Details
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Bug
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Resolution: Unresolved
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P2: Important
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None
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5.9.4
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None
Description
Hi,
DNS prefetching can be disabled (and is disabled by default, actually) when using Qt WebKit [1]. However, when a web page explicitely enables DNS prefetching by using a header (X-DNS-Prefetch-Control: on) or a HTML meta http-equiv tag (<meta http-equiv="x-dns-prefetch-control" content="on">), DNS prefetching is enabled anyway, ignoring the settings in QWebSettings (this behavior is inherited from upstream). The following snippet of code shows what is happening (Source/WebCore/dom/Document.cpp):
4729 void Document::initDNSPrefetch()
4730 {
4731 Settings* settings = this->settings();
4732
4733 m_haveExplicitlyDisabledDNSPrefetch = false;
4734 m_isDNSPrefetchEnabled = settings && settings->dnsPrefetchingEnabled() && securityOrigin()->protocol() == "http";
4735
4736 // Inherit DNS prefetch opt-out from parent frame
4737 if (Document* parent = parentDocument()) {
4738 if (!parent->isDNSPrefetchEnabled())
4739 m_isDNSPrefetchEnabled = false;
4740 }
4741 }
4742
4743 void Document::parseDNSPrefetchControlHeader(const String& dnsPrefetchControl)
4744 {
4745 if (equalIgnoringCase(dnsPrefetchControl, "on") && !m_haveExplicitlyDisabledDNSPrefetch) {
4746 m_isDNSPrefetchEnabled = true;
4747 return;
4748 }
4749
4750 m_isDNSPrefetchEnabled = false;
4751 m_haveExplicitlyDisabledDNSPrefetch = true;
4752 }
On line 4734, when initializing DNS prefetching, the settings regarding DNS prefetching are taken into account. However, when a website enables DNS prefetching with a header or a HTML meta http-equiv tag, the settings are ignored and DNS prefetching is enabled anyway (line 4746).
To me, that is somewhat unexpected since I instructed QWebSettings to disable DNS prefetching.
Here is a patch that would solve the problem (the patch is written such that the diff with upstream stays small; I can take a different approach if desired):
diff --git a/Source/WebCore/dom/Document.cpp b/Source/WebCore/dom/Document.cpp index af63b141f..3ff5a2f25 100644 --- a/Source/WebCore/dom/Document.cpp +++ b/Source/WebCore/dom/Document.cpp @@ -4743,7 +4743,6 @@ void Document::initDNSPrefetch() void Document::parseDNSPrefetchControlHeader(const String& dnsPrefetchControl) { if (equalIgnoringCase(dnsPrefetchControl, "on") && !m_haveExplicitlyDisabledDNSPrefetch) { - m_isDNSPrefetchEnabled = true; return; }
Would you accept a patch like this? If so, against which git branch should I submit a gerrit review?
I doubt upstream will take such a patch, as the code seems to be written like this intentionally and furthermore, the behavior is actually described in the documentation [2].
Thanks to Jens Mueller for contacting Trojita about this issue [3].
1: https://doc.qt.io/archives/qt-5.5/qwebsettings.html#WebAttribute-enum
2: https://www.chromium.org/developers/design-documents/dns-prefetching#TOC-DNS-Prefetch-Control
3: https://bugs.kde.org/show_bug.cgi?id=390452