Is it possible to make an ASP.NET MVC route based on a subdomain?

You can do it by creating a new route and adding it to the routes collection in RegisterRoutes in your global.asax. Below is a very simple example of a custom Route:

public class ExampleRoute : RouteBase
{

    public override RouteData GetRouteData(HttpContextBase httpContext)
    {
        var url = httpContext.Request.Headers["HOST"];
        var index = url.IndexOf(".");

        if (index < 0)
            return null;

        var subDomain = url.Substring(0, index);

        if (subDomain == "user1")
        {
            var routeData = new RouteData(this, new MvcRouteHandler());
            routeData.Values.Add("controller", "User1"); //Goes to the User1Controller class
            routeData.Values.Add("action", "Index"); //Goes to the Index action on the User1Controller

            return routeData;
        }

        if (subDomain == "user2")
        {
            var routeData = new RouteData(this, new MvcRouteHandler());
            routeData.Values.Add("controller", "User2"); //Goes to the User2Controller class
            routeData.Values.Add("action", "Index"); //Goes to the Index action on the User2Controller

            return routeData;
        }

        return null;
    }

    public override VirtualPathData GetVirtualPath(RequestContext requestContext, RouteValueDictionary values)
    {
        //Implement your formating Url formating here
        return null;
    }
}

To capture the subdomain when using Web API, override the Action Selector to inject a subdomain query parameter. Then use the subdomain query parameter in your controllers' actions like this:

public string Get(string id, string subdomain)

This approach makes debugging convenient since you can specify the query parameter by hand when using localhost instead of the actual host name (see the standard MVC5 routing answer for details). This is the code for Action Selector:

class SubdomainActionSelector : IHttpActionSelector
{
    private readonly IHttpActionSelector defaultSelector;

    public SubdomainActionSelector(IHttpActionSelector defaultSelector)
    {
        this.defaultSelector = defaultSelector;
    }

    public ILookup<string, HttpActionDescriptor> GetActionMapping(HttpControllerDescriptor controllerDescriptor)
    {
        return defaultSelector.GetActionMapping(controllerDescriptor);
    }

    public HttpActionDescriptor SelectAction(HttpControllerContext controllerContext)
    {
        var routeValues = controllerContext.Request.GetRouteData().Values;
        if (!routeValues.ContainsKey("subdomain")) {
            string host = controllerContext.Request.Headers.Host;
            int index = host.IndexOf('.');
            if (index >= 0)
                controllerContext.Request.GetRouteData().Values.Add("subdomain", host.Substring(0, index));
        }
        return defaultSelector.SelectAction(controllerContext);
    }
}

Replace the default Action Selector by adding this to WebApiConfig.Register:

config.Services.Replace(typeof(IHttpActionSelector), new SubdomainActionSelector(config.Services.GetActionSelector()));

This is not my work, but I had to add it on this answer.

Here is a great solution to this problem. Maartin Balliauw wrote code that creates a DomainRoute class that can be used very similarly to the normal routing.

http://blog.maartenballiauw.be/post/2009/05/20/ASPNET-MVC-Domain-Routing.aspx

Sample use would be like this...

routes.Add("DomainRoute", new DomainRoute( 
    "{customer}.example.com", // Domain with parameters 
    "{action}/{id}",    // URL with parameters 
    new { controller = "Home", action = "Index", id = "" }  // Parameter defaults 
))

;

To capture the subdomain while retaining the standard MVC5 routing features, use the following SubdomainRoute class derived from Route.

Additionally, SubdomainRoute allows the subdomain optionally to be specified as a query parameter, making sub.example.com/foo/bar and example.com/foo/bar?subdomain=sub equivalent. This allows you to test before the DNS subdomains are configured. The query parameter (when in use) is propagated through new links generated by Url.Action, etc.

The query parameter also enables local debugging with Visual Studio 2013 without having to configure with netsh or run as Administrator. By default, IIS Express only binds to localhost when non-elevated; it won't bind to synonymous hostnames like sub.localtest.me.

class SubdomainRoute : Route
{
    public SubdomainRoute(string url) : base(url, new MvcRouteHandler()) {}

    public override RouteData GetRouteData(HttpContextBase httpContext)
    {
        var routeData = base.GetRouteData(httpContext);
        if (routeData == null) return null; // Only look at the subdomain if this route matches in the first place.
        string subdomain = httpContext.Request.Params["subdomain"]; // A subdomain specified as a query parameter takes precedence over the hostname.
        if (subdomain == null) {
            string host = httpContext.Request.Headers["Host"];
            int index = host.IndexOf('.');
            if (index >= 0)
                subdomain = host.Substring(0, index);
        }
        if (subdomain != null)
            routeData.Values["subdomain"] = subdomain;
        return routeData;
    }

    public override VirtualPathData GetVirtualPath(RequestContext requestContext, RouteValueDictionary values)
    {
        object subdomainParam = requestContext.HttpContext.Request.Params["subdomain"];
        if (subdomainParam != null)
            values["subdomain"] = subdomainParam;
        return base.GetVirtualPath(requestContext, values);
    }
}

For convenience, call the following MapSubdomainRoute method from your RegisterRoutes method just as you would plain old MapRoute:

static void MapSubdomainRoute(this RouteCollection routes, string name, string url, object defaults = null, object constraints = null)
{
    routes.Add(name, new SubdomainRoute(url) {
        Defaults = new RouteValueDictionary(defaults),
        Constraints = new RouteValueDictionary(constraints),
        DataTokens = new RouteValueDictionary()
    });
}

Finally, to conveniently access the subdomain (either from a true subdomain or a query parameter), it is helpful to create a Controller base class with this Subdomain property:

protected string Subdomain
{
    get { return (string)Request.RequestContext.RouteData.Values["subdomain"]; }
}