Set up a hosting environment for ASP.NET Core on Linux with Nginx, and deploy to it

By Sourabh Shirhatti

This guide explains setting up a production-ready ASP.NET Core environment on an Ubuntu 16.04 Server.

Note: For Ubuntu 14.04, supervisord is recommended as a solution for monitoring the Kestrel process. systemd is not available on Ubuntu 14.04. See previous version of this document

This guide:

  • Places an existing ASP.NET Core application behind a reverse proxy server
  • Sets up the reverse proxy server to forward requests to the Kestrel web server
  • Ensures the web application runs on startup as a daemon
  • Configures a process management tool to help restart the web application

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

Run dotnet publish from the dev environment to package an app into a self-contained directory that can run on the server.

Copy the ASP.NET Core app to the server using whatever tool (SCP, FTP, etc.) integrates into your workflow. Test the app, for example:

  • From the command line, run dotnet yourapp.dll
  • In a browser, navigate to http://<serveraddress>:<port> to verify the app works on Linux.

Configure a reverse proxy server

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

Why use a reverse proxy server?

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

For the purposes of this guide, a single instance of Nginx is used. It runs on the same server, alongside the HTTP server. Based on your requirements, you may choose a different setup.

Because requests are forwarded by reverse proxy, use the ForwardedHeaders middleware from the Microsoft.AspNetCore.HttpOverrides package. This middleware updates Request.Scheme, using the X-Forwarded-Proto header, so that redirect URIs and other security policies work correctly.

When setting up a reverse proxy server, the authentication middleware needs UseForwardedHeaders to run first. This ordering ensures that the authentication middleware can consume the affected values and generate correct redirect URIs.

Invoke the UseForwardedHeaders method (in the Configure method of Startup.cs) before calling UseAuthentication or similar authentication scheme middleware:

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

app.UseAuthentication();

Install Nginx

sudo apt-get install nginx

Note

If you plan to install optional Nginx modules, you may be required to build Nginx from source.

Use apt-get to install Nginx. The installer creates a System V init script that runs Nginx as daemon on system startup. Since Nginx was installed for the first time, explicitly start it by running:

sudo service nginx start

Verify a browser displays the default landing page for Nginx.

Configure Nginx

To configure Nginx as a reverse proxy to forward requests to our ASP.NET Core application, modify /etc/nginx/sites-available/default. Open it in a 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 Nginx configuration file forwards incoming public traffic from port 80 to port 5000.

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://yourhost: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. You can 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

The following is 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
Environment=DOTNET_PRINT_TELEMETRY_MESSAGE=false

[Install]
WantedBy=multi-user.target

Note: If the user www-data is not used by your configuration, the user defined here must be created first and given proper ownership for files.

Save the file, and enable the service.

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. It is also accessible from a remote machine, barring any firewall that might be blocking. Inspecting the response headers, the Server header 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

Linux Security Modules (LSM) is a framework that is part of the Linux kernel since Linux 2.6. LSM supports different implementations of security modules. AppArmor is a LSM that implements a Mandatory Access Control system which allows confining the program to a limited set of resources. Ensure AppArmor is enabled and properly configured.

Configuring our firewall

Close off all external ports that are not in use. Uncomplicated firewall (ufw) provides a front end 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 additional security features, 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 depicted in the following /etc/nginx/nginx.conf file. Examples include 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 the /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 the /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

Add the line add_header X-Frame-Options "SAMEORIGIN"; and save the file, then restart Nginx.

MIME-type sniffing

This header prevents most browsers 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 renders it as "text/html".

Edit the nginx.conf file:

sudo nano /etc/nginx/nginx.conf

Add the line add_header X-Content-Type-Options "nosniff"; and save the file, then restart Nginx.