diff --git a/.cirrus.yml b/.cirrus.yml
index c2a274ea..17dbb186 100644
--- a/.cirrus.yml
+++ b/.cirrus.yml
@@ -1,7 +1,7 @@
 ---
 cirrus-ci_task:
   container:
-    image: toxchat/toktok-stack:0.0.13
+    image: toxchat/toktok-stack:0.0.18
     cpu: 2
     memory: 2G
   configure_script:
@@ -22,7 +22,7 @@ cirrus-ci_task:
 
 cimple_task:
   container:
-    image: toxchat/toktok-stack:0.0.13
+    image: toxchat/toktok-stack:0.0.18
     cpu: 2
     memory: 4G
   configure_script:
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 98a4eabe..979c4963 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -363,6 +363,7 @@ include(CompileGTest)
 #
 unit_test(toxav ring_buffer)
 unit_test(toxav rtp)
+unit_test(toxcore DHT)
 unit_test(toxcore crypto_core)
 unit_test(toxcore mono_time)
 unit_test(toxcore ping_array)
diff --git a/toxcore/BUILD.bazel b/toxcore/BUILD.bazel
index 0fdfe0ae..0bdd9c72 100644
--- a/toxcore/BUILD.bazel
+++ b/toxcore/BUILD.bazel
@@ -153,6 +153,16 @@ cc_library(
     ],
 )
 
+cc_test(
+    name = "DHT_test",
+    size = "small",
+    srcs = ["DHT_test.cc"],
+    deps = [
+        ":DHT",
+        "@com_google_googletest//:gtest_main",
+    ],
+)
+
 cc_library(
     name = "DHT_srcs",
     hdrs = [
diff --git a/toxcore/DHT.h b/toxcore/DHT.h
index 40ce73f1..4f7b012b 100644
--- a/toxcore/DHT.h
+++ b/toxcore/DHT.h
@@ -17,6 +17,10 @@
 
 #include <stdbool.h>
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 /* Maximum number of clients stored per friend. */
 #define MAX_FRIEND_CLIENTS 8
 
@@ -405,4 +409,8 @@ bool dht_non_lan_connected(const DHT *dht);
 
 uint32_t addto_lists(DHT *dht, IP_Port ip_port, const uint8_t *public_key);
 
+#ifdef __cplusplus
+}  // extern "C"
+#endif
+
 #endif
diff --git a/toxcore/DHT_test.cc b/toxcore/DHT_test.cc
new file mode 100644
index 00000000..9ecd3cb9
--- /dev/null
+++ b/toxcore/DHT_test.cc
@@ -0,0 +1,112 @@
+#include "DHT.h"
+
+#include <gtest/gtest.h>
+
+#include <algorithm>
+#include <array>
+
+#include "crypto_core.h"
+
+namespace {
+
+using PublicKey = std::array<uint8_t, CRYPTO_PUBLIC_KEY_SIZE>;
+
+template <typename T, size_t N>
+std::array<T, N> to_array(T const (&arr)[N]) {
+  std::array<T, N> stdarr;
+  memcpy(stdarr.data(), arr, N);
+  return stdarr;
+}
+
+TEST(IdClosest, IdenticalKeysAreSameDistance) {
+  PublicKey pk0;
+  random_bytes(pk0.data(), CRYPTO_PUBLIC_KEY_SIZE);
+
+  PublicKey pk1;
+  random_bytes(pk1.data(), CRYPTO_PUBLIC_KEY_SIZE);
+
+  PublicKey pk2 = pk1;
+
+  EXPECT_EQ(id_closest(pk0.data(), pk1.data(), pk2.data()), 0);
+}
+
+TEST(IdClosest, DistanceIsCommutative) {
+  for (uint32_t i = 0; i < 100; ++i) {
+    PublicKey pk0;
+    random_bytes(pk0.data(), CRYPTO_PUBLIC_KEY_SIZE);
+
+    PublicKey pk1;
+    random_bytes(pk1.data(), CRYPTO_PUBLIC_KEY_SIZE);
+
+    PublicKey pk2;
+    random_bytes(pk2.data(), CRYPTO_PUBLIC_KEY_SIZE);
+
+    ASSERT_NE(pk1, pk2);  // RNG can't produce the same random key twice
+
+    // Two non-equal keys can't have the same distance from any given key.
+    EXPECT_NE(id_closest(pk0.data(), pk1.data(), pk2.data()), 0);
+
+    if (id_closest(pk0.data(), pk1.data(), pk2.data()) == 1) {
+      EXPECT_EQ(id_closest(pk0.data(), pk2.data(), pk1.data()), 2);
+    }
+
+    if (id_closest(pk0.data(), pk1.data(), pk2.data()) == 2) {
+      EXPECT_EQ(id_closest(pk0.data(), pk2.data(), pk1.data()), 1);
+    }
+  }
+}
+
+TEST(IdClosest, SmallXorDistanceIsCloser) {
+  PublicKey const pk0 = {{0xaa}};
+  PublicKey const pk1 = {{0xa0}};
+  PublicKey const pk2 = {{0x0a}};
+
+  EXPECT_EQ(id_closest(pk0.data(), pk1.data(), pk2.data()), 1);
+}
+
+TEST(IdClosest, DistinctKeysCannotHaveTheSameDistance) {
+  PublicKey const pk0 = {{0x06}};
+  PublicKey const pk1 = {{0x00}};
+  PublicKey pk2 = {{0x00}};
+
+  for (uint8_t i = 1; i < 0xff; ++i) {
+    pk2[0] = i;
+    EXPECT_NE(id_closest(pk0.data(), pk1.data(), pk2.data()), 0);
+  }
+}
+
+TEST(AddToList, OverridesKeysWithCloserKeys) {
+  PublicKey const self_pk = {{0xaa}};
+  PublicKey const keys[] = {
+      {{0xa0}},  // closest
+      {{0x0a}},  //
+      {{0x0b}},  //
+      {{0x0c}},  //
+      {{0x0d}},  //
+      {{0xa1}},  // closer than the 4 keys above
+  };
+
+  std::array<Node_format, 4> nodes{};
+
+  EXPECT_TRUE(add_to_list(nodes.data(), nodes.size(), keys[0].data(), IP_Port(), self_pk.data()));
+  EXPECT_TRUE(add_to_list(nodes.data(), nodes.size(), keys[1].data(), IP_Port(), self_pk.data()));
+  EXPECT_TRUE(add_to_list(nodes.data(), nodes.size(), keys[2].data(), IP_Port(), self_pk.data()));
+  EXPECT_TRUE(add_to_list(nodes.data(), nodes.size(), keys[3].data(), IP_Port(), self_pk.data()));
+
+  EXPECT_EQ(to_array(nodes[0].public_key), keys[0]);
+  EXPECT_EQ(to_array(nodes[1].public_key), keys[1]);
+  EXPECT_EQ(to_array(nodes[2].public_key), keys[2]);
+  EXPECT_EQ(to_array(nodes[3].public_key), keys[3]);
+
+  // key 4 is less close than keys 0-3
+  EXPECT_FALSE(add_to_list(nodes.data(), nodes.size(), keys[4].data(), IP_Port(), self_pk.data()));
+  // 5 is closer than all except key 0
+  EXPECT_TRUE(add_to_list(nodes.data(), nodes.size(), keys[5].data(), IP_Port(), self_pk.data()));
+
+  EXPECT_EQ(to_array(nodes[0].public_key), keys[0]);
+  EXPECT_EQ(to_array(nodes[1].public_key), keys[5]);
+  EXPECT_EQ(to_array(nodes[2].public_key), keys[1]);
+  EXPECT_EQ(to_array(nodes[3].public_key), keys[2]);
+}
+
+}  // namespace