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      September 2018

      How To Enable SFTP Without Shell Access on Ubuntu 18.04

      Introduction

      SFTP stands for SSH File Transfer Protocol. As its name suggests, it’s a secure way to transfer files between machines using an encrypted SSH connection. Despite the name, it’s a completely different protocol than FTP (File Transfer Protocol), though it’s widely supported by modern FTP clients.

      SFTP is available by default with no additional configuration on all servers that have SSH access enabled. It’s secure and easy to use, but comes with a disadvantage: in a standard configuration, the SSH server grants file transfer access and terminal shell access to all users with an account on the system.

      In some cases, you might want only certain users to be allowed file transfers and no SSH access. In this tutorial, we’ll set up the SSH daemon to limit SFTP access to one directory with no SSH access allowed on per-user basis.

      Prerequisites

      To follow this tutorial, you will need access to an Ubuntu 18.04 server. This server should have a non-root user with sudo privileges, as well as a firewall enabled. For help with setting this up, follow our Initial Server Setup Guide for Ubuntu 18.04.

      Step 1 — Creating a New User

      First, create a new user who will be granted only file transfer access to the server. Here, we’re using the username sammyfiles, but you can use any username you like.

      You’ll be prompted to create a password for the account, followed by some information about the user. The user information is optional, so you can press ENTER to leave those fields blank.

      You have now created a new user that will be granted access to the restricted directory. In the next step we will create the directory for file transfers and set up the necessary permissions.

      Step 2 — Creating a Directory for File Transfers

      In order to restrict SFTP access to one directory, we first have to make sure the directory complies with the SSH server’s permissions requirements, which are very particular.

      Specifically, the directory itself and all directories above it in the filesystem tree must be owned by root and not writable by anyone else. Consequently, it’s not possible to simply give restricted access to a user’s home directory because home directories are owned by the user, not root.

      Note: Some versions of OpenSSH do not have such strict requirements for the directory structure and ownership, but most modern Linux distributions (including Ubuntu 18.04) do.

      There are a number of ways to work around this ownership issue. In this tutorial, we’ll create and use /var/sftp/uploads as the target upload directory. /var/sftp will be owned by root and will not be writable by other users; the subdirectory /var/sftp/uploads will be owned by sammyfiles, so that user will be able to upload files to it.

      First, create the directories.

      • sudo mkdir -p /var/sftp/uploads

       

      Set the owner of /var/sftp to root.

      • sudo chown root:root /var/sftp

       

      Give root write permissions to the same directory, and give other users only read and execute rights.

      Change the ownership on the uploads directory to sammyfiles.

      • sudo chown sammyfiles:sammyfiles /var/sftp/uploads

       

      Now that the directory structure is in place, we can configure the SSH server itself.

      Step 3 — Restricting Access to One Directory

      In this step, we’ll modify the SSH server configuration to disallow terminal access for sammyfiles but allow file transfer access.

      Open the SSH server configuration file using nano or your favorite text editor.

      • sudo nano /etc/ssh/sshd_config

       

      Scroll to the very bottom of the file and append the following configuration snippet:

      /etc/ssh/sshd_config

      . . .
      
      Match User sammyfiles
      ForceCommand internal-sftp
      PasswordAuthentication yes
      ChrootDirectory /var/sftp
      PermitTunnel no
      AllowAgentForwarding no
      AllowTcpForwarding no
      X11Forwarding no
      

      Then save and close the file.

      Here’s what each of those directives do:

      • Match User tells the SSH server to apply the following commands only to the user specified. Here, we specify sammyfiles.
      • ForceCommand internal-sftp forces the SSH server to run the SFTP server upon login, disallowing shell access.
      • PasswordAuthentication yes allows password authentication for this user.
      • ChrootDirectory /var/sftp/ ensures that the user will not be allowed access to anything beyond the /var/sftp directory.
      • AllowAgentForwarding no, AllowTcpForwarding no. and X11Forwarding no disables port forwarding, tunneling and X11 forwarding for this user.

      This set of commands, starting with Match User, can be copied and repeated for different users too. Make sure to modify the username in the Match User line accordingly.

       

      Note: You can omit the PasswordAuthentication yes line and instead set up SSH key access for increased security. Follow the Copying your Public SSH Key section of the SSH Essentials: Working with SSH Servers, Clients, and Keys tutorial to do so. Make sure to do this before you disable shell access for the user.

      In the next step, we’ll test the configuration by SSHing locally with password access, but if you set up SSH keys, you’ll instead need access to a computer with the user’s keypair.

       

      To apply the configuration changes, restart the service.

      • sudo systemctl restart sshd

       

      You have now configured the SSH server to restrict access to file transfer only for sammyfiles. The last step is testing the configuration to make sure it works as intended.

      Step 4 — Verifying the Configuration

      Let’s ensure that our new sammyfiles user can only transfer files.

      Logging in to the server as sammyfiles using normal shell access should no longer be possible. Let’s try it:

      You’ll see the following message before being returned to your original prompt:

      Error message

      This service allows sftp connections only. Connection to localhost closed.

      This means that sammyfiles can no longer can access the server shell using SSH.

      Next, let’s verify if the user can successfully access SFTP for file transfer.

      • sftp sammyfiles@localhost

       

      Instead of an error message, this command will show a successful login message with an interactive prompt.

      SFTP prompt

      Connected to localhost. sftp>

      You can list the directory contents using ls in the prompt:

      This will show the uploads directory that was created in the previous step and return you to the sftp> prompt.

      SFTP file list output

      uploads

      To verify that the user is indeed restricted to this directory and cannot access any directory above it, you can try changing the directory to the one above it.

      This command will not give an error, but listing the directory contents as before will show no change, proving that the user was not able to switch to the parent directory.

      You have now verified that the restricted configuration works as intended. The newly created sammyfiles user can access the server only using the SFTP protocol for file transfer and has no ability to access the full shell.

      Conclusion

      You’ve restricted a user to SFTP-only access to a single directory on a server without full shell access. While this tutorial uses only one directory and one user for brevity, you can extend this example to multiple users and multiple directories.

      The SSH server allows more complex configuration schemes, including limiting access to groups or multiple users at once, or even limited access to certain IP addresses. You can find examples of additional configuration options and explanation of possible directives in the OpenSSH Cookbook. If you run into any issues with SSH, you can debug and fix them with this troubleshooting SSH series.

      How To Serve Flask Applications with uWSGI and Nginx on Ubuntu 18.04


      Introduction

      In this guide, you will build a Python application using the Flask microframework on Ubuntu 18.04. The bulk of this article will be about how to set up the uWSGI application server and how to launch the application and configure Nginx to act as a front-end reverse proxy.

      Prerequisites

      Before starting this guide, you should have:

      • A server with Ubuntu 18.04 installed and a non-root user with sudo privileges. Follow our initial server setup guide for guidance.
      • Nginx installed, following Steps 1 and 2 of How To Install Nginx on Ubuntu 18.04.
      • A domain name configured to point to your server. You can purchase one on Namecheap or get one for free on Freenom. You can learn how to point domains to DigitalOcean by following the relevant documentation on domains and DNS. Be sure to create the following DNS records:

        • An A record with your_domain pointing to your server’s public IP address.
        • An A record with www.your_domain pointing to your server’s public IP address.
      • Familiarity with uWSGI, our application server, and the WSGI specification. This discussion of definitions and concepts goes over both in detail.

      Step 1 — Installing the Components from the Ubuntu Repositories

      Our first step will be to install all of the pieces that we need from the Ubuntu repositories. We will install pip, the Python package manager, to manage our Python components. We will also get the Python development files necessary to build uWSGI.

      First, let’s update the local package index and install the packages that will allow us to build our Python environment. These will include python3-pip, along with a few more packages and development tools necessary for a robust programming environment:

      • sudo apt update
      • sudo apt install python3-pip python3-dev build-essential libssl-dev libffi-dev python3-setuptools

      With these packages in place, let’s move on to creating a virtual environment for our project.

      Step 2 — Creating a Python Virtual Environment

      Next, we’ll set up a virtual environment in order to isolate our Flask application from the other Python files on the system.

      Start by installing the python3-venv package, which will install the venv module:

      • sudo apt install python3-venv

      Next, let’s make a parent directory for our Flask project. Move into the directory after you create it:

      • mkdir ~/myproject
      • cd ~/myproject

      Create a virtual environment to store your Flask project’s Python requirements by typing:

      • python3.6 -m venv myprojectenv

      This will install a local copy of Python and pip into a directory called myprojectenv within your project directory.

      Before installing applications within the virtual environment, you need to activate it. Do so by typing:

      • source myprojectenv/bin/activate

      Your prompt will change to indicate that you are now operating within the virtual environment. It will look something like this (myprojectenv)user@host:~/myproject$.

      Step 3 — Setting Up a Flask Application

      Now that you are in your virtual environment, you can install Flask and uWSGI and get started on designing your application.

      First, let’s install wheel with the local instance of pip to ensure that our packages will install even if they are missing wheel archives:

      Note


      Regardless of which version of Python you are using, when the virtual environment is activated, you should use the pip command (not pip3).

      Next, let's install Flask and uWSGI:

      Creating a Sample App

      Now that you have Flask available, you can create a simple application. Flask is a microframework. It does not include many of the tools that more full-featured frameworks might, and exists mainly as a module that you can import into your projects to assist you in initializing a web application.

      While your application might be more complex, we'll create our Flask app in a single file, called myproject.py:

      • nano ~/myproject/myproject.py

      The application code will live in this file. It will import Flask and instantiate a Flask object. You can use this to define the functions that should be run when a specific route is requested:

      ~/myproject/myproject.py

      from flask import Flask
      app = Flask(__name__)
      
      @app.route("/")
      def hello():
          return "<h1 style='color:blue'>Hello There!</h1>"
      
      if __name__ == "__main__":
          app.run(host='0.0.0.0')
      

      This basically defines what content to present when the root domain is accessed. Save and close the file when you're finished.

      If you followed the initial server setup guide, you should have a UFW firewall enabled. To test the application, you need to allow access to port 5000:

      Now, you can test your Flask app by typing:

      You will see output like the following, including a helpful warning reminding you not to use this server setup in production:

      Output

      * Serving Flask app "myproject" (lazy loading) * Environment: production WARNING: Do not use the development server in a production environment. Use a production WSGI server instead. * Debug mode: off * Running on http://0.0.0.0:5000/ (Press CTRL+C to quit)

      Visit your server's IP address followed by :5000 in your web browser:

      http://your_server_ip:5000
      

      You should see something like this:

      Flask sample app

      When you are finished, hit CTRL-C in your terminal window to stop the Flask development server.

      Creating the WSGI Entry Point

      Next, let's create a file that will serve as the entry point for our application. This will tell our uWSGI server how to interact with it.

      Let's call the file wsgi.py:

      In this file, let's import the Flask instance from our application and then run it:

      ~/myproject/wsgi.py

      from myproject import app
      
      if __name__ == "__main__":
          app.run()
      

      Save and close the file when you are finished.

      Step 4 — Configuring uWSGI

      Your application is now written with an entry point established. We can now move on to configuring uWSGI.

      Testing uWSGI Serving

      Let's test to make sure that uWSGI can serve our application.

      We can do this by simply passing it the name of our entry point. This is constructed by the name of the module (minus the .py extension) plus the name of the callable within the application. In our case, this is wsgi:app.

      Let's also specify the socket, so that it will be started on a publicly available interface, as well as the protocol, so that it will use HTTP instead of the uwsgi binary protocol. We'll use the same port number, 5000, that we opened earlier:

      • uwsgi --socket 0.0.0.0:5000 --protocol=http -w wsgi:app

      Visit your server's IP address with :5000 appended to the end in your web browser again:

      http://your_server_ip:5000
      

      You should see your application's output again:

      Flask sample app

      When you have confirmed that it's functioning properly, press CTRL-C in your terminal window.

      We're now done with our virtual environment, so we can deactivate it:

      Any Python commands will now use the system's Python environment again.

      Creating a uWSGI Configuration File

      You have tested that uWSGI is able to serve your application, but ultimately you will want something more robust for long-term usage. You can create a uWSGI configuration file with the relevant options for this.

      Let's place that file in our project directory and call it myproject.ini:

      • nano ~/myproject/myproject.ini

      Inside, we will start off with the [uwsgi] header so that uWSGI knows to apply the settings. We'll specify two things: the module itself, by referring to the wsgi.py file minus the extension, and the callable within the file, app:

      ~/myproject/myproject.ini

      [uwsgi]
      module = wsgi:app
      

      Next, we'll tell uWSGI to start up in master mode and spawn five worker processes to serve actual requests:

      ~/myproject/myproject.ini

      [uwsgi]
      module = wsgi:app
      
      master = true
      processes = 5
      

      When you were testing, you exposed uWSGI on a network port. However, you're going to be using Nginx to handle actual client connections, which will then pass requests to uWSGI. Since these components are operating on the same computer, a Unix socket is preferable because it is faster and more secure. Let's call the socket myproject.sock and place it in this directory.

      Let's also change the permissions on the socket. We'll be giving the Nginx group ownership of the uWSGI process later on, so we need to make sure the group owner of the socket can read information from it and write to it. We will also clean up the socket when the process stops by adding the vacuum option:

      ~/myproject/myproject.ini

      [uwsgi]
      module = wsgi:app
      
      master = true
      processes = 5
      
      socket = myproject.sock
      chmod-socket = 660
      vacuum = true
      

      The last thing we'll do is set the die-on-term option. This can help ensure that the init system and uWSGI have the same assumptions about what each process signal means. Setting this aligns the two system components, implementing the expected behavior:

      ~/myproject/myproject.ini

      [uwsgi]
      module = wsgi:app
      
      master = true
      processes = 5
      
      socket = myproject.sock
      chmod-socket = 660
      vacuum = true
      
      die-on-term = true
      

      You may have noticed that we did not specify a protocol like we did from the command line. That is because by default, uWSGI speaks using the uwsgi protocol, a fast binary protocol designed to communicate with other servers. Nginx can speak this protocol natively, so it's better to use this than to force communication by HTTP.

      When you are finished, save and close the file.

      Step 5 — Creating a systemd Unit File

      Next, let's create the systemd service unit file. Creating a systemd unit file will allow Ubuntu's init system to automatically start uWSGI and serve the Flask application whenever the server boots.

      Create a unit file ending in .service within the /etc/systemd/system directory to begin:

      • sudo nano /etc/systemd/system/myproject.service

      Inside, we'll start with the [Unit] section, which is used to specify metadata and dependencies. Let's put a description of our service here and tell the init system to only start this after the networking target has been reached:

      /etc/systemd/system/myproject.service

      [Unit]
      Description=uWSGI instance to serve myproject
      After=network.target
      

      Next, let's open up the [Service] section. This will specify the user and group that we want the process to run under. Let's give our regular user account ownership of the process since it owns all of the relevant files. Let's also give group ownership to the www-data group so that Nginx can communicate easily with the uWSGI processes. Remember to replace the username here with your username:

      /etc/systemd/system/myproject.service

      [Unit]
      Description=uWSGI instance to serve myproject
      After=network.target
      
      [Service]
      User=sammy
      Group=www-data
      

      Next, let's map out the working directory and set the PATH environmental variable so that the init system knows that the executables for the process are located within our virtual environment. Let's also specify the command to start the service. Systemd requires that we give the full path to the uWSGI executable, which is installed within our virtual environment. We will pass the name of the .ini configuration file we created in our project directory.

      Remember to replace the username and project paths with your own information:

      /etc/systemd/system/myproject.service

      [Unit]
      Description=uWSGI instance to serve myproject
      After=network.target
      
      [Service]
      User=sammy
      Group=www-data
      WorkingDirectory=/home/sammy/myproject
      Environment="PATH=/home/sammy/myproject/myprojectenv/bin"
      ExecStart=/home/sammy/myproject/myprojectenv/bin/uwsgi --ini myproject.ini
      

      Finally, let's add an [Install] section. This will tell systemd what to link this service to if we enable it to start at boot. We want this service to start when the regular multi-user system is up and running:

      /etc/systemd/system/myproject.service

      [Unit]
      Description=uWSGI instance to serve myproject
      After=network.target
      
      [Service]
      User=sammy
      Group=www-data
      WorkingDirectory=/home/sammy/myproject
      Environment="PATH=/home/sammy/myproject/myprojectenv/bin"
      ExecStart=/home/sammy/myproject/myprojectenv/bin/uwsgi --ini myproject.ini
      
      [Install]
      WantedBy=multi-user.target
      

      With that, our systemd service file is complete. Save and close it now.

      We can now start the uWSGI service we created and enable it so that it starts at boot:

      • sudo systemctl start myproject
      • sudo systemctl enable myproject

      Let's check the status:

      • sudo systemctl status myproject

      You should see output like this:

      Output

      ● myproject.service - uWSGI instance to serve myproject Loaded: loaded (/etc/systemd/system/myproject.service; enabled; vendor preset: enabled) Active: active (running) since Fri 2018-07-13 14:28:39 UTC; 46s ago Main PID: 30360 (uwsgi) Tasks: 6 (limit: 1153) CGroup: /system.slice/myproject.service ├─30360 /home/sammy/myproject/myprojectenv/bin/uwsgi --ini myproject.ini ├─30378 /home/sammy/myproject/myprojectenv/bin/uwsgi --ini myproject.ini ├─30379 /home/sammy/myproject/myprojectenv/bin/uwsgi --ini myproject.ini ├─30380 /home/sammy/myproject/myprojectenv/bin/uwsgi --ini myproject.ini ├─30381 /home/sammy/myproject/myprojectenv/bin/uwsgi --ini myproject.ini └─30382 /home/sammy/myproject/myprojectenv/bin/uwsgi --ini myproject.ini

      If you see any errors, be sure to resolve them before continuing with the tutorial.

      Step 6 — Configuring Nginx to Proxy Requests

      Our uWSGI application server should now be up and running, waiting for requests on the socket file in the project directory. Let's configure Nginx to pass web requests to that socket using the uwsgi protocol.

      Begin by creating a new server block configuration file in Nginx's sites-available directory. Let's call this myproject to keep in line with the rest of the guide:

      • sudo nano /etc/nginx/sites-available/myproject

      Open up a server block and tell Nginx to listen on the default port 80. Let's also tell it to use this block for requests for our server's domain name:

      /etc/nginx/sites-available/myproject

      server {
          listen 80;
          server_name your_domain www.your_domain;
      }
      

      Next, let's add a location block that matches every request. Within this block, we'll include the uwsgi_params file that specifies some general uWSGI parameters that need to be set. We'll then pass the requests to the socket we defined using the uwsgi_pass directive:

      /etc/nginx/sites-available/myproject

      server {
          listen 80;
          server_name your_domain www.your_domain;
      
          location / {
              include uwsgi_params;
              uwsgi_pass unix:/home/sammy/myproject/myproject.sock;
          }
      }
      

      Save and close the file when you're finished.

      To enable the Nginx server block configuration you've just created, link the file to the sites-enabled directory:

      • sudo ln -s /etc/nginx/sites-available/myproject /etc/nginx/sites-enabled

      With the file in that directory, we can test for syntax errors by typing:

      If this returns without indicating any issues, restart the Nginx process to read the new configuration:

      • sudo systemctl restart nginx

      Finally, let's adjust the firewall again. We no longer need access through port 5000, so we can remove that rule. We can then allow access to the Nginx server:

      • sudo ufw delete allow 5000
      • sudo ufw allow 'Nginx Full'

      You should now be able to navigate to your server's domain name in your web browser:

      http://your_domain
      

      You should see your application output:

      Flask sample app

      If you encounter any errors, trying checking the following:

      • sudo less /var/log/nginx/error.log: checks the Nginx error logs.
      • sudo less /var/log/nginx/access.log: checks the Nginx access logs.
      • sudo journalctl -u nginx: checks the Nginx process logs.
      • sudo journalctl -u myproject: checks your Flask app's uWSGI logs.

      Step 7 — Securing the Application

      To ensure that traffic to your server remains secure, let's get an SSL certificate for your domain. There are multiple ways to do this, including getting a free certificate from Let's Encrypt, generating a self-signed certificate, or buying one from another provider and configuring Nginx to use it by following Steps 2 through 6 of  How to Create a Self-signed SSL Certificate for Nginx in Ubuntu 18.04. We will go with option one for the sake of expediency.

      First, add the Certbot Ubuntu repository:

      • sudo add-apt-repository ppa:certbot/certbot

      You'll need to press ENTER to accept.

      Next, install Certbot's Nginx package with apt:

      • sudo apt install python-certbot-nginx

      Certbot provides a variety of ways to obtain SSL certificates through plugins. The Nginx plugin will take care of reconfiguring Nginx and reloading the config whenever necessary. To use this plugin, type the following:

      • sudo certbot --nginx -d your_domain -d www.your_domain

      This runs certbot with the --nginx plugin, using -d to specify the names we'd like the certificate to be valid for.

      If this is your first time running certbot, you will be prompted to enter an email address and agree to the terms of service. After doing so, certbot will communicate with the Let's Encrypt server, then run a challenge to verify that you control the domain you're requesting a certificate for.

      If that's successful, certbot will ask how you'd like to configure your HTTPS settings.

      Output

      Please choose whether or not to redirect HTTP traffic to HTTPS, removing HTTP access. ------------------------------------------------------------------------------- 1: No redirect - Make no further changes to the webserver configuration. 2: Redirect - Make all requests redirect to secure HTTPS access. Choose this for new sites, or if you're confident your site works on HTTPS. You can undo this change by editing your web server's configuration. ------------------------------------------------------------------------------- Select the appropriate number [1-2] then [enter] (press 'c' to cancel):

      Select your choice then hit ENTER. The configuration will be updated, and Nginx will reload to pick up the new settings. certbot will wrap up with a message telling you the process was successful and where your certificates are stored:

      Output

      IMPORTANT NOTES: - Congratulations! Your certificate and chain have been saved at: /etc/letsencrypt/live/your_domain/fullchain.pem Your key file has been saved at: /etc/letsencrypt/live/your_domain/privkey.pem Your cert will expire on 2018-07-23. To obtain a new or tweaked version of this certificate in the future, simply run certbot again with the "certonly" option. To non-interactively renew *all* of your certificates, run "certbot renew" - Your account credentials have been saved in your Certbot configuration directory at /etc/letsencrypt. You should make a secure backup of this folder now. This configuration directory will also contain certificates and private keys obtained by Certbot so making regular backups of this folder is ideal. - If you like Certbot, please consider supporting our work by: Donating to ISRG / Let's Encrypt: https://letsencrypt.org/donate Donating to EFF: https://eff.org/donate-le

      If you followed the Nginx installation instructions in the prerequisites, you will no longer need the redundant HTTP profile allowance:

      • sudo ufw delete allow 'Nginx HTTP'

      To verify the configuration, let's navigate once again to your domain, using https://:

      https://your_domain
      

      You should see your application output once again, along with your browser's security indicator, which should indicate that the site is secured.

      Conclusion

      In this guide, you created and secured a simple Flask application within a Python virtual environment. You created a WSGI entry point so that any WSGI-capable application server can interface with it, and then configured the uWSGI app server to provide this function. Afterwards, you created a systemd service file to automatically launch the application server on boot. You also created an Nginx server block that passes web client traffic to the application server, relaying external requests, and secured traffic to your server with Let's Encrypt.

      Flask is a very simple, but extremely flexible framework meant to provide your applications with functionality without being too restrictive about structure and design. You can use the general stack described in this guide to serve the flask applications that you design.



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      How To Serve Flask Applications with Gunicorn and Nginx on Ubuntu 18.04


      Introduction

      In this guide, you will build a Python application using the Flask microframework on Ubuntu 18.04. The bulk of this article will be about how to set up the Gunicorn application server and how to launch the application and configure Nginx to act as a front-end reverse proxy.

      Prerequisites

      Before starting this guide, you should have:

      • A server with Ubuntu 18.04 installed and a non-root user with sudo privileges. Follow our initial server setup guide for guidance.
      • Nginx installed, following Steps 1 and 2 of How To Install Nginx on Ubuntu 18.04.
      • A domain name configured to point to your server. You can purchase one on Namecheap or get one for free on Freenom. You can learn how to point domains to DigitalOcean by following the relevant documentation on domains and DNS. Be sure to create the following DNS records:

        • An A record with your_domain pointing to your server’s public IP address.
        • An A record with www.your_domain pointing to your server’s public IP address.
      • Familiarity with the WSGI specification, which the Gunicorn server will use to communicate with your Flask application. This discussion covers WSGI in more detail.

      Step 1 — Installing the Components from the Ubuntu Repositories

      Our first step will be to install all of the pieces we need from the Ubuntu repositories. This includes pip, the Python package manager, which will manage our Python components. We will also get the Python development files necessary to build some of the Gunicorn components.

      First, let’s update the local package index and install the packages that will allow us to build our Python environment. These will include python3-pip, along with a few more packages and development tools necessary for a robust programming environment:

      • sudo apt update
      • sudo apt install python3-pip python3-dev build-essential libssl-dev libffi-dev python3-setuptools

      With these packages in place, let’s move on to creating a virtual environment for our project.

      Step 2 — Creating a Python Virtual Environment

      Next, we’ll set up a virtual environment in order to isolate our Flask application from the other Python files on the system.

      Start by installing the python3-venv package, which will install the venv module:

      • sudo apt install python3-venv

      Next, let’s make a parent directory for our Flask project. Move into the directory after you create it:

      • mkdir ~/myproject
      • cd ~/myproject

      Create a virtual environment to store your Flask project’s Python requirements by typing:

      • python3.6 -m venv myprojectenv

      This will install a local copy of Python and pip into a directory called myprojectenv within your project directory.

      Before installing applications within the virtual environment, you need to activate it. Do so by typing:

      • source myprojectenv/bin/activate

      Your prompt will change to indicate that you are now operating within the virtual environment. It will look something like this: (myprojectenv)user@host:~/myproject$.

      Step 3 — Setting Up a Flask Application

      Now that you are in your virtual environment, you can install Flask and Gunicorn and get started on designing your application.

      First, let’s install wheel with the local instance of pip to ensure that our packages will install even if they are missing wheel archives:

      Note


      Regardless of which version of Python you are using, when the virtual environment is activated, you should use the pip command (not pip3).

      Next, let's install Flask and Gunicorn:

      • pip install gunicorn flask

      Creating a Sample App

      Now that you have Flask available, you can create a simple application. Flask is a microframework. It does not include many of the tools that more full-featured frameworks might, and exists mainly as a module that you can import into your projects to assist you in initializing a web application.

      While your application might be more complex, we'll create our Flask app in a single file, called myproject.py:

      • nano ~/myproject/myproject.py

      The application code will live in this file. It will import Flask and instantiate a Flask object. You can use this to define the functions that should be run when a specific route is requested:

      ~/myproject/myproject.py

      from flask import Flask
      app = Flask(__name__)
      
      @app.route("/")
      def hello():
          return "<h1 style='color:blue'>Hello There!</h1>"
      
      if __name__ == "__main__":
          app.run(host='0.0.0.0')
      

      This basically defines what content to present when the root domain is accessed. Save and close the file when you're finished.

      If you followed the initial server setup guide, you should have a UFW firewall enabled. To test the application, you need to allow access to port 5000:

      Now you can test your Flask app by typing:

      You will see output like the following, including a helpful warning reminding you not to use this server setup in production:

      Output

      * Serving Flask app "myproject" (lazy loading) * Environment: production WARNING: Do not use the development server in a production environment. Use a production WSGI server instead. * Debug mode: off * Running on http://0.0.0.0:5000/ (Press CTRL+C to quit)

      Visit your server's IP address followed by :5000 in your web browser:

      http://your_server_ip:5000
      

      You should see something like this:

      Flask sample app

      When you are finished, hit CTRL-C in your terminal window to stop the Flask development server.

      Creating the WSGI Entry Point

      Next, let's create a file that will serve as the entry point for our application. This will tell our Gunicorn server how to interact with the application.

      Let's call the file wsgi.py:

      In this file, let's import the Flask instance from our application and then run it:

      ~/myproject/wsgi.py

      from myproject import app
      
      if __name__ == "__main__":
          app.run()
      

      Save and close the file when you are finished.

      Step 4 — Configuring Gunicorn

      Your application is now written with an entry point established. We can now move on to configuring Gunicorn.

      Before moving on, we should check that Gunicorn can serve the application correctly.

      We can do this by simply passing it the name of our entry point. This is constructed as the name of the module (minus the .py extension), plus the name of the callable within the application. In our case, this is wsgi:app.

      We'll also specify the interface and port to bind to so that the application will be started on a publicly available interface:

      • cd ~/myproject
      • gunicorn --bind 0.0.0.0:5000 wsgi:app

      You should see output like the following:

      Output

      [2018-07-13 19:35:13 +0000] [28217] [INFO] Starting gunicorn 19.9.0 [2018-07-13 19:35:13 +0000] [28217] [INFO] Listening at: http://0.0.0.0:5000 (28217) [2018-07-13 19:35:13 +0000] [28217] [INFO] Using worker: sync [2018-07-13 19:35:13 +0000] [28220] [INFO] Booting worker with pid: 28220

      Visit your server's IP address with :5000 appended to the end in your web browser again:

      http://your_server_ip:5000
      

      You should see your application's output:

      Flask sample app

      When you have confirmed that it's functioning properly, press CTRL-C in your terminal window.

      We're now done with our virtual environment, so we can deactivate it:

      Any Python commands will now use the system's Python environment again.

      Next, let's create the systemd service unit file. Creating a systemd unit file will allow Ubuntu's init system to automatically start Gunicorn and serve the Flask application whenever the server boots.

      Create a unit file ending in .service within the /etc/systemd/system directory to begin:

      • sudo nano /etc/systemd/system/myproject.service

      Inside, we'll start with the [Unit] section, which is used to specify metadata and dependencies. Let's put a description of our service here and tell the init system to only start this after the networking target has been reached:

      /etc/systemd/system/myproject.service

      [Unit]
      Description=Gunicorn instance to serve myproject
      After=network.target
      

      Next, let's open up the [Service] section. This will specify the user and group that we want the process to run under. Let's give our regular user account ownership of the process since it owns all of the relevant files. Let's also give group ownership to the www-data group so that Nginx can communicate easily with the Gunicorn processes. Remember to replace the username here with your username:

      /etc/systemd/system/myproject.service

      [Unit]
      Description=Gunicorn instance to serve myproject
      After=network.target
      
      [Service]
      User=sammy
      Group=www-data
      

      Next, let's map out the working directory and set the PATH environmental variable so that the init system knows that the executables for the process are located within our virtual environment. Let's also specify the command to start the service. This command will do the following:

      • Start 3 worker processes (though you should adjust this as necessary)
      • Create and bind to a Unix socket file, myproject.sock, within our project directory. We'll set an umask value of 007 so that the socket file is created giving access to the owner and group, while restricting other access
      • Specify the WSGI entry point file name, along with the Python callable within that file (wsgi:app)

      Systemd requires that we give the full path to the Gunicorn executable, which is installed within our virtual environment.

      Remember to replace the username and project paths with your own information:

      /etc/systemd/system/myproject.service

      [Unit]
      Description=Gunicorn instance to serve myproject
      After=network.target
      
      [Service]
      User=sammy
      Group=www-data
      WorkingDirectory=/home/sammy/myproject
      Environment="PATH=/home/sammy/myproject/myprojectenv/bin"
      ExecStart=/home/sammy/myproject/myprojectenv/bin/gunicorn --workers 3 --bind unix:myproject.sock -m 007 wsgi:app
      

      Finally, let's add an [Install] section. This will tell systemd what to link this service to if we enable it to start at boot. We want this service to start when the regular multi-user system is up and running:

      /etc/systemd/system/myproject.service

      [Unit]
      Description=Gunicorn instance to serve myproject
      After=network.target
      
      [Service]
      User=sammy
      Group=www-data
      WorkingDirectory=/home/sammy/myproject
      Environment="PATH=/home/sammy/myproject/myprojectenv/bin"
      ExecStart=/home/sammy/myproject/myprojectenv/bin/gunicorn --workers 3 --bind unix:myproject.sock -m 007 wsgi:app
      
      [Install]
      WantedBy=multi-user.target
      

      With that, our systemd service file is complete. Save and close it now.

      We can now start the Gunicorn service we created and enable it so that it starts at boot:

      • sudo systemctl start myproject
      • sudo systemctl enable myproject

      Let's check the status:

      • sudo systemctl status myproject

      You should see output like this:

      Output

      ● myproject.service - Gunicorn instance to serve myproject Loaded: loaded (/etc/systemd/system/myproject.service; enabled; vendor preset: enabled) Active: active (running) since Fri 2018-07-13 14:28:39 UTC; 46s ago Main PID: 28232 (gunicorn) Tasks: 4 (limit: 1153) CGroup: /system.slice/myproject.service ├─28232 /home/sammy/myproject/myprojectenv/bin/python3.6 /home/sammy/myproject/myprojectenv/bin/gunicorn --workers 3 --bind unix:myproject.sock -m 007 ├─28250 /home/sammy/myproject/myprojectenv/bin/python3.6 /home/sammy/myproject/myprojectenv/bin/gunicorn --workers 3 --bind unix:myproject.sock -m 007 ├─28251 /home/sammy/myproject/myprojectenv/bin/python3.6 /home/sammy/myproject/myprojectenv/bin/gunicorn --workers 3 --bind unix:myproject.sock -m 007 └─28252 /home/sammy/myproject/myprojectenv/bin/python3.6 /home/sammy/myproject/myprojectenv/bin/gunicorn --workers 3 --bind unix:myproject.sock -m 007

      If you see any errors, be sure to resolve them before continuing with the tutorial.

      Step 5 — Configuring Nginx to Proxy Requests

      Our Gunicorn application server should now be up and running, waiting for requests on the socket file in the project directory. Let's now configure Nginx to pass web requests to that socket by making some small additions to its configuration file.

      Begin by creating a new server block configuration file in Nginx's sites-available directory. Let's call this myproject to keep in line with the rest of the guide:

      • sudo nano /etc/nginx/sites-available/myproject

      Open up a server block and tell Nginx to listen on the default port 80. Let's also tell it to use this block for requests for our server's domain name:

      /etc/nginx/sites-available/myproject

      server {
          listen 80;
          server_name your_domain www.your_domain;
      }
      

      Next, let's add a location block that matches every request. Within this block, we'll include the proxy_params file that specifies some general proxying parameters that need to be set. We'll then pass the requests to the socket we defined using the proxy_pass directive:

      /etc/nginx/sites-available/myproject

      server {
          listen 80;
          server_name your_domain www.your_domain;
      
          location / {
              include proxy_params;
              proxy_pass http://unix:/home/sammy/myproject/myproject.sock;
          }
      }
      

      Save and close the file when you're finished.

      To enable the Nginx server block configuration you've just created, link the file to the sites-enabled directory:

      • sudo ln -s /etc/nginx/sites-available/myproject /etc/nginx/sites-enabled

      With the file in that directory, you can test for syntax errors:

      If this returns without indicating any issues, restart the Nginx process to read the new configuration:

      • sudo systemctl restart nginx

      Finally, let's adjust the firewall again. We no longer need access through port 5000, so we can remove that rule. We can then allow full access to the Nginx server:

      • sudo ufw delete allow 5000
      • sudo ufw allow 'Nginx Full'

      You should now be able to navigate to your server's domain name in your web browser:

      http://your_domain
      

      You should see your application's output:

      Flask sample app

      If you encounter any errors, trying checking the following:

      • sudo less /var/log/nginx/error.log: checks the Nginx error logs.
      • sudo less /var/log/nginx/access.log: checks the Nginx access logs.
      • sudo journalctl -u nginx: checks the Nginx process logs.
      • sudo journalctl -u myproject: checks your Flask app's Gunicorn logs.

      Step 6 — Securing the Application

      To ensure that traffic to your server remains secure, let's get an SSL certificate for your domain. There are multiple ways to do this, including getting a free certificate from Let's Encrypt, generating a self-signed certificate, or buying one from another provider and configuring Nginx to use it by following Steps 2 through 6 of  How to Create a Self-signed SSL Certificate for Nginx in Ubuntu 18.04. We will go with option one for the sake of expediency.

      First, add the Certbot Ubuntu repository:

      • sudo add-apt-repository ppa:certbot/certbot

      You'll need to press ENTER to accept.

      Install Certbot's Nginx package with apt:

      • sudo apt install python-certbot-nginx

      Certbot provides a variety of ways to obtain SSL certificates through plugins. The Nginx plugin will take care of reconfiguring Nginx and reloading the config whenever necessary. To use this plugin, type the following:

      • sudo certbot --nginx -d your_domain -d www.your_domain

      This runs certbot with the --nginx plugin, using -d to specify the names we'd like the certificate to be valid for.

      If this is your first time running certbot, you will be prompted to enter an email address and agree to the terms of service. After doing so, certbot will communicate with the Let's Encrypt server, then run a challenge to verify that you control the domain you're requesting a certificate for.

      If that's successful, certbot will ask how you'd like to configure your HTTPS settings:

      Output

      Please choose whether or not to redirect HTTP traffic to HTTPS, removing HTTP access. ------------------------------------------------------------------------------- 1: No redirect - Make no further changes to the webserver configuration. 2: Redirect - Make all requests redirect to secure HTTPS access. Choose this for new sites, or if you're confident your site works on HTTPS. You can undo this change by editing your web server's configuration. ------------------------------------------------------------------------------- Select the appropriate number [1-2] then [enter] (press 'c' to cancel):

      Select your choice then hit ENTER. The configuration will be updated, and Nginx will reload to pick up the new settings. certbot will wrap up with a message telling you the process was successful and where your certificates are stored:

      Output

      IMPORTANT NOTES: - Congratulations! Your certificate and chain have been saved at: /etc/letsencrypt/live/your_domain/fullchain.pem Your key file has been saved at: /etc/letsencrypt/live/your_domain/privkey.pem Your cert will expire on 2018-07-23. To obtain a new or tweaked version of this certificate in the future, simply run certbot again with the "certonly" option. To non-interactively renew *all* of your certificates, run "certbot renew" - Your account credentials have been saved in your Certbot configuration directory at /etc/letsencrypt. You should make a secure backup of this folder now. This configuration directory will also contain certificates and private keys obtained by Certbot so making regular backups of this folder is ideal. - If you like Certbot, please consider supporting our work by: Donating to ISRG / Let's Encrypt: https://letsencrypt.org/donate Donating to EFF: https://eff.org/donate-le

      If you followed the Nginx installation instructions in the prerequisites, you will no longer need the redundant HTTP profile allowance:

      • sudo ufw delete allow 'Nginx HTTP'

      To verify the configuration, navigate once again to your domain, using https://:

      https://your_domain
      

      You should see your application output once again, along with your browser's security indicator, which should indicate that the site is secured.

      Conclusion

      In this guide, you created and secured a simple Flask application within a Python virtual environment. You created a WSGI entry point so that any WSGI-capable application server can interface with it, and then configured the Gunicorn app server to provide this function. Afterwards, you created a systemd service file to automatically launch the application server on boot. You also created an Nginx server block that passes web client traffic to the application server, relaying external requests, and secured traffic to your server with Let's Encrypt.

      Flask is a very simple, but extremely flexible framework meant to provide your applications with functionality without being too restrictive about structure and design. You can use the general stack described in this guide to serve the flask applications that you design.



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