From 3563388d4d0579054e6a1dd8a43fab9e833dd687 Mon Sep 17 00:00:00 2001
From: Ashley Harrison <ashley.harrison@grafana.com>
Date: Thu, 13 Nov 2025 03:32:14 +0000
Subject: [PATCH] Frontend Service: Client side redirect to custom domain
 (#113717)

* handle redirect in frontend service

* add comments
---
 pkg/middleware/validate_host.go  | 10 ++++++++++
 pkg/services/frontend/index.html | 10 ++++++++++
 2 files changed, 20 insertions(+)

diff --git a/pkg/middleware/validate_host.go b/pkg/middleware/validate_host.go
index eb077b8e06c..ebdf96b4274 100644
--- a/pkg/middleware/validate_host.go
+++ b/pkg/middleware/validate_host.go
@@ -32,6 +32,16 @@ func ValidateHostHeader(cfg *setting.Cfg) web.Handler {
 		}
 
 		if !strings.EqualFold(h, cfg.Domain) {
+			// the normal redirecting logic doesn't work when running as frontend service, since it has no knowledge of the custom domain.
+			// instead, we modify the single tenant `/bootdata` call with a 204 response and a header indicating the domain to redirect to
+			// this is safe because only the frontend service calls `/bootdata`
+			// the redirect is then handled client side.
+			// see pkg/services/frontend/index.html
+			if c.Req.URL.Path == "/bootdata" {
+				c.Resp.Header().Set("Redirect-Domain", cfg.Domain)
+				c.Resp.WriteHeader(204)
+				return
+			}
 			hostRedirectCounter.Inc()
 			c.Logger.Info("Enforcing Host header", "hosted", c.Req.Host, "expected", cfg.Domain)
 			c.Redirect(strings.TrimSuffix(cfg.AppURL, "/")+c.Req.RequestURI, 301)
diff --git a/pkg/services/frontend/index.html b/pkg/services/frontend/index.html
index f937a987ca4..c8ba00870b9 100644
--- a/pkg/services/frontend/index.html
+++ b/pkg/services/frontend/index.html
@@ -220,6 +220,16 @@
           }
 
           const resp = await fetch(bootDataUrl);
+
+          // manual redirect for custom domains
+          // see pkg/middleware/validate_host.go
+          if (resp.status === 204) {
+            const redirectDomain = resp.headers.get('Redirect-Domain');
+            if (redirectDomain) {
+              window.location.hostname = redirectDomain;
+              return;
+            }
+          }
           const textResponse = await resp.text();
 
           let rawBootData;