diff --git a/configure.ac b/configure.ac
index 1fb88b3c7f..bad4f047d4 100644
--- a/configure.ac
+++ b/configure.ac
@@ -427,6 +427,22 @@ if test "$tor_cv_c_c99_designated_init" != "yes"; then
   AC_MSG_ERROR([Your compiler doesn't support c99 designated initializers. This is required as of Tor 0.2.6.x])
 fi
 
+saved_CFLAGS="$CFLAGS"
+CFLAGS="$CFLAGS -Werror"
+AC_CACHE_CHECK([for __attribute__((fallthrough))],
+      tor_cv_c_attr_fallthrough,
+      [AC_COMPILE_IFELSE(
+         [AC_LANG_PROGRAM([extern int x; void fn(void) ;],
+	       [[ switch (x) { case 1: fn(); __attribute__((fallthrough));
+                               case 2: fn(); break; } ]])],
+	 [tor_cv_c_attr_fallthrough=yes],
+	 [tor_cv_c_attr_fallthrough=no] )])
+CFLAGS="$saved_CFLAGS"
+
+if test "$tor_cv_c_attr_fallthrough" == "yes"; then
+  AC_DEFINE(HAVE_ATTR_FALLTHROUGH, [1], [defined if we have the fallthrough attribute.])
+fi
+
 TORUSER=_tor
 AC_ARG_WITH(tor-user,
         AS_HELP_STRING(--with-tor-user=NAME, [specify username for tor daemon]),
diff --git a/src/core/proto/proto_socks.c b/src/core/proto/proto_socks.c
index 6fd08b2273..697fc683c5 100644
--- a/src/core/proto/proto_socks.c
+++ b/src/core/proto/proto_socks.c
@@ -1072,7 +1072,10 @@ parse_socks_client(const uint8_t *data, size_t datalen,
           log_info(LD_NET, "SOCKS 5 client: need authentication.");
           *drain_out = -1;
           return 2;
-        /* fall through */
+        default:
+          /* This wasn't supposed to be exhaustive; there are other
+           * authentication methods too. */
+          ;
       }
 
       *reason = tor_strdup("server doesn't support any of our available "
diff --git a/src/ext/ed25519/donna/ed25519_tor.c b/src/ext/ed25519/donna/ed25519_tor.c
index 7f5ab398d8..a5bb6f4e21 100644
--- a/src/ext/ed25519/donna/ed25519_tor.c
+++ b/src/ext/ed25519/donna/ed25519_tor.c
@@ -35,6 +35,9 @@
 #define ED25519_FN(fn)         ED25519_FN2(fn,ED25519_SUFFIX)
 
 #include "orconfig.h"
+
+#include "lib/cc/compat_compiler.h"
+
 #include "ed25519-donna.h"
 #include "ed25519_donna_tor.h"
 #include "ed25519-randombytes.h"
@@ -366,4 +369,3 @@ ed25519_donna_scalarmult_with_group_order(unsigned char *out,
 }
 
 #include "test-internals.c"
-
diff --git a/src/feature/dirauth/keypin.c b/src/feature/dirauth/keypin.c
index 6f6cfc01f1..edf5ba5833 100644
--- a/src/feature/dirauth/keypin.c
+++ b/src/feature/dirauth/keypin.c
@@ -265,7 +265,7 @@ keypin_add_or_replace_entry_in_map(keypin_ent_t *ent)
     }
     tor_free(ent2);
     r = -1;
-    /* Fall through */
+    /* Note lack of return here: we fall through to the next line. */
   }
 
   keypin_add_entry_to_map(ent);
diff --git a/src/feature/relay/dns.c b/src/feature/relay/dns.c
index da0cbb1df4..9639193685 100644
--- a/src/feature/relay/dns.c
+++ b/src/feature/relay/dns.c
@@ -546,7 +546,7 @@ send_resolved_cell,(edge_connection_t *conn, uint8_t answer_type,
         break;
       } else {
         answer_type = RESOLVED_TYPE_ERROR;
-        /* fall through. */
+        /* We let this fall through and treat it as an error. */
       }
       /* Falls through. */
     case RESOLVED_TYPE_ERROR_TRANSIENT:
diff --git a/src/lib/cc/compat_compiler.h b/src/lib/cc/compat_compiler.h
index 1bb8c54a0c..015b5af320 100644
--- a/src/lib/cc/compat_compiler.h
+++ b/src/lib/cc/compat_compiler.h
@@ -50,6 +50,12 @@
 #define CHECK_SCANF(formatIdx, firstArg)
 #endif /* defined(__GNUC__) */
 
+#if defined(HAVE_ATTR_FALLTHROUGH)
+#define FALLTHROUGH __attribute__((fallthrough))
+#else
+#define FALLTHROUGH
+#endif
+
 /* What GCC do we have? */
 #ifdef __GNUC__
 #define GCC_VERSION (__GNUC__ * 100 + __GNUC_MINOR__)
diff --git a/src/lib/crypt_ops/crypto_digest_nss.c b/src/lib/crypt_ops/crypto_digest_nss.c
index 7e7464273e..3092a08015 100644
--- a/src/lib/crypt_ops/crypto_digest_nss.c
+++ b/src/lib/crypt_ops/crypto_digest_nss.c
@@ -91,7 +91,7 @@ library_supports_digest(digest_algorithm_t alg)
   switch (alg) {
     case DIGEST_SHA1: /* Fall through */
     case DIGEST_SHA256: /* Fall through */
-    case DIGEST_SHA512: /* Fall through */
+    case DIGEST_SHA512:
       return true;
     case DIGEST_SHA3_256: /* Fall through */
     case DIGEST_SHA3_512: /* Fall through */
diff --git a/src/lib/net/address.c b/src/lib/net/address.c
index ac4350b8fe..d623cdd131 100644
--- a/src/lib/net/address.c
+++ b/src/lib/net/address.c
@@ -929,6 +929,7 @@ tor_addr_copy_tight(tor_addr_t *dest, const tor_addr_t *src)
       break;
     case AF_INET6:
       memcpy(dest->addr.in6_addr.s6_addr, src->addr.in6_addr.s6_addr, 16);
+      break;
     case AF_UNSPEC:
       break;
       // LCOV_EXCL_START