Since my app has background tasks, I use the Flask context. For the context to work, the Flask setting SERVER_NAME should be set.
When the SERVER_NAME is set the incoming requests are checked to match this value or the route isn’t found. When placing an nginx (or other webserver in front), the SERVER_NAME should also include the port and the reverse proxy should handle the rewrite stuff, hiding the port number from the outside world (which it does).
For session cookies to work in modern browsers, the URL name passed by the proxy should be the same as the SERVER_NAME, otherwise the browser refuses to send the cookies. This can be solved by adding the official hostname in the /etc/hosts and setting it to 127.0.0.1.
There is one thing that I haven’t figured out yet and it is the URL in the background tasks. url_for() is used with the _external option to generate URLs in the mail it sends out. But that URL includes the port, which is different from the 443 port used by my nginx instance.
Removing the port from the SERVER_NAME makes the stuff described in the first paragraph fail.
So what are my best options for handling the url_for in the mail. Create a separate config setting? Create my own url_for?
Okay, this is a classic problem with Flask apps behind reverse proxies, especially when dealing with background tasks and URL generation. Here\’s a breakdown of the situation and the best options, combining practicality with best practices:\n\n**Understanding the Core Issue**\n\nYou\’re facing a mismatch between the internal Flask environment (where the port is necessary for correct routing and session handling) and the external, user-facing URL (where the port should be hidden by the reverse proxy). `url_for(_external=True)` picks up the `SERVER_NAME` and generates URLs including the internal port, which is undesirable for external use (like in emails).\n\n**Prioritized Options (Best to Worst)**\n\nHere\’s a breakdown of options, ranked by effectiveness, maintainability, and adherence to best practices:\n\n1. **Recommended: Configure the Proxy to Forward the Correct Host Header and Use `PREFERRED_URL_SCHEME`:**\n\n * **Explanation:** This is the *cleanest* and most robust solution if your reverse proxy supports it. The key is to ensure the proxy *correctly* forwards the original host and scheme (HTTP/HTTPS) to your Flask application. Then, you use Flask\’s `PREFERRED_URL_SCHEME` to handle HTTPS.\n\n * **Implementation:**\n\n * **Nginx Configuration (Example):** Make sure your Nginx configuration includes these crucial lines within your server block:\n\n “`nginx\n server {\n listen 443 ssl;\n server_name yourdomain.com; # Replace with your actual domain\n\n location / {\n proxy_pass http://127.0.0.1:5000; # Or your Flask app\’s address\n proxy_set_header Host $host; # Important: Pass the original host\n proxy_set_header X-Real-IP $remote_addr; # Important for logging\n proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;\n proxy_set_header X-Forwarded-Proto $scheme; # Crucial for HTTPS detection\n }\n }\n “`\n\n * **Flask Configuration:** Set the `PREFERRED_URL_SCHEME` in your Flask configuration:\n\n “`python\n app = Flask(__name__)\n app.config[\’SERVER_NAME\’] = \’yourdomain.com:5000\’ #Flask needs the internal port\n app.config[\’PREFERRED_URL_SCHEME\’] = \’https\’ #Force HTTPS\n “`\n\n * **Explanation:** `proxy_set_header Host $host;` is *critical*. It tells the proxy to forward the *original* host header that the user\’s browser sent. `X-Forwarded-Proto $scheme;` ensures Flask knows if the original request was HTTPS.\n\n * **Why this is the best:**\n\n * **Minimal Code Changes:** Relies on correct configuration rather than code hacks.\n * **Future-Proof:** If you change your domain or move to a different hosting setup, the configuration is more adaptable.\n * **Handles HTTPS Correctly:** The `PREFERRED_URL_SCHEME` ensures HTTPS URLs are always generated when behind the proxy.\n * **Correct Session Management:** By using the real domain name for the context, sessions work seamlessly.\n\n2. **Subclass `Flask` and Override `url_for` (If the Above Doesn\’t Work):**\n\n * **Explanation:** This is a more direct, code-based solution. You create a custom `Flask` class and override the `url_for` method to strip the port when `_external=True`.\n\n * **Implementation:**\n\n “`python\n from flask import Flask, url_for\n\n class MyFlask(Flask):\n def url_for(self, endpoint, **values):\n url = super().url_for(endpoint, **values)\n if \’_external\’ in values and values[\’_external\’] is True:\n from urllib.parse import urlparse, urlunparse\n\n parsed_url = urlparse(url)\n # Reconstruct the URL without the port if it\’s the default HTTP/HTTPS port\n if (parsed_url.port == 80 and parsed_url.scheme == \’http\’) or \\\n (parsed_url.port == 443 and parsed_url.scheme == \’https\’):\n url = urlunparse((parsed_url.scheme, parsed_url.hostname, parsed_url.path, parsed_url.params, parsed_url.query, parsed_url.fragment))\n elif parsed_url.port == 5000 and parsed_url.scheme == \’http\’: #If the port is explicityl set to 5000, remove the port. This allows to run Flask standalone without a reverse proxy\n url = urlunparse((parsed_url.scheme, parsed_url.hostname, parsed_url.path, parsed_url.params, parsed_url.query, parsed_url.fragment))\n\n return url\n\n app = MyFlask(__name__)\n app.config[\’SERVER_NAME\’] = \’yourdomain.com:5000\’ #Flask needs the internal port\n #No need for app.config[\’PREFERRED_URL_SCHEME\’]\n “`\n\n * **How to Use:** Instantiate `MyFlask` instead of `Flask`. The overridden `url_for` will now strip the port when `_external=True`, *only* if the port is the default HTTP/HTTPS port or port 5000.\n\n * **Why this is good:**\n\n * **Relatively targeted:** Only modifies `url_for` behavior.\n * **Handles default ports:** Won\’t strip the port if it\’s a non-standard port that *should* be included.\n * **Works without reverse proxy:** If Flask is standalone, port 5000 will be removed (assuming the URL is on port 5000)\n\n * **Caveats:**\n\n * **More code:** More to maintain.\n * **Less Flexible:** If you need to handle port stripping in more complex ways, the logic becomes harder to manage.\n * **Not as clear:** Less obvious to someone reading your code why the `Flask` class is subclassed.\n\n3. **Create a Separate Config Setting for External URLs (and Modify `url_for`):**\n\n * **Explanation:** Introduce a new configuration variable, e.g., `EXTERNAL_URL_BASE`, that contains the base URL without the port. Then, modify `url_for` to use this setting when generating external URLs.\n\n * **Implementation:**\n\n “`python\n from flask import Flask, url_for, current_app\n\n def external_url_for(endpoint, **values):\n \”\”\”Custom function to generate external URLs using the EXTERNAL_URL_BASE.\”\”\”\n base_url = current_app.config.get(\’EXTERNAL_URL_BASE\’)\n if not base_url:\n raise ValueError(\”EXTERNAL_URL_BASE must be set in the Flask config\”)\n\n relative_url = url_for(endpoint, **values)\n return f\”{base_url}{relative_url}\”\n\n app = Flask(__name__)\n app.config[\’SERVER_NAME\’] = \’yourdomain.com:5000\’ #Flask needs the internal port\n app.config[\’EXTERNAL_URL_BASE\’] = \’https://yourdomain.com\’\n\n # Example usage (in your background task):\n # url = external_url_for(\’my_route\’, _external=True, some_param=\’value\’)\n “`\n\n * **Why this is a viable option:**\n\n * **Explicit:** Makes it very clear where the external URL is coming from.\n * **Simple:** Relatively easy to understand.\n\n * **Drawbacks:**\n\n * **Requires changing all `url_for` calls:** You have to replace every instance of `url_for(_external=True)` with `external_url_for`. This is error-prone.\n * **Less maintainable:** If your URL structure changes, you have to update this config value.\n * **Not DRY (Don\’t Repeat Yourself):** You\’re essentially duplicating URL configuration.\n\n4. **Modify `/etc/hosts` (Discouraged):**\n\n * **Explanation:** Add your official hostname to `/etc/hosts` pointing to `127.0.0.1`.\n\n * **Why this is the *least* desirable:**\n\n * **Environment-Specific:** Only works on the specific server where you modify the file. Breaks portability.\n * **Not Scalable:** Doesn\’t work well in a multi-server environment.\n * **Can Cause Confusion:** Makes debugging harder. Developers may not realize this change has been made.\n * **Overrides DNS:** Circumvents your DNS settings, which can have unintended consequences.\n * **Only for development/testing:** This is acceptable only for local development/testing on your own machine, not for production.\n\n**Recommendation Summary**\n\n* **Start with Option 1 (Reverse Proxy Configuration and `PREFERRED_URL_SCHEME`).** It\’s the cleanest and most maintainable approach. This is the *correct* way to handle this situation.\n* If, and *only if*, you absolutely cannot configure your reverse proxy correctly, then move to **Option 2** (Subclassing `Flask` and Overriding `url_for`).\n* Avoid **Option 3** unless you have a very specific reason and understand the drawbacks.\n* Never use **Option 4** in a production environment.\n\n**Important Considerations:**\n\n* **Testing:** Thoroughly test your URL generation in your background tasks after implementing any of these solutions.\n* **Security:** Always use HTTPS in production. Ensure your reverse proxy is configured to handle SSL/TLS correctly.\n* **Logging:** Enable detailed logging in your Flask app and reverse proxy to help diagnose any URL-related issues.\n\nBy prioritizing the reverse proxy configuration and leveraging `PREFERRED_URL_SCHEME`, you can achieve a robust and maintainable solution for handling external URLs in your Flask application. Good luck!\n
