Move proxy-detection code to Firefox-specific code

Related commit:
- https://github.com/uBlockOrigin/uBlock-issues/issues/911

The motivation is to avoid executing code which is
unnecessary on platforms not supporting the browser.dns
API.

platform/firefox/vapi-webrequest.js

@@ -49,6 +49,27 @@
     const reAsciiHostname  = /^https?:\/\/[0-9a-z_.:@-]+[/?#]/;
     const parsedURL = new URL('about:blank');
 
+    // Canonical name-uncloaking feature.
+    let cnameUncloak = browser.dns instanceof Object;
+    let cnameUncloakProxied = false;
+
+    // https://github.com/uBlockOrigin/uBlock-issues/issues/911
+    //   We detect here whether network requests are proxied, and if so,
+    //   de-aliasing of hostnames will be disabled to avoid possible
+    //   DNS leaks.
+    const proxyDetector = function(details) {
+        if ( details.proxyInfo instanceof Object ) {
+            cnameUncloak = false;
+            proxyDetectorTryCount = 0;
+        }
+        if ( proxyDetectorTryCount === 0 ) {
+            browser.webRequest.onHeadersReceived.removeListener(proxyDetector);
+            return;
+        }
+        proxyDetectorTryCount -= 1;
+    };
+    let proxyDetectorTryCount = 0;
+
     // Related issues:
     // - https://github.com/gorhill/uBlock/issues/1327
     // - https://github.com/uBlockOrigin/uBlock-issues/issues/128
@@ -68,13 +89,15 @@
             this.cnameMaxTTL = 120;
             this.cnameReplayFullURL = false;
             this.cnameFlushTime = Date.now() + this.cnameMaxTTL * 60000;
-            this.cnameUncloak = browser.dns instanceof Object;
         }
         setOptions(options) {
             super.setOptions(options);
             if ( 'cnameUncloak' in options ) {
-                this.cnameUncloak = browser.dns instanceof Object &&
+                cnameUncloak = browser.dns instanceof Object &&
-                                    options.cnameUncloak !== false;
+                               options.cnameUncloak !== false;
+            }
+            if ( 'cnameUncloakProxied' in options ) {
+                cnameUncloakProxied = options.cnameUncloakProxied === true;
             }
             if ( 'cnameIgnoreList' in options ) {
                 this.cnameIgnoreList =
@@ -100,6 +123,21 @@
             }
             this.cnames.clear(); this.cnames.set('', '');
             this.cnameFlushTime = Date.now() + this.cnameMaxTTL * 60000;
+            // https://github.com/uBlockOrigin/uBlock-issues/issues/911
+            //   Install/remove proxy detector.
+            const wrohr = browser.webRequest.onHeadersReceived;
+            if ( cnameUncloak === false || cnameUncloakProxied ) {
+                if ( wrohr.hasListener(proxyDetector) ) {
+                    wrohr.removeListener(proxyDetector);
+                }
+            } else if ( wrohr.hasListener(proxyDetector) === false ) {
+                wrohr.addListener(
+                    proxyDetector,
+                    { urls: [ '*://*/*' ] },
+                    [ 'blocking' ]
+                );
+            }
+            proxyDetectorTryCount = 32;
         }
         normalizeDetails(details) {
             if ( mustPunycode && !reAsciiHostname.test(details.url) ) {
@@ -226,7 +264,7 @@
         }
         onBeforeSuspendableRequest(details) {
             const r = super.onBeforeSuspendableRequest(details);
-            if ( this.cnameUncloak === false ) { return r; }
+            if ( cnameUncloak === false ) { return r; }
             if ( r !== undefined ) {
                 if (
                     r.cancel === true ||

src/js/storage.js

@@ -133,26 +133,18 @@
 };
 
 self.addEventListener('hiddenSettingsChanged', ( ) => {
-    self.log.verbosity = µBlock.hiddenSettings.consoleLogLevel;
+    const µbhs = µBlock.hiddenSettings;
+    self.log.verbosity = µbhs.consoleLogLevel;
     vAPI.net.setOptions({
-        cnameIgnoreList: µBlock.hiddenSettings.cnameIgnoreList,
+        cnameIgnoreList: µbhs.cnameIgnoreList,
-        cnameIgnore1stParty: µBlock.hiddenSettings.cnameIgnore1stParty,
+        cnameIgnore1stParty: µbhs.cnameIgnore1stParty,
-        cnameIgnoreExceptions: µBlock.hiddenSettings.cnameIgnoreExceptions,
+        cnameIgnoreExceptions: µbhs.cnameIgnoreExceptions,
-        cnameIgnoreRootDocument: µBlock.hiddenSettings.cnameIgnoreRootDocument,
+        cnameIgnoreRootDocument: µbhs.cnameIgnoreRootDocument,
-        cnameMaxTTL: µBlock.hiddenSettings.cnameMaxTTL,
+        cnameMaxTTL: µbhs.cnameMaxTTL,
-        cnameReplayFullURL: µBlock.hiddenSettings.cnameReplayFullURL,
+        cnameReplayFullURL: µbhs.cnameReplayFullURL,
-        cnameUncloak: µBlock.hiddenSettings.cnameUncloak,
+        cnameUncloak: µbhs.cnameUncloak,
+        cnameUncloakProxied: µbhs.cnameUncloakProxied,
     });
-    // https://github.com/uBlockOrigin/uBlock-issues/issues/911
-    //   See uBO's onHeadersReceived() listener.
-    if (
-        µBlock.hiddenSettings.cnameUncloak === false ||
-        µBlock.hiddenSettings.cnameUncloakProxied === true
-    ) {
-        µBlock.proxyDNS = false;
-    } else {
-        µBlock.proxyDNS = undefined;
-    }
 });
 
 /******************************************************************************/

src/js/traffic.js

@@ -441,17 +441,6 @@ const onHeadersReceived = function(details) {
     const isRootDoc = requestType === 'main_frame';
     const isDoc = isRootDoc || requestType === 'sub_frame';
 
-    // https://github.com/uBlockOrigin/uBlock-issues/issues/911
-    //   We detect here whether network requests are proxied, and if so,
-    //   de-aliasing of hostnames will be disabled to avoid possible
-    //   DNS leaks.
-    if ( isRootDoc && µb.proxyDNS === undefined ) {
-        µb.proxyDNS = details.proxyInfo instanceof Object;
-        if ( µb.proxyDNS ) {
-            vAPI.net.setOptions({ cnameUncloak: false });
-        }
-    }
-
     let pageStore = µb.pageStoreFromTabId(fctxt.tabId);
     if ( pageStore === null ) {
         if ( isRootDoc === false ) { return; }