import TTLCache
import threading
from flask_cors import CORS

app = Flask(__name__)

# Enable CORS for all routes and origins
CORS(app, resources={r"/proxy": {"origins": "*"}})

# Configuration
PROXY_BASE_URL = "https://yourdomain.com/proxy"

# Create a cache with a Time-To-Live (TTL) of 5 minutes for playlists and segments
playlist_cache = TTLCache(maxsize=1000, ttl=300)  # 5 minutes TTL
segment_cache = TTLCache(maxsize=10000, ttl=300)   # 5 minutes TTL

# Lock for thread-safe cache access
cache_lock = threading.Lock()

# Helper function to determine if the URL points to an M3U8 file
def is_m3u8(url):
    return url.endswith('.m3u8')

# Helper function to extract filename from URL
def get_filename(url):
    return url.split('/')[-1] or 'file'

@app.route('/proxy')
def proxy():
    target_url = request.args.get('url')
    if not target_url:
        abort(400, description="Missing 'url' parameter.")

    filename = get_filename(target_url)

    if is_m3u8(target_url):
        with cache_lock:
            cached_playlist = playlist_cache.get(target_url)
        if cached_playlist:
            resp = make_response(cached_playlist)
            resp.headers['Content-Type'] = 'application/vnd.apple.mpegurl'
            resp.headers['Content-Disposition'] = f'inline; filename="{filename}"'
            return resp

        try:
            # Fetch the M3U8 playlist
            response = requests.get(target_url, timeout=10)
            response.raise_for_status()
        except requests.RequestException as e:
            abort(502, description=f"Error fetching the URL: {e}")

        try:
            # Parse the M3U8 playlist
            playlist = m3u8.loads(response.text)
        except Exception as e:
            abort(500, description=f"Error parsing M3U8 file: {e}")

        base_url = response.url  # Final URL after redirects

        # Modify each segment URI to route through the proxy
        for segment in playlist.segments:
            absolute_uri = urljoin(base_url, segment.uri)
            proxied_uri = f"{PROXY_BASE_URL}?{urlencode({'url': absolute_uri})}"
            segment.uri = proxied_uri

        # Handle variant playlists if present
        if playlist.is_variant:
            for playlist_variant in playlist.playlists:
                original_uri = playlist_variant.uri
                absolute_uri = urljoin(base_url, original_uri)
                proxied_uri = f"{PROXY_BASE_URL}?{urlencode({'url': absolute_uri})}"
                playlist_variant.uri = proxied_uri

        # Serialize the modified playlist
        modified_playlist = playlist.dumps()

        # Cache the modified playlist
        with cache_lock:
            playlist_cache[target_url] = modified_playlist

        resp = make_response(modified_playlist)
        resp.headers['Content-Type'] = 'application/vnd.apple.mpegurl'
        resp.headers['Content-Length'] = len(modified_playlist)
        resp.headers['Content-Disposition'] = f'inline; filename="{filename}"'
        return resp

    else:
        with cache_lock:
            cached_segment = segment_cache.get(target_url)
        if cached_segment:
            resp = Response(cached_segment, mimetype='video/MP2T')
            resp.headers['Content-Disposition'] = f'inline; filename="{filename}"'
            return resp

        try:
            # Fetch the media segment
            response = requests.get(target_url, timeout=10)
            response.raise_for_status()
        except requests.RequestException as e:
            abort(502, description=f"Error fetching the URL: {e}")

        content_type = response.headers.get('Content-Type', '')
        mime_type = 'video/MP2T' if target_url.endswith('.ts') else (content_type or 'application/octet-stream')

        content = response.content

        with cache_lock:
            segment_cache[target_url] = content

        resp = Response(content, mimetype=mime_type)
        resp.headers['Content-Disposition'] = f'inline; filename="{filename}"'
        resp.headers['Content-Length'] = len(content)
        return resp

if __name__ == '__main__':
    # Run the Flask app
    # For production, consider using a production-ready server like Gunicorn
    app.run(host='0.0.0.0', port=8000, debug=True)

References

  1. W3C. (2014). Cross-Origin Resource Sharing (CORS). https://www.w3.org/TR/cors/
  2. Apple Inc. (2017). HTTP Live Streaming. https://developer.apple.com/streaming/
  3. Cachetools Documentation. https://cachetools.readthedocs.io/en/stable/
  4. Flask Documentation. https://flask.palletsprojects.com/
  5. Gunicorn Documentation. https://gunicorn.org/