diff --git a/Source/WebCore/ChangeLog b/Source/WebCore/ChangeLog
index 714ea2c3fad6..82c23476379a 100644
--- a/Source/WebCore/ChangeLog
+++ b/Source/WebCore/ChangeLog
@@ -1,3 +1,20 @@
+2018-02-23  Philippe Normand  <pnormand@igalia.com>
+
+        [GStreamer] HTTP totalBytes query returns 0 after seeking (sometimes)
+        https://bugs.webkit.org/show_bug.cgi?id=183002
+
+        Reviewed by Xabier Rodriguez-Calvar.
+
+        * platform/graphics/gstreamer/WebKitWebSourceGStreamer.cpp:
+        (webkit_web_src_init): Initialize member variables. Also no need
+        to set the appsrc size at that point.
+        (webKitWebSrcStop): There is no need to reset the size when
+        seeking. Size should in most cases represent the Content-Length
+        response attribute, even when seeking.
+        (webKitWebSrcStart): No need to reset the size attribute.
+        (webKitWebSrcQueryWithParent): Let appsrc handle DURATION queries.
+        (CachedResourceStreamingClient::responseReceived): Emit duration notification one time only.
+
 2018-02-23  Philippe Normand  <pnormand@igalia.com>
 
         [GStreamer] media/video-src-blob-using-open-panel.html crashes in Debug
diff --git a/Source/WebCore/platform/graphics/gstreamer/WebKitWebSourceGStreamer.cpp b/Source/WebCore/platform/graphics/gstreamer/WebKitWebSourceGStreamer.cpp
index acf6ace1125c..f08030ed1095 100644
--- a/Source/WebCore/platform/graphics/gstreamer/WebKitWebSourceGStreamer.cpp
+++ b/Source/WebCore/platform/graphics/gstreamer/WebKitWebSourceGStreamer.cpp
@@ -83,6 +83,7 @@ struct _WebKitWebSrcPrivate {
     bool didPassAccessControlCheck;
 
     guint64 offset;
+    bool haveSize;
     guint64 size;
     gboolean seekable;
     bool paused;
@@ -204,6 +205,9 @@ static void webkit_web_src_init(WebKitWebSrc* src)
 
     priv->notifier = MainThreadNotifier<MainThreadSourceNotification>::create();
 
+    priv->haveSize = FALSE;
+    priv->size = 0;
+
     priv->appsrc = GST_APP_SRC(gst_element_factory_make("appsrc", nullptr));
     if (!priv->appsrc) {
         GST_ERROR_OBJECT(src, "Failed to create appsrc");
@@ -246,7 +250,6 @@ static void webkit_web_src_init(WebKitWebSrc* src)
     gst_base_src_set_automatic_eos(GST_BASE_SRC(priv->appsrc), FALSE);
 
     gst_app_src_set_caps(priv->appsrc, nullptr);
-    gst_app_src_set_size(priv->appsrc, -1);
 }
 
 static void webKitWebSrcDispose(GObject* object)
@@ -360,7 +363,6 @@ static void webKitWebSrcStop(WebKitWebSrc* src)
     priv->offset = 0;
 
     if (!wasSeeking) {
-        priv->size = 0;
         priv->requestedOffset = 0;
         priv->player = nullptr;
         priv->seekable = FALSE;
@@ -446,8 +448,6 @@ static void webKitWebSrcStart(WebKitWebSrc* src)
     request.setAllowCookies(true);
     request.setFirstPartyForCookies(url);
 
-    priv->size = 0;
-
     request.setHTTPReferrer(priv->player->referrer());
 
     if (priv->httpMethod.get())
@@ -563,18 +563,6 @@ static gboolean webKitWebSrcQueryWithParent(GstPad* pad, GstObject* parent, GstQ
     gboolean result = FALSE;
 
     switch (GST_QUERY_TYPE(query)) {
-    case GST_QUERY_DURATION: {
-        GstFormat format;
-
-        gst_query_parse_duration(query, &format, nullptr);
-
-        GST_LOG_OBJECT(src, "duration query in format %s, current size: %lu", gst_format_get_name(format), priv->size);
-        if (format == GST_FORMAT_BYTES && priv->size > 0) {
-            gst_query_set_duration(query, format, priv->size);
-            result = TRUE;
-        }
-        break;
-    }
     case GST_QUERY_URI: {
         gst_query_set_uri(query, priv->originalURI.data());
         if (!priv->redirectedURI.isNull())
@@ -792,14 +780,17 @@ void CachedResourceStreamingClient::responseReceived(PlatformMediaResource&, con
     if (length > 0 && priv->requestedOffset && response.httpStatusCode() == 206)
         length += priv->requestedOffset;
 
-    priv->size = length >= 0 ? length : 0;
     priv->seekable = length > 0 && g_ascii_strcasecmp("none", response.httpHeaderField(HTTPHeaderName::AcceptRanges).utf8().data());
 
-    GST_DEBUG_OBJECT(src, "Size: %" G_GINT64_FORMAT ", seekable: %s", priv->size, priv->seekable ? "yes" : "no");
+    GST_DEBUG_OBJECT(src, "Size: %lld, seekable: %s", length, priv->seekable ? "yes" : "no");
     // notify size/duration
-    if (length > 0)
-        gst_app_src_set_size(priv->appsrc, length);
-    else {
+    if (length > 0) {
+        if (!priv->haveSize || (static_cast<long long>(priv->size) != length)) {
+            priv->haveSize = TRUE;
+            priv->size = length;
+            gst_app_src_set_size(priv->appsrc, length);
+        }
+    } else {
         gst_app_src_set_size(priv->appsrc, -1);
         if (!priv->seekable)
             gst_app_src_set_stream_type(priv->appsrc, GST_APP_STREAM_TYPE_STREAM);