15 files changed, 615 insertions, 76 deletions

diff --git a/src/or/circuitbuild.c b/src/or/circuitbuild.c
index 2e7ea2f79a..69a8a9c5ec 100644
--- a/src/or/circuitbuild.c
+++ b/src/or/circuitbuild.c
@@ -28,6 +28,7 @@
 #include "connection_edge.h"
 #include "connection_or.h"
 #include "control.h"
+#include "crypto.h"
 #include "directory.h"
 #include "entrynodes.h"
 #include "main.h"
@@ -38,14 +39,14 @@
 #include "onion_tap.h"
 #include "onion_fast.h"
 #include "policies.h"
-#include "transports.h"
 #include "relay.h"
+#include "rendcommon.h"
 #include "rephist.h"
 #include "router.h"
 #include "routerlist.h"
 #include "routerparse.h"
 #include "routerset.h"
-#include "crypto.h"
+#include "transports.h"
 
 static channel_t * channel_connect_for_circuit(const tor_addr_t *addr,
                                                uint16_t port,
@@ -1996,7 +1997,9 @@ onion_pick_cpath_exit(origin_circuit_t *circ, extend_info_t *exit_ei)
   cpath_build_state_t *state = circ->build_state;
 
   if (state->onehop_tunnel) {
-    log_debug(LD_CIRC, "Launching a one-hop circuit for dir tunnel.");
+    log_debug(LD_CIRC, "Launching a one-hop circuit for dir tunnel%s.",
+              (rend_allow_non_anonymous_connection(get_options()) ?
+               ", or intro or rendezvous connection" : ""));
     state->desired_path_len = 1;
   } else {
     int r = new_route_len(circ->base_.purpose, exit_ei, nodelist_get_list());
diff --git a/src/or/circuitstats.c b/src/or/circuitstats.c
index f4db64ebca..fe8860e9c9 100644
--- a/src/or/circuitstats.c
+++ b/src/or/circuitstats.c
@@ -21,6 +21,8 @@
 #include "control.h"
 #include "main.h"
 #include "networkstatus.h"
+#include "rendclient.h"
+#include "rendservice.h"
 #include "statefile.h"
 
 #undef log
@@ -81,12 +83,14 @@ get_circuit_build_timeout_ms(void)
 
 /**
  * This function decides if CBT learning should be disabled. It returns
- * true if one or more of the following four conditions are met:
+ * true if one or more of the following conditions are met:
  *
  *  1. If the cbtdisabled consensus parameter is set.
  *  2. If the torrc option LearnCircuitBuildTimeout is false.
  *  3. If we are a directory authority
  *  4. If we fail to write circuit build time history to our state file.
+ *  5. If we are compiled or configured in Tor2web mode
+ *  6. If we are configured in Single Onion mode
  */
 int
 circuit_build_times_disabled(void)
@@ -94,14 +98,30 @@ circuit_build_times_disabled(void)
   if (unit_tests) {
     return 0;
   } else {
+    const or_options_t *options = get_options();
     int consensus_disabled = networkstatus_get_param(NULL, "cbtdisabled",
                                                      0, 0, 1);
-    int config_disabled = !get_options()->LearnCircuitBuildTimeout;
-    int dirauth_disabled = get_options()->AuthoritativeDir;
+    int config_disabled = !options->LearnCircuitBuildTimeout;
+    int dirauth_disabled = options->AuthoritativeDir;
     int state_disabled = did_last_state_file_write_fail() ? 1 : 0;
+    /* LearnCircuitBuildTimeout and Tor2webMode/OnionServiceSingleHopMode are
+     * incompatible in two ways:
+     *
+     * - LearnCircuitBuildTimeout results in a low CBT, which
+     *   Single Onion use of one-hop intro and rendezvous circuits lowers
+     *   much further, producing *far* too many timeouts.
+     *
+     * - The adaptive CBT code does not update its timeout estimate
+     *   using build times for single-hop circuits.
+     *
+     * If we fix both of these issues someday, we should test
+     * these modes with LearnCircuitBuildTimeout on again. */
+    int tor2web_disabled = rend_client_allow_non_anonymous_connection(options);
+    int single_onion_disabled = rend_service_allow_non_anonymous_connection(
+                                                                      options);
 
     if (consensus_disabled || config_disabled || dirauth_disabled ||
-           state_disabled) {
+        state_disabled || tor2web_disabled || single_onion_disabled) {
 #if 0
       log_debug(LD_CIRC,
                "CircuitBuildTime learning is disabled. "
diff --git a/src/or/config.c b/src/or/config.c
index 136958c6aa..f7f1f571ce 100644
--- a/src/or/config.c
+++ b/src/or/config.c
@@ -18,6 +18,7 @@
 #include "circuitlist.h"
 #include "circuitmux.h"
 #include "circuitmux_ewma.h"
+#include "circuitstats.h"
 #include "config.h"
 #include "connection.h"
 #include "connection_edge.h"
@@ -297,6 +298,8 @@ static config_var_t option_vars_[] = {
   V(HidServAuth,                 LINELIST, NULL),
   V(CloseHSClientCircuitsImmediatelyOnTimeout, BOOL, "0"),
   V(CloseHSServiceRendCircuitsImmediatelyOnTimeout, BOOL, "0"),
+  V(OnionServiceSingleHopMode,   BOOL,     "0"),
+  V(OnionServiceNonAnonymousMode,BOOL,     "0"),
   V(HTTPProxy,                   STRING,   NULL),
   V(HTTPProxyAuthenticator,      STRING,   NULL),
   V(HTTPSProxy,                  STRING,   NULL),
@@ -1558,10 +1561,10 @@ options_act(const or_options_t *old_options)
   if (consider_adding_dir_servers(options, old_options) < 0)
     return -1;
 
-#ifdef NON_ANONYMOUS_MODE_ENABLED
-  log_warn(LD_GENERAL, "This copy of Tor was compiled to run in a "
-      "non-anonymous mode. It will provide NO ANONYMITY.");
-#endif
+  if (rend_non_anonymous_mode_enabled(options)) {
+    log_warn(LD_GENERAL, "This copy of Tor was compiled or configured to run "
+             "in a non-anonymous mode. It will provide NO ANONYMITY.");
+  }
 
 #ifdef ENABLE_TOR2WEB_MODE
 /* LCOV_EXCL_START */
@@ -1723,8 +1726,27 @@ options_act(const or_options_t *old_options)
 
   monitor_owning_controller_process(options->OwningControllerProcess);
 
+  /* We must create new keys after we poison the directories, because our
+   * poisoning code checks for existing keys, and refuses to modify their
+   * directories. */
+
+  /* If we use the insecure OnionServiceSingleHopMode, make sure we poison any
+     new hidden service directories, so that we never accidentally launch the
+     non-anonymous hidden services thinking they are anonymous. */
+  if (running_tor && rend_service_allow_non_anonymous_connection(options)) {
+    if (options->RendConfigLines && !num_rend_services()) {
+      log_warn(LD_BUG,"Error: hidden services configured, but not parsed.");
+      return -1;
+    }
+    if (rend_service_poison_new_single_onion_dirs(NULL) < 0) {
+      log_warn(LD_GENERAL,"Failed to mark new hidden services as Single "
+               "Onion.");
+      return -1;
+    }
+  }
+
   /* reload keys as needed for rendezvous services. */
-  if (rend_service_load_all_keys()<0) {
+  if (rend_service_load_all_keys(NULL)<0) {
     log_warn(LD_GENERAL,"Error loading rendezvous service keys");
     return -1;
   }
@@ -2796,6 +2818,88 @@ warn_about_relative_paths(or_options_t *options)
   }
 }
 
+/* Validate options related to OnionServiceSingleHopMode.
+ * Modifies some options that are incompatible with OnionServiceSingleHopMode.
+ * On failure returns -1, and sets *msg to an error string.
+ * Returns 0 on success. */
+STATIC int
+options_validate_single_onion(or_options_t *options, char **msg)
+{
+  /* You must set OnionServiceNonAnonymousMode to 1 to use
+   * OnionServiceSingleHopMode */
+  if (options->OnionServiceSingleHopMode &&
+      !rend_service_non_anonymous_mode_enabled(options)) {
+    REJECT("OnionServiceSingleHopMode does not provide any server anonymity. "
+           "It must be used with OnionServiceNonAnonymousMode set to 1.");
+  }
+
+  /* If you have OnionServiceNonAnonymousMode set, you must use
+   * OnionServiceSingleHopMode. */
+  if (rend_service_non_anonymous_mode_enabled(options) &&
+      !options->OnionServiceSingleHopMode) {
+    REJECT("OnionServiceNonAnonymousMode does not provide any server "
+           "anonymity. It must be used with OnionServiceSingleHopMode set to "
+           "1.");
+  }
+
+  /* If you run an anonymous client with an active Single Onion service, the
+   * client loses anonymity. */
+  const int client_port_set = (options->SocksPort_set ||
+                               options->TransPort_set ||
+                               options->NATDPort_set ||
+                               options->DNSPort_set);
+  if (options->OnionServiceSingleHopMode && client_port_set &&
+      !options->Tor2webMode) {
+    REJECT("OnionServiceSingleHopMode is incompatible with using Tor as an "
+           "anonymous client. Please set Socks/Trans/NATD/DNSPort to 0, or "
+           "OnionServiceSingleHopMode to 0, or use the non-anonymous "
+           "Tor2webMode.");
+  }
+
+  /* If you run a hidden service in non-anonymous mode, the hidden service
+   * loses anonymity, even if SOCKSPort / Tor2web mode isn't used. */
+  if (!options->OnionServiceSingleHopMode && options->RendConfigLines
+      && options->Tor2webMode) {
+    REJECT("Non-anonymous (Tor2web) mode is incompatible with using Tor as a "
+           "hidden service. Please remove all HiddenServiceDir lines, or use "
+           "a version of tor compiled without --enable-tor2web-mode, or use "
+           "the non-anonymous OnionServiceSingleHopMode.");
+  }
+
+  if (options->OnionServiceSingleHopMode
+      && options->UseEntryGuards) {
+    /* Single Onion services do not (and should not) use entry guards
+     * in any meaningful way.  Further, Single Onions causes the hidden
+     * service code to do things which break the path bias
+     * detector, and it's far easier to turn off entry guards (and
+     * thus the path bias detector with it) than to figure out how to
+     * make a piece of code which cannot possibly help Single Onions,
+     * compatible with OnionServiceSingleHopMode.
+     */
+    log_notice(LD_CONFIG,
+               "OnionServiceSingleHopMode is enabled; disabling "
+               "UseEntryGuards.");
+    options->UseEntryGuards = 0;
+  }
+
+  /* Check if existing hidden service keys were created with a different
+   * setting of OnionServiceNonAnonymousMode, and refuse to launch if they
+   * have. We'll poison new keys in options_act() just before we create them.
+   */
+  if (rend_service_list_verify_single_onion_poison(NULL, options) < 0) {
+    log_warn(LD_GENERAL, "We are configured with OnionServiceSingleHopMode "
+             "%d, but one or more hidden service keys were created in %s "
+             "mode. This is not allowed.",
+             rend_service_non_anonymous_mode_enabled(options) ? 1 : 0,
+             rend_service_non_anonymous_mode_enabled(options) ?
+             "an anonymous" : "a non-anonymous"
+             );
+    return -1;
+  }
+
+  return 0;
+}
+
 /** Return 0 if every setting in <b>options</b> is reasonable, is a
  * permissible transition from <b>old_options</b>, and none of the
  * testing-only settings differ from <b>default_options</b> unless in
@@ -3291,25 +3395,11 @@ options_validate(or_options_t *old_options, or_options_t *options,
     options->PredictedPortsRelevanceTime = MAX_PREDICTED_CIRCS_RELEVANCE;
   }
 
-#ifdef ENABLE_TOR2WEB_MODE
-  if (options->Tor2webMode && options->LearnCircuitBuildTimeout) {
-    /* LearnCircuitBuildTimeout and Tor2webMode are incompatible in
-     * two ways:
-     *
-     * - LearnCircuitBuildTimeout results in a low CBT, which
-     *   Tor2webMode's use of one-hop rendezvous circuits lowers
-     *   much further, producing *far* too many timeouts.
-     *
-     * - The adaptive CBT code does not update its timeout estimate
-     *   using build times for single-hop circuits.
-     *
-     * If we fix both of these issues someday, we should test
-     * Tor2webMode with LearnCircuitBuildTimeout on again. */
-    log_notice(LD_CONFIG,"Tor2webMode is enabled; turning "
-               "LearnCircuitBuildTimeout off.");
-    options->LearnCircuitBuildTimeout = 0;
-  }
+  /* Check the Single Onion Service options */
+  if (options_validate_single_onion(options, msg) < 0)
+    return -1;
 
+#ifdef ENABLE_TOR2WEB_MODE
   if (options->Tor2webMode && options->UseEntryGuards) {
     /* tor2web mode clients do not (and should not) use entry guards
      * in any meaningful way.  Further, tor2web mode causes the hidden
@@ -3353,6 +3443,17 @@ options_validate(or_options_t *old_options, or_options_t *options,
     return -1;
   }
 
+  /* OnionServiceSingleHopMode: one hop between the onion service server and
+   * intro and rendezvous points */
+  if (options->OnionServiceSingleHopMode) {
+    log_warn(LD_CONFIG,
+             "OnionServiceSingleHopMode is set. Every hidden service on this "
+             "tor instance is NON-ANONYMOUS. If OnionServiceSingleHopMode is "
+             "disabled, Tor will refuse to launch hidden services from the "
+             "same directories, to protect against config errors. This "
+             "setting is for experimental use only.");
+  }
+
   if (!options->LearnCircuitBuildTimeout && options->CircuitBuildTimeout &&
       options->CircuitBuildTimeout < RECOMMENDED_MIN_CIRCUIT_BUILD_TIMEOUT) {
     log_warn(LD_CONFIG,
@@ -4295,6 +4396,19 @@ options_transition_allowed(const or_options_t *old,
     return -1;
   }
 
+  if (old->OnionServiceSingleHopMode != new_val->OnionServiceSingleHopMode) {
+    *msg = tor_strdup("While Tor is running, changing "
+                      "OnionServiceSingleHopMode is not allowed.");
+    return -1;
+  }
+
+  if (old->OnionServiceNonAnonymousMode !=
+      new_val->OnionServiceNonAnonymousMode) {
+    *msg = tor_strdup("While Tor is running, changing "
+                      "OnionServiceNonAnonymousMode is not allowed.");
+    return -1;
+  }
+
   if (old->DisableDebuggerAttachment &&
       !new_val->DisableDebuggerAttachment) {
     *msg = tor_strdup("While Tor is running, disabling "
diff --git a/src/or/config.h b/src/or/config.h
index 7db66a31b9..208659acb7 100644
--- a/src/or/config.h
+++ b/src/or/config.h
@@ -166,6 +166,8 @@ extern struct config_format_t options_format;
 STATIC port_cfg_t *port_cfg_new(size_t namelen);
 STATIC void port_cfg_free(port_cfg_t *port);
 STATIC void or_options_free(or_options_t *options);
+STATIC int options_validate_single_onion(or_options_t *options,
+                                         char **msg);
 STATIC int options_validate(or_options_t *old_options,
                             or_options_t *options,
                             or_options_t *default_options,
diff --git a/src/or/connection_edge.c b/src/or/connection_edge.c
index dc6b0930ca..8ad0f0c71d 100644
--- a/src/or/connection_edge.c
+++ b/src/or/connection_edge.c
@@ -27,6 +27,7 @@
 #include "control.h"
 #include "dns.h"
 #include "dnsserv.h"
+#include "directory.h"
 #include "dirserv.h"
 #include "hibernate.h"
 #include "main.h"
@@ -2271,6 +2272,7 @@ connection_ap_handshake_send_begin(entry_connection_t *ap_conn)
   char payload[CELL_PAYLOAD_SIZE];
   int payload_len;
   int begin_type;
+  const or_options_t *options = get_options();
   origin_circuit_t *circ;
   edge_connection_t *edge_conn = ENTRY_TO_EDGE_CONN(ap_conn);
   connection_t *base_conn = TO_CONN(edge_conn);
@@ -2314,10 +2316,31 @@ connection_ap_handshake_send_begin(entry_connection_t *ap_conn)
 
   begin_type = ap_conn->use_begindir ?
                  RELAY_COMMAND_BEGIN_DIR : RELAY_COMMAND_BEGIN;
+
+  /* Check that circuits are anonymised, based on their type. */
   if (begin_type == RELAY_COMMAND_BEGIN) {
-#ifndef NON_ANONYMOUS_MODE_ENABLED
-    tor_assert(circ->build_state->onehop_tunnel == 0);
-#endif
+    /* This connection is a standard OR connection.
+     * Make sure its path length is anonymous, or that we're in a
+     * non-anonymous mode. */
+    assert_circ_anonymity_ok(circ, options);
+  } else if (begin_type == RELAY_COMMAND_BEGIN_DIR) {
+    /* This connection is a begindir directory connection.
+     * Look at the linked directory connection to access the directory purpose.
+     * (This must be non-NULL, because we're doing begindir.) */
+    tor_assert(base_conn->linked);
+    connection_t *linked_dir_conn_base = base_conn->linked_conn;
+    tor_assert(linked_dir_conn_base);
+    /* Sensitive directory connections must have an anonymous path length.
+     * Otherwise, directory connections are typically one-hop.
+     * This matches the earlier check for directory connection path anonymity
+     * in directory_initiate_command_rend(). */
+    if (is_sensitive_dir_purpose(linked_dir_conn_base->purpose)) {
+      assert_circ_anonymity_ok(circ, options);
+    }
+  } else {
+    /* This code was written for the two connection types BEGIN and BEGIN_DIR
+     */
+    tor_assert_unreached();
   }
 
   if (connection_edge_send_command(edge_conn, begin_type,
diff --git a/src/or/directory.c b/src/or/directory.c
index d37b5c2e0f..52b14b9bae 100644
--- a/src/or/directory.c
+++ b/src/or/directory.c
@@ -1085,7 +1085,7 @@ directory_initiate_command(const tor_addr_t *or_addr, uint16_t or_port,
  * <b>dir_purpose</b> reveals sensitive information about a Tor
  * instance's client activities.  (Such connections must be performed
  * through normal three-hop Tor circuits.) */
-static int
+int
 is_sensitive_dir_purpose(uint8_t dir_purpose)
 {
   return ((dir_purpose == DIR_PURPOSE_HAS_FETCHED_RENDDESC_V2) ||
@@ -1140,12 +1140,10 @@ directory_initiate_command_rend(const tor_addr_port_t *or_addr_port,
 
   log_debug(LD_DIR, "Initiating %s", dir_conn_purpose_to_string(dir_purpose));
 
-#ifndef NON_ANONYMOUS_MODE_ENABLED
-  tor_assert(!(is_sensitive_dir_purpose(dir_purpose) &&
-               !anonymized_connection));
-#else
-  (void)is_sensitive_dir_purpose;
-#endif
+  if (is_sensitive_dir_purpose(dir_purpose)) {
+    tor_assert(anonymized_connection ||
+               rend_non_anonymous_mode_enabled(options));
+  }
 
   /* use encrypted begindir connections for everything except relays
    * this provides better protection for directory fetches */
diff --git a/src/or/directory.h b/src/or/directory.h
index f04e7ab315..9477948aa0 100644
--- a/src/or/directory.h
+++ b/src/or/directory.h
@@ -132,7 +132,10 @@ int download_status_get_n_failures(const download_status_t *dls);
 int download_status_get_n_attempts(const download_status_t *dls);
 time_t download_status_get_next_attempt_at(const download_status_t *dls);
 
+/* Yes, these two functions are confusingly similar.
+ * Let's sort that out in #20077. */
 int purpose_needs_anonymity(uint8_t dir_purpose, uint8_t router_purpose);
+int is_sensitive_dir_purpose(uint8_t dir_purpose);
 
 #ifdef TOR_UNIT_TESTS
 /* Used only by directory.c and test_dir.c */
diff --git a/src/or/main.c b/src/or/main.c
index 03c2b7ed58..76f9f38d3c 100644
--- a/src/or/main.c
+++ b/src/or/main.c
@@ -2833,11 +2833,6 @@ tor_init(int argc, char *argv[])
                  "Expect more bugs than usual.");
   }
 
-#ifdef NON_ANONYMOUS_MODE_ENABLED
-  log_warn(LD_GENERAL, "This copy of Tor was compiled to run in a "
-      "non-anonymous mode. It will provide NO ANONYMITY.");
-#endif
-
   if (network_init()<0) {
     log_err(LD_BUG,"Error initializing network; exiting.");
     return -1;
@@ -2849,6 +2844,14 @@ tor_init(int argc, char *argv[])
     return -1;
   }
 
+  /* The options are now initialised */
+  const or_options_t *options = get_options();
+
+  if (rend_non_anonymous_mode_enabled(options)) {
+    log_warn(LD_GENERAL, "This copy of Tor was compiled or configured to run "
+             "in a non-anonymous mode. It will provide NO ANONYMITY.");
+  }
+
 #ifndef _WIN32
   if (geteuid()==0)
     log_warn(LD_GENERAL,"You are running Tor as root. You don't need to, "
diff --git a/src/or/or.h b/src/or/or.h
index 574f184a13..dd3ab8a03a 100644
--- a/src/or/or.h
+++ b/src/or/or.h
@@ -3701,6 +3701,27 @@ typedef struct {
    * they reach the normal circuit-build timeout. */
   int CloseHSServiceRendCircuitsImmediatelyOnTimeout;
 
+  /** Onion Services in OnionServiceSingleHopMode make one-hop (direct)
+   * circuits between the onion service server, and the introduction and
+   * rendezvous points. (Onion service descriptors are still posted using
+   * 3-hop paths, to avoid onion service directories blocking the service.)
+   * This option makes every hidden service instance hosted by
+   * this tor instance a Single Onion Service. One-hop circuits make Single
+   * Onion servers easily locatable, but clients remain location-anonymous.
+   * OnionServiceSingleHopMode requires OnionServiceNonAnonymousMode to be set
+   * to 1.
+   * Use rend_service_allow_non_anonymous_connection() or
+   * rend_service_reveal_startup_time() instead of using this option directly.
+   */
+  int OnionServiceSingleHopMode;
+  /* Makes hidden service clients and servers non-anonymous on this tor
+   * instance. Allows the non-anonymous OnionServiceSingleHopMode. Enables
+   * direct connections in the hidden service protocol.
+   * Use rend_service_non_anonymous_mode() instead of using this option
+   * directly.
+   */
+  int OnionServiceNonAnonymousMode;
+
   int ConnLimit; /**< Demanded minimum number of simultaneous connections. */
   int ConnLimit_; /**< Maximum allowed number of simultaneous connections. */
   int ConnLimit_high_thresh; /**< start trying to lower socket usage if we
diff --git a/src/or/rendclient.c b/src/or/rendclient.c
index 3a742fec0a..9d16e3b716 100644
--- a/src/or/rendclient.c
+++ b/src/or/rendclient.c
@@ -134,6 +134,7 @@ int
 rend_client_send_introduction(origin_circuit_t *introcirc,
                               origin_circuit_t *rendcirc)
 {
+  const or_options_t *options = get_options();
   size_t payload_len;
   int r, v3_shift = 0;
   char payload[RELAY_PAYLOAD_SIZE];
@@ -150,10 +151,8 @@ rend_client_send_introduction(origin_circuit_t *introcirc,
   tor_assert(rendcirc->rend_data);
   tor_assert(!rend_cmp_service_ids(introcirc->rend_data->onion_address,
                                    rendcirc->rend_data->onion_address));
-#ifndef NON_ANONYMOUS_MODE_ENABLED
-  tor_assert(!(introcirc->build_state->onehop_tunnel));
-  tor_assert(!(rendcirc->build_state->onehop_tunnel));
-#endif
+  assert_circ_anonymity_ok(introcirc, options);
+  assert_circ_anonymity_ok(rendcirc, options);
 
   r = rend_cache_lookup_entry(introcirc->rend_data->onion_address, -1,
                               &entry);
@@ -387,6 +386,7 @@ int
 rend_client_introduction_acked(origin_circuit_t *circ,
                                const uint8_t *request, size_t request_len)
 {
+  const or_options_t *options = get_options();
   origin_circuit_t *rendcirc;
   (void) request; // XXXX Use this.
 
@@ -398,10 +398,9 @@ rend_client_introduction_acked(origin_circuit_t *circ,
     return -1;
   }
 
+  tor_assert(circ->build_state);
   tor_assert(circ->build_state->chosen_exit);
-#ifndef NON_ANONYMOUS_MODE_ENABLED
-  tor_assert(!(circ->build_state->onehop_tunnel));
-#endif
+  assert_circ_anonymity_ok(circ, options);
   tor_assert(circ->rend_data);
 
   /* For path bias: This circuit was used successfully. Valid
@@ -416,9 +415,7 @@ rend_client_introduction_acked(origin_circuit_t *circ,
     log_info(LD_REND,"Received ack. Telling rend circ...");
     rendcirc = circuit_get_ready_rend_circ_by_rend_data(circ->rend_data);
     if (rendcirc) { /* remember the ack */
-#ifndef NON_ANONYMOUS_MODE_ENABLED
-      tor_assert(!(rendcirc->build_state->onehop_tunnel));
-#endif
+      assert_circ_anonymity_ok(rendcirc, options);
       circuit_change_purpose(TO_CIRCUIT(rendcirc),
                              CIRCUIT_PURPOSE_C_REND_READY_INTRO_ACKED);
       /* Set timestamp_dirty, because circuit_expire_building expects
@@ -1552,3 +1549,35 @@ rend_parse_service_authorization(const or_options_t *options,
   return res;
 }
 
+/* Can Tor client code make direct (non-anonymous) connections to introduction
+ * or rendezvous points?
+ * Returns true if tor was compiled with NON_ANONYMOUS_MODE_ENABLED, and is
+ * configured in Tor2web mode. */
+int
+rend_client_allow_non_anonymous_connection(const or_options_t *options)
+{
+  /* Tor2web support needs to be compiled in to a tor binary. */
+#ifdef NON_ANONYMOUS_MODE_ENABLED
+  /* Tor2web */
+  return options->Tor2webMode ? 1 : 0;
+#else
+  (void)options;
+  return 0;
+#endif
+}
+
+/* At compile-time, was non-anonymous mode enabled via
+ * NON_ANONYMOUS_MODE_ENABLED ? */
+int
+rend_client_non_anonymous_mode_enabled(const or_options_t *options)
+{
+  (void)options;
+  /* Tor2web support needs to be compiled in to a tor binary. */
+#ifdef NON_ANONYMOUS_MODE_ENABLED
+  /* Tor2web */
+  return 1;
+#else
+  return 0;
+#endif
+}
+
diff --git a/src/or/rendclient.h b/src/or/rendclient.h
index e90dac07ab..b8f8c2f871 100644
--- a/src/or/rendclient.h
+++ b/src/or/rendclient.h
@@ -51,5 +51,8 @@ rend_service_authorization_t *rend_client_lookup_service_authorization(
                                                 const char *onion_address);
 void rend_service_authorization_free_all(void);
 
+int rend_client_allow_non_anonymous_connection(const or_options_t *options);
+int rend_client_non_anonymous_mode_enabled(const or_options_t *options);
+
 #endif
 
diff --git a/src/or/rendcommon.c b/src/or/rendcommon.c
index 01b0766cf0..12dd0f44e5 100644
--- a/src/or/rendcommon.c
+++ b/src/or/rendcommon.c
@@ -1067,3 +1067,51 @@ rend_auth_decode_cookie(const char *cookie_in, uint8_t *cookie_out,
   return res;
 }
 
+/* Is this a rend client or server that allows direct (non-anonymous)
+ * connections?
+ * Clients must be specifically compiled and configured in this mode.
+ * Onion services can be configured to start in this mode.
+ * Prefer rend_client_allow_non_anonymous_connection() or
+ * rend_service_allow_non_anonymous_connection() whenever possible, so that
+ * checks are specific to Single Onion Services or Tor2web. */
+int
+rend_allow_non_anonymous_connection(const or_options_t* options)
+{
+  return (rend_client_allow_non_anonymous_connection(options)
+          || rend_service_allow_non_anonymous_connection(options));
+}
+
+/* Is this a rend client or server in non-anonymous mode?
+ * Clients must be specifically compiled in this mode.
+ * Onion services can be configured to start in this mode.
+ * Prefer rend_client_non_anonymous_mode_enabled() or
+ * rend_service_non_anonymous_mode_enabled() whenever possible, so that checks
+ * are specific to Single Onion Services or Tor2web. */
+int
+rend_non_anonymous_mode_enabled(const or_options_t *options)
+{
+  return (rend_client_non_anonymous_mode_enabled(options)
+          || rend_service_non_anonymous_mode_enabled(options));
+}
+
+/* Make sure that tor only builds one-hop circuits when they would not
+ * compromise user anonymity.
+ *
+ * One-hop circuits are permitted in Tor2webMode or OnionServiceSingleHopMode.
+ *
+ * Tor2webMode and OnionServiceSingleHopMode are also allowed to make
+ * multi-hop circuits. For example, single onion HSDir circuits are 3-hop to
+ * prevent denial of service.
+ */
+void
+assert_circ_anonymity_ok(origin_circuit_t *circ,
+                         const or_options_t *options)
+{
+  tor_assert(options);
+  tor_assert(circ);
+  tor_assert(circ->build_state);
+
+  if (circ->build_state->onehop_tunnel) {
+    tor_assert(rend_allow_non_anonymous_connection(options));
+  }
+}
diff --git a/src/or/rendcommon.h b/src/or/rendcommon.h
index 88cf512f4a..090e6f25e0 100644
--- a/src/or/rendcommon.h
+++ b/src/or/rendcommon.h
@@ -77,5 +77,11 @@ int rend_auth_decode_cookie(const char *cookie_in,
                             rend_auth_type_t *auth_type_out,
                             char **err_msg_out);
 
+int rend_allow_non_anonymous_connection(const or_options_t* options);
+int rend_non_anonymous_mode_enabled(const or_options_t *options);
+
+void assert_circ_anonymity_ok(origin_circuit_t *circ,
+                              const or_options_t *options);
+
 #endif
 
diff --git a/src/or/rendservice.c b/src/or/rendservice.c
index fa147ba98e..0ba9205c6e 100644
--- a/src/or/rendservice.c
+++ b/src/or/rendservice.c
@@ -20,6 +20,7 @@
 #include "main.h"
 #include "networkstatus.h"
 #include "nodelist.h"
+#include "policies.h"
 #include "rendclient.h"
 #include "rendcommon.h"
 #include "rendservice.h"
@@ -949,6 +950,208 @@ rend_service_update_descriptor(rend_service_t *service)
   }
 }
 
+static const char *sos_poison_fname = "non_anonymous_hidden_service";
+
+/* Allocate and return a string containing the path to the single onion
+ * poison file in service.
+ * The caller must free this path.
+ * Returns NULL if there is no directory for service. */
+static char *
+sos_poison_path(const rend_service_t *service)
+{
+  char *poison_path;
+
+  tor_assert(service->directory);
+
+  tor_asprintf(&poison_path, "%s%s%s",
+               service->directory, PATH_SEPARATOR, sos_poison_fname);
+
+  return poison_path;
+}
+
+/** Return True if hidden services <b>service> has been poisoned by single
+ * onion mode. */
+static int
+service_is_single_onion_poisoned(const rend_service_t *service)
+{
+  char *poison_fname = NULL;
+  file_status_t fstatus;
+
+  if (!service->directory) {
+    return 0;
+  }
+
+  poison_fname = sos_poison_path(service);
+  tor_assert(poison_fname);
+
+  fstatus = file_status(poison_fname);
+  tor_free(poison_fname);
+
+  /* If this fname is occupied, the hidden service has been poisoned. */
+  if (fstatus == FN_FILE || fstatus == FN_EMPTY) {
+    return 1;
+  }
+
+  return 0;
+}
+
+/* Return 1 if the private key file for service exists and has a non-zero size,
+ * and 0 otherwise. */
+static int
+rend_service_private_key_exists(const rend_service_t *service)
+{
+  char *private_key_path = rend_service_path(service, private_key_fname);
+  const file_status_t private_key_status = file_status(private_key_path);
+  tor_free(private_key_path);
+  /* Only non-empty regular private key files could have been used before. */
+  return private_key_status == FN_FILE;
+}
+
+/** Check the single onion service poison state of all existing hidden service
+ * directories:
+ * - If each service is poisoned, and we are in OnionServiceSingleHopMode,
+ *   return 0,
+ * - If each service is not poisoned, and we are not in
+ *   OnionServiceSingleHopMode, return 0,
+ * - Otherwise, the poison state is invalid, and a service that was created in
+ *   one mode is being used in the other, return -1.
+ * Hidden service directories without keys are not checked for consistency.
+ * When their keys are created, they will be poisoned (if needed).
+ * If a <b>service_list</b> is provided, treat it
+ * as the list of hidden services (used in unittests). */
+int
+rend_service_list_verify_single_onion_poison(const smartlist_t *service_list,
+                                             const or_options_t *options)
+{
+  const smartlist_t *s_list;
+  /* If no special service list is provided, then just use the global one. */
+  if (!service_list) {
+    if (!rend_service_list) { /* No global HS list. Nothing to see here. */
+      return 0;
+    }
+
+    s_list = rend_service_list;
+  } else {
+    s_list = service_list;
+  }
+
+  int consistent = 1;
+  SMARTLIST_FOREACH_BEGIN(s_list, const rend_service_t *, s) {
+    if (service_is_single_onion_poisoned(s) !=
+        rend_service_allow_non_anonymous_connection(options) &&
+        rend_service_private_key_exists(s)) {
+      consistent = 0;
+    }
+  } SMARTLIST_FOREACH_END(s);
+
+  return consistent ? 0 : -1;
+}
+
+/*** Helper for rend_service_poison_new_single_onion_dirs(). When in single
+ * onion mode, add a file to this hidden service directory that marks it as a
+ * single onion hidden service. Returns 0 when a directory is successfully
+ * poisoned, or if it is already poisoned. Returns -1 on a failure to read
+ * the directory or write the poison file, or if there is an existing private
+ * key file in the directory. (The service should have been poisoned when the
+ * key was created.) */
+static int
+poison_new_single_onion_hidden_service_dir(const rend_service_t *service)
+{
+  /* We must only poison directories if we're in Single Onion mode */
+  tor_assert(rend_service_allow_non_anonymous_connection(get_options()));
+
+  int fd;
+  int retval = -1;
+  char *poison_fname = NULL;
+
+  if (!service->directory) {
+    log_info(LD_REND, "Ephemeral HS started in OnionServiceSingleHopMode.");
+    return 0;
+  }
+
+  /* Make sure we're only poisoning new hidden service directories */
+  if (rend_service_private_key_exists(service)) {
+    log_warn(LD_BUG, "Tried to single onion poison a service directory after "
+             "the private key was created.");
+    return -1;
+  }
+
+  poison_fname = sos_poison_path(service);
+
+  switch (file_status(poison_fname)) {
+  case FN_DIR:
+  case FN_ERROR:
+    log_warn(LD_FS, "Can't read single onion poison file \"%s\"",
+             poison_fname);
+    goto done;
+  case FN_FILE: /* single onion poison file already exists. NOP. */
+  case FN_EMPTY: /* single onion poison file already exists. NOP. */
+    log_debug(LD_FS, "Tried to re-poison a single onion poisoned file \"%s\"",
+              poison_fname);
+    break;
+  case FN_NOENT:
+    fd = tor_open_cloexec(poison_fname, O_RDWR|O_CREAT|O_TRUNC, 0600);
+    if (fd < 0) {
+      log_warn(LD_FS, "Could not create single onion poison file %s",
+               poison_fname);
+      goto done;
+    }
+    close(fd);
+    break;
+  default:
+    tor_assert(0);
+  }
+
+  retval = 0;
+
+ done:
+  tor_free(poison_fname);
+
+  return retval;
+}
+
+/** We just got launched in OnionServiceSingleHopMode. That's a non-anoymous
+ * mode for hidden services; hence we should mark all new hidden service
+ * directories appropriately so that they are never launched as
+ * location-private hidden services again. (New directories don't have private
+ * key files.)
+ * If a <b>service_list</b> is provided, treat it as the list of hidden
+ * services (used in unittests).
+ * Return 0 on success, -1 on fail. */
+int
+rend_service_poison_new_single_onion_dirs(const smartlist_t *service_list)
+{
+  /* We must only poison directories if we're in Single Onion mode */
+  tor_assert(rend_service_allow_non_anonymous_connection(get_options()));
+
+  const smartlist_t *s_list;
+  /* If no special service list is provided, then just use the global one. */
+  if (!service_list) {
+    if (!rend_service_list) { /* No global HS list. Nothing to see here. */
+      return 0;
+    }
+
+    s_list = rend_service_list;
+  } else {
+    s_list = service_list;
+  }
+
+  SMARTLIST_FOREACH_BEGIN(s_list, const rend_service_t *, s) {
+    if (!rend_service_private_key_exists(s)) {
+      if (poison_new_single_onion_hidden_service_dir(s) < 0) {
+        return -1;
+      }
+    }
+  } SMARTLIST_FOREACH_END(s);
+
+  /* The keys for these services are linked to the server IP address */
+  log_notice(LD_REND, "The configured onion service directories have been "
+             "used in single onion mode. They can not be used for anonymous "
+             "hidden services.");
+
+  return 0;
+}
+
 /** Load and/or generate private keys for all hidden services, possibly
  * including keys for client authorization.  Return 0 on success, -1 on
  * failure. */
@@ -1372,6 +1575,19 @@ rend_check_authorization(rend_service_t *service,
   return 1;
 }
 
+/* Can this service make a direct connection to ei?
+ * It must be a single onion service, and the firewall rules must allow ei. */
+static int
+rend_service_use_direct_connection(const or_options_t* options,
+                                   const extend_info_t* ei)
+{
+  /* The prefer_ipv6 argument to fascist_firewall_allows_address_addr is
+   * ignored, because pref_only is 0. */
+  return (rend_service_allow_direct_connection(options) &&
+          fascist_firewall_allows_address_addr(&ei->addr, ei->port,
+                                               FIREWALL_OR_CONNECTION, 0, 0));
+}
+
 /******
  * Handle cells
  ******/
@@ -1421,9 +1637,7 @@ rend_service_receive_introduction(origin_circuit_t *circuit,
     goto err;
   }
 
-#ifndef NON_ANONYMOUS_MODE_ENABLED
-  tor_assert(!(circuit->build_state->onehop_tunnel));
-#endif
+  assert_circ_anonymity_ok(circuit, options);
   tor_assert(circuit->rend_data);
 
   /* We'll use this in a bazillion log messages */
@@ -1627,6 +1841,11 @@ rend_service_receive_introduction(origin_circuit_t *circuit,
   for (i=0;i<MAX_REND_FAILURES;i++) {
     int flags = CIRCLAUNCH_NEED_CAPACITY | CIRCLAUNCH_IS_INTERNAL;
     if (circ_needs_uptime) flags |= CIRCLAUNCH_NEED_UPTIME;
+    /* A Single Onion Service only uses a direct connection if its
+     * firewall rules permit direct connections to the address. */
+    if (rend_service_use_direct_connection(options, rp)) {
+      flags = flags | CIRCLAUNCH_ONEHOP_TUNNEL;
+    }
     launched = circuit_launch_by_extend_info(
                         CIRCUIT_PURPOSE_S_CONNECT_REND, rp, flags);
 
@@ -1739,7 +1958,10 @@ find_rp_for_intro(const rend_intro_cell_t *intro,
       goto err;
     }
 
-    rp = extend_info_from_node(node, 0);
+    /* Are we in single onion mode? */
+    const int allow_direct = rend_service_allow_non_anonymous_connection(
+                                                                get_options());
+    rp = extend_info_from_node(node, allow_direct);
     if (!rp) {
       if (err_msg_out) {
         tor_asprintf(&err_msg,
@@ -1764,6 +1986,10 @@ find_rp_for_intro(const rend_intro_cell_t *intro,
     goto err;
   }
 
+  /* rp is always set here: extend_info_dup guarantees a non-NULL result, and
+   * the other cases goto err. */
+  tor_assert(rp);
+
   /* Make sure the RP we are being asked to connect to is _not_ a private
    * address unless it's allowed. Let's avoid to build a circuit to our
    * second middle node and fail right after when extending to the RP. */
@@ -2538,6 +2764,10 @@ rend_service_relaunch_rendezvous(origin_circuit_t *oldcirc)
   log_info(LD_REND,"Reattempting rendezvous circuit to '%s'",
            safe_str(extend_info_describe(oldstate->chosen_exit)));
 
+  /* You'd think Single Onion Services would want to retry the rendezvous
+   * using a direct connection. But if it's blocked by a firewall, or the
+   * service is IPv6-only, or the rend point avoiding becoming a one-hop
+   * proxy, we need a 3-hop connection. */
   newcirc = circuit_launch_by_extend_info(CIRCUIT_PURPOSE_S_CONNECT_REND,
                             oldstate->chosen_exit,
                             CIRCLAUNCH_NEED_CAPACITY|CIRCLAUNCH_IS_INTERNAL);
@@ -2566,6 +2796,11 @@ rend_service_launch_establish_intro(rend_service_t *service,
                                     rend_intro_point_t *intro)
 {
   origin_circuit_t *launched;
+  int flags = CIRCLAUNCH_NEED_UPTIME|CIRCLAUNCH_IS_INTERNAL;
+
+  if (rend_service_allow_direct_connection(get_options())) {
+    flags = flags | CIRCLAUNCH_ONEHOP_TUNNEL;
+  }
 
   log_info(LD_REND,
            "Launching circuit to introduction point %s for service %s",
@@ -2576,8 +2811,7 @@ rend_service_launch_establish_intro(rend_service_t *service,
 
   ++service->n_intro_circuits_launched;
   launched = circuit_launch_by_extend_info(CIRCUIT_PURPOSE_S_ESTABLISH_INTRO,
-                             intro->extend_info,
-                             CIRCLAUNCH_NEED_UPTIME|CIRCLAUNCH_IS_INTERNAL);
+                             intro->extend_info, flags);
 
   if (!launched) {
     log_info(LD_REND,
@@ -2651,9 +2885,7 @@ rend_service_intro_has_opened(origin_circuit_t *circuit)
   int reason = END_CIRC_REASON_TORPROTOCOL;
 
   tor_assert(circuit->base_.purpose == CIRCUIT_PURPOSE_S_ESTABLISH_INTRO);
-#ifndef NON_ANONYMOUS_MODE_ENABLED
-  tor_assert(!(circuit->build_state->onehop_tunnel));
-#endif
+  assert_circ_anonymity_ok(circuit, get_options());
   tor_assert(circuit->cpath);
   tor_assert(circuit->rend_data);
 
@@ -2720,6 +2952,7 @@ rend_service_intro_has_opened(origin_circuit_t *circuit)
   log_info(LD_REND,
            "Established circuit %u as introduction point for service %s",
            (unsigned)circuit->base_.n_circ_id, serviceid);
+  circuit_log_path(LOG_INFO, LD_REND, circuit);
 
   /* Use the intro key instead of the service key in ESTABLISH_INTRO. */
   crypto_pk_t *intro_key = circuit->intro_key;
@@ -2849,9 +3082,7 @@ rend_service_rendezvous_has_opened(origin_circuit_t *circuit)
   tor_assert(circuit->base_.purpose == CIRCUIT_PURPOSE_S_CONNECT_REND);
   tor_assert(circuit->cpath);
   tor_assert(circuit->build_state);
-#ifndef NON_ANONYMOUS_MODE_ENABLED
-  tor_assert(!(circuit->build_state->onehop_tunnel));
-#endif
+  assert_circ_anonymity_ok(circuit, get_options());
   tor_assert(circuit->rend_data);
 
   /* Declare the circuit dirty to avoid reuse, and for path-bias */
@@ -2871,6 +3102,7 @@ rend_service_rendezvous_has_opened(origin_circuit_t *circuit)
            "Done building circuit %u to rendezvous with "
            "cookie %s for service %s",
            (unsigned)circuit->base_.n_circ_id, hexcookie, serviceid);
+  circuit_log_path(LOG_INFO, LD_REND, circuit);
 
   /* Clear the 'in-progress HS circ has timed out' flag for
    * consistency with what happens on the client side; this line has
@@ -3547,7 +3779,8 @@ rend_consider_services_intro_points(void)
        * pick it again in the next iteration. */
       smartlist_add(exclude_nodes, (void*)node);
       intro = tor_malloc_zero(sizeof(rend_intro_point_t));
-      intro->extend_info = extend_info_from_node(node, 0);
+      intro->extend_info = extend_info_from_node(node,
+                                rend_service_allow_direct_connection(options));
       intro->intro_key = crypto_pk_new();
       const int fail = crypto_pk_generate_key(intro->intro_key);
       tor_assert(!fail);
@@ -3592,8 +3825,9 @@ rend_consider_services_upload(time_t now)
 {
   int i;
   rend_service_t *service;
-  int rendpostperiod = get_options()->RendPostPeriod;
-  int rendinitialpostdelay = (get_options()->TestingTorNetwork ?
+  const or_options_t *options = get_options();
+  int rendpostperiod = options->RendPostPeriod;
+  int rendinitialpostdelay = (options->TestingTorNetwork ?
                               MIN_REND_INITIAL_POST_DELAY_TESTING :
                               MIN_REND_INITIAL_POST_DELAY);
 
@@ -3604,6 +3838,12 @@ rend_consider_services_upload(time_t now)
        * the descriptor is stable before being published. See comment below. */
       service->next_upload_time =
         now + rendinitialpostdelay + crypto_rand_int(2*rendpostperiod);
+      /* Single Onion Services prioritise availability over hiding their
+       * startup time, as their IP address is publicly discoverable anyway.
+       */
+      if (rend_service_reveal_startup_time(options)) {
+        service->next_upload_time = now + rendinitialpostdelay;
+      }
     }
     /* Does every introduction points have been established? */
     unsigned int intro_points_ready =
@@ -3844,11 +4084,30 @@ rend_service_set_connection_addr_port(edge_connection_t *conn,
     return -2;
 }
 
-/* Stub that should be replaced with the #17178 version of the function
- * when merging. */
+/* Do the options allow onion services to make direct (non-anonymous)
+ * connections to introduction or rendezvous points?
+ * Returns true if tor is in OnionServiceSingleHopMode. */
 int
-rend_service_allow_direct_connection(const or_options_t *options)
+rend_service_allow_non_anonymous_connection(const or_options_t *options)
 {
-  (void)options;
-  return 0;
+  return options->OnionServiceSingleHopMode ? 1 : 0;
+}
+
+/* Do the options allow us to reveal the exact startup time of the onion
+ * service?
+ * Single Onion Services prioritise availability over hiding their
+ * startup time, as their IP address is publicly discoverable anyway.
+ * Returns true if tor is in OnionServiceSingleHopMode. */
+int
+rend_service_reveal_startup_time(const or_options_t *options)
+{
+  return rend_service_allow_non_anonymous_connection(options);
+}
+
+/* Is non-anonymous mode enabled using the OnionServiceNonAnonymousMode
+ * config option? */
+int
+rend_service_non_anonymous_mode_enabled(const or_options_t *options)
+{
+  return options->OnionServiceNonAnonymousMode ? 1 : 0;
 }
diff --git a/src/or/rendservice.h b/src/or/rendservice.h
index d39d7bde5b..0cf448e207 100644
--- a/src/or/rendservice.h
+++ b/src/or/rendservice.h
@@ -164,6 +164,11 @@ void rend_service_port_config_free(rend_service_port_config_t *p);
 
 void rend_authorized_client_free(rend_authorized_client_t *client);
 
+int rend_service_list_verify_single_onion_poison(
+                                              const smartlist_t *service_list,
+                                              const or_options_t *options);
+int rend_service_poison_new_single_onion_dirs(const smartlist_t *service_list);
+
 /** Return value from rend_service_add_ephemeral. */
 typedef enum {
   RSAE_BADAUTH = -5, /**< Invalid auth_type/auth_clients */
@@ -187,7 +192,9 @@ void directory_post_to_hs_dir(rend_service_descriptor_t *renddesc,
                               const char *service_id, int seconds_valid);
 void rend_service_desc_has_uploaded(const rend_data_t *rend_data);
 
-int rend_service_allow_direct_connection(const or_options_t *options);
+int rend_service_allow_non_anonymous_connection(const or_options_t *options);
+int rend_service_reveal_startup_time(const or_options_t *options);
+int rend_service_non_anonymous_mode_enabled(const or_options_t *options);
 
 #endif