providers/proxy: add option to override host header with property mappings

internal/outpost/proxyv2/application/claims.go

@@ -3,6 +3,7 @@ package application
 type ProxyClaims struct {
 	UserAttributes  map[string]interface{} `json:"user_attributes"`
 	BackendOverride string                 `json:"backend_override"`
+	HostHeader      string                 `json:"host_header"`
 	IsSuperuser     bool                   `json:"is_superuser"`
 }
 

internal/outpost/proxyv2/application/mode_proxy.go

@@ -74,13 +74,18 @@ func (a *Application) proxyModifyRequest(ou *url.URL) func(req *http.Request) {
 		r.URL.Scheme = ou.Scheme
 		r.URL.Host = ou.Host
 		claims := a.getClaimsFromSession(r)
-		if claims != nil && claims.Proxy != nil && claims.Proxy.BackendOverride != "" {
-			u, err := url.Parse(claims.Proxy.BackendOverride)
-			if err != nil {
-				a.log.WithField("backend_override", claims.Proxy.BackendOverride).WithError(err).Warning("failed parse user backend override")
-			} else {
-				r.URL.Scheme = u.Scheme
-				r.URL.Host = u.Host
+		if claims != nil && claims.Proxy != nil {
+			if claims.Proxy.BackendOverride != "" {
+				u, err := url.Parse(claims.Proxy.BackendOverride)
+				if err != nil {
+					a.log.WithField("backend_override", claims.Proxy.BackendOverride).WithError(err).Warning("failed parse user backend override")
+				} else {
+					r.URL.Scheme = u.Scheme
+					r.URL.Host = u.Host
+				}
+			}
+			if claims.Proxy.HostHeader != "" {
+				r.Host = claims.Proxy.HostHeader
 			}
 		}
 		a.log.WithField("upstream_url", r.URL.String()).Trace("final upstream url")

website/docs/add-secure-apps/providers/proxy/index.md

@@ -152,3 +152,17 @@ return {
 ```
 
 Afterwards, edit the _Proxy provider_ and add this new mapping. The expression is only evaluated when the user logs into the application.
+
+## Host header:ak-version[2025.6.1]
+
+By default, the proxy provider will use forwarded Host header received from the client. Starting with authentik 2025.6.1, it is possible to dynamically adjust the Host header with a property mapping.
+
+```python
+return {
+    "ak_proxy": {
+        "host_header": "my-internal-host-header"
+    }
+}
+```
+
+Afterwards, edit the _Proxy provider_ and add this new mapping. The expression is only evaluated when the user logs into the application.