Add Via header to passed-through requests in Kestrel engine. Closes #1948

DevProxy.Integration.Tests/FakeOrigin.cs

@@ -24,6 +24,7 @@ namespace DevProxy.Integration.Tests;
 ///   POST /echo           → 200 echoes request body  (X-Echo-Length header)
 ///   GET  /status/{code}  → {code} "status {code}"
 ///   GET  /headers        → 200, reflects X-Probe request header in body
+///   GET  /via            → 200, reflects the Via request header values in body
 ///   GET  /sse            → 200 text/event-stream, 5 events flushed ~50ms apart
 ///   GET  /big/{n}        → 200, n bytes of 'A' (large finite body)
 ///   GET  /json           → 200 application/json, a 2-element array
@@ -94,6 +95,9 @@ public static async Task<FakeOrigin> StartAsync()
             return Results.Text($"probe={probe}");
         });
 
+        app.MapGet("/via", (HttpContext ctx) =>
+            Results.Text(string.Join(", ", ctx.Request.Headers.Via.ToArray())));
+
         app.MapGet("/sse", async (HttpContext ctx) =>
         {
             ctx.Response.Headers.ContentType = "text/event-stream";

DevProxy.Integration.Tests/PlainHttpIntegrationTests.cs

@@ -4,6 +4,7 @@
 
 using System.Net;
 using System.Text;
+using DevProxy.Abstractions.Utils;
 using Xunit;
 
 namespace DevProxy.Integration.Tests;
@@ -130,4 +131,34 @@ public async Task Get_RequestHeader_ReachesOrigin()
 
         Assert.Equal("probe=abc123", body);
     }
+
+    [Fact]
+    public async Task Get_PassedThrough_AddsViaHeader()
+    {
+        await using var origin = await FakeOrigin.StartAsync();
+        await using var proxy = await KestrelProxyHarness.StartAsync(origin.Host);
+        using var client = proxy.CreateHttpClient();
+
+        using var response = await client.GetAsync(
+            new Uri($"http://{origin.Host}/via"));
+        var body = await response.Content.ReadAsStringAsync();
+
+        Assert.Equal($"1.1 dev-proxy/{ProxyUtils.ProductVersion}", body);
+    }
+
+    [Fact]
+    public async Task Get_PassedThrough_AppendsToExistingViaHeader()
+    {
+        await using var origin = await FakeOrigin.StartAsync();
+        await using var proxy = await KestrelProxyHarness.StartAsync(origin.Host);
+        using var client = proxy.CreateHttpClient();
+
+        using var request = new HttpRequestMessage(
+            HttpMethod.Get, new Uri($"http://{origin.Host}/via"));
+        _ = request.Headers.TryAddWithoutValidation("Via", "1.0 upstream-proxy");
+        using var response = await client.SendAsync(request);
+        var body = await response.Content.ReadAsStringAsync();
+
+        Assert.Equal($"1.0 upstream-proxy, 1.1 dev-proxy/{ProxyUtils.ProductVersion}", body);
+    }
 }

DevProxy.Proxy.Kestrel/Internal/PluginPipeline.cs

@@ -6,6 +6,7 @@
 using System.Globalization;
 using DevProxy.Abstractions.Plugins;
 using DevProxy.Abstractions.Proxy;
+using DevProxy.Abstractions.Utils;
 using DevProxy.Proxy.Kestrel.Http;
 using Microsoft.Extensions.Logging;
 
@@ -125,10 +126,16 @@ public async Task<RequestPhase> RunRequestAsync(CanonicalProxySession session, C
             }
 
             _logger.LogRequest("Passed through", MessageType.PassedThrough, loggingContext);
+            AddProxyHeader(request);
         }
         return RequestPhase.Watched;
     }
 
+    // RFC 9110 §7.6.3: a proxy MUST send a Via header in each message it forwards.
+    // Appended as a new field so any existing Via value is preserved.
+    private static void AddProxyHeader(Abstractions.Proxy.Http.IHttpRequest request) =>
+        request.Headers.Add("Via", $"{request.HttpVersion.ToString(2)} dev-proxy/{ProxyUtils.ProductVersion}");
+
     public Task RunResponseAsync(CanonicalProxySession session, CancellationToken ct)
         => RunResponseCoreAsync(session, betweenPhases: null, ct);