Add config option "USE_HOST_HEADER"

Resolves go-gitea#32554

This option can be set to make Gitea always use the "Host" request header for construction of absolute URLs.

modules/httplib/url.go

@@ -69,11 +69,19 @@ func GuessCurrentHostURL(ctx context.Context) string {
 	// 1. The reverse proxy is configured correctly, it passes "X-Forwarded-Proto/Host" headers. Perfect, Gitea can handle it correctly.
 	// 2. The reverse proxy is not configured correctly, doesn't pass "X-Forwarded-Proto/Host" headers, eg: only one "proxy_pass http://gitea:3000" in Nginx.
 	// 3. There is no reverse proxy.
-	// Without an extra config option, Gitea is impossible to distinguish between case 2 and case 3,
-	// then case 2 would result in wrong guess like guessed AppURL becomes "http://gitea:3000/", which is not accessible by end users.
-	// So in the future maybe it should introduce a new config option, to let site admin decide how to guess the AppURL.
+	// With the "USE_HOST_HEADER" config option disabled (default), Gitea is impossible to distinguish between case 2 and case 3,
+	// When enabling "USE_HOST_HEADER", any reverse proxies must be configured to properly pass "X-Forwarded-Proto/Host" headers,
+	// otherwise this would result in wrong guess like guessed AppURL becomes "http://gitea:3000/", which is not accessible by end users.
 	reqScheme := getRequestScheme(req)
 	if reqScheme == "" {
+		if setting.UseHostHeader && req.Host != "" {
+			if req.TLS == nil {
+				return "http://" + req.Host
+			} else {
+				return "https://" + req.Host
+			}
+		}
+
 		return strings.TrimSuffix(setting.AppURL, setting.AppSubURL+"/")
 	}
 	// X-Forwarded-Host has many problems: non-standard, not well-defined (X-Forwarded-Port or not), conflicts with Host header.

modules/httplib/url_test.go

@@ -5,6 +5,7 @@ package httplib
 
 import (
 	"context"
+	"crypto/tls"
 	"net/http"
 	"testing"
 
@@ -39,6 +40,32 @@ func TestIsRelativeURL(t *testing.T) {
 	}
 }
 
+func TestGuessCurrentHostURL(t *testing.T) {
+	defer test.MockVariableValue(&setting.AppURL, "http://cfg-host/sub/")()
+	defer test.MockVariableValue(&setting.AppSubURL, "/sub")()
+
+	ctx := context.Background()
+	assert.Equal(t, "http://cfg-host", GuessCurrentHostURL(ctx))
+
+	ctx = context.WithValue(ctx, RequestContextKey, &http.Request{
+		Host: "localhost:3000",
+	})
+	assert.Equal(t, "http://cfg-host", GuessCurrentHostURL(ctx))
+
+	defer test.MockVariableValue(&setting.UseHostHeader, true)()
+
+	ctx = context.WithValue(ctx, RequestContextKey, &http.Request{
+		Host: "localhost:3000",
+	})
+	assert.Equal(t, "http://localhost:3000", GuessCurrentHostURL(ctx))
+
+	ctx = context.WithValue(ctx, RequestContextKey, &http.Request{
+		Host: "localhost",
+		TLS:  &tls.ConnectionState{},
+	})
+	assert.Equal(t, "https://localhost", GuessCurrentHostURL(ctx))
+}
+
 func TestMakeAbsoluteURL(t *testing.T) {
 	defer test.MockVariableValue(&setting.Protocol, "http")()
 	defer test.MockVariableValue(&setting.AppURL, "http://cfg-host/sub/")()

modules/setting/server.go

@@ -50,6 +50,10 @@ var (
 	AppSubURL string
 	// UseSubURLPath makes Gitea handle requests with sub-path like "/sub-path/owner/repo/...", to make it easier to debug sub-path related problems without a reverse proxy.
 	UseSubURLPath bool
+	// UseHostHeader makes Gitea always use the "Host" request header for construction of absolute URLs.
+	// This requires any reverse proxy to properly pass headers like "X-Forwarded-Proto" and "Host".
+	// It maps to ini:"USE_HOST_HEADER" in [server] and defaults to false
+	UseHostHeader bool
 	// AppDataPath is the default path for storing data.
 	// It maps to ini:"APP_DATA_PATH" in [server] and defaults to AppWorkPath + "/data"
 	AppDataPath string
@@ -277,6 +281,7 @@ func loadServerFrom(rootCfg ConfigProvider) {
 	// This value is empty if site does not have sub-url.
 	AppSubURL = strings.TrimSuffix(appURL.Path, "/")
 	UseSubURLPath = sec.Key("USE_SUB_URL_PATH").MustBool(false)
+	UseHostHeader = sec.Key("USE_HOST_HEADER").MustBool(false)
 	StaticURLPrefix = strings.TrimSuffix(sec.Key("STATIC_URL_PREFIX").MustString(AppSubURL), "/")
 
 	// Check if Domain differs from AppURL domain than update it to AppURL's domain