[Chromoting] Hook up host talkgadget policy checks.

remoting/host/dns_blackhole_checker.h

Index: remoting/host/dns_blackhole_checker.h
diff --git a/remoting/host/dns_blackhole_checker.h b/remoting/host/dns_blackhole_checker.h
new file mode 100644
index 0000000000000000000000000000000000000000..4ff75b4c8879b5b1fc358ffe4bfa13d07d308281
--- /dev/null
+++ b/remoting/host/dns_blackhole_checker.h
@@ -0,0 +1,53 @@
+// 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.
+
+#ifndef REMOTING_HOST_DNS_BLACKHOLE_CHECKER_H_
+#define REMOTING_HOST_DNS_BLACKHOLE_CHECKER_H_
+
+#include "net/url_request/url_fetcher_delegate.h"
+
+#include "base/callback.h"
+
+namespace remoting {
+
+// This is the default prefix that is prepended to the kTalkGadgetUrl to form
+// the complete talkgadget URL used by the host. Policy settings allow admins
+// to change the prefix that is used.
+extern const char kDefaultHostTalkGadgetPrefix[];
+
+class ChromotingHostContext;
+
+class DnsBlackholeChecker : public net::URLFetcherDelegate {
+ public:
+  DnsBlackholeChecker(ChromotingHostContext* context,
+                      std::string talkgadget_prefix);
+  virtual ~DnsBlackholeChecker();
+
+  // net::URLFetcherDelegate interface.
+  virtual void OnURLFetchComplete(const net::URLFetcher* source) OVERRIDE;
+
+  // Initiates a check the verify that the host talkgadget has not been "DNS
+  // blackholed" to prevent connections. If this is called again before the
+  // callback has been called, then the second call is ignored.
+  void CheckForDnsBlackhole(const base::Callback<void(bool)>& callback);
+
+ private:
+  ChromotingHostContext* context_;
+
+  // URL fetcher used to verify access to the host talkgadget.
+  scoped_ptr<net::URLFetcher> url_fetcher_;
+
+  // The string pre-pended to '.talkgadget.google.com' to create the full
+  // talkgadget domain name for the host.
+  std::string talkgadget_prefix_;
+
+  // Called with the results of the connection check.
+  base::Callback<void(bool)> callback_;
+
+  DISALLOW_COPY_AND_ASSIGN(DnsBlackholeChecker);
+};
+
+}  // namespace remoting
+
+#endif  // REMOTING_HOST_DNS_BLACKHOLE_CHECKER_H_