GUACAMOLE-1852: Send a desktop scale factor to the RDP server.

src/protocols/rdp/channels/disp.c

@@ -224,6 +224,23 @@ void guac_rdp_disp_update_size(guac_rdp_disp* disp,
     }
 
     else if (settings->resize_method == GUAC_RESIZE_DISPLAY_UPDATE) {
+
+        /*
+         * Forward any requested desktop scale factor to the server so the
+         * remote session keeps scaling its UI across dynamic resizes. Both
+         * fields default to 0 (no scaling requested), which the server ignores
+         * per [MS-RDPEDISP]. When set, the desktop scale factor MUST be paired
+         * with a protocol-valid device scale factor (100, 140, or 180) or both
+         * are ignored.
+         */
+        UINT32 desktop_scale_factor = 0;
+        UINT32 device_scale_factor = 0;
+        if (settings->desktop_scale_factor != 0) {
+            desktop_scale_factor = settings->desktop_scale_factor;
+            device_scale_factor =
+                guac_rdp_get_device_scale_factor(settings->desktop_scale_factor);
+        }
+
         DISPLAY_CONTROL_MONITOR_LAYOUT monitors[1] = {{
             .Flags  = 0x1, /* DISPLAYCONTROL_MONITOR_PRIMARY */
             .Left = 0,
@@ -233,8 +250,8 @@ void guac_rdp_disp_update_size(guac_rdp_disp* disp,
             .PhysicalWidth = 0,
             .PhysicalHeight = 0,
             .Orientation = 0,
-            .DesktopScaleFactor = 0,
-            .DeviceScaleFactor = 0
+            .DesktopScaleFactor = desktop_scale_factor,
+            .DeviceScaleFactor = device_scale_factor
         }};
 
         /* Send display update notification if display channel is connected */

src/protocols/rdp/settings.c

@@ -153,6 +153,7 @@ const char* GUAC_RDP_CLIENT_ARGS[] = {
 
     "force-lossless",
     "normalize-clipboard",
+    "desktop-scale-factor",
     NULL
 };
 
@@ -740,6 +741,17 @@ enum RDP_ARGS_IDX {
      */
     IDX_NORMALIZE_CLIPBOARD,
 
+    /**
+     * The desktop scale factor to request from the RDP server, as a percentage
+     * between 100 and 500. When set, the remote Windows session is asked to
+     * scale its UI accordingly (the equivalent of the Windows display scaling
+     * setting), keeping UI and text comfortably sized even when a
+     * high-resolution (device-pixel) framebuffer is requested. If omitted or
+     * set to 0, no scale factor is sent and the server uses its default (no
+     * scaling).
+     */
+    IDX_DESKTOP_SCALE_FACTOR,
+
     RDP_ARGS_COUNT
 };
 
@@ -911,6 +923,45 @@ guac_rdp_settings* guac_rdp_parse_args(guac_user* user,
             settings->height,
             settings->resolution);
 
+    /*
+     * Desktop scale factor to request from the RDP server, as a percentage. A
+     * value of 0 (the default) disables the feature entirely, preserving the
+     * historical behavior of never asking the server to scale its session UI.
+     * Any non-zero value is clamped to the range permitted by [MS-RDPBCGR]
+     * (100-500), as values outside that range are silently ignored by the
+     * server.
+     */
+    settings->desktop_scale_factor =
+        guac_user_parse_args_int(user, GUAC_RDP_CLIENT_ARGS, argv,
+                IDX_DESKTOP_SCALE_FACTOR, 0);
+
+    if (settings->desktop_scale_factor != 0) {
+
+        if (settings->desktop_scale_factor < 100) {
+            guac_user_log(user, GUAC_LOG_WARNING,
+                    "Requested desktop scale factor of %i%% is below the "
+                    "minimum supported by RDP. Clamping to 100%%.",
+                    settings->desktop_scale_factor);
+            settings->desktop_scale_factor = 100;
+        }
+
+        else if (settings->desktop_scale_factor > 500) {
+            guac_user_log(user, GUAC_LOG_WARNING,
+                    "Requested desktop scale factor of %i%% is above the "
+                    "maximum supported by RDP. Clamping to 500%%.",
+                    settings->desktop_scale_factor);
+            settings->desktop_scale_factor = 500;
+        }
+
+        guac_user_log(user, GUAC_LOG_DEBUG,
+                "Requesting desktop scale factor of %i%% "
+                "(device scale factor %i%%)",
+                settings->desktop_scale_factor,
+                guac_rdp_get_device_scale_factor(
+                    settings->desktop_scale_factor));
+
+    }
+
     /* Lossless compression */
     settings->lossless =
         guac_user_parse_args_boolean(user, GUAC_RDP_CLIENT_ARGS, argv,
@@ -1544,6 +1595,27 @@ int guac_rdp_get_depth(freerdp* rdp) {
 #endif
 }
 
+int guac_rdp_get_device_scale_factor(int desktop_scale_factor) {
+
+    /*
+     * Per [MS-RDPBCGR] and [MS-RDPEDISP], the device scale factor MUST be one
+     * of exactly 100, 140, or 180 percent. Any other value causes both the
+     * device scale factor AND the accompanying desktop scale factor to be
+     * ignored by the server, so we always map to one of these three values.
+     * The thresholds mirror the behavior of native RDP clients (mstsc /
+     * FreeRDP): scales below 120% use 100, 120%-160% use 140, and anything
+     * higher uses 180.
+     */
+    if (desktop_scale_factor < 120)
+        return 100;
+
+    if (desktop_scale_factor <= 160)
+        return 140;
+
+    return 180;
+
+}
+
 void guac_rdp_push_settings(guac_client* client,
         guac_rdp_settings* guac_settings, freerdp* rdp) {
 
@@ -1567,6 +1639,19 @@ void guac_rdp_push_settings(guac_client* client,
     freerdp_settings_set_string(rdp_settings, FreeRDP_AlternateShell, guac_strdup(guac_settings->initial_program));
     freerdp_settings_set_uint32(rdp_settings, FreeRDP_KeyboardLayout, guac_settings->server_layout->freerdp_keyboard_layout);
 
+    /*
+     * Request server-side UI scaling if a desktop scale factor was specified.
+     * These values are sent to the server within the client core data
+     * ([MS-RDPBCGR] TS_UD_CS_CORE) during connection negotiation. The desktop
+     * scale factor MUST be paired with a protocol-valid device scale factor
+     * (100, 140, or 180), otherwise the server ignores both values.
+     */
+    if (guac_settings->desktop_scale_factor != 0) {
+        freerdp_settings_set_uint32(rdp_settings, FreeRDP_DesktopScaleFactor,
+                guac_settings->desktop_scale_factor);
+        freerdp_settings_set_uint32(rdp_settings, FreeRDP_DeviceScaleFactor,
+                guac_rdp_get_device_scale_factor(guac_settings->desktop_scale_factor));
+    }
 
     /* Performance flags */
     /* Explicitly set flag value */
@@ -1803,6 +1888,19 @@ void guac_rdp_push_settings(guac_client* client,
     rdp_settings->AlternateShell = guac_strdup(guac_settings->initial_program);
     rdp_settings->KeyboardLayout = guac_settings->server_layout->freerdp_keyboard_layout;
 
+    /*
+     * Request server-side UI scaling if a desktop scale factor was specified.
+     * These values are sent to the server within the client core data
+     * ([MS-RDPBCGR] TS_UD_CS_CORE) during connection negotiation. The desktop
+     * scale factor MUST be paired with a protocol-valid device scale factor
+     * (100, 140, or 180), otherwise the server ignores both values.
+     */
+    if (guac_settings->desktop_scale_factor != 0) {
+        rdp_settings->DesktopScaleFactor = guac_settings->desktop_scale_factor;
+        rdp_settings->DeviceScaleFactor =
+            guac_rdp_get_device_scale_factor(guac_settings->desktop_scale_factor);
+    }
+
     /* Performance flags */
     /* Explicitly set flag value */
     rdp_settings->PerformanceFlags = guac_rdp_get_performance_flags(guac_settings);

src/protocols/rdp/settings.h

@@ -217,6 +217,19 @@ typedef struct guac_rdp_settings {
      */
     int resolution;
 
+    /**
+     * The desktop scale factor to request from the RDP server, as a
+     * percentage. When set to a value between 100 and 500, the remote Windows
+     * session is asked to scale its UI accordingly (the equivalent of the
+     * Windows "Scale and layout" / display scaling setting). This value is sent
+     * to the server as the [MS-RDPBCGR] desktopScaleFactor (paired with a
+     * protocol-valid deviceScaleFactor), allowing a high-resolution
+     * (device-pixel) framebuffer to remain razor sharp while keeping UI and
+     * text at a comfortable size. A value of 0 (the default) disables the
+     * feature, and no scale factor is sent to the server.
+     */
+    int desktop_scale_factor;
+
     /**
      * Whether all graphical updates for this connection should use lossless
      * compression only.
@@ -753,6 +766,25 @@ extern const char* GUAC_RDP_CLIENT_ARGS[];
 void guac_rdp_push_settings(guac_client* client,
         guac_rdp_settings* guac_settings, freerdp* rdp);
 
+/**
+ * Returns the RDP device scale factor that should accompany the given desktop
+ * scale factor. Per [MS-RDPBCGR] (Client Core Data) and [MS-RDPEDISP]
+ * (DISPLAYCONTROL_MONITOR_LAYOUT), the device scale factor MUST be exactly
+ * 100, 140, or 180 percent, otherwise both the device scale factor AND the
+ * accompanying desktop scale factor are ignored by the server. The mapping
+ * used here mirrors the behavior of native clients (mstsc / FreeRDP):
+ * a desktop scale below 120% maps to 100, 120%-160% maps to 140, and anything
+ * higher maps to 180.
+ *
+ * @param desktop_scale_factor
+ *     The desktop scale factor, as a percentage (typically 100-500).
+ *
+ * @return
+ *     The protocol-valid device scale factor (100, 140, or 180) to pair with
+ *     the given desktop scale factor.
+ */
+int guac_rdp_get_device_scale_factor(int desktop_scale_factor);
+
 /**
  * Returns the width of the RDP session display.
  *