Merge branch 'ticket30924_042_04_squashed' into ticket30924_042_04_squashed_merged

29 files changed, 1645 insertions, 168 deletions

diff --git a/src/app/config/config.c b/src/app/config/config.c
index 0cbc223d02..1babb0d4f8 100644
--- a/src/app/config/config.c
+++ b/src/app/config/config.c
@@ -496,6 +496,11 @@ static const config_var_t option_vars_[] = {
   VAR("HiddenServiceMaxStreamsCloseCircuit",LINELIST_S, RendConfigLines, NULL),
   VAR("HiddenServiceNumIntroductionPoints", LINELIST_S, RendConfigLines, NULL),
   VAR("HiddenServiceExportCircuitID", LINELIST_S,  RendConfigLines, NULL),
+  VAR("HiddenServiceEnableIntroDoSDefense", LINELIST_S, RendConfigLines, NULL),
+  VAR("HiddenServiceEnableIntroDoSRatePerSec",
+      LINELIST_S, RendConfigLines, NULL),
+  VAR("HiddenServiceEnableIntroDoSBurstPerSec",
+      LINELIST_S, RendConfigLines, NULL),
   VAR("HiddenServiceStatistics", BOOL, HiddenServiceStatistics_option, "1"),
   V(HidServAuth,                 LINELIST, NULL),
   V(ClientOnionAuthDir,          FILENAME, NULL),
diff --git a/src/core/or/or.h b/src/core/or/or.h
index ab258629a6..990cfacbc0 100644
--- a/src/core/or/or.h
+++ b/src/core/or/or.h
@@ -843,6 +843,10 @@ typedef struct protover_summary_flags_t {
   /** True iff this router has a protocol list that allows clients to
    * negotiate hs circuit setup padding. Requires Padding>=2. */
   unsigned int supports_hs_setup_padding : 1;
+
+  /** True iff this router has a protocol list that allows it to support the
+   * ESTABLISH_INTRO DoS cell extension. Requires HSIntro>=5. */
+  unsigned int supports_establish_intro_dos_extension : 1;
 } protover_summary_flags_t;
 
 typedef struct routerinfo_t routerinfo_t;
diff --git a/src/core/or/or_circuit_st.h b/src/core/or/or_circuit_st.h
index 8f319585a6..f3eb861613 100644
--- a/src/core/or/or_circuit_st.h
+++ b/src/core/or/or_circuit_st.h
@@ -72,6 +72,10 @@ struct or_circuit_t {
    * buffer stats to disk. */
   uint64_t total_cell_waiting_time;
 
+  /** If set, the DoS defenses are enabled on this circuit meaning that the
+   * introduce2_bucket is initialized and used. */
+  unsigned int introduce2_dos_defense_enabled : 1;
+
   /** INTRODUCE2 cell bucket controlling how much can go on this circuit. Only
    * used if this is a service introduction circuit at the intro point
    * (purpose = CIRCUIT_PURPOSE_INTRO_POINT). */
diff --git a/src/core/or/protover.c b/src/core/or/protover.c
index ccd33fabf7..905c5e9ed3 100644
--- a/src/core/or/protover.c
+++ b/src/core/or/protover.c
@@ -392,7 +392,7 @@ protover_get_supported_protocols(void)
     "Desc=1-2 "
     "DirCache=1-2 "
     "HSDir=1-2 "
-    "HSIntro=3-4 "
+    "HSIntro=3-5 "
     "HSRend=1-2 "
     "Link=1-5 "
 #ifdef HAVE_WORKING_TOR_TLS_GET_TLSSECRETS
diff --git a/src/core/or/versions.c b/src/core/or/versions.c
index 06417bb4eb..2c32b529f7 100644
--- a/src/core/or/versions.c
+++ b/src/core/or/versions.c
@@ -450,7 +450,9 @@ memoize_protover_summary(protover_summary_flags_t *out,
                                     PROTOVER_HS_RENDEZVOUS_POINT_V3);
   out->supports_hs_setup_padding =
     protocol_list_supports_protocol(protocols, PRT_PADDING,
-              PROTOVER_HS_SETUP_PADDING);
+                                    PROTOVER_HS_SETUP_PADDING);
+  out->supports_establish_intro_dos_extension =
+    protocol_list_supports_protocol(protocols, PRT_HSINTRO, 5);
 
   protover_summary_flags_t *new_cached = tor_memdup(out, sizeof(*out));
   cached = strmap_set(protover_summary_map, protocols, new_cached);
diff --git a/src/feature/hs/hs_cell.c b/src/feature/hs/hs_cell.c
index 69f1ccbef4..547dda3e16 100644
--- a/src/feature/hs/hs_cell.c
+++ b/src/feature/hs/hs_cell.c
@@ -473,10 +473,110 @@ introduce1_set_legacy_id(trn_cell_introduce1_t *cell,
   }
 }
 
+/* Build and add to the given DoS cell extension the given parameter type and
+ * value. */
+static void
+build_establish_intro_dos_param(trn_cell_extension_dos_t *dos_ext,
+                                uint8_t param_type, uint64_t param_value)
+{
+  trn_cell_extension_dos_param_t *dos_param =
+    trn_cell_extension_dos_param_new();
+
+  /* Extra safety. We should never send an unknown parameter type. */
+  tor_assert(param_type == TRUNNEL_DOS_PARAM_TYPE_INTRO2_RATE_PER_SEC ||
+             param_type == TRUNNEL_DOS_PARAM_TYPE_INTRO2_BURST_PER_SEC);
+
+  trn_cell_extension_dos_param_set_type(dos_param, param_type);
+  trn_cell_extension_dos_param_set_value(dos_param, param_value);
+  trn_cell_extension_dos_add_params(dos_ext, dos_param);
+
+  /* Not freeing the trunnel object because it is now owned by dos_ext. */
+}
+
+/* Build the DoS defense cell extension and put it in the given extensions
+ * object. This can't fail. */
+static void
+build_establish_intro_dos_extension(const hs_service_config_t *service_config,
+                                    trn_cell_extension_t *extensions)
+{
+  ssize_t ret, dos_ext_encoded_len;
+  uint8_t *field_array;
+  trn_cell_extension_field_t *field;
+  trn_cell_extension_dos_t *dos_ext;
+
+  tor_assert(service_config);
+  tor_assert(extensions);
+
+  /* We are creating a cell extension field of the type DoS. */
+  field = trn_cell_extension_field_new();
+  trn_cell_extension_field_set_field_type(field,
+                                          TRUNNEL_CELL_EXTENSION_TYPE_DOS);
+
+  /* Build DoS extension field. We will put in two parameters. */
+  dos_ext = trn_cell_extension_dos_new();
+  trn_cell_extension_dos_set_n_params(dos_ext, 2);
+
+  /* Build DoS parameter INTRO2 rate per second. */
+  build_establish_intro_dos_param(dos_ext,
+                                  TRUNNEL_DOS_PARAM_TYPE_INTRO2_RATE_PER_SEC,
+                                  service_config->intro_dos_rate_per_sec);
+  /* Build DoS parameter INTRO2 burst per second. */
+  build_establish_intro_dos_param(dos_ext,
+                                  TRUNNEL_DOS_PARAM_TYPE_INTRO2_BURST_PER_SEC,
+                                  service_config->intro_dos_burst_per_sec);
+
+  /* Set the field with the encoded DoS extension. */
+  dos_ext_encoded_len = trn_cell_extension_dos_encoded_len(dos_ext);
+  /* Set length field and the field array size length. */
+  trn_cell_extension_field_set_field_len(field, dos_ext_encoded_len);
+  trn_cell_extension_field_setlen_field(field, dos_ext_encoded_len);
+  /* Encode the DoS extension into the cell extension field. */
+  field_array = trn_cell_extension_field_getarray_field(field);
+  ret = trn_cell_extension_dos_encode(field_array,
+                 trn_cell_extension_field_getlen_field(field), dos_ext);
+  tor_assert(ret == dos_ext_encoded_len);
+
+  /* Finally, encode field into the cell extension. */
+  trn_cell_extension_add_fields(extensions, field);
+
+  /* We've just add an extension field to the cell extensions so increment the
+   * total number. */
+  trn_cell_extension_set_num(extensions,
+                             trn_cell_extension_get_num(extensions) + 1);
+
+  /* Cleanup. DoS extension has been encoded at this point. */
+  trn_cell_extension_dos_free(dos_ext);
+}
+
 /* ========== */
 /* Public API */
 /* ========== */
 
+/* Allocate and build all the ESTABLISH_INTRO cell extension. The given
+ * extensions pointer is always set to a valid cell extension object. */
+STATIC trn_cell_extension_t *
+build_establish_intro_extensions(const hs_service_config_t *service_config,
+                                 const hs_service_intro_point_t *ip)
+{
+  trn_cell_extension_t *extensions;
+
+  tor_assert(service_config);
+  tor_assert(ip);
+
+  extensions = trn_cell_extension_new();
+  trn_cell_extension_set_num(extensions, 0);
+
+  /* If the defense has been enabled service side (by the operator with a
+   * torrc option) and the intro point does support it. */
+  if (service_config->has_dos_defense_enabled &&
+      ip->support_intro2_dos_defense) {
+    /* This function takes care to increment the number of extensions. */
+    build_establish_intro_dos_extension(service_config, extensions);
+  }
+
+  return extensions;
+}
+
 /* Build an ESTABLISH_INTRO cell with the given circuit nonce and intro point
  * object. The encoded cell is put in cell_out that MUST at least be of the
  * size of RELAY_PAYLOAD_SIZE. Return the encoded cell length on success else
@@ -484,15 +584,17 @@ introduce1_set_legacy_id(trn_cell_introduce1_t *cell,
  * legacy cell creation. */
 ssize_t
 hs_cell_build_establish_intro(const char *circ_nonce,
+                              const hs_service_config_t *service_config,
                               const hs_service_intro_point_t *ip,
                               uint8_t *cell_out)
 {
   ssize_t cell_len = -1;
   uint16_t sig_len = ED25519_SIG_LEN;
-  trn_cell_extension_t *ext;
   trn_cell_establish_intro_t *cell = NULL;
+  trn_cell_extension_t *extensions;
 
   tor_assert(circ_nonce);
+  tor_assert(service_config);
   tor_assert(ip);
 
   /* Quickly handle the legacy IP. */
@@ -505,11 +607,12 @@ hs_cell_build_establish_intro(const char *circ_nonce,
     goto done;
   }
 
+  /* Build the extensions, if any. */
+  extensions = build_establish_intro_extensions(service_config, ip);
+
   /* Set extension data. None used here. */
-  ext = trn_cell_extension_new();
-  trn_cell_extension_set_num(ext, 0);
   cell = trn_cell_establish_intro_new();
-  trn_cell_establish_intro_set_extensions(cell, ext);
+  trn_cell_establish_intro_set_extensions(cell, extensions);
   /* Set signature size. Array is then allocated in the cell. We need to do
    * this early so we can use trunnel API to get the signature length. */
   trn_cell_establish_intro_set_sig_len(cell, sig_len);
diff --git a/src/feature/hs/hs_cell.h b/src/feature/hs/hs_cell.h
index 9569de535e..864b6fda5f 100644
--- a/src/feature/hs/hs_cell.h
+++ b/src/feature/hs/hs_cell.h
@@ -79,6 +79,7 @@ typedef struct hs_cell_introduce2_data_t {
 
 /* Build cell API. */
 ssize_t hs_cell_build_establish_intro(const char *circ_nonce,
+                                      const hs_service_config_t *config,
                                       const hs_service_intro_point_t *ip,
                                       uint8_t *cell_out);
 ssize_t hs_cell_build_rendezvous1(const uint8_t *rendezvous_cookie,
@@ -105,5 +106,15 @@ int hs_cell_parse_rendezvous2(const uint8_t *payload, size_t payload_len,
 /* Util API. */
 void hs_cell_introduce1_data_clear(hs_cell_introduce1_data_t *data);
 
+#ifdef TOR_UNIT_TESTS
+
+#include "trunnel/hs/cell_common.h"
+
+STATIC trn_cell_extension_t *
+build_establish_intro_extensions(const hs_service_config_t *service_config,
+                                 const hs_service_intro_point_t *ip);
+
+#endif /* defined(TOR_UNIT_TESTS) */
+
 #endif /* !defined(TOR_HS_CELL_H) */
 
diff --git a/src/feature/hs/hs_circuit.c b/src/feature/hs/hs_circuit.c
index a5aa2fb228..5e213b5aba 100644
--- a/src/feature/hs/hs_circuit.c
+++ b/src/feature/hs/hs_circuit.c
@@ -317,7 +317,7 @@ send_establish_intro(const hs_service_t *service,
 
   /* Encode establish intro cell. */
   cell_len = hs_cell_build_establish_intro(circ->cpath->prev->rend_circ_nonce,
-                                           ip, payload);
+                                           &service->config, ip, payload);
   if (cell_len < 0) {
     log_warn(LD_REND, "Unable to encode ESTABLISH_INTRO cell for service %s "
                       "on circuit %u. Closing circuit.",
diff --git a/src/feature/hs/hs_config.c b/src/feature/hs/hs_config.c
index 87f6257591..7424d7d3ce 100644
--- a/src/feature/hs/hs_config.c
+++ b/src/feature/hs/hs_config.c
@@ -218,6 +218,9 @@ config_has_invalid_options(const config_line_t *line_,
 
   const char *opts_exclude_v2[] = {
     "HiddenServiceExportCircuitID",
+    "HiddenServiceEnableIntroDoSDefense",
+    "HiddenServiceEnableIntroDoSRatePerSec",
+    "HiddenServiceEnableIntroDoSBurstPerSec",
     NULL /* End marker. */
   };
 
@@ -276,6 +279,15 @@ config_validate_service(const hs_service_config_t *config)
     goto invalid;
   }
 
+  /* DoS validation values. */
+  if (config->has_dos_defense_enabled &&
+      (config->intro_dos_burst_per_sec < config->intro_dos_rate_per_sec)) {
+    log_warn(LD_CONFIG, "Hidden service DoS defenses burst (%" PRIu32 ") can "
+                        "not be smaller than the rate value (%" PRIu32 ").",
+             config->intro_dos_burst_per_sec, config->intro_dos_rate_per_sec);
+    goto invalid;
+  }
+
   /* Valid. */
   return 0;
  invalid:
@@ -296,6 +308,8 @@ config_service_v3(const config_line_t *line_,
 {
   int have_num_ip = 0;
   bool export_circuit_id = false; /* just to detect duplicate options */
+  bool dos_enabled = false, dos_rate_per_sec = false;
+  bool dos_burst_per_sec = false;
   const char *dup_opt_seen = NULL;
   const config_line_t *line;
 
@@ -334,6 +348,52 @@ config_service_v3(const config_line_t *line_,
       export_circuit_id = true;
       continue;
     }
+    if (!strcasecmp(line->key, "HiddenServiceEnableIntroDoSDefense")) {
+      config->has_dos_defense_enabled =
+        (unsigned int) helper_parse_uint64(line->key, line->value,
+                                           HS_CONFIG_V3_DOS_DEFENSE_DEFAULT,
+                                           1, &ok);
+      if (!ok || dos_enabled) {
+        if (dos_enabled) {
+          dup_opt_seen = line->key;
+        }
+        goto err;
+      }
+      dos_enabled = true;
+      continue;
+    }
+    if (!strcasecmp(line->key, "HiddenServiceEnableIntroDoSRatePerSec")) {
+      config->intro_dos_rate_per_sec =
+        (unsigned int) helper_parse_uint64(line->key, line->value,
+                              HS_CONFIG_V3_DOS_DEFENSE_RATE_PER_SEC_MIN,
+                              HS_CONFIG_V3_DOS_DEFENSE_RATE_PER_SEC_MAX, &ok);
+      if (!ok || dos_rate_per_sec) {
+        if (dos_rate_per_sec) {
+          dup_opt_seen = line->key;
+        }
+        goto err;
+      }
+      dos_rate_per_sec = true;
+      log_info(LD_REND, "Service INTRO2 DoS defenses rate set to: %" PRIu32,
+               config->intro_dos_rate_per_sec);
+      continue;
+    }
+    if (!strcasecmp(line->key, "HiddenServiceEnableIntroDoSBurstPerSec")) {
+      config->intro_dos_burst_per_sec =
+        (unsigned int) helper_parse_uint64(line->key, line->value,
+                              HS_CONFIG_V3_DOS_DEFENSE_BURST_PER_SEC_MIN,
+                              HS_CONFIG_V3_DOS_DEFENSE_BURST_PER_SEC_MAX, &ok);
+      if (!ok || dos_burst_per_sec) {
+        if (dos_burst_per_sec) {
+          dup_opt_seen = line->key;
+        }
+        goto err;
+      }
+      dos_burst_per_sec = true;
+      log_info(LD_REND, "Service INTRO2 DoS defenses burst set to: %" PRIu32,
+               config->intro_dos_burst_per_sec);
+      continue;
+    }
   }
 
   /* We do not load the key material for the service at this stage. This is
diff --git a/src/feature/hs/hs_config.h b/src/feature/hs/hs_config.h
index 040e451f13..beefc7a613 100644
--- a/src/feature/hs/hs_config.h
+++ b/src/feature/hs/hs_config.h
@@ -15,6 +15,15 @@
 #define HS_CONFIG_MAX_STREAMS_PER_RDV_CIRCUIT 65535
 /* Maximum number of intro points per version 3 services. */
 #define HS_CONFIG_V3_MAX_INTRO_POINTS 20
+/* Default value for the introduction DoS defenses. The MIN/MAX are inclusive
+ * meaning they can be used as valid values. */
+#define HS_CONFIG_V3_DOS_DEFENSE_DEFAULT 0
+#define HS_CONFIG_V3_DOS_DEFENSE_RATE_PER_SEC_DEFAULT 25
+#define HS_CONFIG_V3_DOS_DEFENSE_RATE_PER_SEC_MIN 0
+#define HS_CONFIG_V3_DOS_DEFENSE_RATE_PER_SEC_MAX INT32_MAX
+#define HS_CONFIG_V3_DOS_DEFENSE_BURST_PER_SEC_DEFAULT 200
+#define HS_CONFIG_V3_DOS_DEFENSE_BURST_PER_SEC_MIN 0
+#define HS_CONFIG_V3_DOS_DEFENSE_BURST_PER_SEC_MAX INT32_MAX
 
 /* API */
 
diff --git a/src/feature/hs/hs_dos.c b/src/feature/hs/hs_dos.c
index a4586dd700..0ae36017f8 100644
--- a/src/feature/hs/hs_dos.c
+++ b/src/feature/hs/hs_dos.c
@@ -45,24 +45,26 @@
  * introduction DoS defense. Disabled by default. */
 #define HS_DOS_INTRODUCE_ENABLED_DEFAULT 0
 
-/* Consensus parameters. */
-static uint32_t hs_dos_introduce_rate_per_sec =
+/* Consensus parameters. The ESTABLISH_INTRO DoS cell extension have higher
+ * priority than these values. If no extension is sent, these are used only by
+ * the introduction point. */
+static uint32_t consensus_param_introduce_rate_per_sec =
   HS_DOS_INTRODUCE_DEFAULT_CELL_RATE_PER_SEC;
-static uint32_t hs_dos_introduce_burst_per_sec =
+static uint32_t consensus_param_introduce_burst_per_sec =
   HS_DOS_INTRODUCE_DEFAULT_CELL_BURST_PER_SEC;
-static uint32_t hs_dos_introduce_enabled =
+static uint32_t consensus_param_introduce_defense_enabled =
   HS_DOS_INTRODUCE_ENABLED_DEFAULT;
 
-static uint32_t
-get_param_intro_dos_enabled(const networkstatus_t *ns)
+STATIC uint32_t
+get_intro2_enable_consensus_param(const networkstatus_t *ns)
 {
   return networkstatus_get_param(ns, "HiddenServiceEnableIntroDoSDefense",
                                  HS_DOS_INTRODUCE_ENABLED_DEFAULT, 0, 1);
 }
 
 /* Return the parameter for the introduction rate per sec. */
-static uint32_t
-get_param_rate_per_sec(const networkstatus_t *ns)
+STATIC uint32_t
+get_intro2_rate_consensus_param(const networkstatus_t *ns)
 {
   return networkstatus_get_param(ns, "HiddenServiceEnableIntroDoSRatePerSec",
                                  HS_DOS_INTRODUCE_DEFAULT_CELL_RATE_PER_SEC,
@@ -70,8 +72,8 @@ get_param_rate_per_sec(const networkstatus_t *ns)
 }
 
 /* Return the parameter for the introduction burst per sec. */
-static uint32_t
-get_param_burst_per_sec(const networkstatus_t *ns)
+STATIC uint32_t
+get_intro2_burst_consensus_param(const networkstatus_t *ns)
 {
   return networkstatus_get_param(ns, "HiddenServiceEnableIntroDoSBurstPerSec",
                                  HS_DOS_INTRODUCE_DEFAULT_CELL_BURST_PER_SEC,
@@ -88,10 +90,13 @@ update_intro_circuits(void)
   smartlist_t *intro_circs = hs_circuitmap_get_all_intro_circ_relay_side();
 
   SMARTLIST_FOREACH_BEGIN(intro_circs, circuit_t *, circ) {
+    /* Defenses might have been enabled or disabled. */
+    TO_OR_CIRCUIT(circ)->introduce2_dos_defense_enabled =
+      consensus_param_introduce_defense_enabled;
     /* Adjust the rate/burst value that might have changed. */
     token_bucket_ctr_adjust(&TO_OR_CIRCUIT(circ)->introduce2_bucket,
-                            hs_dos_get_intro2_rate(),
-                            hs_dos_get_intro2_burst());
+                            consensus_param_introduce_rate_per_sec,
+                            consensus_param_introduce_burst_per_sec);
   } SMARTLIST_FOREACH_END(circ);
 
   smartlist_free(intro_circs);
@@ -101,9 +106,12 @@ update_intro_circuits(void)
 static void
 set_consensus_parameters(const networkstatus_t *ns)
 {
-  hs_dos_introduce_rate_per_sec = get_param_rate_per_sec(ns);
-  hs_dos_introduce_burst_per_sec = get_param_burst_per_sec(ns);
-  hs_dos_introduce_enabled = get_param_intro_dos_enabled(ns);
+  consensus_param_introduce_rate_per_sec =
+    get_intro2_rate_consensus_param(ns);
+  consensus_param_introduce_burst_per_sec =
+    get_intro2_burst_consensus_param(ns);
+  consensus_param_introduce_defense_enabled =
+    get_intro2_enable_consensus_param(ns);
 
   /* The above might have changed which means we need to go through all
    * introduction circuits (relay side) and update the token buckets. */
@@ -114,18 +122,20 @@ set_consensus_parameters(const networkstatus_t *ns)
  * Public API.
  */
 
-/* Return the INTRODUCE2 cell rate per second. */
-uint32_t
-hs_dos_get_intro2_rate(void)
-{
-  return hs_dos_introduce_rate_per_sec;
-}
-
-/* Return the INTRODUCE2 cell burst per second. */
-uint32_t
-hs_dos_get_intro2_burst(void)
+/* Initialize the INTRODUCE2 token bucket for the DoS defenses using the
+ * consensus/default values. We might get a cell extension that changes those
+ * later but if we don't, the default or consensus parameters are used. */
+void
+hs_dos_setup_default_intro2_defenses(or_circuit_t *circ)
 {
-  return hs_dos_introduce_burst_per_sec;
+  tor_assert(circ);
+
+  circ->introduce2_dos_defense_enabled =
+    consensus_param_introduce_defense_enabled;
+  token_bucket_ctr_init(&circ->introduce2_bucket,
+                        consensus_param_introduce_rate_per_sec,
+                        consensus_param_introduce_burst_per_sec,
+                        (uint32_t) approx_time());
 }
 
 /* Called when the consensus has changed. We might have new consensus
@@ -149,8 +159,10 @@ hs_dos_can_send_intro2(or_circuit_t *s_intro_circ)
 {
   tor_assert(s_intro_circ);
 
-  /* Always allowed if the defense is disabled. */
-  if (!hs_dos_introduce_enabled) {
+  /* Allow to send the cell if the DoS defenses are disabled on the circuit.
+   * This can be set by the consensus, the ESTABLISH_INTRO cell extension or
+   * the hardcoded values in tor code. */
+  if (!s_intro_circ->introduce2_dos_defense_enabled) {
     return true;
   }
 
diff --git a/src/feature/hs/hs_dos.h b/src/feature/hs/hs_dos.h
index 9fba00b52b..6647b24be0 100644
--- a/src/feature/hs/hs_dos.h
+++ b/src/feature/hs/hs_dos.h
@@ -20,16 +20,18 @@ void hs_dos_init(void);
 /* Consensus. */
 void hs_dos_consensus_has_changed(const networkstatus_t *ns);
 
+/* Introduction Point. */
 bool hs_dos_can_send_intro2(or_circuit_t *s_intro_circ);
-
-/* Getters. */
-uint32_t hs_dos_get_intro2_rate(void);
-uint32_t hs_dos_get_intro2_burst(void);
+void hs_dos_setup_default_intro2_defenses(or_circuit_t *circ);
 
 #ifdef HS_DOS_PRIVATE
 
 #ifdef TOR_UNIT_TESTS
 
+STATIC uint32_t get_intro2_enable_consensus_param(const networkstatus_t *ns);
+STATIC uint32_t get_intro2_rate_consensus_param(const networkstatus_t *ns);
+STATIC uint32_t get_intro2_burst_consensus_param(const networkstatus_t *ns);
+
 #endif /* define(TOR_UNIT_TESTS) */
 
 #endif /* defined(HS_DOS_PRIVATE) */
diff --git a/src/feature/hs/hs_intropoint.c b/src/feature/hs/hs_intropoint.c
index 2c105f0b60..90a7f28943 100644
--- a/src/feature/hs/hs_intropoint.c
+++ b/src/feature/hs/hs_intropoint.c
@@ -26,6 +26,7 @@
 
 #include "feature/hs/hs_circuitmap.h"
 #include "feature/hs/hs_common.h"
+#include "feature/hs/hs_config.h"
 #include "feature/hs/hs_descriptor.h"
 #include "feature/hs/hs_dos.h"
 #include "feature/hs/hs_intropoint.h"
@@ -181,6 +182,185 @@ hs_intro_send_intro_established_cell,(or_circuit_t *circ))
   return ret;
 }
 
+/* Validate the cell DoS extension parameters. Return true iff they've been
+ * bound check and can be used. Else return false. See proposal 305 for
+ * details and reasons about this validation. */
+STATIC bool
+cell_dos_extension_parameters_are_valid(uint64_t intro2_rate_per_sec,
+                                        uint64_t intro2_burst_per_sec)
+{
+  bool ret = false;
+
+  /* Check that received value is not below the minimum. Don't check if minimum
+     is set to 0, since the param is a positive value and gcc will complain. */
+#if HS_CONFIG_V3_DOS_DEFENSE_RATE_PER_SEC_MIN > 0
+  if (intro2_rate_per_sec < HS_CONFIG_V3_DOS_DEFENSE_RATE_PER_SEC_MIN) {
+    log_fn(LOG_PROTOCOL_WARN, LD_REND,
+           "Intro point DoS defenses rate per second is "
+           "too small. Received value: %" PRIu64, intro2_rate_per_sec);
+    goto end;
+  }
+#endif
+
+  /* Check that received value is not above maximum */
+  if (intro2_rate_per_sec > HS_CONFIG_V3_DOS_DEFENSE_RATE_PER_SEC_MAX) {
+    log_fn(LOG_PROTOCOL_WARN, LD_REND,
+           "Intro point DoS defenses rate per second is "
+           "too big. Received value: %" PRIu64, intro2_rate_per_sec);
+    goto end;
+  }
+
+  /* Check that received value is not below the minimum */
+#if HS_CONFIG_V3_DOS_DEFENSE_BURST_PER_SEC_MIN > 0
+  if (intro2_burst_per_sec < HS_CONFIG_V3_DOS_DEFENSE_BURST_PER_SEC_MIN) {
+    log_fn(LOG_PROTOCOL_WARN, LD_REND,
+           "Intro point DoS defenses burst per second is "
+           "too small. Received value: %" PRIu64, intro2_burst_per_sec);
+    goto end;
+  }
+#endif
+
+  /* Check that received value is not above maximum */
+  if (intro2_burst_per_sec > HS_CONFIG_V3_DOS_DEFENSE_BURST_PER_SEC_MAX) {
+    log_fn(LOG_PROTOCOL_WARN, LD_REND,
+           "Intro point DoS defenses burst per second is "
+           "too big. Received value: %" PRIu64, intro2_burst_per_sec);
+    goto end;
+  }
+
+  /* In a rate limiting scenario, burst can never be smaller than the rate. At
+   * best it can be equal. */
+  if (intro2_burst_per_sec < intro2_rate_per_sec) {
+    log_info(LD_REND, "Intro point DoS defenses burst is smaller than rate. "
+                      "Rate: %" PRIu64 " vs Burst: %" PRIu64,
+             intro2_rate_per_sec, intro2_burst_per_sec);
+    goto end;
+  }
+
+  /* Passing validation. */
+  ret = true;
+
+ end:
+  return ret;
+}
+
+/* Parse the cell DoS extension and apply defenses on the given circuit if
+ * validation passes. If the cell extension is malformed or contains unusable
+ * values, the DoS defenses is disabled on the circuit. */
+static void
+handle_establish_intro_cell_dos_extension(
+                                const trn_cell_extension_field_t *field,
+                                or_circuit_t *circ)
+{
+  ssize_t ret;
+  uint64_t intro2_rate_per_sec = 0, intro2_burst_per_sec = 0;
+  trn_cell_extension_dos_t *dos = NULL;
+
+  tor_assert(field);
+  tor_assert(circ);
+
+  ret = trn_cell_extension_dos_parse(&dos,
+                 trn_cell_extension_field_getconstarray_field(field),
+                 trn_cell_extension_field_getlen_field(field));
+  if (ret < 0) {
+    goto end;
+  }
+
+  for (size_t i = 0; i < trn_cell_extension_dos_get_n_params(dos); i++) {
+    const trn_cell_extension_dos_param_t *param =
+      trn_cell_extension_dos_getconst_params(dos, i);
+    if (BUG(param == NULL)) {
+      goto end;
+    }
+
+    switch (trn_cell_extension_dos_param_get_type(param)) {
+    case TRUNNEL_DOS_PARAM_TYPE_INTRO2_RATE_PER_SEC:
+      intro2_rate_per_sec = trn_cell_extension_dos_param_get_value(param);
+      break;
+    case TRUNNEL_DOS_PARAM_TYPE_INTRO2_BURST_PER_SEC:
+      intro2_burst_per_sec = trn_cell_extension_dos_param_get_value(param);
+      break;
+    default:
+      goto end;
+    }
+  }
+
+  /* A value of 0 is valid in the sense that we accept it but we still disable
+   * the defenses so return false. */
+  if (intro2_rate_per_sec == 0 || intro2_burst_per_sec == 0) {
+    log_info(LD_REND, "Intro point DoS defenses parameter set to 0. "
+                      "Disabling INTRO2 DoS defenses on circuit id %u",
+             circ->p_circ_id);
+    circ->introduce2_dos_defense_enabled = 0;
+    goto end;
+  }
+
+  /* If invalid, we disable the defense on the circuit. */
+  if (!cell_dos_extension_parameters_are_valid(intro2_rate_per_sec,
+                                               intro2_burst_per_sec)) {
+    circ->introduce2_dos_defense_enabled = 0;
+    log_info(LD_REND, "Disabling INTRO2 DoS defenses on circuit id %u",
+             circ->p_circ_id);
+    goto end;
+  }
+
+  /* We passed validation, enable defenses and apply rate/burst. */
+  circ->introduce2_dos_defense_enabled = 1;
+
+  /* Initialize the INTRODUCE2 token bucket for the rate limiting. */
+  token_bucket_ctr_init(&circ->introduce2_bucket,
+                        (uint32_t) intro2_rate_per_sec,
+                        (uint32_t) intro2_burst_per_sec,
+                        (uint32_t) approx_time());
+  log_info(LD_REND, "Intro point DoS defenses enabled. Rate is %" PRIu64
+                    " and Burst is %" PRIu64,
+           intro2_rate_per_sec, intro2_burst_per_sec);
+
+ end:
+  trn_cell_extension_dos_free(dos);
+  return;
+}
+
+/* Parse every cell extension in the given ESTABLISH_INTRO cell. */
+static void
+handle_establish_intro_cell_extensions(
+                            const trn_cell_establish_intro_t *parsed_cell,
+                            or_circuit_t *circ)
+{
+  const trn_cell_extension_t *extensions;
+
+  tor_assert(parsed_cell);
+  tor_assert(circ);
+
+  extensions = trn_cell_establish_intro_getconst_extensions(parsed_cell);
+  if (extensions == NULL) {
+    goto end;
+  }
+
+  /* Go over all extensions. */
+  for (size_t idx = 0; idx < trn_cell_extension_get_num(extensions); idx++) {
+    const trn_cell_extension_field_t *field =
+      trn_cell_extension_getconst_fields(extensions, idx);
+    if (BUG(field == NULL)) {
+      /* The number of extensions should match the number of fields. */
+      break;
+    }
+
+    switch (trn_cell_extension_field_get_field_type(field)) {
+    case TRUNNEL_CELL_EXTENSION_TYPE_DOS:
+      /* After this, the circuit should be set for DoS defenses. */
+      handle_establish_intro_cell_dos_extension(field, circ);
+      break;
+    default:
+      /* Unknown extension. Skip over. */
+      break;
+    }
+  }
+
+ end:
+  return;
+}
+
 /** We received an ESTABLISH_INTRO <b>parsed_cell</b> on <b>circ</b>. It's
  *  well-formed and passed our verifications. Perform appropriate actions to
  *  establish an intro point. */
@@ -193,6 +373,13 @@ handle_verified_establish_intro_cell(or_circuit_t *circ,
   get_auth_key_from_cell(&auth_key, RELAY_COMMAND_ESTABLISH_INTRO,
                          parsed_cell);
 
+  /* Setup INTRODUCE2 defenses on the circuit. Must be done before parsing the
+   * cell extension that can possibly change the defenses' values. */
+  hs_dos_setup_default_intro2_defenses(circ);
+
+  /* Handle cell extension if any. */
+  handle_establish_intro_cell_extensions(parsed_cell, circ);
+
   /* Then notify the hidden service that the intro point is established by
      sending an INTRO_ESTABLISHED cell */
   if (hs_intro_send_intro_established_cell(circ)) {
@@ -204,9 +391,6 @@ handle_verified_establish_intro_cell(or_circuit_t *circ,
   hs_circuitmap_register_intro_circ_v3_relay_side(circ, &auth_key);
   /* Repurpose this circuit into an intro circuit. */
   circuit_change_purpose(TO_CIRCUIT(circ), CIRCUIT_PURPOSE_INTRO_POINT);
-  /* Initialize the INTRODUCE2 token bucket for the rate limiting. */
-  token_bucket_ctr_init(&circ->introduce2_bucket, hs_dos_get_intro2_rate(),
-                        hs_dos_get_intro2_burst(), (uint32_t) approx_time());
 
   return 0;
 }
diff --git a/src/feature/hs/hs_intropoint.h b/src/feature/hs/hs_intropoint.h
index e82575f052..94ebf021e4 100644
--- a/src/feature/hs/hs_intropoint.h
+++ b/src/feature/hs/hs_intropoint.h
@@ -57,6 +57,9 @@ STATIC int handle_introduce1(or_circuit_t *client_circ,
                              const uint8_t *request, size_t request_len);
 STATIC int validate_introduce1_parsed_cell(const trn_cell_introduce1_t *cell);
 STATIC int circuit_is_suitable_for_introduce1(const or_circuit_t *circ);
+STATIC bool cell_dos_extension_parameters_are_valid(
+                                          uint64_t intro2_rate_per_sec,
+                                          uint64_t intro2_burst_per_sec);
 
 #endif /* defined(HS_INTROPOINT_PRIVATE) */
 
diff --git a/src/feature/hs/hs_service.c b/src/feature/hs/hs_service.c
index 2835912742..7021190903 100644
--- a/src/feature/hs/hs_service.c
+++ b/src/feature/hs/hs_service.c
@@ -242,6 +242,9 @@ set_service_default_config(hs_service_config_t *c,
   c->is_single_onion = 0;
   c->dir_group_readable = 0;
   c->is_ephemeral = 0;
+  c->has_dos_defense_enabled = HS_CONFIG_V3_DOS_DEFENSE_DEFAULT;
+  c->intro_dos_rate_per_sec = HS_CONFIG_V3_DOS_DEFENSE_RATE_PER_SEC_DEFAULT;
+  c->intro_dos_burst_per_sec = HS_CONFIG_V3_DOS_DEFENSE_BURST_PER_SEC_DEFAULT;
 }
 
 /* From a service configuration object config, clear everything from it
@@ -489,6 +492,10 @@ service_intro_point_new(const node_t *node)
     }
   }
 
+  /* Flag if this intro point supports the INTRO2 dos defenses. */
+  ip->support_intro2_dos_defense =
+    node_supports_establish_intro_dos_extension(node);
+
   /* Finally, copy onion key from the node. */
   memcpy(&ip->onion_key, node_get_curve25519_onion_key(node),
          sizeof(ip->onion_key));
diff --git a/src/feature/hs/hs_service.h b/src/feature/hs/hs_service.h
index 22aa00b2d7..c4bbb293bb 100644
--- a/src/feature/hs/hs_service.h
+++ b/src/feature/hs/hs_service.h
@@ -76,6 +76,10 @@ typedef struct hs_service_intro_point_t {
    * circuit associated with this intro point has received. This is used to
    * prevent replay attacks. */
   replaycache_t *replay_cache;
+
+  /* Support the INTRO2 DoS defense. If set, the DoS extension described by
+   * proposal 305 is sent. */
+  unsigned int support_intro2_dos_defense : 1;
 } hs_service_intro_point_t;
 
 /* Object handling introduction points of a service. */
@@ -241,6 +245,11 @@ typedef struct hs_service_config_t {
 
   /* Does this service export the circuit ID of its clients? */
   hs_circuit_id_protocol_t circuit_id_protocol;
+
+  /* DoS defenses. For the ESTABLISH_INTRO cell extension. */
+  unsigned int has_dos_defense_enabled : 1;
+  uint32_t intro_dos_rate_per_sec;
+  uint32_t intro_dos_burst_per_sec;
 } hs_service_config_t;
 
 /* Service state. */
diff --git a/src/feature/nodelist/nodelist.c b/src/feature/nodelist/nodelist.c
index 6ae8d2bcb1..bd80fc1b4b 100644
--- a/src/feature/nodelist/nodelist.c
+++ b/src/feature/nodelist/nodelist.c
@@ -1106,7 +1106,7 @@ node_ed25519_id_matches(const node_t *node, const ed25519_public_key_t *id)
 /** Dummy object that should be unreturnable.  Used to ensure that
  * node_get_protover_summary_flags() always returns non-NULL. */
 static const protover_summary_flags_t zero_protover_flags = {
-  0,0,0,0,0,0,0,0
+  0,0,0,0,0,0,0,0,0
 };
 
 /** Return the protover_summary_flags for a given node. */
@@ -1166,6 +1166,17 @@ node_supports_ed25519_hs_intro(const node_t *node)
   return node_get_protover_summary_flags(node)->supports_ed25519_hs_intro;
 }
 
+/** Return true iff <b>node</b> supports the DoS ESTABLISH_INTRO cell
+ * extenstion. */
+int
+node_supports_establish_intro_dos_extension(const node_t *node)
+{
+  tor_assert(node);
+
+  return node_get_protover_summary_flags(node)->
+                           supports_establish_intro_dos_extension;
+}
+
 /** Return true iff <b>node</b> supports to be a rendezvous point for hidden
  * service version 3 (HSRend=2). */
 int
diff --git a/src/feature/nodelist/nodelist.h b/src/feature/nodelist/nodelist.h
index 84ab5f7a54..af144c197f 100644
--- a/src/feature/nodelist/nodelist.h
+++ b/src/feature/nodelist/nodelist.h
@@ -76,6 +76,7 @@ int node_supports_ed25519_link_authentication(const node_t *node,
 int node_supports_v3_hsdir(const node_t *node);
 int node_supports_ed25519_hs_intro(const node_t *node);
 int node_supports_v3_rendezvous_point(const node_t *node);
+int node_supports_establish_intro_dos_extension(const node_t *node);
 const uint8_t *node_get_rsa_id_digest(const node_t *node);
 smartlist_t *node_get_link_specifier_smartlist(const node_t *node,
                                                bool direct_conn);
diff --git a/src/feature/rend/rendmid.c b/src/feature/rend/rendmid.c
index bc86df632c..06471b2a7f 100644
--- a/src/feature/rend/rendmid.c
+++ b/src/feature/rend/rendmid.c
@@ -118,8 +118,7 @@ rend_mid_establish_intro_legacy(or_circuit_t *circ, const uint8_t *request,
   /* Now, set up this circuit. */
   circuit_change_purpose(TO_CIRCUIT(circ), CIRCUIT_PURPOSE_INTRO_POINT);
   hs_circuitmap_register_intro_circ_v2_relay_side(circ, (uint8_t *)pk_digest);
-  token_bucket_ctr_init(&circ->introduce2_bucket, hs_dos_get_intro2_rate(),
-                        hs_dos_get_intro2_burst(), (uint32_t) approx_time());
+  hs_dos_setup_default_intro2_defenses(circ);
 
   log_info(LD_REND,
            "Established introduction point on circuit %u for service %s",
diff --git a/src/test/test_hs_cell.c b/src/test/test_hs_cell.c
index cdcbe23e69..403509fbc8 100644
--- a/src/test/test_hs_cell.c
+++ b/src/test/test_hs_cell.c
@@ -20,6 +20,7 @@
 #include "feature/hs/hs_service.h"
 
 /* Trunnel. */
+#include "trunnel/hs/cell_common.h"
 #include "trunnel/hs/cell_establish_intro.h"
 
 /** We simulate the creation of an outgoing ESTABLISH_INTRO cell, and then we
@@ -38,11 +39,13 @@ test_gen_establish_intro_cell(void *arg)
   /* Create outgoing ESTABLISH_INTRO cell and extract its payload so that we
      attempt to parse it. */
   {
+    hs_service_config_t config;
+    memset(&config, 0, sizeof(config));
     /* We only need the auth key pair here. */
     hs_service_intro_point_t *ip = service_intro_point_new(NULL);
     /* Auth key pair is generated in the constructor so we are all set for
      * using this IP object. */
-    ret = hs_cell_build_establish_intro(circ_nonce, ip, buf);
+    ret = hs_cell_build_establish_intro(circ_nonce, &config, ip, buf);
     service_intro_point_free(ip);
     tt_u64_op(ret, OP_GT, 0);
   }
@@ -97,6 +100,9 @@ test_gen_establish_intro_cell_bad(void *arg)
   trn_cell_establish_intro_t *cell = NULL;
   char circ_nonce[DIGEST_LEN] = {0};
   hs_service_intro_point_t *ip = NULL;
+  hs_service_config_t config;
+
+  memset(&config, 0, sizeof(config));
 
   MOCK(ed25519_sign_prefixed, mock_ed25519_sign_prefixed);
 
@@ -108,7 +114,7 @@ test_gen_establish_intro_cell_bad(void *arg)
   cell = trn_cell_establish_intro_new();
   tt_assert(cell);
   ip = service_intro_point_new(NULL);
-  cell_len = hs_cell_build_establish_intro(circ_nonce, ip, NULL);
+  cell_len = hs_cell_build_establish_intro(circ_nonce, &config, ip, NULL);
   service_intro_point_free(ip);
   expect_log_msg_containing("Unable to make signature for "
                             "ESTABLISH_INTRO cell.");
@@ -120,11 +126,97 @@ test_gen_establish_intro_cell_bad(void *arg)
   UNMOCK(ed25519_sign_prefixed);
 }
 
+static void
+test_gen_establish_intro_dos_ext(void *arg)
+{
+  ssize_t ret;
+  hs_service_config_t config;
+  hs_service_intro_point_t *ip = NULL;
+  trn_cell_extension_t *extensions = NULL;
+  trn_cell_extension_dos_t *dos = NULL;
+
+  (void) arg;
+
+  memset(&config, 0, sizeof(config));
+  ip = service_intro_point_new(NULL);
+  tt_assert(ip);
+  ip->support_intro2_dos_defense = 1;
+
+  /* Case 1: No DoS parameters so no extension to be built. */
+  extensions = build_establish_intro_extensions(&config, ip);
+  tt_int_op(trn_cell_extension_get_num(extensions), OP_EQ, 0);
+  trn_cell_extension_free(extensions);
+  extensions = NULL;
+
+  /* Case 2: Enable the DoS extension. Parameter set to 0 should indicate to
+   * disable the defense on the intro point but there should be an extension
+   * nonetheless in the cell. */
+  config.has_dos_defense_enabled = 1;
+  extensions = build_establish_intro_extensions(&config, ip);
+  tt_int_op(trn_cell_extension_get_num(extensions), OP_EQ, 1);
+  /* Validate the extension. */
+  const trn_cell_extension_field_t *field =
+    trn_cell_extension_getconst_fields(extensions, 0);
+  tt_int_op(trn_cell_extension_field_get_field_type(field), OP_EQ,
+            TRUNNEL_CELL_EXTENSION_TYPE_DOS);
+  ret = trn_cell_extension_dos_parse(&dos,
+                 trn_cell_extension_field_getconstarray_field(field),
+                 trn_cell_extension_field_getlen_field(field));
+  tt_int_op(ret, OP_EQ, 19);
+  /* Rate per sec param. */
+  const trn_cell_extension_dos_param_t *param =
+    trn_cell_extension_dos_getconst_params(dos, 0);
+  tt_int_op(trn_cell_extension_dos_param_get_type(param), OP_EQ,
+            TRUNNEL_DOS_PARAM_TYPE_INTRO2_RATE_PER_SEC);
+  tt_u64_op(trn_cell_extension_dos_param_get_value(param), OP_EQ, 0);
+  /* Burst per sec param. */
+  param = trn_cell_extension_dos_getconst_params(dos, 1);
+  tt_int_op(trn_cell_extension_dos_param_get_type(param), OP_EQ,
+            TRUNNEL_DOS_PARAM_TYPE_INTRO2_BURST_PER_SEC);
+  tt_u64_op(trn_cell_extension_dos_param_get_value(param), OP_EQ, 0);
+  trn_cell_extension_dos_free(dos); dos = NULL;
+  trn_cell_extension_free(extensions); extensions = NULL;
+
+  /* Case 3: Enable the DoS extension. Parameter set to some normal values. */
+  config.has_dos_defense_enabled = 1;
+  config.intro_dos_rate_per_sec = 42;
+  config.intro_dos_burst_per_sec = 250;
+  extensions = build_establish_intro_extensions(&config, ip);
+  tt_int_op(trn_cell_extension_get_num(extensions), OP_EQ, 1);
+  /* Validate the extension. */
+  field = trn_cell_extension_getconst_fields(extensions, 0);
+  tt_int_op(trn_cell_extension_field_get_field_type(field), OP_EQ,
+            TRUNNEL_CELL_EXTENSION_TYPE_DOS);
+  ret = trn_cell_extension_dos_parse(&dos,
+                 trn_cell_extension_field_getconstarray_field(field),
+                 trn_cell_extension_field_getlen_field(field));
+  tt_int_op(ret, OP_EQ, 19);
+  /* Rate per sec param. */
+  param = trn_cell_extension_dos_getconst_params(dos, 0);
+  tt_int_op(trn_cell_extension_dos_param_get_type(param), OP_EQ,
+            TRUNNEL_DOS_PARAM_TYPE_INTRO2_RATE_PER_SEC);
+  tt_u64_op(trn_cell_extension_dos_param_get_value(param), OP_EQ, 42);
+  /* Burst per sec param. */
+  param = trn_cell_extension_dos_getconst_params(dos, 1);
+  tt_int_op(trn_cell_extension_dos_param_get_type(param), OP_EQ,
+            TRUNNEL_DOS_PARAM_TYPE_INTRO2_BURST_PER_SEC);
+  tt_u64_op(trn_cell_extension_dos_param_get_value(param), OP_EQ, 250);
+  trn_cell_extension_dos_free(dos); dos = NULL;
+  trn_cell_extension_free(extensions); extensions = NULL;
+
+ done:
+  service_intro_point_free(ip);
+  trn_cell_extension_dos_free(dos);
+  trn_cell_extension_free(extensions);
+}
+
 struct testcase_t hs_cell_tests[] = {
   { "gen_establish_intro_cell", test_gen_establish_intro_cell, TT_FORK,
     NULL, NULL },
   { "gen_establish_intro_cell_bad", test_gen_establish_intro_cell_bad, TT_FORK,
     NULL, NULL },
+  { "gen_establish_intro_dos_ext", test_gen_establish_intro_dos_ext, TT_FORK,
+    NULL, NULL },
 
   END_OF_TESTCASES
 };
diff --git a/src/test/test_hs_config.c b/src/test/test_hs_config.c
index c2c556307d..2b3afbb6e9 100644
--- a/src/test/test_hs_config.c
+++ b/src/test/test_hs_config.c
@@ -489,6 +489,111 @@ test_staging_service_v3(void *arg)
   hs_free_all();
 }
 
+static void
+test_dos_parameters(void *arg)
+{
+  int ret;
+
+  (void) arg;
+
+  hs_init();
+
+  /* Valid configuration. */
+  {
+    const char *conf =
+      "HiddenServiceDir /tmp/tor-test-hs-RANDOM/hs3\n"
+      "HiddenServiceVersion 3\n"
+      "HiddenServicePort 22 1.1.1.1:22\n"
+      "HiddenServiceEnableIntroDoSDefense 1\n"
+      "HiddenServiceEnableIntroDoSRatePerSec 42\n"
+      "HiddenServiceEnableIntroDoSBurstPerSec 87\n";
+
+    setup_full_capture_of_logs(LOG_INFO);
+    ret = helper_config_service(conf, 0);
+    tt_int_op(ret, OP_EQ, 0);
+    expect_log_msg_containing("Service INTRO2 DoS defenses rate set to: 42");
+    expect_log_msg_containing("Service INTRO2 DoS defenses burst set to: 87");
+    teardown_capture_of_logs();
+  }
+
+  /* Invalid rate. Value of 2^37. Max allowed is 2^31. */
+  {
+    const char *conf =
+      "HiddenServiceDir /tmp/tor-test-hs-RANDOM/hs3\n"
+      "HiddenServiceVersion 3\n"
+      "HiddenServicePort 22 1.1.1.1:22\n"
+      "HiddenServiceEnableIntroDoSDefense 1\n"
+      "HiddenServiceEnableIntroDoSRatePerSec 137438953472\n"
+      "HiddenServiceEnableIntroDoSBurstPerSec 87\n";
+
+    setup_full_capture_of_logs(LOG_WARN);
+    ret = helper_config_service(conf, 0);
+    tt_int_op(ret, OP_EQ, -1);
+    expect_log_msg_containing("HiddenServiceEnableIntroDoSRatePerSec must "
+                              "be between 0 and 2147483647, "
+                              "not 137438953472");
+    teardown_capture_of_logs();
+  }
+
+  /* Invalid burst. Value of 2^38. Max allowed is 2^31. */
+  {
+    const char *conf =
+      "HiddenServiceDir /tmp/tor-test-hs-RANDOM/hs3\n"
+      "HiddenServiceVersion 3\n"
+      "HiddenServicePort 22 1.1.1.1:22\n"
+      "HiddenServiceEnableIntroDoSDefense 1\n"
+      "HiddenServiceEnableIntroDoSRatePerSec 42\n"
+      "HiddenServiceEnableIntroDoSBurstPerSec 274877906944\n";
+
+    setup_full_capture_of_logs(LOG_WARN);
+    ret = helper_config_service(conf, 0);
+    tt_int_op(ret, OP_EQ, -1);
+    expect_log_msg_containing("HiddenServiceEnableIntroDoSBurstPerSec must "
+                              "be between 0 and 2147483647, "
+                              "not 274877906944");
+    teardown_capture_of_logs();
+  }
+
+  /* Burst is smaller than rate. */
+  {
+    const char *conf =
+      "HiddenServiceDir /tmp/tor-test-hs-RANDOM/hs3\n"
+      "HiddenServiceVersion 3\n"
+      "HiddenServicePort 22 1.1.1.1:22\n"
+      "HiddenServiceEnableIntroDoSDefense 1\n"
+      "HiddenServiceEnableIntroDoSRatePerSec 42\n"
+      "HiddenServiceEnableIntroDoSBurstPerSec 27\n";
+
+    setup_full_capture_of_logs(LOG_WARN);
+    ret = helper_config_service(conf, 0);
+    tt_int_op(ret, OP_EQ, -1);
+    expect_log_msg_containing("Hidden service DoS defenses burst (27) can "
+                              "not be smaller than the rate value (42).");
+    teardown_capture_of_logs();
+  }
+
+  /* Negative value. */
+  {
+    const char *conf =
+      "HiddenServiceDir /tmp/tor-test-hs-RANDOM/hs3\n"
+      "HiddenServiceVersion 3\n"
+      "HiddenServicePort 22 1.1.1.1:22\n"
+      "HiddenServiceEnableIntroDoSDefense 1\n"
+      "HiddenServiceEnableIntroDoSRatePerSec -1\n"
+      "HiddenServiceEnableIntroDoSBurstPerSec 42\n";
+
+    setup_full_capture_of_logs(LOG_WARN);
+    ret = helper_config_service(conf, 0);
+    tt_int_op(ret, OP_EQ, -1);
+    expect_log_msg_containing("HiddenServiceEnableIntroDoSRatePerSec must be "
+                              "between 0 and 2147483647, not -1");
+    teardown_capture_of_logs();
+  }
+
+ done:
+  hs_free_all();
+}
+
 struct testcase_t hs_config_tests[] = {
   /* Invalid service not specific to any version. */
   { "invalid_service", test_invalid_service, TT_FORK,
@@ -512,6 +617,10 @@ struct testcase_t hs_config_tests[] = {
   { "staging_service_v3", test_staging_service_v3, TT_FORK,
     NULL, NULL },
 
+  /* Test HS DoS parameters. */
+  { "dos_parameters", test_dos_parameters, TT_FORK,
+    NULL, NULL },
+
   END_OF_TESTCASES
 };
 
diff --git a/src/test/test_hs_dos.c b/src/test/test_hs_dos.c
index 3dfa057a4a..f68639e24a 100644
--- a/src/test/test_hs_dos.c
+++ b/src/test/test_hs_dos.c
@@ -8,6 +8,8 @@
 
 #define CIRCUITLIST_PRIVATE
 #define NETWORKSTATUS_PRIVATE
+#define HS_DOS_PRIVATE
+#define HS_INTROPOINT_PRIVATE
 
 #include "test/test.h"
 #include "test/test_helpers.h"
@@ -20,6 +22,7 @@
 #include "core/or/or_circuit_st.h"
 
 #include "feature/hs/hs_dos.h"
+#include "feature/hs/hs_intropoint.h"
 #include "feature/nodelist/networkstatus.h"
 
 static void
@@ -57,9 +60,8 @@ test_can_send_intro2(void *arg)
 
   /* Make that circuit a service intro point. */
   circuit_change_purpose(TO_CIRCUIT(or_circ), CIRCUIT_PURPOSE_INTRO_POINT);
-  /* Initialize the INTRODUCE2 token bucket for the rate limiting. */
-  token_bucket_ctr_init(&or_circ->introduce2_bucket, hs_dos_get_intro2_rate(),
-                        hs_dos_get_intro2_burst(), now);
+  hs_dos_setup_default_intro2_defenses(or_circ);
+  or_circ->introduce2_dos_defense_enabled = 1;
 
   /* Brand new circuit, we should be able to send INTRODUCE2 cells. */
   tt_int_op(true, OP_EQ, hs_dos_can_send_intro2(or_circ));
@@ -71,13 +73,13 @@ test_can_send_intro2(void *arg)
     tt_int_op(true, OP_EQ, hs_dos_can_send_intro2(or_circ));
   }
   tt_uint_op(token_bucket_ctr_get(&or_circ->introduce2_bucket), OP_EQ,
-             hs_dos_get_intro2_burst() - 10);
+             get_intro2_burst_consensus_param(NULL) - 10);
 
   /* Fully refill the bucket minus 1 cell. */
   update_approx_time(++now);
   tt_int_op(true, OP_EQ, hs_dos_can_send_intro2(or_circ));
   tt_uint_op(token_bucket_ctr_get(&or_circ->introduce2_bucket), OP_EQ,
-             hs_dos_get_intro2_burst() - 1);
+             get_intro2_burst_consensus_param(NULL) - 1);
 
   /* Receive an INTRODUCE2 at each second. We should have the bucket full
    * since at every second it gets refilled. */
@@ -87,18 +89,18 @@ test_can_send_intro2(void *arg)
   }
   /* Last check if we can send the cell decrements the bucket so minus 1. */
   tt_uint_op(token_bucket_ctr_get(&or_circ->introduce2_bucket), OP_EQ,
-             hs_dos_get_intro2_burst() - 1);
+             get_intro2_burst_consensus_param(NULL) - 1);
 
   /* Manually reset bucket for next test. */
   token_bucket_ctr_reset(&or_circ->introduce2_bucket, now);
   tt_uint_op(token_bucket_ctr_get(&or_circ->introduce2_bucket), OP_EQ,
-             hs_dos_get_intro2_burst());
+             get_intro2_burst_consensus_param(NULL));
 
   /* Do a full burst in the current second which should empty the bucket and
    * we shouldn't be allowed to send one more cell after that. We go minus 1
    * cell else the very last check if we can send the INTRO2 cell returns
    * false because the bucket goes down to 0. */
-  for (uint32_t i = 0; i < hs_dos_get_intro2_burst() - 1; i++) {
+  for (uint32_t i = 0; i < get_intro2_burst_consensus_param(NULL) - 1; i++) {
     tt_int_op(true, OP_EQ, hs_dos_can_send_intro2(or_circ));
   }
   tt_uint_op(token_bucket_ctr_get(&or_circ->introduce2_bucket), OP_EQ, 1);
@@ -116,7 +118,7 @@ test_can_send_intro2(void *arg)
   update_approx_time(++now);
   tt_int_op(true, OP_EQ, hs_dos_can_send_intro2(or_circ));
   tt_uint_op(token_bucket_ctr_get(&or_circ->introduce2_bucket), OP_EQ,
-             hs_dos_get_intro2_rate() - 1);
+             get_intro2_rate_consensus_param(NULL) - 1);
 
  done:
   circuit_free_(TO_CIRCUIT(or_circ));
@@ -125,10 +127,50 @@ test_can_send_intro2(void *arg)
   free_mock_consensus();
 }
 
+static void
+test_validate_dos_extension_params(void *arg)
+{
+  bool ret;
+
+  (void) arg;
+
+  /* Validate the default values. */
+  ret = cell_dos_extension_parameters_are_valid(
+                                      get_intro2_rate_consensus_param(NULL),
+                                      get_intro2_burst_consensus_param(NULL));
+  tt_assert(ret);
+
+  /* Valid custom rate/burst. */
+  ret = cell_dos_extension_parameters_are_valid(17, 42);
+  tt_assert(ret);
+  ret = cell_dos_extension_parameters_are_valid(INT32_MAX, INT32_MAX);
+  tt_assert(ret);
+
+  /* Invalid rate. */
+  ret = cell_dos_extension_parameters_are_valid(UINT64_MAX, 42);
+  tt_assert(!ret);
+
+  /* Invalid burst. */
+  ret = cell_dos_extension_parameters_are_valid(42, UINT64_MAX);
+  tt_assert(!ret);
+
+  /* Value of 0 is valid (but should disable defenses) */
+  ret = cell_dos_extension_parameters_are_valid(0, 0);
+  tt_assert(ret);
+
+  /* Can't have burst smaller than rate. */
+  ret = cell_dos_extension_parameters_are_valid(42, 40);
+  tt_assert(!ret);
+
+ done:
+  return;
+}
+
 struct testcase_t hs_dos_tests[] = {
   { "can_send_intro2", test_can_send_intro2, TT_FORK,
     NULL, NULL },
+  { "validate_dos_extension_params", test_validate_dos_extension_params,
+    TT_FORK, NULL, NULL },
 
   END_OF_TESTCASES
 };
-
diff --git a/src/test/test_hs_intropoint.c b/src/test/test_hs_intropoint.c
index 7b01809f96..feb934d93c 100644
--- a/src/test/test_hs_intropoint.c
+++ b/src/test/test_hs_intropoint.c
@@ -26,6 +26,7 @@
 #include "feature/hs/hs_cell.h"
 #include "feature/hs/hs_circuitmap.h"
 #include "feature/hs/hs_common.h"
+#include "feature/hs/hs_config.h"
 #include "feature/hs/hs_dos.h"
 #include "feature/hs/hs_intropoint.h"
 #include "feature/hs/hs_service.h"
@@ -45,6 +46,9 @@ new_establish_intro_cell(const char *circ_nonce,
   uint8_t buf[RELAY_PAYLOAD_SIZE] = {0};
   trn_cell_establish_intro_t *cell = NULL;
   hs_service_intro_point_t *ip = NULL;
+  hs_service_config_t config;
+
+  memset(&config, 0, sizeof(config));
 
   /* Ensure that *cell_out is NULL such that we can use to check if we need to
    * free `cell` in case of an error. */
@@ -54,7 +58,7 @@ new_establish_intro_cell(const char *circ_nonce,
    * using this IP object. */
   ip = service_intro_point_new(NULL);
   tt_assert(ip);
-  cell_len = hs_cell_build_establish_intro(circ_nonce, ip, buf);
+  cell_len = hs_cell_build_establish_intro(circ_nonce, &config, ip, buf);
   tt_i64_op(cell_len, OP_GT, 0);
 
   cell_len = trn_cell_establish_intro_parse(&cell, buf, sizeof(buf));
@@ -75,12 +79,15 @@ new_establish_intro_encoded_cell(const char *circ_nonce, uint8_t *cell_out)
 {
   ssize_t cell_len = 0;
   hs_service_intro_point_t *ip = NULL;
+  hs_service_config_t config;
+
+  memset(&config, 0, sizeof(config));
 
   /* Auth key pair is generated in the constructor so we are all set for
    * using this IP object. */
   ip = service_intro_point_new(NULL);
   tt_assert(ip);
-  cell_len = hs_cell_build_establish_intro(circ_nonce, ip, cell_out);
+  cell_len = hs_cell_build_establish_intro(circ_nonce, &config, ip, cell_out);
   tt_i64_op(cell_len, OP_GT, 0);
 
  done:
@@ -903,6 +910,153 @@ test_received_introduce1_handling(void *arg)
   UNMOCK(relay_send_command_from_edge_);
 }
 
+static void
+test_received_establish_intro_dos_ext(void *arg)
+{
+  int ret;
+  ssize_t cell_len = 0;
+  uint8_t cell[RELAY_PAYLOAD_SIZE] = {0};
+  char circ_nonce[DIGEST_LEN] = {0};
+  hs_service_intro_point_t *ip = NULL;
+  hs_service_config_t config;
+  or_circuit_t *intro_circ = or_circuit_new(0,NULL);
+
+  (void) arg;
+
+  MOCK(relay_send_command_from_edge_, mock_relay_send_command_from_edge);
+
+  hs_circuitmap_init();
+
+  /* Setup. */
+  crypto_rand(circ_nonce, sizeof(circ_nonce));
+  ip = service_intro_point_new(NULL);
+  tt_assert(ip);
+  ip->support_intro2_dos_defense = 1;
+  memset(&config, 0, sizeof(config));
+  config.has_dos_defense_enabled = 1;
+  config.intro_dos_rate_per_sec = 13;
+  config.intro_dos_burst_per_sec = 42;
+  helper_prepare_circ_for_intro(intro_circ, circ_nonce);
+  /* The INTRO2 bucket should be 0 at this point. */
+  tt_u64_op(token_bucket_ctr_get(&intro_circ->introduce2_bucket), OP_EQ, 0);
+  tt_u64_op(intro_circ->introduce2_bucket.cfg.rate, OP_EQ, 0);
+  tt_int_op(intro_circ->introduce2_bucket.cfg.burst, OP_EQ, 0);
+  tt_int_op(intro_circ->introduce2_dos_defense_enabled, OP_EQ, 0);
+
+  /* Case 1: Build encoded cell. Usable DoS parameters. */
+  cell_len = hs_cell_build_establish_intro(circ_nonce, &config, ip, cell);
+  tt_size_op(cell_len, OP_GT, 0);
+  /* Pass it to the intro point. */
+  ret = hs_intro_received_establish_intro(intro_circ, cell, cell_len);
+  tt_int_op(ret, OP_EQ, 0);
+  /* Should be set to the burst value. */
+  tt_u64_op(token_bucket_ctr_get(&intro_circ->introduce2_bucket), OP_EQ, 42);
+  /* Validate the config of the intro2 bucket. */
+  tt_u64_op(intro_circ->introduce2_bucket.cfg.rate, OP_EQ, 13);
+  tt_int_op(intro_circ->introduce2_bucket.cfg.burst, OP_EQ, 42);
+  tt_int_op(intro_circ->introduce2_dos_defense_enabled, OP_EQ, 1);
+
+  /* Need to reset the circuit in between test cases. */
+  circuit_free_(TO_CIRCUIT(intro_circ));
+  intro_circ = or_circuit_new(0,NULL);
+  helper_prepare_circ_for_intro(intro_circ, circ_nonce);
+
+  /* Case 2: Build encoded cell. Bad DoS parameters. */
+  config.has_dos_defense_enabled = 1;
+  config.intro_dos_rate_per_sec = UINT_MAX;
+  config.intro_dos_burst_per_sec = 13;
+  cell_len = hs_cell_build_establish_intro(circ_nonce, &config, ip, cell);
+  tt_size_op(cell_len, OP_GT, 0);
+  /* Pass it to the intro point. */
+  ret = hs_intro_received_establish_intro(intro_circ, cell, cell_len);
+  tt_int_op(ret, OP_EQ, 0);
+  tt_u64_op(token_bucket_ctr_get(&intro_circ->introduce2_bucket), OP_EQ,
+            HS_CONFIG_V3_DOS_DEFENSE_BURST_PER_SEC_DEFAULT);
+  tt_u64_op(intro_circ->introduce2_bucket.cfg.rate, OP_EQ,
+            HS_CONFIG_V3_DOS_DEFENSE_RATE_PER_SEC_DEFAULT);
+  tt_int_op(intro_circ->introduce2_bucket.cfg.burst, OP_EQ,
+            HS_CONFIG_V3_DOS_DEFENSE_BURST_PER_SEC_DEFAULT);
+  tt_int_op(intro_circ->introduce2_dos_defense_enabled, OP_EQ,
+            HS_CONFIG_V3_DOS_DEFENSE_DEFAULT);
+
+  /* Need to reset the circuit in between test cases. */
+  circuit_free_(TO_CIRCUIT(intro_circ));
+  intro_circ = or_circuit_new(0,NULL);
+  helper_prepare_circ_for_intro(intro_circ, circ_nonce);
+
+  /* Case 3: Build encoded cell. Burst is smaller than rate. Not allowed. */
+  config.has_dos_defense_enabled = 1;
+  config.intro_dos_rate_per_sec = 87;
+  config.intro_dos_burst_per_sec = 45;
+  cell_len = hs_cell_build_establish_intro(circ_nonce, &config, ip, cell);
+  tt_size_op(cell_len, OP_GT, 0);
+  /* Pass it to the intro point. */
+  ret = hs_intro_received_establish_intro(intro_circ, cell, cell_len);
+  tt_int_op(ret, OP_EQ, 0);
+  tt_u64_op(token_bucket_ctr_get(&intro_circ->introduce2_bucket), OP_EQ,
+            HS_CONFIG_V3_DOS_DEFENSE_BURST_PER_SEC_DEFAULT);
+  tt_u64_op(intro_circ->introduce2_bucket.cfg.rate, OP_EQ,
+            HS_CONFIG_V3_DOS_DEFENSE_RATE_PER_SEC_DEFAULT);
+  tt_int_op(intro_circ->introduce2_bucket.cfg.burst, OP_EQ,
+            HS_CONFIG_V3_DOS_DEFENSE_BURST_PER_SEC_DEFAULT);
+  tt_int_op(intro_circ->introduce2_dos_defense_enabled, OP_EQ,
+            HS_CONFIG_V3_DOS_DEFENSE_DEFAULT);
+
+  /* Need to reset the circuit in between test cases. */
+  circuit_free_(TO_CIRCUIT(intro_circ));
+  intro_circ = or_circuit_new(0,NULL);
+  helper_prepare_circ_for_intro(intro_circ, circ_nonce);
+
+  /* Case 4: Build encoded cell. Rate is 0 but burst is not 0. Disables the
+   * defense. */
+  config.has_dos_defense_enabled = 1;
+  config.intro_dos_rate_per_sec = 0;
+  config.intro_dos_burst_per_sec = 45;
+  cell_len = hs_cell_build_establish_intro(circ_nonce, &config, ip, cell);
+  tt_size_op(cell_len, OP_GT, 0);
+  /* Pass it to the intro point. */
+  ret = hs_intro_received_establish_intro(intro_circ, cell, cell_len);
+  tt_int_op(ret, OP_EQ, 0);
+  tt_u64_op(token_bucket_ctr_get(&intro_circ->introduce2_bucket), OP_EQ,
+            HS_CONFIG_V3_DOS_DEFENSE_BURST_PER_SEC_DEFAULT);
+  tt_u64_op(intro_circ->introduce2_bucket.cfg.rate, OP_EQ,
+            HS_CONFIG_V3_DOS_DEFENSE_RATE_PER_SEC_DEFAULT);
+  tt_int_op(intro_circ->introduce2_bucket.cfg.burst, OP_EQ,
+            HS_CONFIG_V3_DOS_DEFENSE_BURST_PER_SEC_DEFAULT);
+  tt_int_op(intro_circ->introduce2_dos_defense_enabled, OP_EQ,
+            HS_CONFIG_V3_DOS_DEFENSE_DEFAULT);
+
+  /* Need to reset the circuit in between test cases. */
+  circuit_free_(TO_CIRCUIT(intro_circ));
+  intro_circ = or_circuit_new(0,NULL);
+  helper_prepare_circ_for_intro(intro_circ, circ_nonce);
+
+  /* Case 5: Build encoded cell. Burst is 0 but rate is not 0. Disables the
+   * defense. */
+  config.has_dos_defense_enabled = 1;
+  config.intro_dos_rate_per_sec = 45;
+  config.intro_dos_burst_per_sec = 0;
+  cell_len = hs_cell_build_establish_intro(circ_nonce, &config, ip, cell);
+  tt_size_op(cell_len, OP_GT, 0);
+  /* Pass it to the intro point. */
+  ret = hs_intro_received_establish_intro(intro_circ, cell, cell_len);
+  tt_int_op(ret, OP_EQ, 0);
+  tt_u64_op(token_bucket_ctr_get(&intro_circ->introduce2_bucket), OP_EQ,
+            HS_CONFIG_V3_DOS_DEFENSE_BURST_PER_SEC_DEFAULT);
+  tt_u64_op(intro_circ->introduce2_bucket.cfg.rate, OP_EQ,
+            HS_CONFIG_V3_DOS_DEFENSE_RATE_PER_SEC_DEFAULT);
+  tt_int_op(intro_circ->introduce2_bucket.cfg.burst, OP_EQ,
+            HS_CONFIG_V3_DOS_DEFENSE_BURST_PER_SEC_DEFAULT);
+  tt_int_op(intro_circ->introduce2_dos_defense_enabled, OP_EQ,
+            HS_CONFIG_V3_DOS_DEFENSE_DEFAULT);
+
+ done:
+  circuit_free_(TO_CIRCUIT(intro_circ));
+  service_intro_point_free(ip);
+  hs_circuitmap_free_all();
+  UNMOCK(relay_send_command_from_edge_);
+}
+
 static void *
 hs_subsystem_setup_fn(const struct testcase_t *tc)
 {
@@ -961,5 +1115,8 @@ struct testcase_t hs_intropoint_tests[] = {
   { "received_introduce1_handling",
     test_received_introduce1_handling, TT_FORK, NULL, &test_setup},
 
+  { "received_establish_intro_dos_ext",
+    test_received_establish_intro_dos_ext, TT_FORK, NULL, &test_setup},
+
   END_OF_TESTCASES
 };
diff --git a/src/trunnel/hs/cell_common.c b/src/trunnel/hs/cell_common.c
index 830af5c78b..1f50961d69 100644
--- a/src/trunnel/hs/cell_common.c
+++ b/src/trunnel/hs/cell_common.c
@@ -28,10 +28,10 @@ int cellcommon_deadcode_dummy__ = 0;
     }                                                            \
   } while (0)
 
-trn_cell_extension_fields_t *
-trn_cell_extension_fields_new(void)
+trn_cell_extension_field_t *
+trn_cell_extension_field_new(void)
 {
-  trn_cell_extension_fields_t *val = trunnel_calloc(1, sizeof(trn_cell_extension_fields_t));
+  trn_cell_extension_field_t *val = trunnel_calloc(1, sizeof(trn_cell_extension_field_t));
   if (NULL == val)
     return NULL;
   return val;
@@ -40,7 +40,7 @@ trn_cell_extension_fields_new(void)
 /** Release all storage held inside 'obj', but do not free 'obj'.
  */
 static void
-trn_cell_extension_fields_clear(trn_cell_extension_fields_t *obj)
+trn_cell_extension_field_clear(trn_cell_extension_field_t *obj)
 {
   (void) obj;
   TRUNNEL_DYNARRAY_WIPE(&obj->field);
@@ -48,62 +48,62 @@ trn_cell_extension_fields_clear(trn_cell_extension_fields_t *obj)
 }
 
 void
-trn_cell_extension_fields_free(trn_cell_extension_fields_t *obj)
+trn_cell_extension_field_free(trn_cell_extension_field_t *obj)
 {
   if (obj == NULL)
     return;
-  trn_cell_extension_fields_clear(obj);
-  trunnel_memwipe(obj, sizeof(trn_cell_extension_fields_t));
+  trn_cell_extension_field_clear(obj);
+  trunnel_memwipe(obj, sizeof(trn_cell_extension_field_t));
   trunnel_free_(obj);
 }
 
 uint8_t
-trn_cell_extension_fields_get_field_type(const trn_cell_extension_fields_t *inp)
+trn_cell_extension_field_get_field_type(const trn_cell_extension_field_t *inp)
 {
   return inp->field_type;
 }
 int
-trn_cell_extension_fields_set_field_type(trn_cell_extension_fields_t *inp, uint8_t val)
+trn_cell_extension_field_set_field_type(trn_cell_extension_field_t *inp, uint8_t val)
 {
   inp->field_type = val;
   return 0;
 }
 uint8_t
-trn_cell_extension_fields_get_field_len(const trn_cell_extension_fields_t *inp)
+trn_cell_extension_field_get_field_len(const trn_cell_extension_field_t *inp)
 {
   return inp->field_len;
 }
 int
-trn_cell_extension_fields_set_field_len(trn_cell_extension_fields_t *inp, uint8_t val)
+trn_cell_extension_field_set_field_len(trn_cell_extension_field_t *inp, uint8_t val)
 {
   inp->field_len = val;
   return 0;
 }
 size_t
-trn_cell_extension_fields_getlen_field(const trn_cell_extension_fields_t *inp)
+trn_cell_extension_field_getlen_field(const trn_cell_extension_field_t *inp)
 {
   return TRUNNEL_DYNARRAY_LEN(&inp->field);
 }
 
 uint8_t
-trn_cell_extension_fields_get_field(trn_cell_extension_fields_t *inp, size_t idx)
+trn_cell_extension_field_get_field(trn_cell_extension_field_t *inp, size_t idx)
 {
   return TRUNNEL_DYNARRAY_GET(&inp->field, idx);
 }
 
 uint8_t
-trn_cell_extension_fields_getconst_field(const trn_cell_extension_fields_t *inp, size_t idx)
+trn_cell_extension_field_getconst_field(const trn_cell_extension_field_t *inp, size_t idx)
 {
-  return trn_cell_extension_fields_get_field((trn_cell_extension_fields_t*)inp, idx);
+  return trn_cell_extension_field_get_field((trn_cell_extension_field_t*)inp, idx);
 }
 int
-trn_cell_extension_fields_set_field(trn_cell_extension_fields_t *inp, size_t idx, uint8_t elt)
+trn_cell_extension_field_set_field(trn_cell_extension_field_t *inp, size_t idx, uint8_t elt)
 {
   TRUNNEL_DYNARRAY_SET(&inp->field, idx, elt);
   return 0;
 }
 int
-trn_cell_extension_fields_add_field(trn_cell_extension_fields_t *inp, uint8_t elt)
+trn_cell_extension_field_add_field(trn_cell_extension_field_t *inp, uint8_t elt)
 {
 #if SIZE_MAX >= UINT8_MAX
   if (inp->field.n_ == UINT8_MAX)
@@ -117,17 +117,17 @@ trn_cell_extension_fields_add_field(trn_cell_extension_fields_t *inp, uint8_t el
 }
 
 uint8_t *
-trn_cell_extension_fields_getarray_field(trn_cell_extension_fields_t *inp)
+trn_cell_extension_field_getarray_field(trn_cell_extension_field_t *inp)
 {
   return inp->field.elts_;
 }
 const uint8_t  *
-trn_cell_extension_fields_getconstarray_field(const trn_cell_extension_fields_t *inp)
+trn_cell_extension_field_getconstarray_field(const trn_cell_extension_field_t *inp)
 {
-  return (const uint8_t  *)trn_cell_extension_fields_getarray_field((trn_cell_extension_fields_t*)inp);
+  return (const uint8_t  *)trn_cell_extension_field_getarray_field((trn_cell_extension_field_t*)inp);
 }
 int
-trn_cell_extension_fields_setlen_field(trn_cell_extension_fields_t *inp, size_t newlen)
+trn_cell_extension_field_setlen_field(trn_cell_extension_field_t *inp, size_t newlen)
 {
   uint8_t *newptr;
 #if UINT8_MAX < SIZE_MAX
@@ -147,7 +147,7 @@ trn_cell_extension_fields_setlen_field(trn_cell_extension_fields_t *inp, size_t
   return -1;
 }
 const char *
-trn_cell_extension_fields_check(const trn_cell_extension_fields_t *obj)
+trn_cell_extension_field_check(const trn_cell_extension_field_t *obj)
 {
   if (obj == NULL)
     return "Object was NULL";
@@ -159,11 +159,11 @@ trn_cell_extension_fields_check(const trn_cell_extension_fields_t *obj)
 }
 
 ssize_t
-trn_cell_extension_fields_encoded_len(const trn_cell_extension_fields_t *obj)
+trn_cell_extension_field_encoded_len(const trn_cell_extension_field_t *obj)
 {
   ssize_t result = 0;
 
-  if (NULL != trn_cell_extension_fields_check(obj))
+  if (NULL != trn_cell_extension_field_check(obj))
      return -1;
 
 
@@ -178,24 +178,24 @@ trn_cell_extension_fields_encoded_len(const trn_cell_extension_fields_t *obj)
   return result;
 }
 int
-trn_cell_extension_fields_clear_errors(trn_cell_extension_fields_t *obj)
+trn_cell_extension_field_clear_errors(trn_cell_extension_field_t *obj)
 {
   int r = obj->trunnel_error_code_;
   obj->trunnel_error_code_ = 0;
   return r;
 }
 ssize_t
-trn_cell_extension_fields_encode(uint8_t *output, const size_t avail, const trn_cell_extension_fields_t *obj)
+trn_cell_extension_field_encode(uint8_t *output, const size_t avail, const trn_cell_extension_field_t *obj)
 {
   ssize_t result = 0;
   size_t written = 0;
   uint8_t *ptr = output;
   const char *msg;
 #ifdef TRUNNEL_CHECK_ENCODED_LEN
-  const ssize_t encoded_len = trn_cell_extension_fields_encoded_len(obj);
+  const ssize_t encoded_len = trn_cell_extension_field_encoded_len(obj);
 #endif
 
-  if (NULL != (msg = trn_cell_extension_fields_check(obj)))
+  if (NULL != (msg = trn_cell_extension_field_check(obj)))
     goto check_failed;
 
 #ifdef TRUNNEL_CHECK_ENCODED_LEN
@@ -252,11 +252,11 @@ trn_cell_extension_fields_encode(uint8_t *output, const size_t avail, const trn_
   return result;
 }
 
-/** As trn_cell_extension_fields_parse(), but do not allocate the
+/** As trn_cell_extension_field_parse(), but do not allocate the
  * output object.
  */
 static ssize_t
-trn_cell_extension_fields_parse_into(trn_cell_extension_fields_t *obj, const uint8_t *input, const size_t len_in)
+trn_cell_extension_field_parse_into(trn_cell_extension_field_t *obj, const uint8_t *input, const size_t len_in)
 {
   const uint8_t *ptr = input;
   size_t remaining = len_in;
@@ -290,15 +290,15 @@ trn_cell_extension_fields_parse_into(trn_cell_extension_fields_t *obj, const uin
 }
 
 ssize_t
-trn_cell_extension_fields_parse(trn_cell_extension_fields_t **output, const uint8_t *input, const size_t len_in)
+trn_cell_extension_field_parse(trn_cell_extension_field_t **output, const uint8_t *input, const size_t len_in)
 {
   ssize_t result;
-  *output = trn_cell_extension_fields_new();
+  *output = trn_cell_extension_field_new();
   if (NULL == *output)
     return -1;
-  result = trn_cell_extension_fields_parse_into(*output, input, len_in);
+  result = trn_cell_extension_field_parse_into(*output, input, len_in);
   if (result < 0) {
-    trn_cell_extension_fields_free(*output);
+    trn_cell_extension_field_free(*output);
     *output = NULL;
   }
   return result;
@@ -322,7 +322,7 @@ trn_cell_extension_clear(trn_cell_extension_t *obj)
 
     unsigned idx;
     for (idx = 0; idx < TRUNNEL_DYNARRAY_LEN(&obj->fields); ++idx) {
-      trn_cell_extension_fields_free(TRUNNEL_DYNARRAY_GET(&obj->fields, idx));
+      trn_cell_extension_field_free(TRUNNEL_DYNARRAY_GET(&obj->fields, idx));
     }
   }
   TRUNNEL_DYNARRAY_WIPE(&obj->fields);
@@ -356,66 +356,66 @@ trn_cell_extension_getlen_fields(const trn_cell_extension_t *inp)
   return TRUNNEL_DYNARRAY_LEN(&inp->fields);
 }
 
-struct trn_cell_extension_fields_st *
+struct trn_cell_extension_field_st *
 trn_cell_extension_get_fields(trn_cell_extension_t *inp, size_t idx)
 {
   return TRUNNEL_DYNARRAY_GET(&inp->fields, idx);
 }
 
- const struct trn_cell_extension_fields_st *
+ const struct trn_cell_extension_field_st *
 trn_cell_extension_getconst_fields(const trn_cell_extension_t *inp, size_t idx)
 {
   return trn_cell_extension_get_fields((trn_cell_extension_t*)inp, idx);
 }
 int
-trn_cell_extension_set_fields(trn_cell_extension_t *inp, size_t idx, struct trn_cell_extension_fields_st * elt)
+trn_cell_extension_set_fields(trn_cell_extension_t *inp, size_t idx, struct trn_cell_extension_field_st * elt)
 {
-  trn_cell_extension_fields_t *oldval = TRUNNEL_DYNARRAY_GET(&inp->fields, idx);
+  trn_cell_extension_field_t *oldval = TRUNNEL_DYNARRAY_GET(&inp->fields, idx);
   if (oldval && oldval != elt)
-    trn_cell_extension_fields_free(oldval);
+    trn_cell_extension_field_free(oldval);
   return trn_cell_extension_set0_fields(inp, idx, elt);
 }
 int
-trn_cell_extension_set0_fields(trn_cell_extension_t *inp, size_t idx, struct trn_cell_extension_fields_st * elt)
+trn_cell_extension_set0_fields(trn_cell_extension_t *inp, size_t idx, struct trn_cell_extension_field_st * elt)
 {
   TRUNNEL_DYNARRAY_SET(&inp->fields, idx, elt);
   return 0;
 }
 int
-trn_cell_extension_add_fields(trn_cell_extension_t *inp, struct trn_cell_extension_fields_st * elt)
+trn_cell_extension_add_fields(trn_cell_extension_t *inp, struct trn_cell_extension_field_st * elt)
 {
 #if SIZE_MAX >= UINT8_MAX
   if (inp->fields.n_ == UINT8_MAX)
     goto trunnel_alloc_failed;
 #endif
-  TRUNNEL_DYNARRAY_ADD(struct trn_cell_extension_fields_st *, &inp->fields, elt, {});
+  TRUNNEL_DYNARRAY_ADD(struct trn_cell_extension_field_st *, &inp->fields, elt, {});
   return 0;
  trunnel_alloc_failed:
   TRUNNEL_SET_ERROR_CODE(inp);
   return -1;
 }
 
-struct trn_cell_extension_fields_st * *
+struct trn_cell_extension_field_st * *
 trn_cell_extension_getarray_fields(trn_cell_extension_t *inp)
 {
   return inp->fields.elts_;
 }
-const struct trn_cell_extension_fields_st *  const  *
+const struct trn_cell_extension_field_st *  const  *
 trn_cell_extension_getconstarray_fields(const trn_cell_extension_t *inp)
 {
-  return (const struct trn_cell_extension_fields_st *  const  *)trn_cell_extension_getarray_fields((trn_cell_extension_t*)inp);
+  return (const struct trn_cell_extension_field_st *  const  *)trn_cell_extension_getarray_fields((trn_cell_extension_t*)inp);
 }
 int
 trn_cell_extension_setlen_fields(trn_cell_extension_t *inp, size_t newlen)
 {
-  struct trn_cell_extension_fields_st * *newptr;
+  struct trn_cell_extension_field_st * *newptr;
 #if UINT8_MAX < SIZE_MAX
   if (newlen > UINT8_MAX)
     goto trunnel_alloc_failed;
 #endif
   newptr = trunnel_dynarray_setlen(&inp->fields.allocated_,
                  &inp->fields.n_, inp->fields.elts_, newlen,
-                 sizeof(inp->fields.elts_[0]), (trunnel_free_fn_t) trn_cell_extension_fields_free,
+                 sizeof(inp->fields.elts_[0]), (trunnel_free_fn_t) trn_cell_extension_field_free,
                  &inp->trunnel_error_code_);
   if (newlen != 0 && newptr == NULL)
     goto trunnel_alloc_failed;
@@ -437,7 +437,7 @@ trn_cell_extension_check(const trn_cell_extension_t *obj)
 
     unsigned idx;
     for (idx = 0; idx < TRUNNEL_DYNARRAY_LEN(&obj->fields); ++idx) {
-      if (NULL != (msg = trn_cell_extension_fields_check(TRUNNEL_DYNARRAY_GET(&obj->fields, idx))))
+      if (NULL != (msg = trn_cell_extension_field_check(TRUNNEL_DYNARRAY_GET(&obj->fields, idx))))
         return msg;
     }
   }
@@ -458,12 +458,12 @@ trn_cell_extension_encoded_len(const trn_cell_extension_t *obj)
   /* Length of u8 num */
   result += 1;
 
-  /* Length of struct trn_cell_extension_fields fields[num] */
+  /* Length of struct trn_cell_extension_field fields[num] */
   {
 
     unsigned idx;
     for (idx = 0; idx < TRUNNEL_DYNARRAY_LEN(&obj->fields); ++idx) {
-      result += trn_cell_extension_fields_encoded_len(TRUNNEL_DYNARRAY_GET(&obj->fields, idx));
+      result += trn_cell_extension_field_encoded_len(TRUNNEL_DYNARRAY_GET(&obj->fields, idx));
     }
   }
   return result;
@@ -500,13 +500,13 @@ trn_cell_extension_encode(uint8_t *output, const size_t avail, const trn_cell_ex
   trunnel_set_uint8(ptr, (obj->num));
   written += 1; ptr += 1;
 
-  /* Encode struct trn_cell_extension_fields fields[num] */
+  /* Encode struct trn_cell_extension_field fields[num] */
   {
 
     unsigned idx;
     for (idx = 0; idx < TRUNNEL_DYNARRAY_LEN(&obj->fields); ++idx) {
       trunnel_assert(written <= avail);
-      result = trn_cell_extension_fields_encode(ptr, avail - written, TRUNNEL_DYNARRAY_GET(&obj->fields, idx));
+      result = trn_cell_extension_field_encode(ptr, avail - written, TRUNNEL_DYNARRAY_GET(&obj->fields, idx));
       if (result < 0)
         goto fail; /* XXXXXXX !*/
       written += result; ptr += result;
@@ -553,18 +553,18 @@ trn_cell_extension_parse_into(trn_cell_extension_t *obj, const uint8_t *input, c
   obj->num = (trunnel_get_uint8(ptr));
   remaining -= 1; ptr += 1;
 
-  /* Parse struct trn_cell_extension_fields fields[num] */
-  TRUNNEL_DYNARRAY_EXPAND(trn_cell_extension_fields_t *, &obj->fields, obj->num, {});
+  /* Parse struct trn_cell_extension_field fields[num] */
+  TRUNNEL_DYNARRAY_EXPAND(trn_cell_extension_field_t *, &obj->fields, obj->num, {});
   {
-    trn_cell_extension_fields_t * elt;
+    trn_cell_extension_field_t * elt;
     unsigned idx;
     for (idx = 0; idx < obj->num; ++idx) {
-      result = trn_cell_extension_fields_parse(&elt, ptr, remaining);
+      result = trn_cell_extension_field_parse(&elt, ptr, remaining);
       if (result < 0)
         goto relay_fail;
       trunnel_assert((size_t)result <= remaining);
       remaining -= result; ptr += result;
-      TRUNNEL_DYNARRAY_ADD(trn_cell_extension_fields_t *, &obj->fields, elt, {trn_cell_extension_fields_free(elt);});
+      TRUNNEL_DYNARRAY_ADD(trn_cell_extension_field_t *, &obj->fields, elt, {trn_cell_extension_field_free(elt);});
     }
   }
   trunnel_assert(ptr + remaining == input + len_in);
diff --git a/src/trunnel/hs/cell_common.h b/src/trunnel/hs/cell_common.h
index c84d17d8e5..beb65e015f 100644
--- a/src/trunnel/hs/cell_common.h
+++ b/src/trunnel/hs/cell_common.h
@@ -8,112 +8,112 @@
 #include <stdint.h>
 #include "trunnel.h"
 
-#if !defined(TRUNNEL_OPAQUE) && !defined(TRUNNEL_OPAQUE_TRN_CELL_EXTENSION_FIELDS)
-struct trn_cell_extension_fields_st {
+#if !defined(TRUNNEL_OPAQUE) && !defined(TRUNNEL_OPAQUE_TRN_CELL_EXTENSION_FIELD)
+struct trn_cell_extension_field_st {
   uint8_t field_type;
   uint8_t field_len;
   TRUNNEL_DYNARRAY_HEAD(, uint8_t) field;
   uint8_t trunnel_error_code_;
 };
 #endif
-typedef struct trn_cell_extension_fields_st trn_cell_extension_fields_t;
+typedef struct trn_cell_extension_field_st trn_cell_extension_field_t;
 #if !defined(TRUNNEL_OPAQUE) && !defined(TRUNNEL_OPAQUE_TRN_CELL_EXTENSION)
 struct trn_cell_extension_st {
   uint8_t num;
-  TRUNNEL_DYNARRAY_HEAD(, struct trn_cell_extension_fields_st *) fields;
+  TRUNNEL_DYNARRAY_HEAD(, struct trn_cell_extension_field_st *) fields;
   uint8_t trunnel_error_code_;
 };
 #endif
 typedef struct trn_cell_extension_st trn_cell_extension_t;
-/** Return a newly allocated trn_cell_extension_fields with all
+/** Return a newly allocated trn_cell_extension_field with all
  * elements set to zero.
  */
-trn_cell_extension_fields_t *trn_cell_extension_fields_new(void);
-/** Release all storage held by the trn_cell_extension_fields in
+trn_cell_extension_field_t *trn_cell_extension_field_new(void);
+/** Release all storage held by the trn_cell_extension_field in
  * 'victim'. (Do nothing if 'victim' is NULL.)
  */
-void trn_cell_extension_fields_free(trn_cell_extension_fields_t *victim);
-/** Try to parse a trn_cell_extension_fields from the buffer in
+void trn_cell_extension_field_free(trn_cell_extension_field_t *victim);
+/** Try to parse a trn_cell_extension_field from the buffer in
  * 'input', using up to 'len_in' bytes from the input buffer. On
  * success, return the number of bytes consumed and set *output to the
- * newly allocated trn_cell_extension_fields_t. On failure, return -2
+ * newly allocated trn_cell_extension_field_t. On failure, return -2
  * if the input appears truncated, and -1 if the input is otherwise
  * invalid.
  */
-ssize_t trn_cell_extension_fields_parse(trn_cell_extension_fields_t **output, const uint8_t *input, const size_t len_in);
+ssize_t trn_cell_extension_field_parse(trn_cell_extension_field_t **output, const uint8_t *input, const size_t len_in);
 /** Return the number of bytes we expect to need to encode the
- * trn_cell_extension_fields in 'obj'. On failure, return a negative
+ * trn_cell_extension_field in 'obj'. On failure, return a negative
  * value. Note that this value may be an overestimate, and can even be
  * an underestimate for certain unencodeable objects.
  */
-ssize_t trn_cell_extension_fields_encoded_len(const trn_cell_extension_fields_t *obj);
-/** Try to encode the trn_cell_extension_fields from 'input' into the
+ssize_t trn_cell_extension_field_encoded_len(const trn_cell_extension_field_t *obj);
+/** Try to encode the trn_cell_extension_field from 'input' into the
  * buffer at 'output', using up to 'avail' bytes of the output buffer.
  * On success, return the number of bytes used. On failure, return -2
  * if the buffer was not long enough, and -1 if the input was invalid.
  */
-ssize_t trn_cell_extension_fields_encode(uint8_t *output, size_t avail, const trn_cell_extension_fields_t *input);
-/** Check whether the internal state of the trn_cell_extension_fields
+ssize_t trn_cell_extension_field_encode(uint8_t *output, size_t avail, const trn_cell_extension_field_t *input);
+/** Check whether the internal state of the trn_cell_extension_field
  * in 'obj' is consistent. Return NULL if it is, and a short message
  * if it is not.
  */
-const char *trn_cell_extension_fields_check(const trn_cell_extension_fields_t *obj);
+const char *trn_cell_extension_field_check(const trn_cell_extension_field_t *obj);
 /** Clear any errors that were set on the object 'obj' by its setter
  * functions. Return true iff errors were cleared.
  */
-int trn_cell_extension_fields_clear_errors(trn_cell_extension_fields_t *obj);
+int trn_cell_extension_field_clear_errors(trn_cell_extension_field_t *obj);
 /** Return the value of the field_type field of the
- * trn_cell_extension_fields_t in 'inp'
+ * trn_cell_extension_field_t in 'inp'
  */
-uint8_t trn_cell_extension_fields_get_field_type(const trn_cell_extension_fields_t *inp);
+uint8_t trn_cell_extension_field_get_field_type(const trn_cell_extension_field_t *inp);
 /** Set the value of the field_type field of the
- * trn_cell_extension_fields_t in 'inp' to 'val'. Return 0 on success;
+ * trn_cell_extension_field_t in 'inp' to 'val'. Return 0 on success;
  * return -1 and set the error code on 'inp' on failure.
  */
-int trn_cell_extension_fields_set_field_type(trn_cell_extension_fields_t *inp, uint8_t val);
+int trn_cell_extension_field_set_field_type(trn_cell_extension_field_t *inp, uint8_t val);
 /** Return the value of the field_len field of the
- * trn_cell_extension_fields_t in 'inp'
+ * trn_cell_extension_field_t in 'inp'
  */
-uint8_t trn_cell_extension_fields_get_field_len(const trn_cell_extension_fields_t *inp);
+uint8_t trn_cell_extension_field_get_field_len(const trn_cell_extension_field_t *inp);
 /** Set the value of the field_len field of the
- * trn_cell_extension_fields_t in 'inp' to 'val'. Return 0 on success;
+ * trn_cell_extension_field_t in 'inp' to 'val'. Return 0 on success;
  * return -1 and set the error code on 'inp' on failure.
  */
-int trn_cell_extension_fields_set_field_len(trn_cell_extension_fields_t *inp, uint8_t val);
+int trn_cell_extension_field_set_field_len(trn_cell_extension_field_t *inp, uint8_t val);
 /** Return the length of the dynamic array holding the field field of
- * the trn_cell_extension_fields_t in 'inp'.
+ * the trn_cell_extension_field_t in 'inp'.
  */
-size_t trn_cell_extension_fields_getlen_field(const trn_cell_extension_fields_t *inp);
+size_t trn_cell_extension_field_getlen_field(const trn_cell_extension_field_t *inp);
 /** Return the element at position 'idx' of the dynamic array field
- * field of the trn_cell_extension_fields_t in 'inp'.
+ * field of the trn_cell_extension_field_t in 'inp'.
  */
-uint8_t trn_cell_extension_fields_get_field(trn_cell_extension_fields_t *inp, size_t idx);
-/** As trn_cell_extension_fields_get_field, but take and return a
- * const pointer
+uint8_t trn_cell_extension_field_get_field(trn_cell_extension_field_t *inp, size_t idx);
+/** As trn_cell_extension_field_get_field, but take and return a const
+ * pointer
  */
-uint8_t trn_cell_extension_fields_getconst_field(const trn_cell_extension_fields_t *inp, size_t idx);
+uint8_t trn_cell_extension_field_getconst_field(const trn_cell_extension_field_t *inp, size_t idx);
 /** Change the element at position 'idx' of the dynamic array field
- * field of the trn_cell_extension_fields_t in 'inp', so that it will
+ * field of the trn_cell_extension_field_t in 'inp', so that it will
  * hold the value 'elt'.
  */
-int trn_cell_extension_fields_set_field(trn_cell_extension_fields_t *inp, size_t idx, uint8_t elt);
+int trn_cell_extension_field_set_field(trn_cell_extension_field_t *inp, size_t idx, uint8_t elt);
 /** Append a new element 'elt' to the dynamic array field field of the
- * trn_cell_extension_fields_t in 'inp'.
+ * trn_cell_extension_field_t in 'inp'.
  */
-int trn_cell_extension_fields_add_field(trn_cell_extension_fields_t *inp, uint8_t elt);
+int trn_cell_extension_field_add_field(trn_cell_extension_field_t *inp, uint8_t elt);
 /** Return a pointer to the variable-length array field field of
  * 'inp'.
  */
-uint8_t * trn_cell_extension_fields_getarray_field(trn_cell_extension_fields_t *inp);
-/** As trn_cell_extension_fields_get_field, but take and return a
- * const pointer
+uint8_t * trn_cell_extension_field_getarray_field(trn_cell_extension_field_t *inp);
+/** As trn_cell_extension_field_get_field, but take and return a const
+ * pointer
  */
-const uint8_t  * trn_cell_extension_fields_getconstarray_field(const trn_cell_extension_fields_t *inp);
+const uint8_t  * trn_cell_extension_field_getconstarray_field(const trn_cell_extension_field_t *inp);
 /** Change the length of the variable-length array field field of
  * 'inp' to 'newlen'.Fill extra elements with 0. Return 0 on success;
  * return -1 and set the error code on 'inp' on failure.
  */
-int trn_cell_extension_fields_setlen_field(trn_cell_extension_fields_t *inp, size_t newlen);
+int trn_cell_extension_field_setlen_field(trn_cell_extension_field_t *inp, size_t newlen);
 /** Return a newly allocated trn_cell_extension with all elements set
  * to zero.
  */
@@ -166,32 +166,32 @@ size_t trn_cell_extension_getlen_fields(const trn_cell_extension_t *inp);
 /** Return the element at position 'idx' of the dynamic array field
  * fields of the trn_cell_extension_t in 'inp'.
  */
-struct trn_cell_extension_fields_st * trn_cell_extension_get_fields(trn_cell_extension_t *inp, size_t idx);
+struct trn_cell_extension_field_st * trn_cell_extension_get_fields(trn_cell_extension_t *inp, size_t idx);
 /** As trn_cell_extension_get_fields, but take and return a const
  * pointer
  */
- const struct trn_cell_extension_fields_st * trn_cell_extension_getconst_fields(const trn_cell_extension_t *inp, size_t idx);
+ const struct trn_cell_extension_field_st * trn_cell_extension_getconst_fields(const trn_cell_extension_t *inp, size_t idx);
 /** Change the element at position 'idx' of the dynamic array field
  * fields of the trn_cell_extension_t in 'inp', so that it will hold
  * the value 'elt'. Free the previous value, if any.
  */
-int trn_cell_extension_set_fields(trn_cell_extension_t *inp, size_t idx, struct trn_cell_extension_fields_st * elt);
+int trn_cell_extension_set_fields(trn_cell_extension_t *inp, size_t idx, struct trn_cell_extension_field_st * elt);
 /** As trn_cell_extension_set_fields, but does not free the previous
  * value.
  */
-int trn_cell_extension_set0_fields(trn_cell_extension_t *inp, size_t idx, struct trn_cell_extension_fields_st * elt);
+int trn_cell_extension_set0_fields(trn_cell_extension_t *inp, size_t idx, struct trn_cell_extension_field_st * elt);
 /** Append a new element 'elt' to the dynamic array field fields of
  * the trn_cell_extension_t in 'inp'.
  */
-int trn_cell_extension_add_fields(trn_cell_extension_t *inp, struct trn_cell_extension_fields_st * elt);
+int trn_cell_extension_add_fields(trn_cell_extension_t *inp, struct trn_cell_extension_field_st * elt);
 /** Return a pointer to the variable-length array field fields of
  * 'inp'.
  */
-struct trn_cell_extension_fields_st * * trn_cell_extension_getarray_fields(trn_cell_extension_t *inp);
+struct trn_cell_extension_field_st * * trn_cell_extension_getarray_fields(trn_cell_extension_t *inp);
 /** As trn_cell_extension_get_fields, but take and return a const
  * pointer
  */
-const struct trn_cell_extension_fields_st *  const  * trn_cell_extension_getconstarray_fields(const trn_cell_extension_t *inp);
+const struct trn_cell_extension_field_st *  const  * trn_cell_extension_getconstarray_fields(const trn_cell_extension_t *inp);
 /** Change the length of the variable-length array field fields of
  * 'inp' to 'newlen'.Fill extra elements with NULL; free removed
  * elements. Return 0 on success; return -1 and set the error code on
diff --git a/src/trunnel/hs/cell_common.trunnel b/src/trunnel/hs/cell_common.trunnel
index 1aa6999de7..7e99cbfa66 100644
--- a/src/trunnel/hs/cell_common.trunnel
+++ b/src/trunnel/hs/cell_common.trunnel
@@ -1,6 +1,6 @@
 /* This file contains common data structure that cells use. */
 
-struct trn_cell_extension_fields {
+struct trn_cell_extension_field {
   u8 field_type;
   u8 field_len;
   u8 field[field_len];
@@ -8,5 +8,5 @@ struct trn_cell_extension_fields {
 
 struct trn_cell_extension {
   u8 num;
-  struct trn_cell_extension_fields fields[num];
+  struct trn_cell_extension_field fields[num];
 };
diff --git a/src/trunnel/hs/cell_establish_intro.c b/src/trunnel/hs/cell_establish_intro.c
index 99ceadbda4..f31404c55f 100644
--- a/src/trunnel/hs/cell_establish_intro.c
+++ b/src/trunnel/hs/cell_establish_intro.c
@@ -36,6 +36,185 @@ ssize_t trn_cell_extension_encoded_len(const trn_cell_extension_t *obj);
 ssize_t trn_cell_extension_encode(uint8_t *output, size_t avail, const trn_cell_extension_t *input);
 const char *trn_cell_extension_check(const trn_cell_extension_t *obj);
 int trn_cell_extension_clear_errors(trn_cell_extension_t *obj);
+trn_cell_extension_dos_param_t *
+trn_cell_extension_dos_param_new(void)
+{
+  trn_cell_extension_dos_param_t *val = trunnel_calloc(1, sizeof(trn_cell_extension_dos_param_t));
+  if (NULL == val)
+    return NULL;
+  return val;
+}
+
+/** Release all storage held inside 'obj', but do not free 'obj'.
+ */
+static void
+trn_cell_extension_dos_param_clear(trn_cell_extension_dos_param_t *obj)
+{
+  (void) obj;
+}
+
+void
+trn_cell_extension_dos_param_free(trn_cell_extension_dos_param_t *obj)
+{
+  if (obj == NULL)
+    return;
+  trn_cell_extension_dos_param_clear(obj);
+  trunnel_memwipe(obj, sizeof(trn_cell_extension_dos_param_t));
+  trunnel_free_(obj);
+}
+
+uint8_t
+trn_cell_extension_dos_param_get_type(const trn_cell_extension_dos_param_t *inp)
+{
+  return inp->type;
+}
+int
+trn_cell_extension_dos_param_set_type(trn_cell_extension_dos_param_t *inp, uint8_t val)
+{
+  inp->type = val;
+  return 0;
+}
+uint64_t
+trn_cell_extension_dos_param_get_value(const trn_cell_extension_dos_param_t *inp)
+{
+  return inp->value;
+}
+int
+trn_cell_extension_dos_param_set_value(trn_cell_extension_dos_param_t *inp, uint64_t val)
+{
+  inp->value = val;
+  return 0;
+}
+const char *
+trn_cell_extension_dos_param_check(const trn_cell_extension_dos_param_t *obj)
+{
+  if (obj == NULL)
+    return "Object was NULL";
+  if (obj->trunnel_error_code_)
+    return "A set function failed on this object";
+  return NULL;
+}
+
+ssize_t
+trn_cell_extension_dos_param_encoded_len(const trn_cell_extension_dos_param_t *obj)
+{
+  ssize_t result = 0;
+
+  if (NULL != trn_cell_extension_dos_param_check(obj))
+     return -1;
+
+
+  /* Length of u8 type */
+  result += 1;
+
+  /* Length of u64 value */
+  result += 8;
+  return result;
+}
+int
+trn_cell_extension_dos_param_clear_errors(trn_cell_extension_dos_param_t *obj)
+{
+  int r = obj->trunnel_error_code_;
+  obj->trunnel_error_code_ = 0;
+  return r;
+}
+ssize_t
+trn_cell_extension_dos_param_encode(uint8_t *output, const size_t avail, const trn_cell_extension_dos_param_t *obj)
+{
+  ssize_t result = 0;
+  size_t written = 0;
+  uint8_t *ptr = output;
+  const char *msg;
+#ifdef TRUNNEL_CHECK_ENCODED_LEN
+  const ssize_t encoded_len = trn_cell_extension_dos_param_encoded_len(obj);
+#endif
+
+  if (NULL != (msg = trn_cell_extension_dos_param_check(obj)))
+    goto check_failed;
+
+#ifdef TRUNNEL_CHECK_ENCODED_LEN
+  trunnel_assert(encoded_len >= 0);
+#endif
+
+  /* Encode u8 type */
+  trunnel_assert(written <= avail);
+  if (avail - written < 1)
+    goto truncated;
+  trunnel_set_uint8(ptr, (obj->type));
+  written += 1; ptr += 1;
+
+  /* Encode u64 value */
+  trunnel_assert(written <= avail);
+  if (avail - written < 8)
+    goto truncated;
+  trunnel_set_uint64(ptr, trunnel_htonll(obj->value));
+  written += 8; ptr += 8;
+
+
+  trunnel_assert(ptr == output + written);
+#ifdef TRUNNEL_CHECK_ENCODED_LEN
+  {
+    trunnel_assert(encoded_len >= 0);
+    trunnel_assert((size_t)encoded_len == written);
+  }
+
+#endif
+
+  return written;
+
+ truncated:
+  result = -2;
+  goto fail;
+ check_failed:
+  (void)msg;
+  result = -1;
+  goto fail;
+ fail:
+  trunnel_assert(result < 0);
+  return result;
+}
+
+/** As trn_cell_extension_dos_param_parse(), but do not allocate the
+ * output object.
+ */
+static ssize_t
+trn_cell_extension_dos_param_parse_into(trn_cell_extension_dos_param_t *obj, const uint8_t *input, const size_t len_in)
+{
+  const uint8_t *ptr = input;
+  size_t remaining = len_in;
+  ssize_t result = 0;
+  (void)result;
+
+  /* Parse u8 type */
+  CHECK_REMAINING(1, truncated);
+  obj->type = (trunnel_get_uint8(ptr));
+  remaining -= 1; ptr += 1;
+
+  /* Parse u64 value */
+  CHECK_REMAINING(8, truncated);
+  obj->value = trunnel_ntohll(trunnel_get_uint64(ptr));
+  remaining -= 8; ptr += 8;
+  trunnel_assert(ptr + remaining == input + len_in);
+  return len_in - remaining;
+
+ truncated:
+  return -2;
+}
+
+ssize_t
+trn_cell_extension_dos_param_parse(trn_cell_extension_dos_param_t **output, const uint8_t *input, const size_t len_in)
+{
+  ssize_t result;
+  *output = trn_cell_extension_dos_param_new();
+  if (NULL == *output)
+    return -1;
+  result = trn_cell_extension_dos_param_parse_into(*output, input, len_in);
+  if (result < 0) {
+    trn_cell_extension_dos_param_free(*output);
+    *output = NULL;
+  }
+  return result;
+}
 trn_cell_establish_intro_t *
 trn_cell_establish_intro_new(void)
 {
@@ -561,6 +740,296 @@ trn_cell_establish_intro_parse(trn_cell_establish_intro_t **output, const uint8_
   }
   return result;
 }
+trn_cell_extension_dos_t *
+trn_cell_extension_dos_new(void)
+{
+  trn_cell_extension_dos_t *val = trunnel_calloc(1, sizeof(trn_cell_extension_dos_t));
+  if (NULL == val)
+    return NULL;
+  return val;
+}
+
+/** Release all storage held inside 'obj', but do not free 'obj'.
+ */
+static void
+trn_cell_extension_dos_clear(trn_cell_extension_dos_t *obj)
+{
+  (void) obj;
+  {
+
+    unsigned idx;
+    for (idx = 0; idx < TRUNNEL_DYNARRAY_LEN(&obj->params); ++idx) {
+      trn_cell_extension_dos_param_free(TRUNNEL_DYNARRAY_GET(&obj->params, idx));
+    }
+  }
+  TRUNNEL_DYNARRAY_WIPE(&obj->params);
+  TRUNNEL_DYNARRAY_CLEAR(&obj->params);
+}
+
+void
+trn_cell_extension_dos_free(trn_cell_extension_dos_t *obj)
+{
+  if (obj == NULL)
+    return;
+  trn_cell_extension_dos_clear(obj);
+  trunnel_memwipe(obj, sizeof(trn_cell_extension_dos_t));
+  trunnel_free_(obj);
+}
+
+uint8_t
+trn_cell_extension_dos_get_n_params(const trn_cell_extension_dos_t *inp)
+{
+  return inp->n_params;
+}
+int
+trn_cell_extension_dos_set_n_params(trn_cell_extension_dos_t *inp, uint8_t val)
+{
+  inp->n_params = val;
+  return 0;
+}
+size_t
+trn_cell_extension_dos_getlen_params(const trn_cell_extension_dos_t *inp)
+{
+  return TRUNNEL_DYNARRAY_LEN(&inp->params);
+}
+
+struct trn_cell_extension_dos_param_st *
+trn_cell_extension_dos_get_params(trn_cell_extension_dos_t *inp, size_t idx)
+{
+  return TRUNNEL_DYNARRAY_GET(&inp->params, idx);
+}
+
+ const struct trn_cell_extension_dos_param_st *
+trn_cell_extension_dos_getconst_params(const trn_cell_extension_dos_t *inp, size_t idx)
+{
+  return trn_cell_extension_dos_get_params((trn_cell_extension_dos_t*)inp, idx);
+}
+int
+trn_cell_extension_dos_set_params(trn_cell_extension_dos_t *inp, size_t idx, struct trn_cell_extension_dos_param_st * elt)
+{
+  trn_cell_extension_dos_param_t *oldval = TRUNNEL_DYNARRAY_GET(&inp->params, idx);
+  if (oldval && oldval != elt)
+    trn_cell_extension_dos_param_free(oldval);
+  return trn_cell_extension_dos_set0_params(inp, idx, elt);
+}
+int
+trn_cell_extension_dos_set0_params(trn_cell_extension_dos_t *inp, size_t idx, struct trn_cell_extension_dos_param_st * elt)
+{
+  TRUNNEL_DYNARRAY_SET(&inp->params, idx, elt);
+  return 0;
+}
+int
+trn_cell_extension_dos_add_params(trn_cell_extension_dos_t *inp, struct trn_cell_extension_dos_param_st * elt)
+{
+#if SIZE_MAX >= UINT8_MAX
+  if (inp->params.n_ == UINT8_MAX)
+    goto trunnel_alloc_failed;
+#endif
+  TRUNNEL_DYNARRAY_ADD(struct trn_cell_extension_dos_param_st *, &inp->params, elt, {});
+  return 0;
+ trunnel_alloc_failed:
+  TRUNNEL_SET_ERROR_CODE(inp);
+  return -1;
+}
+
+struct trn_cell_extension_dos_param_st * *
+trn_cell_extension_dos_getarray_params(trn_cell_extension_dos_t *inp)
+{
+  return inp->params.elts_;
+}
+const struct trn_cell_extension_dos_param_st *  const  *
+trn_cell_extension_dos_getconstarray_params(const trn_cell_extension_dos_t *inp)
+{
+  return (const struct trn_cell_extension_dos_param_st *  const  *)trn_cell_extension_dos_getarray_params((trn_cell_extension_dos_t*)inp);
+}
+int
+trn_cell_extension_dos_setlen_params(trn_cell_extension_dos_t *inp, size_t newlen)
+{
+  struct trn_cell_extension_dos_param_st * *newptr;
+#if UINT8_MAX < SIZE_MAX
+  if (newlen > UINT8_MAX)
+    goto trunnel_alloc_failed;
+#endif
+  newptr = trunnel_dynarray_setlen(&inp->params.allocated_,
+                 &inp->params.n_, inp->params.elts_, newlen,
+                 sizeof(inp->params.elts_[0]), (trunnel_free_fn_t) trn_cell_extension_dos_param_free,
+                 &inp->trunnel_error_code_);
+  if (newlen != 0 && newptr == NULL)
+    goto trunnel_alloc_failed;
+  inp->params.elts_ = newptr;
+  return 0;
+ trunnel_alloc_failed:
+  TRUNNEL_SET_ERROR_CODE(inp);
+  return -1;
+}
+const char *
+trn_cell_extension_dos_check(const trn_cell_extension_dos_t *obj)
+{
+  if (obj == NULL)
+    return "Object was NULL";
+  if (obj->trunnel_error_code_)
+    return "A set function failed on this object";
+  {
+    const char *msg;
+
+    unsigned idx;
+    for (idx = 0; idx < TRUNNEL_DYNARRAY_LEN(&obj->params); ++idx) {
+      if (NULL != (msg = trn_cell_extension_dos_param_check(TRUNNEL_DYNARRAY_GET(&obj->params, idx))))
+        return msg;
+    }
+  }
+  if (TRUNNEL_DYNARRAY_LEN(&obj->params) != obj->n_params)
+    return "Length mismatch for params";
+  return NULL;
+}
+
+ssize_t
+trn_cell_extension_dos_encoded_len(const trn_cell_extension_dos_t *obj)
+{
+  ssize_t result = 0;
+
+  if (NULL != trn_cell_extension_dos_check(obj))
+     return -1;
+
+
+  /* Length of u8 n_params */
+  result += 1;
+
+  /* Length of struct trn_cell_extension_dos_param params[n_params] */
+  {
+
+    unsigned idx;
+    for (idx = 0; idx < TRUNNEL_DYNARRAY_LEN(&obj->params); ++idx) {
+      result += trn_cell_extension_dos_param_encoded_len(TRUNNEL_DYNARRAY_GET(&obj->params, idx));
+    }
+  }
+  return result;
+}
+int
+trn_cell_extension_dos_clear_errors(trn_cell_extension_dos_t *obj)
+{
+  int r = obj->trunnel_error_code_;
+  obj->trunnel_error_code_ = 0;
+  return r;
+}
+ssize_t
+trn_cell_extension_dos_encode(uint8_t *output, const size_t avail, const trn_cell_extension_dos_t *obj)
+{
+  ssize_t result = 0;
+  size_t written = 0;
+  uint8_t *ptr = output;
+  const char *msg;
+#ifdef TRUNNEL_CHECK_ENCODED_LEN
+  const ssize_t encoded_len = trn_cell_extension_dos_encoded_len(obj);
+#endif
+
+  if (NULL != (msg = trn_cell_extension_dos_check(obj)))
+    goto check_failed;
+
+#ifdef TRUNNEL_CHECK_ENCODED_LEN
+  trunnel_assert(encoded_len >= 0);
+#endif
+
+  /* Encode u8 n_params */
+  trunnel_assert(written <= avail);
+  if (avail - written < 1)
+    goto truncated;
+  trunnel_set_uint8(ptr, (obj->n_params));
+  written += 1; ptr += 1;
+
+  /* Encode struct trn_cell_extension_dos_param params[n_params] */
+  {
+
+    unsigned idx;
+    for (idx = 0; idx < TRUNNEL_DYNARRAY_LEN(&obj->params); ++idx) {
+      trunnel_assert(written <= avail);
+      result = trn_cell_extension_dos_param_encode(ptr, avail - written, TRUNNEL_DYNARRAY_GET(&obj->params, idx));
+      if (result < 0)
+        goto fail; /* XXXXXXX !*/
+      written += result; ptr += result;
+    }
+  }
+
+
+  trunnel_assert(ptr == output + written);
+#ifdef TRUNNEL_CHECK_ENCODED_LEN
+  {
+    trunnel_assert(encoded_len >= 0);
+    trunnel_assert((size_t)encoded_len == written);
+  }
+
+#endif
+
+  return written;
+
+ truncated:
+  result = -2;
+  goto fail;
+ check_failed:
+  (void)msg;
+  result = -1;
+  goto fail;
+ fail:
+  trunnel_assert(result < 0);
+  return result;
+}
+
+/** As trn_cell_extension_dos_parse(), but do not allocate the output
+ * object.
+ */
+static ssize_t
+trn_cell_extension_dos_parse_into(trn_cell_extension_dos_t *obj, const uint8_t *input, const size_t len_in)
+{
+  const uint8_t *ptr = input;
+  size_t remaining = len_in;
+  ssize_t result = 0;
+  (void)result;
+
+  /* Parse u8 n_params */
+  CHECK_REMAINING(1, truncated);
+  obj->n_params = (trunnel_get_uint8(ptr));
+  remaining -= 1; ptr += 1;
+
+  /* Parse struct trn_cell_extension_dos_param params[n_params] */
+  TRUNNEL_DYNARRAY_EXPAND(trn_cell_extension_dos_param_t *, &obj->params, obj->n_params, {});
+  {
+    trn_cell_extension_dos_param_t * elt;
+    unsigned idx;
+    for (idx = 0; idx < obj->n_params; ++idx) {
+      result = trn_cell_extension_dos_param_parse(&elt, ptr, remaining);
+      if (result < 0)
+        goto relay_fail;
+      trunnel_assert((size_t)result <= remaining);
+      remaining -= result; ptr += result;
+      TRUNNEL_DYNARRAY_ADD(trn_cell_extension_dos_param_t *, &obj->params, elt, {trn_cell_extension_dos_param_free(elt);});
+    }
+  }
+  trunnel_assert(ptr + remaining == input + len_in);
+  return len_in - remaining;
+
+ truncated:
+  return -2;
+ relay_fail:
+  trunnel_assert(result < 0);
+  return result;
+ trunnel_alloc_failed:
+  return -1;
+}
+
+ssize_t
+trn_cell_extension_dos_parse(trn_cell_extension_dos_t **output, const uint8_t *input, const size_t len_in)
+{
+  ssize_t result;
+  *output = trn_cell_extension_dos_new();
+  if (NULL == *output)
+    return -1;
+  result = trn_cell_extension_dos_parse_into(*output, input, len_in);
+  if (result < 0) {
+    trn_cell_extension_dos_free(*output);
+    *output = NULL;
+  }
+  return result;
+}
 trn_cell_intro_established_t *
 trn_cell_intro_established_new(void)
 {
diff --git a/src/trunnel/hs/cell_establish_intro.h b/src/trunnel/hs/cell_establish_intro.h
index 1908645aa6..1924d9cab6 100644
--- a/src/trunnel/hs/cell_establish_intro.h
+++ b/src/trunnel/hs/cell_establish_intro.h
@@ -10,6 +10,17 @@
 
 struct trn_cell_extension_st;
 #define TRUNNEL_SHA3_256_LEN 32
+#define TRUNNEL_CELL_EXTENSION_TYPE_DOS 1
+#define TRUNNEL_DOS_PARAM_TYPE_INTRO2_RATE_PER_SEC 1
+#define TRUNNEL_DOS_PARAM_TYPE_INTRO2_BURST_PER_SEC 2
+#if !defined(TRUNNEL_OPAQUE) && !defined(TRUNNEL_OPAQUE_TRN_CELL_EXTENSION_DOS_PARAM)
+struct trn_cell_extension_dos_param_st {
+  uint8_t type;
+  uint64_t value;
+  uint8_t trunnel_error_code_;
+};
+#endif
+typedef struct trn_cell_extension_dos_param_st trn_cell_extension_dos_param_t;
 #if !defined(TRUNNEL_OPAQUE) && !defined(TRUNNEL_OPAQUE_TRN_CELL_ESTABLISH_INTRO)
 struct trn_cell_establish_intro_st {
   const uint8_t *start_cell;
@@ -26,6 +37,14 @@ struct trn_cell_establish_intro_st {
 };
 #endif
 typedef struct trn_cell_establish_intro_st trn_cell_establish_intro_t;
+#if !defined(TRUNNEL_OPAQUE) && !defined(TRUNNEL_OPAQUE_TRN_CELL_EXTENSION_DOS)
+struct trn_cell_extension_dos_st {
+  uint8_t n_params;
+  TRUNNEL_DYNARRAY_HEAD(, struct trn_cell_extension_dos_param_st *) params;
+  uint8_t trunnel_error_code_;
+};
+#endif
+typedef struct trn_cell_extension_dos_st trn_cell_extension_dos_t;
 #if !defined(TRUNNEL_OPAQUE) && !defined(TRUNNEL_OPAQUE_TRN_CELL_INTRO_ESTABLISHED)
 struct trn_cell_intro_established_st {
   struct trn_cell_extension_st *extensions;
@@ -33,6 +52,62 @@ struct trn_cell_intro_established_st {
 };
 #endif
 typedef struct trn_cell_intro_established_st trn_cell_intro_established_t;
+/** Return a newly allocated trn_cell_extension_dos_param with all
+ * elements set to zero.
+ */
+trn_cell_extension_dos_param_t *trn_cell_extension_dos_param_new(void);
+/** Release all storage held by the trn_cell_extension_dos_param in
+ * 'victim'. (Do nothing if 'victim' is NULL.)
+ */
+void trn_cell_extension_dos_param_free(trn_cell_extension_dos_param_t *victim);
+/** Try to parse a trn_cell_extension_dos_param from the buffer in
+ * 'input', using up to 'len_in' bytes from the input buffer. On
+ * success, return the number of bytes consumed and set *output to the
+ * newly allocated trn_cell_extension_dos_param_t. On failure, return
+ * -2 if the input appears truncated, and -1 if the input is otherwise
+ * invalid.
+ */
+ssize_t trn_cell_extension_dos_param_parse(trn_cell_extension_dos_param_t **output, const uint8_t *input, const size_t len_in);
+/** Return the number of bytes we expect to need to encode the
+ * trn_cell_extension_dos_param in 'obj'. On failure, return a
+ * negative value. Note that this value may be an overestimate, and
+ * can even be an underestimate for certain unencodeable objects.
+ */
+ssize_t trn_cell_extension_dos_param_encoded_len(const trn_cell_extension_dos_param_t *obj);
+/** Try to encode the trn_cell_extension_dos_param from 'input' into
+ * the buffer at 'output', using up to 'avail' bytes of the output
+ * buffer. On success, return the number of bytes used. On failure,
+ * return -2 if the buffer was not long enough, and -1 if the input
+ * was invalid.
+ */
+ssize_t trn_cell_extension_dos_param_encode(uint8_t *output, size_t avail, const trn_cell_extension_dos_param_t *input);
+/** Check whether the internal state of the
+ * trn_cell_extension_dos_param in 'obj' is consistent. Return NULL if
+ * it is, and a short message if it is not.
+ */
+const char *trn_cell_extension_dos_param_check(const trn_cell_extension_dos_param_t *obj);
+/** Clear any errors that were set on the object 'obj' by its setter
+ * functions. Return true iff errors were cleared.
+ */
+int trn_cell_extension_dos_param_clear_errors(trn_cell_extension_dos_param_t *obj);
+/** Return the value of the type field of the
+ * trn_cell_extension_dos_param_t in 'inp'
+ */
+uint8_t trn_cell_extension_dos_param_get_type(const trn_cell_extension_dos_param_t *inp);
+/** Set the value of the type field of the
+ * trn_cell_extension_dos_param_t in 'inp' to 'val'. Return 0 on
+ * success; return -1 and set the error code on 'inp' on failure.
+ */
+int trn_cell_extension_dos_param_set_type(trn_cell_extension_dos_param_t *inp, uint8_t val);
+/** Return the value of the value field of the
+ * trn_cell_extension_dos_param_t in 'inp'
+ */
+uint64_t trn_cell_extension_dos_param_get_value(const trn_cell_extension_dos_param_t *inp);
+/** Set the value of the value field of the
+ * trn_cell_extension_dos_param_t in 'inp' to 'val'. Return 0 on
+ * success; return -1 and set the error code on 'inp' on failure.
+ */
+int trn_cell_extension_dos_param_set_value(trn_cell_extension_dos_param_t *inp, uint64_t val);
 /** Return a newly allocated trn_cell_establish_intro with all
  * elements set to zero.
  */
@@ -216,6 +291,90 @@ const uint8_t  * trn_cell_establish_intro_getconstarray_sig(const trn_cell_estab
  * -1 and set the error code on 'inp' on failure.
  */
 int trn_cell_establish_intro_setlen_sig(trn_cell_establish_intro_t *inp, size_t newlen);
+/** Return a newly allocated trn_cell_extension_dos with all elements
+ * set to zero.
+ */
+trn_cell_extension_dos_t *trn_cell_extension_dos_new(void);
+/** Release all storage held by the trn_cell_extension_dos in
+ * 'victim'. (Do nothing if 'victim' is NULL.)
+ */
+void trn_cell_extension_dos_free(trn_cell_extension_dos_t *victim);
+/** Try to parse a trn_cell_extension_dos from the buffer in 'input',
+ * using up to 'len_in' bytes from the input buffer. On success,
+ * return the number of bytes consumed and set *output to the newly
+ * allocated trn_cell_extension_dos_t. On failure, return -2 if the
+ * input appears truncated, and -1 if the input is otherwise invalid.
+ */
+ssize_t trn_cell_extension_dos_parse(trn_cell_extension_dos_t **output, const uint8_t *input, const size_t len_in);
+/** Return the number of bytes we expect to need to encode the
+ * trn_cell_extension_dos in 'obj'. On failure, return a negative
+ * value. Note that this value may be an overestimate, and can even be
+ * an underestimate for certain unencodeable objects.
+ */
+ssize_t trn_cell_extension_dos_encoded_len(const trn_cell_extension_dos_t *obj);
+/** Try to encode the trn_cell_extension_dos from 'input' into the
+ * buffer at 'output', using up to 'avail' bytes of the output buffer.
+ * On success, return the number of bytes used. On failure, return -2
+ * if the buffer was not long enough, and -1 if the input was invalid.
+ */
+ssize_t trn_cell_extension_dos_encode(uint8_t *output, size_t avail, const trn_cell_extension_dos_t *input);
+/** Check whether the internal state of the trn_cell_extension_dos in
+ * 'obj' is consistent. Return NULL if it is, and a short message if
+ * it is not.
+ */
+const char *trn_cell_extension_dos_check(const trn_cell_extension_dos_t *obj);
+/** Clear any errors that were set on the object 'obj' by its setter
+ * functions. Return true iff errors were cleared.
+ */
+int trn_cell_extension_dos_clear_errors(trn_cell_extension_dos_t *obj);
+/** Return the value of the n_params field of the
+ * trn_cell_extension_dos_t in 'inp'
+ */
+uint8_t trn_cell_extension_dos_get_n_params(const trn_cell_extension_dos_t *inp);
+/** Set the value of the n_params field of the
+ * trn_cell_extension_dos_t in 'inp' to 'val'. Return 0 on success;
+ * return -1 and set the error code on 'inp' on failure.
+ */
+int trn_cell_extension_dos_set_n_params(trn_cell_extension_dos_t *inp, uint8_t val);
+/** Return the length of the dynamic array holding the params field of
+ * the trn_cell_extension_dos_t in 'inp'.
+ */
+size_t trn_cell_extension_dos_getlen_params(const trn_cell_extension_dos_t *inp);
+/** Return the element at position 'idx' of the dynamic array field
+ * params of the trn_cell_extension_dos_t in 'inp'.
+ */
+struct trn_cell_extension_dos_param_st * trn_cell_extension_dos_get_params(trn_cell_extension_dos_t *inp, size_t idx);
+/** As trn_cell_extension_dos_get_params, but take and return a const
+ * pointer
+ */
+ const struct trn_cell_extension_dos_param_st * trn_cell_extension_dos_getconst_params(const trn_cell_extension_dos_t *inp, size_t idx);
+/** Change the element at position 'idx' of the dynamic array field
+ * params of the trn_cell_extension_dos_t in 'inp', so that it will
+ * hold the value 'elt'. Free the previous value, if any.
+ */
+int trn_cell_extension_dos_set_params(trn_cell_extension_dos_t *inp, size_t idx, struct trn_cell_extension_dos_param_st * elt);
+/** As trn_cell_extension_dos_set_params, but does not free the
+ * previous value.
+ */
+int trn_cell_extension_dos_set0_params(trn_cell_extension_dos_t *inp, size_t idx, struct trn_cell_extension_dos_param_st * elt);
+/** Append a new element 'elt' to the dynamic array field params of
+ * the trn_cell_extension_dos_t in 'inp'.
+ */
+int trn_cell_extension_dos_add_params(trn_cell_extension_dos_t *inp, struct trn_cell_extension_dos_param_st * elt);
+/** Return a pointer to the variable-length array field params of
+ * 'inp'.
+ */
+struct trn_cell_extension_dos_param_st * * trn_cell_extension_dos_getarray_params(trn_cell_extension_dos_t *inp);
+/** As trn_cell_extension_dos_get_params, but take and return a const
+ * pointer
+ */
+const struct trn_cell_extension_dos_param_st *  const  * trn_cell_extension_dos_getconstarray_params(const trn_cell_extension_dos_t *inp);
+/** Change the length of the variable-length array field params of
+ * 'inp' to 'newlen'.Fill extra elements with NULL; free removed
+ * elements. Return 0 on success; return -1 and set the error code on
+ * 'inp' on failure.
+ */
+int trn_cell_extension_dos_setlen_params(trn_cell_extension_dos_t *inp, size_t newlen);
 /** Return a newly allocated trn_cell_intro_established with all
  * elements set to zero.
  */
diff --git a/src/trunnel/hs/cell_establish_intro.trunnel b/src/trunnel/hs/cell_establish_intro.trunnel
index 011ee62a15..e30938f6c2 100644
--- a/src/trunnel/hs/cell_establish_intro.trunnel
+++ b/src/trunnel/hs/cell_establish_intro.trunnel
@@ -39,3 +39,26 @@ struct trn_cell_intro_established {
   /* Extension(s). Reserved fields. */
   struct trn_cell_extension extensions;
 };
+
+/*
+ * ESTABLISH_INTRO cell extensions.
+ */
+
+const TRUNNEL_CELL_EXTENSION_TYPE_DOS = 0x01;
+
+/* DoS Parameter types. */
+const TRUNNEL_DOS_PARAM_TYPE_INTRO2_RATE_PER_SEC = 0x01;
+const TRUNNEL_DOS_PARAM_TYPE_INTRO2_BURST_PER_SEC = 0x02;
+
+/*
+ * DoS Parameters Extension. See proposal 305 for more details.
+ */
+struct trn_cell_extension_dos_param {
+  u8 type;
+  u64 value;
+};
+
+struct trn_cell_extension_dos {
+  u8 n_params;
+  struct trn_cell_extension_dos_param params[n_params];
+};