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diff --git a/src/shrpx_dns_tracker.h b/src/shrpx_dns_tracker.h
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+/*
+ * nghttp2 - HTTP/2 C Library
+ *
+ * Copyright (c) 2016 Tatsuhiro Tsujikawa
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining
+ * a copy of this software and associated documentation files (the
+ * "Software"), to deal in the Software without restriction, including
+ * without limitation the rights to use, copy, modify, merge, publish,
+ * distribute, sublicense, and/or sell copies of the Software, and to
+ * permit persons to whom the Software is furnished to do so, subject to
+ * the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be
+ * included in all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+ * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+ * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+ * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
+ * LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
+ * OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+ * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+#ifndef SHRPX_DNS_TRACKER_H
+#define SHRPX_DNS_TRACKER_H
+
+#include "shrpx.h"
+
+#include <map>
+
+#include "shrpx_dual_dns_resolver.h"
+
+using namespace nghttp2;
+
+namespace shrpx {
+
+struct DNSQuery {
+  DNSQuery(StringRef host, CompleteCb cb)
+      : host(std::move(host)),
+        cb(std::move(cb)),
+        dlnext(nullptr),
+        dlprev(nullptr),
+        status(DNSResolverStatus::IDLE),
+        in_qlist(false) {}
+
+  // Host name we lookup for.
+  StringRef host;
+  // Callback function called when name lookup finished.  This
+  // callback is not called if name lookup finishes within
+  // DNSTracker::resolve().
+  CompleteCb cb;
+  DNSQuery *dlnext, *dlprev;
+  DNSResolverStatus status;
+  // true if this object is in linked list ResolverEntry::qlist.
+  bool in_qlist;
+};
+
+struct ResolverEntry {
+  // Host name this entry lookups for.
+  ImmutableString host;
+  // DNS resolver.  Only non-nullptr if status is
+  // DNSResolverStatus::RUNNING.
+  std::unique_ptr<DualDNSResolver> resolv;
+  // DNSQuery interested in this name lookup result.  The result is
+  // notified to them all.
+  DList<DNSQuery> qlist;
+  // Use the same enum with DNSResolverStatus
+  DNSResolverStatus status;
+  // result and its expiry time
+  Address result;
+  // time point when cached result expires.
+  ev_tstamp expiry;
+};
+
+class DNSTracker {
+public:
+  DNSTracker(struct ev_loop *loop, int family);
+  ~DNSTracker();
+
+  // Lookups host name described in |dnsq|.  If name lookup finishes
+  // within this function (either it came from /etc/hosts, host name
+  // is numeric, lookup result is cached, etc), it returns
+  // DNSResolverStatus::OK or DNSResolverStatus::ERROR.  If lookup is
+  // successful, DNSResolverStatus::OK is returned, and |result| is
+  // filled.  If lookup failed, DNSResolverStatus::ERROR is returned.
+  // If name lookup is being done background, it returns
+  // DNSResolverStatus::RUNNING.  Its completion is notified by
+  // calling dnsq->cb.
+  DNSResolverStatus resolve(Address *result, DNSQuery *dnsq);
+  // Cancels name lookup requested by |dnsq|.
+  void cancel(DNSQuery *dnsq);
+  // Removes expired entries from ents_.
+  void gc();
+  // Starts GC timer.
+  void start_gc_timer();
+
+private:
+  ResolverEntry make_entry(std::unique_ptr<DualDNSResolver> resolv,
+                           ImmutableString host, DNSResolverStatus status,
+                           const Address *result);
+
+  void update_entry(ResolverEntry &ent, std::unique_ptr<DualDNSResolver> resolv,
+                    DNSResolverStatus status, const Address *result);
+
+  void add_to_qlist(ResolverEntry &ent, DNSQuery *dnsq);
+
+  std::map<StringRef, ResolverEntry> ents_;
+  // Periodically iterates ents_, and removes expired entries to avoid
+  // excessive use of memory.  Since only backend API can potentially
+  // increase memory consumption, interval could be very long.
+  ev_timer gc_timer_;
+  struct ev_loop *loop_;
+  // IP version preference.
+  int family_;
+};
+
+} // namespace shrpx
+
+#endif // SHRPX_DNS_TRACKER_H