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diff --git a/src/feature/relay/dns_structs.h b/src/feature/relay/dns_structs.h
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+++ b/src/feature/relay/dns_structs.h
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+/* Copyright (c) 2003-2004, Roger Dingledine.
+ * Copyright (c) 2004-2006, Roger Dingledine, Nick Mathewson.
+ * Copyright (c) 2007-2018, The Tor Project, Inc. */
+/* See LICENSE for licensing information */
+
+/**
+ * \file dns_structs.h
+ *
+ * \brief Structures used in dns.c. Exposed to dns.c, and to the unit tests
+ * that declare DNS_PRIVATE.
+ */
+
+#ifndef TOR_DNS_STRUCTS_H
+#define TOR_DNS_STRUCTS_H
+
+/** Longest hostname we're willing to resolve. */
+#define MAX_ADDRESSLEN 256
+
+/** Linked list of connections waiting for a DNS answer. */
+typedef struct pending_connection_t {
+  edge_connection_t *conn;
+  struct pending_connection_t *next;
+} pending_connection_t;
+
+/** Value of 'magic' field for cached_resolve_t.  Used to try to catch bad
+ * pointers and memory stomping. */
+#define CACHED_RESOLVE_MAGIC 0x1234F00D
+
+/* Possible states for a cached resolve_t */
+/** We are waiting for the resolver system to tell us an answer here.
+ * When we get one, or when we time out, the state of this cached_resolve_t
+ * will become "DONE" and we'll possibly add a CACHED
+ * entry. This cached_resolve_t will be in the hash table so that we will
+ * know not to launch more requests for this addr, but rather to add more
+ * connections to the pending list for the addr. */
+#define CACHE_STATE_PENDING 0
+/** This used to be a pending cached_resolve_t, and we got an answer for it.
+ * Now we're waiting for this cached_resolve_t to expire.  This should
+ * have no pending connections, and should not appear in the hash table. */
+#define CACHE_STATE_DONE 1
+/** We are caching an answer for this address. This should have no pending
+ * connections, and should appear in the hash table. */
+#define CACHE_STATE_CACHED 2
+
+/** @name status values for a single DNS request.
+ *
+ * @{ */
+/** The DNS request is in progress. */
+#define RES_STATUS_INFLIGHT 1
+/** The DNS request finished and gave an answer */
+#define RES_STATUS_DONE_OK 2
+/** The DNS request finished and gave an error */
+#define RES_STATUS_DONE_ERR 3
+/**@}*/
+
+/** A DNS request: possibly completed, possibly pending; cached_resolve
+ * structs are stored at the OR side in a hash table, and as a linked
+ * list from oldest to newest.
+ */
+typedef struct cached_resolve_t {
+  HT_ENTRY(cached_resolve_t) node;
+  uint32_t magic;  /**< Must be CACHED_RESOLVE_MAGIC */
+  char address[MAX_ADDRESSLEN]; /**< The hostname to be resolved. */
+
+  union {
+    uint32_t addr_ipv4; /**< IPv4 addr for <b>address</b>, if successful.
+                         * (In host order.) */
+    int err_ipv4; /**< One of DNS_ERR_*, if IPv4 lookup failed. */
+  } result_ipv4; /**< Outcome of IPv4 lookup */
+  union {
+    struct in6_addr addr_ipv6; /**< IPv6 addr for <b>address</b>, if
+                                * successful */
+    int err_ipv6; /**< One of DNS_ERR_*, if IPv6 lookup failed. */
+  } result_ipv6; /**< Outcome of IPv6 lookup, if any */
+  union {
+    char *hostname; /** A hostname, if PTR lookup happened successfully*/
+    int err_hostname; /** One of DNS_ERR_*, if PTR lookup failed. */
+  } result_ptr;
+  /** @name Status fields
+   *
+   * These take one of the RES_STATUS_* values, depending on the state
+   * of the corresponding lookup.
+   *
+   * @{ */
+  unsigned int res_status_ipv4 : 2;
+  unsigned int res_status_ipv6 : 2;
+  unsigned int res_status_hostname : 2;
+  /**@}*/
+  uint8_t state; /**< Is this cached entry pending/done/informative? */
+
+  time_t expire; /**< Remove items from cache after this time. */
+  uint32_t ttl_ipv4; /**< What TTL did the nameserver tell us? */
+  uint32_t ttl_ipv6; /**< What TTL did the nameserver tell us? */
+  uint32_t ttl_hostname; /**< What TTL did the nameserver tell us? */
+  /** Connections that want to know when we get an answer for this resolve. */
+  pending_connection_t *pending_connections;
+  /** Position of this element in the heap*/
+  int minheap_idx;
+} cached_resolve_t;
+
+#endif /* !defined(TOR_DNS_STRUCTS_H) */
+