enable auto setting of SNI based on host header (#1129)

With this change, setting targetSNI to 'auto' causes anubis to
use the request host name as the SNI name, allowing multiple sites
to use the same anubis instance and same backend, while still securely
connecting to the backend via https.

See https://github.com/TecharoHQ/anubis/issues/424

cmd/anubis/main.go

@@ -68,7 +68,7 @@ var (
 	slogLevel                = flag.String("slog-level", "INFO", "logging level (see https://pkg.go.dev/log/slog#hdr-Levels)")
 	stripBasePrefix          = flag.Bool("strip-base-prefix", false, "if true, strips the base prefix from requests forwarded to the target server")
 	target                   = flag.String("target", "http://localhost:3923", "target to reverse proxy to, set to an empty string to disable proxying when only using auth request")
-	targetSNI                = flag.String("target-sni", "", "if set, the value of the TLS handshake hostname when forwarding requests to the target")
+	targetSNI                = flag.String("target-sni", "", "if set, TLS handshake hostname when forwarding requests to the target, if set to auto, use Host header")
 	targetHost               = flag.String("target-host", "", "if set, the value of the Host header when forwarding requests to the target")
 	targetInsecureSkipVerify = flag.Bool("target-insecure-skip-verify", false, "if true, skips TLS validation for the backend")
 	targetDisableKeepAlive   = flag.Bool("target-disable-keepalive", false, "if true, disables HTTP keep-alive for the backend")
@@ -236,23 +236,28 @@ func makeReverseProxy(target string, targetSNI string, targetHost string, insecu
 
 	if insecureSkipVerify || targetSNI != "" {
 		transport.TLSClientConfig = &tls.Config{}
-		if insecureSkipVerify {
-			slog.Warn("TARGET_INSECURE_SKIP_VERIFY is set to true, TLS certificate validation will not be performed", "target", target)
-			transport.TLSClientConfig.InsecureSkipVerify = true
-		}
-		if targetSNI != "" {
-			transport.TLSClientConfig.ServerName = targetSNI
-		}
+	}
+	if insecureSkipVerify {
+		slog.Warn("TARGET_INSECURE_SKIP_VERIFY is set to true, TLS certificate validation will not be performed", "target", target)
+		transport.TLSClientConfig.InsecureSkipVerify = true
+	}
+	if targetSNI != "" && targetSNI != "auto" {
+		transport.TLSClientConfig.ServerName = targetSNI
 	}
 
 	rp := httputil.NewSingleHostReverseProxy(targetUri)
 	rp.Transport = transport
 
-	if targetHost != "" {
+	if targetHost != "" || targetSNI == "auto" {
 		originalDirector := rp.Director
 		rp.Director = func(req *http.Request) {
 			originalDirector(req)
-			req.Host = targetHost
+			if targetHost != "" {
+				req.Host = targetHost
+			}
+			if targetSNI == "auto" {
+				transport.TLSClientConfig.ServerName = req.Host
+			}
 		}
 	}
 

docs/docs/CHANGELOG.md

@@ -29,6 +29,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Fixes concurrency problems with very old browsers ([#1082](https://github.com/TecharoHQ/anubis/issues/1082)).
 - Randomly use the Refresh header instead of the meta refresh tag in the metarefresh challenge.
 - Update OpenRC service to truncate the runtime directory before starting Anubis.
+- Add option to set `targetSNI` to special keyword 'auto' to indicate that it should be automatically set to the request Host name ([424](https://github.com/TecharoHQ/anubis/issues/424)).
 
 ### Bug Fixes
 

docs/docs/admin/installation.mdx

@@ -123,7 +123,7 @@ If you don't know or understand what these settings mean, ignore them. These are
 | `TARGET_DISABLE_KEEPALIVE`    | `false`       | If `true`, disables HTTP keep-alive for connections to the target backend. Useful for backends that don't handle keep-alive properly.                                                                                                                                                                                                                                                           |
 | `TARGET_HOST`                 | unset         | If set, overrides the Host header in requests forwarded to `TARGET`.                                                                                                                                                                                                                                                                                                                            |
 | `TARGET_INSECURE_SKIP_VERIFY` | `false`       | If `true`, skip TLS certificate validation for targets that listen over `https`. If your backend does not listen over `https`, ignore this setting.                                                                                                                                                                                                                                             |
-| `TARGET_SNI`                  | unset         | If set, overrides the TLS handshake hostname in requests forwarded to `TARGET`.                                                                                                                                                                                                                                                                                                                 |
+| `TARGET_SNI`                  | unset         | If set, TLS handshake hostname when forwarding requests to the `TARGET`. If set to auto, use Host header.                                                                                                                                                                                                                                                                                                                 |
 
 </details>