2 files changed, 54 insertions, 11 deletions
diff --git a/modules/httplib/url.go b/modules/httplib/url.go
index 5d5b64dc0c..f51506ac3b 100644
--- a/modules/httplib/url.go
+++ b/modules/httplib/url.go
@@ -53,28 +53,34 @@ func getRequestScheme(req *http.Request) string {
 	return ""
 }
 
-// GuessCurrentAppURL tries to guess the current full app URL (with sub-path) by http headers. It always has a '/' suffix, exactly the same as setting.AppURL
+// GuessCurrentAppURL tries to guess the current full public URL (with sub-path) by http headers. It always has a '/' suffix, exactly the same as setting.AppURL
+// TODO: should rename it to GuessCurrentPublicURL in the future
 func GuessCurrentAppURL(ctx context.Context) string {
 	return GuessCurrentHostURL(ctx) + setting.AppSubURL + "/"
 }
 
 // GuessCurrentHostURL tries to guess the current full host URL (no sub-path) by http headers, there is no trailing slash.
 func GuessCurrentHostURL(ctx context.Context) string {
-	req, ok := ctx.Value(RequestContextKey).(*http.Request)
-	if !ok {
-		return strings.TrimSuffix(setting.AppURL, setting.AppSubURL+"/")
-	}
-	// If no scheme provided by reverse proxy, then do not guess the AppURL, use the configured one.
+	// Try the best guess to get the current host URL (will be used for public URL) by http headers.
 	// At the moment, if site admin doesn't configure the proxy headers correctly, then Gitea would guess wrong.
 	// There are some cases:
 	// 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.
+	// Without more information, Gitea is impossible to distinguish between case 2 and case 3, then case 2 would result in
+	// wrong guess like guessed public URL becomes "http://gitea:3000/" behind a "https" reverse proxy, which is not accessible by end users.
+	// So we introduced "PUBLIC_URL_DETECTION" option, to control the guessing behavior to satisfy different use cases.
+	req, ok := ctx.Value(RequestContextKey).(*http.Request)
+	if !ok {
+		return strings.TrimSuffix(setting.AppURL, setting.AppSubURL+"/")
+	}
 	reqScheme := getRequestScheme(req)
 	if reqScheme == "" {
+		// if no reverse proxy header, try to use "Host" header for absolute URL
+		if setting.PublicURLDetection == setting.PublicURLAuto && req.Host != "" {
+			return util.Iif(req.TLS == nil, "http://", "https://") + req.Host
+		}
+		// fall back to default AppURL
 		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.
@@ -88,8 +94,8 @@ func GuessCurrentHostDomain(ctx context.Context) string {
 	return util.IfZero(domain, host)
 }
 
-// MakeAbsoluteURL tries to make a link to an absolute URL:
-// * If link is empty, it returns the current app URL.
+// MakeAbsoluteURL tries to make a link to an absolute public URL:
+// * If link is empty, it returns the current public URL.
 // * If link is absolute, it returns the link.
 // * Otherwise, it returns the current host URL + link, the link itself should have correct sub-path (AppSubURL) if needed.
 func MakeAbsoluteURL(ctx context.Context, link string) string {
diff --git a/modules/httplib/url_test.go b/modules/httplib/url_test.go
index d57653646b..0ffb0cac05 100644
--- a/modules/httplib/url_test.go
+++ b/modules/httplib/url_test.go
@@ -5,6 +5,7 @@ package httplib
 
 import (
 	"context"
+	"crypto/tls"
 	"net/http"
 	"testing"
 
@@ -39,6 +40,42 @@ 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")()
+	headersWithProto := http.Header{"X-Forwarded-Proto": {"https"}}
+
+	t.Run("Legacy", func(t *testing.T) {
+		defer test.MockVariableValue(&setting.PublicURLDetection, setting.PublicURLLegacy)()
+
+		assert.Equal(t, "http://cfg-host", GuessCurrentHostURL(t.Context()))
+
+		// legacy: "Host" is not used when there is no "X-Forwarded-Proto" header
+		ctx := context.WithValue(t.Context(), RequestContextKey, &http.Request{Host: "req-host:3000"})
+		assert.Equal(t, "http://cfg-host", GuessCurrentHostURL(ctx))
+
+		// if "X-Forwarded-Proto" exists, then use it and "Host" header
+		ctx = context.WithValue(t.Context(), RequestContextKey, &http.Request{Host: "req-host:3000", Header: headersWithProto})
+		assert.Equal(t, "https://req-host:3000", GuessCurrentHostURL(ctx))
+	})
+
+	t.Run("Auto", func(t *testing.T) {
+		defer test.MockVariableValue(&setting.PublicURLDetection, setting.PublicURLAuto)()
+
+		assert.Equal(t, "http://cfg-host", GuessCurrentHostURL(t.Context()))
+
+		// auto: always use "Host" header, the scheme is determined by "X-Forwarded-Proto" header, or TLS config if no "X-Forwarded-Proto" header
+		ctx := context.WithValue(t.Context(), RequestContextKey, &http.Request{Host: "req-host:3000"})
+		assert.Equal(t, "http://req-host:3000", GuessCurrentHostURL(ctx))
+
+		ctx = context.WithValue(t.Context(), RequestContextKey, &http.Request{Host: "req-host", TLS: &tls.ConnectionState{}})
+		assert.Equal(t, "https://req-host", GuessCurrentHostURL(ctx))
+
+		ctx = context.WithValue(t.Context(), RequestContextKey, &http.Request{Host: "req-host:3000", Header: headersWithProto})
+		assert.Equal(t, "https://req-host:3000", GuessCurrentHostURL(ctx))
+	})
+}
+
 func TestMakeAbsoluteURL(t *testing.T) {
 	defer test.MockVariableValue(&setting.Protocol, "http")()
 	defer test.MockVariableValue(&setting.AppURL, "http://cfg-host/sub/")()