RobotsTxtCore

A Robots.txt middleware for ASP.NET Core. Why is this needed you ask? Because if you need to add dynamic values (such as a configured url from your CMS) you'll need some sort of code to handle that, and this makes it easy.

Installation

.Net CLI

> dotnet add package RobotsTxtCore

NuGet

PM> Install-Package RobotsTxtCore

Changes from 1.0 to 2.0

The first version of this package was only a middleware and had to be configured in the Configure method in the Startup class. This felt fine at the time but as more and more things moved to having configuration as a service and then letting the middleware consume the service I felt like this package got outdated.

So I've made a breaking change and made the middleware consume a IRobotsTxtProvider which in turn takes care of configuration. There is a default provider for static uses (i.e. exactly what the old one did) but doing it this way also let me optimize it quite a lot. A quick benchmark shows that running a thousand requests against /robots.txt is now done in 25% of the time while also lowering allocations about the same.

NuGetReferences	Mean	Error	StdDev	Gen 0	Gen 1	Allocated
RobotsTxtCore 1.1.0	1,169.2 μs	22.62 μs	27.77 μs	691.4063	1.9531	4,242 KB
RobotsTxtCore 2.0.0-preview1	419.8 μs	3.88 μs	3.24 μs	167.9688	-	1,031 KB
RobotsTxtCore 2.0.0-preview2	431.5 μs	2.90 μs	2.57 μs	150.3906	-	922 KB
RobotsTxtCore 2.0.0-preview3	307.4 μs	2.00 μs	1.87 μs	155.2734	-	953 KB

Sure, it was really fast to start with and there are very few sites where /robots.txt gets a ton of traffic but that doesn't mean it's not worth it 😉.

Introducing the IRobotsTxtProvider also allows for easier dynamic usage, like reading settings from a database or switching depending on which environment the code is executing in.

Usage

To specify multiple rules with the fluent interface makes it really easy.

public void ConfigureServices(IServiceCollection services) {
    services.AddStaticRobotsTxt(builder =>
        builder
            .AddSection(section =>
                section
                    .AddComment("Allow Googlebot")
                    .AddUserAgent("Googlebot")
                    .Allow("/")
                )
            .AddSection(section =>
                section
                    .AddComment("Disallow the rest")
                    .AddUserAgent("*")
                    .AddCrawlDelay(TimeSpan.FromSeconds(10))
                    .Disallow("/")
                )
            .AddSitemap("https://example.com/sitemap.xml")
    );
}

public void Configure(IApplicationBuilder app) {
    app.UseRobotsTxt();
}

Output

# Allow Googlebot
User-agent: Googlebot
Allow: /

# Disallow the rest
User-agent: *
Disallow: /
Crawl-delay: 10

Sitemap: https://example.com/sitemap.xml

Or if you just want to deny everyone.

public void ConfigureServices(IServiceCollection services) {
    services.AddStaticRobotsTxt(builder =>
        builder
            .DenyAll()
    );
}

public void Configure(IApplicationBuilder app) {
    app.UseRobotsTxt();
}

Output

User-agent: *
Disallow: /

Since version 3.0 you can call AddStaticRobotsTxt multiple times and specify different settings for different environments. If there is a matching envrionment it will be used, otherwise it will fall back to any configuration without an environment specified.

public void ConfigureServices(IServiceCollection services) {
    services.AddStaticRobotsTxt(builder =>
        builder
            .ForEnvironment("Production")
            .AddSection(section =>
                section
                    .AddComment("Allow Googlebot")
                    .AddUserAgent("Googlebot")
                    .Allow("/")
                )
    );

    services.AddStaticRobotsTxt(builder =>
        builder
            .DenyAll()
    );
}

public void Configure(IApplicationBuilder app) {
    app.UseRobotsTxt();
}

IRobotsTxtProvider

IRobotsTxtProvider allows for dynamicly configuring the Robots.txt output depending on your case. It could be used to read from config, to check a database setting or perhaps which environment your application is currently running in.

public class CoolRobotsTxtProvider : IRobotsTxtProvider {
    private readonly CoolContext _context;

    public CoolRobotsTxtProvider(CoolContext context) {
        _context = context;
    }

    public async Task<RobotsTxtResult> GetResultAsync(CancellationToken cancellationToken) {
        var settings = await _context.Settings.FirstAsync();

        var builder = new RobotsTxtOptionsBuilder();

        RobotsTxtOptions options;
        if(settings.AllowAllRobots)
            options = builder.AllowAll().Build();
        else
            options = builder.DenyAll().Build();

        var content = options.ToString();
        var buffer = Encoding.UTF8.GetBytes(content).AsMemory();
        return new RobotsTxtResult(buffer, settings.RobotsTxtMaxAge);
    }
}