How can I change Kestrel (AspNetCore) listening port by environment variables in netcore3.1

Question

I have aspnetcore3.1 project, and I want to set custom port for Kestrel (other than default 5000). I can possibly do it in Program.cs by adding

Host.CreateDefaultBuilder(args)
.ConfigureWebHostDefaults(webBuilder =>
{
    webBuilder.ConfigureKestrel(options =>
    {
        options.ListenLocalhost(80);
    });
    webBuilder.UseStartup<Startup>();
})

But this cannot be applied to my case, so wondered, how can this be done by environment variable?

Solution

There are multiple ways to achieve this as documented here.

1. ASPNETCORE_URLS environment variable.
2. --urls command-line argument.
3. urls host configuration key.
4. UseUrls extension method.

To achieve this using an environment variable, simply create an environment variable called ASPNETCORE_URLS and set the value to the URL you'd like to use

Typically this would be http://+:<port> or https://+:<port>

Another method which, at the time of writing this answer, isn't described above is via the hostsettings.json file.

You can configure the URL & Port by creating a hostsettings.json configuration file and adding the urls key, then add hostsettings.json to your IConfigurationBuilder when building your WebHostBuilder.

https://docs.microsoft.com/en-us/aspnet/core/fundamentals/host/web-host?view=aspnetcore-3.1#override-configuration

Here is the code snippet from the link, in case this link ever goes dead.

Create your hostsettings.json file containing the urls key with your value(s)

{
    urls: "http://*:5005"
}

Register hostsettings.json in your IConfigurationBuilder

public class Program
{
    public static void Main(string[] args)
    {
        CreateWebHostBuilder(args).Build().Run();
    }

    public static IWebHostBuilder CreateWebHostBuilder(string[] args)
    {
        var config = new ConfigurationBuilder()
            .SetBasePath(Directory.GetCurrentDirectory())
            .AddJsonFile("appsettings.json", optional: false)
            .AddJsonFile("hostsettings.json", optional: true)
            .AddCommandLine(args)
            .Build();

        return WebHost.CreateDefaultBuilder(args)
            .UseUrls("http://*:5000")
            .UseConfiguration(config)
            .UseStartup<Startup>();
    }
}