created configuration-driven proxy filter, which uses a handlebar-lik…

src/ReverseProxy/Configuration/Filters/ConfigSubstitutionFilter.cs

@@ -0,0 +1,58 @@
+using System;
+using System.Collections.Generic;
+using System.Text.RegularExpressions;
+using System.Threading;
+using System.Threading.Tasks;
+using Microsoft.Extensions.Configuration;
+
+namespace Yarp.ReverseProxy.Configuration;
+
+public class ConfigSubstitutionFilter : IProxyConfigFilter
+{
+    // Matches {{env_var_name}} or {{my-name}} or {{123name}} etc.
+    private readonly Regex _exp = new("\\{\\{(\\w+\\-?\\w+?)\\}\\}");
+    private readonly IConfiguration _configuration;
+
+    public ConfigSubstitutionFilter(IConfiguration configuration)
+    {
+        _configuration = configuration;
+    }
+
+    public ValueTask<ClusterConfig> ConfigureClusterAsync(ClusterConfig cluster, CancellationToken cancel)
+    {
+        if (cluster.Destinations is null)
+        {
+            return new ValueTask<ClusterConfig>(cluster);
+        }
+
+        // Each cluster has a dictionary of destinations, which is read-only, so we'll create a new one with our updates 
+        var newDests = new Dictionary<string, DestinationConfig>(StringComparer.OrdinalIgnoreCase);
+
+        foreach (var d in cluster.Destinations)
+        {
+            var origAddress = d.Value.Address;
+            if (_exp.IsMatch(origAddress))
+            {
+                var lookup = _exp.Matches(origAddress)[0].Groups[1].Value;
+                var newAddress = _configuration.GetValue<string>(lookup);
+
+                if (string.IsNullOrWhiteSpace(newAddress))
+                {
+                    throw new System.ArgumentException($"Configuration Filter Error: Substitution for '{lookup}' in cluster '{d.Key}' not found in configuration.");
+                }
+
+                var modifiedDest = d.Value with { Address = newAddress };
+                newDests.Add(d.Key, modifiedDest);
+            }
+            else
+            {
+                newDests.Add(d.Key, d.Value);
+            }
+        }
+
+        return new ValueTask<ClusterConfig>(cluster with { Destinations = newDests });
+    }
+
+    public ValueTask<RouteConfig> ConfigureRouteAsync(RouteConfig route, ClusterConfig? cluster, CancellationToken cancel)
+        => new ValueTask<RouteConfig>(route);
+}

src/ReverseProxy/Configuration/Filters/ConfigSubstitutionFilterProxyBuilderExtensions.cs

@@ -0,0 +1,9 @@
+using Microsoft.Extensions.DependencyInjection;
+
+namespace Yarp.ReverseProxy.Configuration;
+
+public static class ConfigSubstitutionFilterProxyBuilderExtensions
+{
+    public static IReverseProxyBuilder AddConfigurationDrivenProxyFilter(this IReverseProxyBuilder builder)
+        => builder.AddConfigFilter<ConfigSubstitutionFilter>();
+}

src/ReverseProxy/Yarp.ReverseProxy.csproj

@@ -23,7 +23,7 @@
   </ItemGroup>
 
   <ItemGroup>
-    <None Include="README.md" Pack="true" PackagePath="\"/>
+    <None Include="README.md" Pack="true" PackagePath="\" />
   </ItemGroup>
 
   <ItemGroup>