r17323@aud-055: nickm | 2008-07-23 17:58:25 +0200

 Implement most of proposal 110.


svn:r16156

ChangeLog

@@ -3,6 +3,13 @@ Changes in version 0.2.1.3-alpha - 2008-07-xx
     - Send a bootstrap problem "warn" event on the first problem if the
       reason is NO_ROUTE (that is, our network is down).
 
+  o Major features:
+    - Implements most of proposal 110: The first K cells to be send
+      along a circuit are marked as special "early" cells; only K
+      "early" cells will be allowed.  Once this code is universal, we
+      can block certain kinds of DOS attack by only allowing EXTEND
+      commands in these cells.
+
   o Major bugfixes:
     - Try to attach connections immediately upon receiving a RENDEZVOUS2
       or RENDEZVOUS_ESTABLISHED cell. This can save a second or two

doc/TODO

@@ -214,7 +214,7 @@ R d Setting DirPort when acting as bridge will give false Warnings
 
 For 0.2.1.x:
   - Proposals to do:
-    - 110: avoid infinite-length circuits
+    o 110: avoid infinite-length circuits
 R   d 128: families of private bridges
     - 134: handle authority fragmentation.
 

doc/spec/proposals/110-avoid-infinite-circuits.txt

@@ -6,9 +6,11 @@ Author: Roger Dingledine
 Created: 13-Mar-2007
 Status: Accepted
 Target: 0.2.1.x
+Implemented-In: 0.2.1.3-alpha
 
 History:
 
+  Revised 28 July 2008 by nickm: set K.
   Revised 3 July 2008 by nickm: rename from relay_extend to
      relay_early.  Revise to current migration plan.  Allow K cells
      over circuit lifetime, not just at start.
@@ -85,17 +87,22 @@ Migration:
   is not speaking the v2 link protocol, the server relays the cell as
   a RELAY cell.
 
-  In 0.2.1.x, clients begin using RELAY_EARLY cells on v2 connections.
-  This functionality can be safely backported to 0.2.0.x.  Clients
-  should pick a random number betweeen (say) 8 and 10 to send.
+  In 0.2.1.3-alpha, clients begin using RELAY_EARLY cells on v2
+  connections.  This functionality can be safely backported to
+  0.2.0.x.  Clients should pick a random number betweeen (say) K and
+  K-2 to send.
 
-  In 0.2.1.x, servers close any circuit in which more than K
+  In 0.2.1.3-alpha, servers close any circuit in which more than K
   relay_early cells are sent.
 
   Once all versions the do not send RELAY_EARLY cells are obsolete,
   servers can begin to reject any EXTEND requests not sent in a
   RELAY_EARLY cell.
 
+Parameters:
+
+  Let K = 8, for no terribly good reason.
+
 Spec:
 
   [We can formalize this part once we think the design is a good one.]

src/or/circuitlist.c

@@ -347,6 +347,8 @@ origin_circuit_new(void)
 
   circ->next_stream_id = crypto_rand_int(1<<16);
   circ->global_identifier = n_circuits_allocated++;
+  circ->remaining_relay_early_cells = MAX_RELAY_EARLY_CELLS_PER_CIRCUIT;
+  circ->remaining_relay_early_cells -= crypto_rand_int(2);
 
   init_circuit_base(TO_CIRCUIT(circ));
 
@@ -367,6 +369,8 @@ or_circuit_new(circid_t p_circ_id, or_connection_t *p_conn)
   if (p_conn)
     circuit_set_p_circid_orconn(circ, p_circ_id, p_conn);
 
+  circ->remaining_relay_early_cells = MAX_RELAY_EARLY_CELLS_PER_CIRCUIT;
+
   init_circuit_base(TO_CIRCUIT(circ));
 
   return circ;

src/or/command.c

@@ -353,8 +353,8 @@ command_process_created_cell(cell_t *cell, or_connection_t *conn)
   }
 }
 
-/** Process a 'relay' <b>cell</b> that just arrived from <b>conn</b>. Make sure
- * it came in with a recognized circ_id. Pass it on to
+/** Process a 'relay' or 'relay_early' <b>cell</b> that just arrived from
+ * <b>conn</b>. Make sure it came in with a recognized circ_id. Pass it on to
  * circuit_receive_relay_cell() for actual processing.
  */
 static void
@@ -390,6 +390,30 @@ command_process_relay_cell(cell_t *cell, or_connection_t *conn)
   else
     direction = CELL_DIRECTION_IN;
 
+  /* If we have a relay_early cell, make sure that it's outbound, and we've
+   * gotten no more than MAX_RELAY_EARLY_CELLS_PER_CIRCUIT of them. */
+  if (cell->command == CELL_RELAY_EARLY) {
+    if (direction == CELL_DIRECTION_IN) {
+      log_fn(LOG_PROTOCOL_WARN, LD_OR,
+             "Received an inbound RELAY_EARLY cell on circuit %d from %s:%d."
+             "  Closing circuit.",
+             cell->circ_id, conn->_base.address, conn->_base.port);
+      circuit_mark_for_close(circ, END_CIRC_REASON_TORPROTOCOL);
+      return;
+    } else {
+      or_circuit_t *or_circ = TO_OR_CIRCUIT(circ);
+      if (or_circ->remaining_relay_early_cells == 0) {
+        log_fn(LOG_PROTOCOL_WARN, LD_OR,
+               "Received too many RELAY_EARLY cells on circ %d from %s:%d."
+               "  Closing circuit.",
+               cell->circ_id, safe_str(conn->_base.address), conn->_base.port);
+        circuit_mark_for_close(circ, END_CIRC_REASON_TORPROTOCOL);
+        return;
+      }
+      --or_circ->remaining_relay_early_cells;
+    }
+  }
+
   if ((reason = circuit_receive_relay_cell(cell, circ, direction)) < 0) {
     log_fn(LOG_PROTOCOL_WARN,LD_PROTOCOL,"circuit_receive_relay_cell "
            "(%s) failed. Closing.",

src/or/or.h

@@ -1828,6 +1828,10 @@ typedef struct circuit_t {
   struct circuit_t *next; /**< Next circuit in linked list of all circuits. */
 } circuit_t;
 
+/** Largest number of relay_early cells that we can send on a given
+ * circuit. */
+#define MAX_RELAY_EARLY_CELLS_PER_CIRCUIT 8
+
 /** An origin_circuit_t holds data necessary to build and use a circuit.
  */
 typedef struct origin_circuit_t {
@@ -1870,15 +1874,19 @@ typedef struct origin_circuit_t {
    */
   uint8_t rend_desc_version;
 
-  /* The intro key replaces the hidden service's public key if purpose is
-   * S_ESTABLISH_INTRO or S_INTRO, provided that no unversioned rendezvous
-   * descriptor is used. */
-  crypto_pk_env_t *intro_key;
+  /** How many more relay_early cells can we send on this circuit, according
+   * to the specification? */
+  unsigned int remaining_relay_early_cells : 4;
 
   /** The next stream_id that will be tried when we're attempting to
    * construct a new AP stream originating at this circuit. */
   streamid_t next_stream_id;
 
+  /* The intro key replaces the hidden service's public key if purpose is
+   * S_ESTABLISH_INTRO or S_INTRO, provided that no unversioned rendezvous
+   * descriptor is used. */
+  crypto_pk_env_t *intro_key;
+
   /** Quasi-global identifier for this circuit; used for control.c */
   /* XXXX NM This can get re-used after 2**32 circuits. */
   uint32_t global_identifier;
@@ -1946,6 +1954,10 @@ typedef struct or_circuit_t {
   /* ???? move to a subtype or adjunct structure? Wastes 20 bytes -NM */
   char handshake_digest[DIGEST_LEN]; /**< Stores KH for the handshake. */
 
+  /** How many more relay_early cells can we send on this circuit, according
+   * to the specification? */
+  unsigned int remaining_relay_early_cells : 4;
+
   /** True iff this circuit was made with a CREATE_FAST cell. */
   unsigned int is_first_hop : 1;
 } or_circuit_t;

src/or/relay.c

@@ -338,6 +338,7 @@ circuit_package_relay_cell(cell_t *cell, circuit_t *circ,
       log_warn(LD_BUG,"outgoing relay cell has n_conn==NULL. Dropping.");
       return 0; /* just drop it */
     }
+
     relay_set_digest(layer_hint->f_digest, cell);
 
     thishop = layer_hint;
@@ -506,6 +507,22 @@ relay_send_command_from_edge(uint16_t stream_id, circuit_t *circ,
     circ->n_conn->client_used = time(NULL);
   }
 
+  if (cell_direction == CELL_DIRECTION_OUT) {
+    origin_circuit_t *origin_circ = TO_ORIGIN_CIRCUIT(circ);
+    if (origin_circ->remaining_relay_early_cells > 0) {
+      /* If we've got any relay_early cells left, use one.  Don't worry
+       * about the conn protocol version: append_cell_to_circuit_queue will
+       * fix it up. */
+      cell.command = CELL_RELAY_EARLY;
+      --origin_circ->remaining_relay_early_cells;
+      log_debug(LD_OR, "Sending a RELAY_EARLY cell; %d remaining.",
+                (int)origin_circ->remaining_relay_early_cells);
+    } else if (relay_command == RELAY_COMMAND_EXTEND) {
+      log_warn(LD_BUG, "Uh-oh.  We're sending a RELAY_COMMAND_EXTEND cell, "
+               "but we have run out of RELAY_EARLY cells on that circuit.");
+    }
+  }
+
   if (circuit_package_relay_cell(&cell, circ, cell_direction, cpath_layer)
       < 0) {
     log_warn(LD_BUG,"circuit_package_relay_cell failed. Closing.");