Fast-fail WPAD detection.

net/dns/host_resolver_impl.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "net/dns/host_resolver_impl.h"
 
 #if defined(OS_WIN)
 #include <Winsock2.h>
 #elif defined(OS_POSIX)
 #include <netdb.h>
@@ skipping 1437 matching lines @@
       DNS_HISTOGRAM_BY_PRIORITY("AsyncDNS.JobQueueTime", priority(),
                                 queue_time);
       DNS_HISTOGRAM_BY_PRIORITY("AsyncDNS.JobQueueTimeAfterChange", priority(),
                                 queue_time_after_change);
     } else {
       DNS_HISTOGRAM_BY_PRIORITY("DNS.JobQueueTime", priority(), queue_time);
       DNS_HISTOGRAM_BY_PRIORITY("DNS.JobQueueTimeAfterChange", priority(),
                                 queue_time_after_change);
     }
 
+    bool system_only =
+        (key_.host_resolver_flags & HOST_RESOLVER_SYSTEM_ONLY) != 0;
+
     // Caution: Job::Start must not complete synchronously.
-    if (had_dns_config_ && !ResemblesMulticastDNSName(key_.hostname)) {
+    if (!system_only && had_dns_config_ &&
+        !ResemblesMulticastDNSName(key_.hostname)) {
       StartDnsTask();
     } else {
       StartProcTask();
     }
   }
 
   // TODO(szym): Since DnsTransaction does not consume threads, we can increase
   // the limits on |dispatcher_|. But in order to keep the number of WorkerPool
   // threads low, we will need to use an "inner" PrioritizedDispatcher with
   // tighter limits.
@@ skipping 873 matching lines @@
     dns_client_->SetConfig(dns_config);
     num_dns_failures_ = 0;
     if (dns_client_->GetConfig())
       UMA_HISTOGRAM_BOOLEAN("AsyncDNS.DnsClientEnabled", true);
   }
 
   AbortDnsTasks();
 }
 
 }  // namespace net