Running Flask on an Internal Network Interface

This guide explains how to configure a Flask application to run on a specific network interface on a macOS server with multiple NICs.

Prerequisites

macOS server with multiple network interfaces
Python installed
Flask installed (pip install flask)

Step 1: Identify Your Internal Network Interface

First, find the IP address of your internal network interface (the one with a 10.x.x.x address):

ifconfig

Look for an interface with an address that starts with 10.x.x.x and note both the interface name and the exact IP address.

Example output:

en0: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST> mtu 1500
    ...
    inet 192.168.1.5 netmask 0xffffff00 broadcast 192.168.1.255

en1: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST> mtu 1500
    ...
    inet 10.0.0.15 netmask 0xffffff00 broadcast 10.0.0.255

In this example, en1 is the internal network interface with IP 10.0.0.15.

Step 2: Configure Your Flask Application

Basic Flask Application

Create a file named app.py with the following content:

from flask import Flask

app = Flask(__name__)

@app.route('/')
def hello():
    return "Hello, World!"

if __name__ == '__main__':
    # Replace with your actual internal IP address
    app.run(host='10.x.x.x', port=5000, debug=True)

Replace 10.x.x.x with your actual internal IP address (e.g., 10.0.0.15).

Step 3: Run Your Flask Application

Method 1: Directly using Python

python app.py

Method 2: Using Flask CLI

export FLASK_APP=app.py
flask run --host=10.x.x.x --port=5000

Replace 10.x.x.x with your actual internal IP address.

Method 3: Using a Production WSGI Server (Recommended for Production)

Install Gunicorn:

pip install gunicorn

Run with Gunicorn:

gunicorn -b 10.x.x.x:5000 app:app

Step 4: Verify Access

Your Flask application should now be accessible only from your internal network at http://10.x.x.x:5000.

Troubleshooting

Application Not Accessible

Check firewall settings:

sudo /usr/libexec/ApplicationFirewall/socketfilterfw --getglobalstate

If the firewall is active, you may need to add an exception:

sudo /usr/libexec/ApplicationFirewall/socketfilterfw --add /path/to/python

Verify the application is binding to the correct interface:
```
lsof -i :5000
```
Test local connectivity:
```
curl http://10.x.x.x:5000
```

Starting on System Boot

To have your Flask application start when your Mac boots:

Create a launch agent in ~/Library/LaunchAgents/com.yourusername.flaskapp.plist:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
    <key>Label</key>
    <string>com.yourusername.flaskapp</string>
    <key>ProgramArguments</key>
    <array>
        <string>/usr/local/bin/python3</string>
        <string>/path/to/your/app.py</string>
    </array>
    <key>RunAtLoad</key>
    <true/>
    <key>KeepAlive</key>
    <true/>
    <key>StandardErrorPath</key>
    <string>/tmp/flaskapp.err</string>
    <key>StandardOutPath</key>
    <string>/tmp/flaskapp.out</string>
</dict>
</plist>

Load the launch agent:

launchctl load ~/Library/LaunchAgents/com.yourusername.flaskapp.plist

Additional Configuration

Using Environment Variables

For better security, consider using environment variables:

import os
from flask import Flask

app = Flask(__name__)

@app.route('/')
def hello():
    return "Hello, World!"

if __name__ == '__main__':
    # Get values from environment or use defaults
    host = os.environ.get('FLASK_HOST', '10.x.x.x')
    port = int(os.environ.get('FLASK_PORT', 5000))
    debug = os.environ.get('FLASK_DEBUG', 'False').lower() == 'true'
    
    app.run(host=host, port=port, debug=debug)

Then run with:

FLASK_HOST=10.x.x.x FLASK_PORT=5000 FLASK_DEBUG=True python app.py

bigsnarfdude/server_internal_flask.md