diff --git a/src/or/buffers.c b/src/or/buffers.c
index 6ce9c6ac1..33118c2b4 100644
--- a/src/or/buffers.c
+++ b/src/or/buffers.c
@@ -1228,54 +1228,20 @@ fetch_from_buf_socks(buf_t *buf, socks_request_t *req,
   }
 }
 
-/** If there is a complete version 0 control message waiting on buf, then store
- * its contents into *<b>type_out</b>, store its body's length into
- * *<b>len_out</b>, allocate and store a string for its body into
- * *<b>body_out</b>, and return 1.  (body_out will always be NUL-terminated,
- * even if the control message body doesn't end with NUL.)
- *
- * If there is not a complete control message waiting, return 0.
- *
- * Return -1 on error; return -2 on "seems to be control protocol v1."
- */
+/** Return 1 iff buf looks more like it has an (obsolete) v0 controller
+ * command on it than any valid v1 controller command. */
 int
-fetch_from_buf_control0(buf_t *buf, uint32_t *len_out, uint16_t *type_out,
-                        char **body_out, int check_for_v1)
+peek_buf_has_control0_command(buf_t *buf)
 {
-  uint32_t msglen;
-  uint16_t type;
-  char tmp[4];
-
-  tor_assert(buf);
-  tor_assert(len_out);
-  tor_assert(type_out);
-  tor_assert(body_out);
-
-  *len_out = 0;
-  *body_out = NULL;
-
-  if (buf->datalen < 4)
-    return 0;
-
-  peek_from_buf(tmp, 4, buf);
-
-  msglen = ntohs(get_uint16(tmp));
-  type = ntohs(get_uint16(tmp+2));
-  if (type > 255 && check_for_v1)
-    return -2;
-
-  if (buf->datalen < 4 + (unsigned)msglen)
-    return 0;
-
-  *len_out = msglen;
-  *type_out = type;
-  buf_remove_from_front(buf, 4);
-  if (msglen) {
-    *body_out = tor_malloc(msglen+1);
-    fetch_from_buf(*body_out, msglen, buf);
-    (*body_out)[msglen] = '\0';
+  if (buf->datalen >= 4) {
+    char header[4];
+    uint16_t cmd;
+    peek_from_buf(header, sizeof(header), buf);
+    cmd = ntohs(get_uint16(header+2));
+    if (cmd <= 0x14)
+      return 1; /* This is definitely not a v1 control command. */
   }
-  return 1;
+  return 0;
 }
 
 /** Helper: return a pointer to the first instance of <b>c</b> in the
diff --git a/src/or/control.c b/src/or/control.c
index 521b810a8..80ea17bed 100644
--- a/src/or/control.c
+++ b/src/or/control.c
@@ -19,23 +19,6 @@ const char control_c_id[] =
  * See control-spec.txt for full details on protocol.
  */
 
-#if 0
-/* Recognized version 0 error codes.  Do not expand. */
-#define ERR_UNSPECIFIED             0x0000
-#define ERR_INTERNAL                0x0001
-#define ERR_UNRECOGNIZED_TYPE       0x0002
-#define ERR_SYNTAX                  0x0003
-#define ERR_UNRECOGNIZED_CONFIG_KEY 0x0004
-#define ERR_INVALID_CONFIG_VALUE    0x0005
-#define ERR_UNRECOGNIZED_EVENT_CODE 0x0006
-#define ERR_UNAUTHORIZED            0x0007
-#define ERR_REJECTED_AUTHENTICATION 0x0008
-#define ERR_RESOURCE_EXHAUSTED      0x0009
-#define ERR_NO_STREAM               0x000A
-#define ERR_NO_CIRC                 0x000B
-#define ERR_NO_ROUTER               0x000C
-#endif
-
 /* Recognized asynchronous event types.  It's okay to expand this list
  * because it is used both as a list of v0 event types, and as indices
  * into the bitfield to determine which controllers want which events.
@@ -2267,6 +2250,24 @@ connection_control_process_inbuf(control_connection_t *conn)
     conn->incoming_cmd_cur_len = 0;
   }
 
+  if (conn->_base.state == CONTROL_CONN_STATE_NEEDAUTH_V1 &&
+      peek_buf_has_control0_command(conn->_base.inbuf)) {
+    /* Detect v0 commands and send a "no more v0" message. */
+    size_t body_len;
+    char buf[128];
+    set_uint16(buf+2, htons(0x0000)); /* type == error */
+    set_uint16(buf+4, htons(0x0001)); /* code == internal error */
+    strlcpy(buf+6, "The v0 control protocol is not supported by Tor 0.2.0.x "
+            "and later; use Tor 0.1.2.x or upgrade your controller",
+            sizeof(buf)-6);
+    body_len = 2+strlen(buf+6)+2; /* code, msg, nul. */
+    set_uint16(buf+0, htons(body_len));
+    connection_write_to_buf(buf, 4+body_len, TO_CONN(conn));
+    connection_mark_for_close(TO_CONN(conn));
+    conn->_base.hold_open_until_flushed = 1;
+    return 0;
+  }
+
  again:
   while (1) {
     size_t last_idx;
diff --git a/src/or/or.h b/src/or/or.h
index 7b5fd6bd5..b05458f4e 100644
--- a/src/or/or.h
+++ b/src/or/or.h
@@ -1923,8 +1923,7 @@ int fetch_from_buf_http(buf_t *buf,
                         int force_complete);
 int fetch_from_buf_socks(buf_t *buf, socks_request_t *req,
                          int log_sockstype, int safe_socks);
-int fetch_from_buf_control0(buf_t *buf, uint32_t *len_out, uint16_t *type_out,
-                            char **body_out, int check_for_v1);
+int peek_buf_has_control0_command(buf_t *buf);
 int fetch_from_buf_line(buf_t *buf, char *data_out, size_t *data_len);
 int fetch_from_buf_line_lf(buf_t *buf, char *data_out, size_t *data_len);