Fast-fail WPAD detection.

net/proxy/proxy_script_decider.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/proxy/proxy_script_decider.h"
 
 #include "base/bind.h"
 #include "base/bind_helpers.h"
 #include "base/compiler_specific.h"
 #include "base/format_macros.h"
 #include "base/logging.h"
+#include "base/metrics/histogram.h"
 #include "base/strings/string_util.h"
 #include "base/strings/utf_string_conversions.h"
 #include "base/values.h"
 #include "net/base/net_errors.h"
 #include "net/proxy/dhcp_proxy_script_fetcher.h"
 #include "net/proxy/dhcp_proxy_script_fetcher_factory.h"
 #include "net/proxy/proxy_script_fetcher.h"
+#include "net/url_request/url_request_context.h"
 
 namespace net {
 
 namespace {
 
 bool LooksLikePacScript(const base::string16& script) {
   // Note: this is only an approximation! It may not always work correctly,
   // however it is very likely that legitimate scripts have this exact string,
   // since they must minimally define a function of this name. Conversely, a
   // file not containing the string is not likely to be a PAC script.
   //
   // An exact test would have to load the script in a javascript evaluator.
   return script.find(ASCIIToUTF16("FindProxyForURL")) != base::string16::npos;
 }
 
 }
 
 // This is the hard-coded location used by the DNS portion of web proxy
 // auto-discovery.
 //
 // Note that we not use DNS devolution to find the WPAD host, since that could
 // be dangerous should our top level domain registry  become out of date.
 //
 // Instead we directly resolve "wpad", and let the operating system apply the
 // DNS suffix search paths. This is the same approach taken by Firefox, and
 // compatibility hasn't been an issue.
 //
 // For more details, also check out this comment:
 // http://code.google.com/p/chromium/issues/detail?id=18575#c20
-static const char kWpadUrl[] = "http://wpad/wpad.dat";
+namespace {
+const char kWpadUrl[] = "http://wpad/wpad.dat";
+const int kQuickCheckDelayMs = 1000;
+};
 
 base::Value* ProxyScriptDecider::PacSource::NetLogCallback(
     const GURL* effective_pac_url,
     NetLog::LogLevel /* log_level */) const {
   base::DictionaryValue* dict = new base::DictionaryValue();
   std::string source;
   switch (type) {
     case PacSource::WPAD_DHCP:
       source = "WPAD DHCP";
       break;
@@ skipping 16 matching lines @@
     NetLog* net_log)
     : resolver_(NULL),
       proxy_script_fetcher_(proxy_script_fetcher),
       dhcp_proxy_script_fetcher_(dhcp_proxy_script_fetcher),
       current_pac_source_index_(0u),
       pac_mandatory_(false),
       next_state_(STATE_NONE),
       net_log_(BoundNetLog::Make(
           net_log, NetLog::SOURCE_PROXY_SCRIPT_DECIDER)),
       fetch_pac_bytes_(false) {
+  if (proxy_script_fetcher && proxy_script_fetcher->GetRequestContext() &&
+      proxy_script_fetcher->GetRequestContext()->host_resolver())
+    host_resolver_.reset(new SingleRequestHostResolver(
+        proxy_script_fetcher->GetRequestContext()->host_resolver()));

[szym, 2013/09/11 14:06:22]

Ok, so just triple-checking here.

This will make the query using the DnsClient if enabled. If "wpad" is served via
NetBIOS or LLMNR then this request will have to fail with DNS twice.

That said, I don't suspect DNS queries for "wpad" are particularly slow (unless
there is a deep suffix search list and domain name devolution enabled).

To be sure, we could add a measurement for how long do DNS queries for "wpad"
take.

[Elly Fong-Jones, 2013/09/11 20:20:37]

Which component is doing the LLMNR and NetBIOS queries?

[szym, 2013/09/11 21:29:21]

If DnsClient is enabled, the call flow is:
HostResolver::Resolve
  DnsClient...DnsTransaction...Start  // fails
  getaddrinfo
    DNSAPI::QueryDns  // fails
    LLMNR/NetBIOS     // maybe succeeds

[cbentzel, 2013/09/12 13:44:30]

I thought we had decided to do a SingleRequestHostResolver which would only do
the getaddrinfo based resolution for WPAD. I think this is necessary
particularly if we roll out the built-in DNS resolver widely on Windows.

[szym, 2013/09/12 16:03:25]

I don't know how often wpad is served via LLMNR/NetBIOS, but this channel is
always tried last (after DNS). So disabling DnsClient for this
HostResolver::Resolve request only helps if the DNS query for wpad takes a long
time. This could happen if the latency to the DNS server is significant.

Do we suspect that the DNS queries take a long time? If so, we should certainly
create a custom HostResolver and subsequently stuff the response in the global
HostResolver's cache.

[szym, 2013/09/12 18:01:52]

Stuffing the cache is a little bit more work.

// in ctor
host_resolver_(HostResolver::CreateDefaultResolver(net_log))
single_request_resolver_(host_resolver_.get())

// in DoQuickCheck
host_resolver_->SetDnsClientEnabled(false);

// in DoQuickCheckComplete
if (result == OK) {
  HostCache* cache =
proxy_script_fetcher_->GetRequestContext()->host_resolver()->GetHostCache();
  if (cache) {
    cache->Set(HostCache::Key("wpad", ADDRESS_FAMILY_..., 0),
               HostCache::Entry(wpad_addresses_));
  }
}

 }
 
 ProxyScriptDecider::~ProxyScriptDecider() {
   if (next_state_ != STATE_NONE)
     Cancel();
 }
 
 int ProxyScriptDecider::Start(
     const ProxyConfig& config, const base::TimeDelta wait_delay,
     bool fetch_pac_bytes, const CompletionCallback& callback) {
@@ skipping 70 matching lines @@
     State state = next_state_;
     next_state_ = STATE_NONE;
     switch (state) {
       case STATE_WAIT:
         DCHECK_EQ(OK, rv);
         rv = DoWait();
         break;
       case STATE_WAIT_COMPLETE:
         rv = DoWaitComplete(rv);
         break;
+      case STATE_QUICK_CHECK:
+        DCHECK_EQ(OK, rv);
+        rv = DoQuickCheck();
+        break;
+      case STATE_QUICK_CHECK_COMPLETE:
+        rv = DoQuickCheckComplete(rv);
+        break;
       case STATE_FETCH_PAC_SCRIPT:
         DCHECK_EQ(OK, rv);
         rv = DoFetchPacScript();
         break;
       case STATE_FETCH_PAC_SCRIPT_COMPLETE:
         rv = DoFetchPacScriptComplete(rv);
         break;
       case STATE_VERIFY_PAC_SCRIPT:
         DCHECK_EQ(OK, rv);
         rv = DoVerifyPacScript();
@@ skipping 29 matching lines @@
   net_log_.BeginEvent(NetLog::TYPE_PROXY_SCRIPT_DECIDER_WAIT);
   return ERR_IO_PENDING;
 }
 
 int ProxyScriptDecider::DoWaitComplete(int result) {
   DCHECK_EQ(OK, result);
   if (wait_delay_.ToInternalValue() != 0) {
     net_log_.EndEventWithNetErrorCode(NetLog::TYPE_PROXY_SCRIPT_DECIDER_WAIT,
                                       result);
   }
-  next_state_ = GetStartState();
+  next_state_ = STATE_QUICK_CHECK;
   return OK;
 }
 
+int ProxyScriptDecider::DoQuickCheck() {
+  if (host_resolver_.get() == NULL) {
+    // If we have no resolver, skip QuickCheck altogether.
+    next_state_ = GetStartState();
+    return OK;
+  }
+
+  quick_check_start_time_ = base::Time::Now();
+  HostResolver::RequestInfo reqinfo(HostPortPair("wpad", 80));
+  CompletionCallback callback = base::Bind(
+      &ProxyScriptDecider::OnIOCompletion,
+      base::Unretained(this));
+
+  // We use HIGHEST here because proxy decision blocks doing any other requests.
+  int rv = host_resolver_->Resolve(reqinfo, HIGHEST, &wpad_addresses_,
+                                   callback, net_log_);
+
+  // We can't get an error response - the name is known to be valid, and we
+  // don't cache negative dns responses.
+  DCHECK(rv == OK || rv == ERR_IO_PENDING);
+
+  if (rv == OK) {
+    next_state_ = GetStartState();
+  } else {
+    quick_check_timer_.Start(FROM_HERE,
+                             base::TimeDelta::FromMilliseconds(
+                                kQuickCheckDelayMs),
+                             base::Bind(callback, ERR_NAME_NOT_RESOLVED));
+    next_state_ = STATE_QUICK_CHECK_COMPLETE;
+  }
+  return rv;
+}
+
+int ProxyScriptDecider::DoQuickCheckComplete(int result) {
+  base::TimeDelta delta = base::Time::Now() - quick_check_start_time_;
+  if (result == OK)
+    UMA_HISTOGRAM_TIMES("Net.WpadQuickCheckSuccess", delta);
+  else
+    UMA_HISTOGRAM_TIMES("Net.WpadQuickCheckFailure", delta);
+  host_resolver_->Cancel();
+  quick_check_timer_.Stop();
+  if (result == OK)
+    next_state_ = GetStartState();
+  return result;
+}
+
 int ProxyScriptDecider::DoFetchPacScript() {
   DCHECK(fetch_pac_bytes_);
 
   next_state_ = STATE_FETCH_PAC_SCRIPT_COMPLETE;
 
   const PacSource& pac_source = current_pac_source();
 
   GURL effective_pac_url;
   DetermineURL(pac_source, &effective_pac_url);
 
@@ skipping 167 matching lines @@
   }
 
   // This is safe to call in any state.
   if (dhcp_proxy_script_fetcher_)
     dhcp_proxy_script_fetcher_->Cancel();
 
   DidComplete();
 }
 
 }  // namespace net