diff --git a/netwerk/cache2/AppCacheStorage.cpp b/netwerk/cache2/AppCacheStorage.cpp
index 75763237d01..69c03fb0f18 100644
--- a/netwerk/cache2/AppCacheStorage.cpp
+++ b/netwerk/cache2/AppCacheStorage.cpp
@@ -84,6 +84,13 @@ NS_IMETHODIMP AppCacheStorage::AsyncOpenURI(nsIURI *aURI,
   return NS_OK;
 }
 
+NS_IMETHODIMP AppCacheStorage::Exists(nsIURI *aURI, const nsACString & aIdExtension,
+                                      bool *aResult)
+{
+  *aResult = false;
+  return NS_ERROR_NOT_AVAILABLE;
+}
+
 NS_IMETHODIMP AppCacheStorage::AsyncDoomURI(nsIURI *aURI, const nsACString & aIdExtension,
                                             nsICacheEntryDoomCallback* aCallback)
 {
diff --git a/netwerk/cache2/CacheStorage.cpp b/netwerk/cache2/CacheStorage.cpp
index e7350fbf9d5..6844be36a7f 100644
--- a/netwerk/cache2/CacheStorage.cpp
+++ b/netwerk/cache2/CacheStorage.cpp
@@ -104,6 +104,26 @@ NS_IMETHODIMP CacheStorage::AsyncOpenURI(nsIURI *aURI,
   return NS_OK;
 }
 
+
+NS_IMETHODIMP CacheStorage::Exists(nsIURI *aURI, const nsACString & aIdExtension,
+                                   bool *aResult)
+{
+  NS_ENSURE_ARG(aURI);
+  NS_ENSURE_ARG(aResult);
+
+  if (!CacheStorageService::Self())
+    return NS_ERROR_NOT_INITIALIZED;
+
+  nsresult rv;
+
+  nsCOMPtr<nsIURI> noRefURI;
+  rv = aURI->CloneIgnoringRef(getter_AddRefs(noRefURI));
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  return CacheStorageService::Self()->CheckStorageEntry(
+    this, noRefURI, aIdExtension, aResult);
+}
+
 NS_IMETHODIMP CacheStorage::AsyncDoomURI(nsIURI *aURI, const nsACString & aIdExtension,
                                          nsICacheEntryDoomCallback* aCallback)
 {
diff --git a/netwerk/cache2/CacheStorageService.cpp b/netwerk/cache2/CacheStorageService.cpp
index ffbcdf80761..d5649393ac9 100644
--- a/netwerk/cache2/CacheStorageService.cpp
+++ b/netwerk/cache2/CacheStorageService.cpp
@@ -1341,6 +1341,67 @@ CacheStorageService::AddStorageEntry(nsCSubstring const& aContextKey,
   return NS_OK;
 }
 
+nsresult
+CacheStorageService::CheckStorageEntry(CacheStorage const* aStorage,
+                                       nsIURI* aURI,
+                                       const nsACString & aIdExtension,
+                                       bool* aResult)
+{
+  nsresult rv;
+
+  nsAutoCString contextKey;
+  CacheFileUtils::AppendKeyPrefix(aStorage->LoadInfo(), contextKey);
+
+  if (!aStorage->WriteToDisk()) {
+    AppendMemoryStorageID(contextKey);
+  }
+
+#ifdef PR_LOGGING
+  nsAutoCString uriSpec;
+  aURI->GetAsciiSpec(uriSpec);
+  LOG(("CacheStorageService::CheckStorageEntry [uri=%s, eid=%s, contextKey=%s]",
+    uriSpec.get(), aIdExtension.BeginReading(), contextKey.get()));
+#endif
+
+  {
+    mozilla::MutexAutoLock lock(mLock);
+
+    NS_ENSURE_FALSE(mShutdown, NS_ERROR_NOT_INITIALIZED);
+
+    nsAutoCString entryKey;
+    rv = CacheEntry::HashingKey(EmptyCString(), aIdExtension, aURI, entryKey);
+    NS_ENSURE_SUCCESS(rv, rv);
+
+    CacheEntryTable* entries;
+    if ((*aResult = sGlobalEntryTables->Get(contextKey, &entries)) &&
+        entries->GetWeak(entryKey, aResult)) {
+      LOG(("  found in hash tables"));
+      return NS_OK;
+    }
+  }
+
+  if (!aStorage->WriteToDisk()) {
+    // Memory entry, nothing more to do.
+    LOG(("  not found in hash tables"));
+    return NS_OK;
+  }
+
+  // Disk entry, not found in the hashtable, check the index.
+  nsAutoCString fileKey;
+  rv = CacheEntry::HashingKey(contextKey, aIdExtension, aURI, fileKey);
+
+  CacheIndex::EntryStatus status;
+  rv = CacheIndex::HasEntry(fileKey, &status);
+  if (NS_FAILED(rv) || status == CacheIndex::DO_NOT_KNOW) {
+    LOG(("  index doesn't know, rv=0x%08x", rv));
+    return NS_ERROR_NOT_AVAILABLE;
+  }
+
+  *aResult = status == CacheIndex::EXISTS;
+  LOG(("  %sfound in index", *aResult ? "" : "not "));
+  return NS_OK;
+}
+
 namespace { // anon
 
 class CacheEntryDoomByKeyCallback : public CacheFileIOListener
diff --git a/netwerk/cache2/CacheStorageService.h b/netwerk/cache2/CacheStorageService.h
index 3bc5a9c63b9..75f828b77d0 100644
--- a/netwerk/cache2/CacheStorageService.h
+++ b/netwerk/cache2/CacheStorageService.h
@@ -159,6 +159,15 @@ private:
                            bool aReplace,
                            CacheEntryHandle** aResult);
 
+  /**
+   * Check existance of an entry.  This may throw NS_ERROR_NOT_AVAILABLE
+   * when the information cannot be obtained synchronously w/o blocking.
+   */
+  nsresult CheckStorageEntry(CacheStorage const* aStorage,
+                             nsIURI* aURI,
+                             const nsACString & aIdExtension,
+                             bool* aResult);
+
   /**
    * Removes the entry from the related entry hash table, if still present
    * and returns it.
diff --git a/netwerk/cache2/OldWrappers.cpp b/netwerk/cache2/OldWrappers.cpp
index 23c4c9b80fa..a039861d644 100644
--- a/netwerk/cache2/OldWrappers.cpp
+++ b/netwerk/cache2/OldWrappers.cpp
@@ -943,6 +943,12 @@ NS_IMETHODIMP _OldStorage::AsyncOpenURI(nsIURI *aURI,
   return NS_OK;
 }
 
+NS_IMETHODIMP _OldStorage::Exists(nsIURI *aURI, const nsACString & aIdExtension,
+                                   bool *aResult)
+{
+  return NS_ERROR_NOT_AVAILABLE;
+}
+
 NS_IMETHODIMP _OldStorage::AsyncDoomURI(nsIURI *aURI, const nsACString & aIdExtension,
                                         nsICacheEntryDoomCallback* aCallback)
 {
diff --git a/netwerk/cache2/nsICacheStorage.idl b/netwerk/cache2/nsICacheStorage.idl
index ca6f948b085..f8d681d300e 100644
--- a/netwerk/cache2/nsICacheStorage.idl
+++ b/netwerk/cache2/nsICacheStorage.idl
@@ -14,7 +14,7 @@ interface nsICacheStorageVisitor;
  * in-mem+on-disk, in-mem+on-disk+app-cache or just a specific
  * app-cache storage.
  */
-[scriptable, uuid(d983ba0c-433f-4017-abc1-93af737c82e4)]
+[scriptable, uuid(d9006881-a536-4ce3-bc48-e7f94b40a690)]
 interface nsICacheStorage : nsISupports
 {
   /**
@@ -77,6 +77,15 @@ interface nsICacheStorage : nsISupports
                     in uint32_t aFlags,
                     in nsICacheEntryOpenCallback aCallback);
 
+  /**
+   * Synchronously check on existance of an entry.  In case of disk entries
+   * this uses information from the cache index.  When the index data are not
+   * up to date or index is still building, NS_ERROR_NOT_AVAILABLE is thrown.
+   * The same error may throw any storage implementation that cannot determine
+   * entry state without blocking the caller.
+   */
+  boolean exists(in nsIURI aURI, in ACString aIdExtension);
+
   /**
    * Asynchronously removes an entry belonging to the URI from the cache.
    */
diff --git a/netwerk/test/unit/test_cache2-24-exists.js b/netwerk/test/unit/test_cache2-24-exists.js
new file mode 100644
index 00000000000..6000bffd9ad
--- /dev/null
+++ b/netwerk/test/unit/test_cache2-24-exists.js
@@ -0,0 +1,33 @@
+Components.utils.import('resource://gre/modules/LoadContextInfo.jsm');
+
+function run_test()
+{
+  do_get_profile();
+
+  var mc = new MultipleCallbacks(2, function() {
+    var mem = getCacheStorage("memory");
+    var disk = getCacheStorage("disk");
+
+    do_check_true(disk.exists(createURI("http://m1/"), ""));
+    do_check_true(mem.exists(createURI("http://m1/"), ""));
+    do_check_false(mem.exists(createURI("http://m2/"), ""));
+    do_check_true(disk.exists(createURI("http://d1/"), ""));
+    do_check_throws_nsIException(() => disk.exists(createURI("http://d2/"), ""), 'NS_ERROR_NOT_AVAILABLE');
+
+    finish_cache2_test();
+  });
+
+  asyncOpenCacheEntry("http://d1/", "disk", Ci.nsICacheStorage.OPEN_NORMALLY, LoadContextInfo.default,
+    new OpenCallback(NEW | WAITFORWRITE, "meta", "data", function(entry) {
+      mc.fired();
+    })
+  );
+
+  asyncOpenCacheEntry("http://m1/", "memory", Ci.nsICacheStorage.OPEN_NORMALLY, LoadContextInfo.default,
+    new OpenCallback(NEW | WAITFORWRITE, "meta", "data", function(entry) {
+      mc.fired();
+    })
+  );
+
+  do_test_pending();
+}
diff --git a/netwerk/test/unit/xpcshell.ini b/netwerk/test/unit/xpcshell.ini
index eae2ba94d68..d2b67b72b62 100644
--- a/netwerk/test/unit/xpcshell.ini
+++ b/netwerk/test/unit/xpcshell.ini
@@ -58,6 +58,9 @@ skip-if = os == "android"
 [test_cache2-21-anon-storage.js]
 [test_cache2-22-anon-visit.js]
 [test_cache2-23-read-over-chunk.js]
+[test_cache2-24-exists.js]
+# Bug 675039, comment 6: "The difference is that the memory cache is disabled in Armv6 builds."
+skip-if = os == "android"
 [test_304_responses.js]
 # Bug 675039: test hangs on Android-armv6 
 skip-if = os == "android"