Lookup FQDN of Application Gateway for backend App Services Web App - asp.net core

AzureSettings.cs

public class AzureSettings
{
    public IAzure Azure { get; set; }
}

Lookup.cs

public string Get(string id)
{
    var forwardedFor = HttpContext.Request.Headers["X-Forwarded-For"];
    var gatewayIp = forwardedFor == StringValues.Empty ? null 
        : forwardedFor.Last().Split(':').First();

    if (string.IsNullOrEmpty(gatewayIp)) //no gw 
    {
        return HttpContext.Request.Host.Value;
    }

    var fqdn = GetFqdn(gatewayIp);

    //TODO: if needed, do a DNS lookup on the CNAME set to this FQDN or use a custom hashtable lookup

    return fqdn;
}

// make sure you target the resource group of the app. Store in config settings

public string GetFqdn(string gatewayIp)
{
    
    var ips = azure.Networks.Manager.PublicIPAddresses.ListByResourceGroup("gw-testing");
    var publicIp = ips.First(ip => ip.IPAddress == gatewayIp);

    return publicIp.Fqdn;
}

Startup.cs

// Add to startup. azureauth.properties file in root
// see https://docs.microsoft.com/en-us/azure/virtual-machines/windows/csharp

// install nuget Microsoft.Azure.Management.Fluent

public void ConfigureServices(IServiceCollection services)
{
    // Add framework services.
    services.AddMvc();
    services.AddOptions();

    services.Configure<AzureSettings>(s =>
    {
        var credentials = SdkContext.AzureCredentialsFactory.FromFile(Path.Combine(ContentRootPath, "azureauth.properties"));
        var azure = Azure
            .Configure()
            .WithLogLevel(HttpLoggingDelegatingHandler.Level.Basic)
            .Authenticate(credentials)
            .WithDefaultSubscription();

        s.Azure = azure;
    });
}