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We will take an existing ASP.NET Core application and place it behind a reverse-proxy server. We will then setup the reverse-proxy server to forward requests to our Kestrel web server.

Additionally we will ensure our web application runs on startup as a daemon and configure a process management tool to help restart our web application in the event of a crash to guarantee high availability.

Prerequisites

1. Access to an Ubuntu 16.04 Server with a standard user account with sudo privilege.

2. An existing ASP.NET Core application.

Copy over your app

2

Before we proceed, copy your ASP.NET Core application to your server using whatever tool (SCP, FTP, etc) integrates into your workflow. Try and run the app and navigate to http://<serveraddress>:<port> in your browser to see if the application runs fine on Linux. I recommend you have a working app before proceeding.

Configure a reverse proxy server

A reverse proxy is a common setup for serving dynamic web applications. The reverse proxy terminates the HTTP request and forwards it to the ASP.NET application.

Why use a reverse-proxy server?

Kestrel is great for serving dynamic content from ASP.NET, however the web serving parts aren’t as feature rich as full-featured servers like IIS, Apache or Nginx. A reverse proxy-server can allow you to offload work like serving static content, caching requests, compressing requests, and SSL termination from the HTTP server. The reverse proxy server may reside on a dedicated machine or may be deployed alongside an HTTP server.

For the purposes of this guide, we are going to use a single instance of Nginx that runs on the same server alongside your HTTP server. However, based on your requirements you may choose a different setup.

When setting up a reverse-proxy server other than IIS, you must call app.UseIdentity (in Configure) before any other external providers.

Add UseForwardedHeaders to Configure before calling app.UseFacebookAuthentication or similar:

            app.UseForwardedHeaders(new ForwardedHeadersOptions
            {
                ForwardedHeaders = ForwardedHeaders.XForwardedFor | ForwardedHeaders.XForwardedProto
            });

Install Nginx

sudo apt-get install nginx

We are going to apt-get to install Nginx. The installer also creates a System V init script that runs Nginx as daemon on system startup. Since we just installed Nginx for the first time, we can explicitly start it by running

sudo service nginx start

At this point you should be able to navigate to your browser and see the default landing page for Nginx.

Configure Nginx

We will now configure Nginx as a reverse proxy to forward requests to our ASP.NET application

We will be modifying the /etc/nginx/sites-available/default, so open it up in your favorite text editor and replace the contents with the following.

server {
    listen 80;
    location / {
        proxy_pass http://localhost:5000;
        proxy_http_version 1.1;
        proxy_set_header Upgrade $http_upgrade;
        proxy_set_header Connection keep-alive;
        proxy_set_header Host $host;
        proxy_cache_bypass $http_upgrade;
    }
}

This is one of the simplest configuration files for Nginx that forwards incoming public traffic on your port 80 to a port 5000 that your web application will listen on.

Once you have completed making changes to your nginx configuration you can run sudo nginx -t to verify the syntax of your configuration files. If the configuration file test is successful you can ask nginx to pick up the changes by running sudo nginx -s reload.

Monitoring our application

Nginx is now setup to forward requests made to http://localhost:80 on to the ASP.NET Core application running on Kestrel at http://127.0.0.1:5000. However, Nginx is not set up to manage the Kestrel process. We will use systemd and create a service file to start and monitor the underlying web app. systemd is an init system that provides many powerful features for starting, stopping and managing processes.

Create the service file

Create the service definition file

    sudo nano /etc/systemd/system/kestrel-hellomvc.service

An example service file for our application.

[Unit]
    Description=Example .NET Web API Application running on Ubuntu

    [Service]
    WorkingDirectory=/var/aspnetcore/hellomvc
    ExecStart=/usr/bin/dotnet /var/aspnetcore/hellomvc/hellomvc.dll
    Restart=always
    RestartSec=10                                          # Restart service after 10 seconds if dotnet service crashes
    SyslogIdentifier=dotnet-example
    User=www-data
    Environment=ASPNETCORE_ENVIRONMENT=Production 

    [Install]
    WantedBy=multi-user.target

1

    systemctl enable kestrel-hellomvc.service

Start the service and verify that it is running.

    systemctl start kestrel-hellomvc.service
    systemctl status kestrel-hellomvc.service

    ● kestrel-hellomvc.service - Example .NET Web API Application running on Ubuntu
        Loaded: loaded (/etc/systemd/system/kestrel-hellomvc.service; enabled)
        Active: active (running) since Thu 2016-10-18 04:09:35 NZDT; 35s ago
    Main PID: 9021 (dotnet)
        CGroup: /system.slice/kestrel-hellomvc.service
                └─9021 /usr/local/bin/dotnet /var/aspnetcore/hellomvc/hellomvc.dll

With the reverse proxy configured and Kestrel managed through systemd, the web application is fully configured and can be accessed from a browser on the local machine at http://localhost. Inspecting the response headers, the Server still shows the ASP.NET Core application being served by Kestrel.

    HTTP/1.1 200 OK
    Date: Tue, 11 Oct 2016 16:22:23 GMT
    Server: Kestrel
    Keep-Alive: timeout=5, max=98
    Connection: Keep-Alive
    Transfer-Encoding: chunked

Viewing logs

Since the web application using Kestrel is managed using systemd, all events and processes are logged to a centralized journal. However, this journal includes all entries for all services and processes managed by systemd. To view the kestrel-hellomvc.service specific items, use the following command.

    sudo journalctl -fu kestrel-hellomvc.service

For further filtering, time options such as --since today, --until 1 hour ago or a combination of these can reduce the amount of entries returned.

    sudo journalctl -fu kestrel-hellomvc.service --since "2016-10-18" --until "2016-10-18 04:00"

Securing our application

Enable AppArmor

AppArmor is enabled and properly configured.

Configuring our firewall

Close off all external ports that are not in use. Uncomplicated firewall (ufw) provides a frontend for iptables by providing a command-line interface for configuring the firewall. Verify that ufw is configured to allow traffic on any ports you need.

sudo apt-get install ufw
sudo ufw enable

sudo ufw allow 80/tcp
sudo ufw allow 443/tcp

Securing Nginx

The default distribution of Nginx doesn't enable SSL. To enable all the security features we require, we will build from source.

Download the source and install the build dependencies

# Install the build dependencies
sudo apt-get update
sudo apt-get install build-essential zlib1g-dev libpcre3-dev libssl-dev libxslt1-dev libxml2-dev libgd2-xpm-dev libgeoip-dev libgoogle-perftools-dev libperl-dev

# Download nginx 1.10.0 or latest
wget http://www.nginx.org/download/nginx-1.10.0.tar.gz
tar zxf nginx-1.10.0.tar.gz

Change the Nginx response name

Edit src/http/ngx_http_header_filter_module.c

static char ngx_http_server_string[] = "Server: Your Web Server" CRLF;
static char ngx_http_server_full_string[] = "Server: Your Web Server" CRLF;

Configure the options and build

The PCRE library is required for regular expressions. Regular expressions are used in the location directive for the ngx_http_rewrite_module. The http_ssl_module adds HTTPS protocol support.

Consider using a web application firewall like ModSecurity to harden your application.

./configure
--with-pcre=../pcre-8.38
--with-zlib=../zlib-1.2.8
--with-http_ssl_module
--with-stream
--with-mail=dynamic

Configure SSL

• Configure your server to listen to HTTPS traffic on port 443 by specifying a valid certificate issued by a trusted Certificate Authority (CA).

• Harden your security by employing some of the practices suggested below like choosing a stronger cipher and redirecting all traffic over HTTP to HTTPS.

• Adding an HTTP Strict-Transport-Security (HSTS) header ensures all subsequent requests made by the client are over HTTPS only.

• Do not add the Strict-Transport-Security header or chose an appropriate max-age if you plan to disable SSL in the future.

Add /etc/nginx/proxy.conf configuration file.

proxy_redirect 			off;
proxy_set_header 		Host 			$host;
proxy_set_header		X-Real-IP 		$remote_addr;
proxy_set_header		X-Forwarded-For	$proxy_add_x_forwarded_for;
proxy_set_header    X-Forwarded-Proto $scheme;
client_max_body_size 	10m;
client_body_buffer_size 128k;
proxy_connect_timeout 	90;
proxy_send_timeout 		90;
proxy_read_timeout 		90;
proxy_buffers			32 4k;

Edit /etc/nginx/nginx.conf configuration file. The example contains both http and server sections in one configuration file.

http {
    include    /etc/nginx/proxy.conf;
    limit_req_zone $binary_remote_addr zone=one:10m rate=5r/s;
    server_tokens off;

    sendfile on;
    keepalive_timeout 29; # Adjust to the lowest possible value that makes sense for your use case.
    client_body_timeout 10; client_header_timeout 10; send_timeout 10;

    upstream hellomvc{
        server localhost:5000;
    }

    server {
        listen *:80;
        add_header Strict-Transport-Security max-age=15768000;
        return 301 https://$host$request_uri;
    }

    server {
        listen *:443    ssl;
        server_name     example.com;
        ssl_certificate /etc/ssl/certs/testCert.crt;
        ssl_certificate_key /etc/ssl/certs/testCert.key;
        ssl_protocols TLSv1.1 TLSv1.2;
        ssl_prefer_server_ciphers on;
        ssl_ciphers "EECDH+AESGCM:EDH+AESGCM:AES256+EECDH:AES256+EDH";
        ssl_ecdh_curve secp384r1;
        ssl_session_cache shared:SSL:10m;
        ssl_session_tickets off;
        ssl_stapling on; #ensure your cert is capable
        ssl_stapling_verify on; #ensure your cert is capable

        add_header Strict-Transport-Security "max-age=63072000; includeSubdomains; preload";
        add_header X-Frame-Options DENY;
        add_header X-Content-Type-Options nosniff;

        #Redirects all traffic
        location / {
            proxy_pass  http://hellomvc;
            limit_req   zone=one burst=10;
        }
    }
}

Secure Nginx from clickjacking

Clickjacking is a malicious technique to collect an infected user's clicks. Clickjacking tricks the victim (visitor) into clicking on an infected site. Use X-FRAME-OPTIONS to secure your site.

Edit the nginx.conf file.

    sudo nano /etc/nginx/nginx.conf

 and save the file, then restart Nginx.

MIME-type sniffing

This header prevents Internet Explorer from MIME-sniffing a response away from the declared content-type as the header instructs the browser not to override the response content type. With the nosniff option, if the server says the content is text/html, the browser will render it as text/html.

Edit the nginx.conf file.

    sudo nano /etc/nginx/nginx.conf

 and save the file, then restart Nginx.