3 Tor Protocol Specification
8 Note: This document aims to specify Tor as implemented in 0.1.2.x
9 and earlier. Future versions of Tor may implement improved protocols, and
10 compatibility is not guaranteed.
12 This specification is not a design document; most design criteria
13 are not examined. For more information on why Tor acts as it does,
18 0.1. Notation and encoding
22 K -- a key for a symmetric cypher.
24 a|b -- concatenation of 'a' and 'b'.
26 [A0 B1 C2] -- a three-byte sequence, containing the bytes with
27 hexadecimal values A0, B1, and C2, in that order.
29 All numeric values are encoded in network (big-endian) order.
31 H(m) -- a cryptographic hash of m.
33 0.2. Security parameters
35 Tor uses a stream cipher, a public-key cipher, the Diffie-Hellman
36 protocol, and a hash function.
38 KEY_LEN -- the length of the stream cipher's key, in bytes.
40 PK_ENC_LEN -- the length of a public-key encrypted message, in bytes.
41 PK_PAD_LEN -- the number of bytes added in padding for public-key
42 encryption, in bytes. (The largest number of bytes that can be encrypted
43 in a single public-key operation is therefore PK_ENC_LEN-PK_PAD_LEN.)
45 DH_LEN -- the number of bytes used to represent a member of the
47 DH_SEC_LEN -- the number of bytes used in a Diffie-Hellman private key (x).
49 HASH_LEN -- the length of the hash function's output, in bytes.
51 PAYLOAD_LEN -- The longest allowable cell payload, in bytes. (509)
53 CELL_LEN -- The length of a Tor cell, in bytes.
57 For a stream cipher, we use 128-bit AES in counter mode, with an IV of all
60 For a public-key cipher, we use RSA with 1024-bit keys and a fixed
61 exponent of 65537. We use OAEP-MGF1 padding, with SHA-1 as its digest
62 function. We leave optional the "Label" parameter unset. (For OAEP
63 padding, see ftp://ftp.rsasecurity.com/pub/pkcs/pkcs-1/pkcs-1v2-1.pdf)
65 [Nick, what does "we leave optional the Label parameter unset" mean? -RD]
67 For Diffie-Hellman, we use a generator (g) of 2. For the modulus (p), we
68 use the 1024-bit safe prime from rfc2409 section 6.2 whose hex
71 "FFFFFFFFFFFFFFFFC90FDAA22168C234C4C6628B80DC1CD129024E08"
72 "8A67CC74020BBEA63B139B22514A08798E3404DDEF9519B3CD3A431B"
73 "302B0A6DF25F14374FE1356D6D51C245E485B576625E7EC6F44C42E9"
74 "A637ED6B0BFF5CB6F406B7EDEE386BFB5A899FA5AE9F24117C4B1FE6"
75 "49286651ECE65381FFFFFFFFFFFFFFFF"
77 As an optimization, implementations SHOULD choose DH private keys (x) of
78 320 bits. Implementations that do this MUST never use any DH key more
80 [May other implementations reuse their DH keys?? -RD]
81 [Probably not. Conceivably, you could get away with changing DH keys once
82 per second, but there are too many oddball attacks for me to be
83 comfortable that this is safe. -NM]
85 For a hash function, we use SHA-1.
88 DH_LEN=128; DH_SEC_LEN=40.
89 PK_ENC_LEN=128; PK_PAD_LEN=42.
92 When we refer to "the hash of a public key", we mean the SHA-1 hash of the
93 DER encoding of an ASN.1 RSA public key (as specified in PKCS.1).
95 All "random" values should be generated with a cryptographically strong
96 random number generator, unless otherwise noted.
98 The "hybrid encryption" of a byte sequence M with a public key PK is
100 1. If M is less than PK_ENC_LEN-PK_PAD_LEN, pad and encrypt M with PK.
101 2. Otherwise, generate a KEY_LEN byte random key K.
102 Let M1 = the first PK_ENC_LEN-PK_PAD_LEN-KEY_LEN bytes of M,
103 and let M2 = the rest of M.
104 Pad and encrypt K|M1 with PK. Encrypt M2 with our stream cipher,
105 using the key K. Concatenate these encrypted values.
106 [XXX Note that this "hybrid encryption" approach does not prevent
107 an attacker from adding or removing bytes to the end of M. It also
108 allows attackers to modify the bytes not covered by the OAEP --
109 see Goldberg's PET2006 paper for details. We will add a MAC to this
112 0.4. Other parameter values
118 Tor is a distributed overlay network designed to anonymize
119 low-latency TCP-based applications such as web browsing, secure shell,
120 and instant messaging. Clients choose a path through the network and
121 build a ``circuit'', in which each node (or ``onion router'' or ``OR'')
122 in the path knows its predecessor and successor, but no other nodes in
123 the circuit. Traffic flowing down the circuit is sent in fixed-size
124 ``cells'', which are unwrapped by a symmetric key at each node (like
125 the layers of an onion) and relayed downstream.
129 Every Tor server has multiple public/private keypairs:
131 - A long-term signing-only "Identity key" used to sign documents and
132 certificates, and used to establish server identity.
133 - A medium-term "Onion key" used to decrypt onion skins when accepting
134 circuit extend attempts. (See 5.1.) Old keys MUST be accepted for at
135 least one week after they are no longer advertised. Because of this,
136 servers MUST retain old keys for a while after they're rotated.
137 - A short-term "Connection key" used to negotiate TLS connections.
138 Tor implementations MAY rotate this key as often as they like, and
139 SHOULD rotate this key at least once a day.
141 Tor servers are also identified by "nicknames"; these are specified in
146 Connections between two Tor servers, or between a client and a server,
147 use TLS/SSLv3 for link authentication and encryption. All
148 implementations MUST support the SSLv3 ciphersuite
149 "SSL_DHE_RSA_WITH_3DES_EDE_CBC_SHA", and SHOULD support the TLS
150 ciphersuite "TLS_DHE_RSA_WITH_AES_128_CBC_SHA" if it is available.
152 There are three acceptable ways to perform a TLS handshake when
153 connecting to a Tor server: "certificates up-front", "renegotiation", and
154 "backwards-compatible renegotiation". ("Backwards-compatible
155 renegotiation" is, as the name implies, compatible with both other
158 Before Tor 0.2.0.21, only "certificates up-front" was supported. In Tor
159 0.2.0.21 or later, "backwards-compatible renegotiation" is used.
161 In "certificates up-front", the connection initiator always sends a
162 two-certificate chain, consisting of an X.509 certificate using a
163 short-term connection public key and a second, self- signed X.509
164 certificate containing its identity key. The other party sends a similar
165 certificate chain. The initiator's ClientHello MUST NOT include any
166 ciphersuites other than:
167 TLS_DHE_RSA_WITH_AES_256_CBC_SHA
168 TLS_DHE_RSA_WITH_AES_128_CBC_SHA
169 SSL_DHE_RSA_WITH_3DES_EDE_CBC_SHA
170 SSL_DHE_DSS_WITH_3DES_EDE_CBC_SHA
172 In "renegotiation", the connection initiator sends no certificates, and
173 the responder sends a single connection certificate. Once the TLS
174 handshake is complete, the initiator renegotiates the handshake, with each
175 parties sending a two-certificate chain as in "certificates up-front".
176 The initiator's ClientHello MUST include at least once ciphersuite not in
179 In "backwards-compatible renegotiation", the connection initiator's
180 ClientHello MUST include at least one ciphersuite other than those listed
181 above. The connection responder examines the initiator's ciphersuite list
182 to see whether it includes any ciphers other than those included in the
183 list above. If extra ciphers are included, the responder proceeds as in
184 "renegotiation": it sends a single certificate and does not request
185 client certificates. Otherwise (in the case that no extra ciphersuites
186 are included in the ClientHello) the responder proceeds as in
187 "certificates up-front": it requests client certificates, and sends a
188 two-certificate chain. In either case, once the responder has sent its
189 certificate or certificates, the initiator counts them. If two
190 certificates have been sent, it proceeds as in "certificates up-front";
191 otherwise, it proceeds as in "renegotiation".
193 All new implementations of the Tor server protocol MUST support
194 "backwards-compatible renegotiation"; clients SHOULD do this too. If
195 this is not possible, new client implementations MUST support both
196 "renegotiation" and "certificates up-front" and use the router's
197 published link protocols list (see dir-spec.txt on the "protocols" entry)
198 to decide which to use.
200 In all of the above handshake variants, certificates sent in the clear
201 SHOULD NOT include any strings to identify the host as a Tor server. In
202 the "renegotation" and "backwards-compatible renegotiation", the
203 initiator SHOULD chose a list of ciphersuites and TLS extensions chosen
204 to mimic one used by a popular web browser.
206 Responders MUST NOT select any TLS ciphersuite that lacks ephemeral keys,
207 or whose symmetric keys are less then KEY_LEN bits, or whose digests are
208 less than HASH_LEN bits. Responders SHOULD NOT select any SSLv3
209 ciphersuite other than those listed above.
211 Even though the connection protocol is identical, we will think of the
212 initiator as either an onion router (OR) if it is willing to relay
213 traffic for other Tor users, or an onion proxy (OP) if it only handles
214 local requests. Onion proxies SHOULD NOT provide long-term-trackable
215 identifiers in their handshakes.
217 In all handshake variants, once all certificates are exchanged, all
218 parties receiving certificates must confirm that the identity key is as
219 expected. (When initiating a connection, the expected identity key is
220 the one given in the directory; when creating a connection because of an
221 EXTEND cell, the expected identity key is the one given in the cell.) If
222 the key is not as expected, the party must close the connection.
224 When connecting to an OR, all parties SHOULD reject the connection if that
225 OR has a malformed or missing certificate. When accepting an incoming
226 connection, an OR SHOULD NOT reject incoming connections from parties with
227 malformed or missing certificates. (However, an OR should not believe
228 that an incoming connection is from another OR unless the certificates
229 are present and well-formed.)
231 [Before version 0.1.2.8-rc, ORs rejected incoming connections from ORs and
232 OPs alike if their certificates were missing or malformed.]
234 Once a TLS connection is established, the two sides send cells
235 (specified below) to one another. Cells are sent serially. All
236 cells are CELL_LEN bytes long. Cells may be sent embedded in TLS
237 records of any size or divided across TLS records, but the framing
238 of TLS records MUST NOT leak information about the type or contents
241 TLS connections are not permanent. Either side MAY close a connection
242 if there are no circuits running over it and an amount of time
243 (KeepalivePeriod, defaults to 5 minutes) has passed since the last time
244 any traffic was transmitted over the TLS connection. Clients SHOULD
245 also hold a TLS connection with no circuits open, if it is likely that a
246 circuit will be built soon using that connection.
248 (As an exception, directory servers may try to stay connected to all of
249 the ORs -- though this will be phased out for the Tor 0.1.2.x release.)
251 3. Cell Packet format
253 The basic unit of communication for onion routers and onion
254 proxies is a fixed-width "cell".
256 On a version 1 connection, each cell contains the following
261 Payload (padded with 0 bytes) [PAYLOAD_LEN bytes]
263 On a version 2 connection, all cells are as in version 1 connections,
264 except for the initial VERSIONS cell, whose format is:
266 Circuit [2 octets; set to 0]
267 Command [1 octet; set to 7 for VERSIONS]
268 Length [2 octets; big-endian integer]
269 Payload [Length bytes]
271 The CircID field determines which circuit, if any, the cell is
274 The 'Command' field holds one of the following values:
275 0 -- PADDING (Padding) (See Sec 7.2)
276 1 -- CREATE (Create a circuit) (See Sec 5.1)
277 2 -- CREATED (Acknowledge create) (See Sec 5.1)
278 3 -- RELAY (End-to-end data) (See Sec 5.5 and 6)
279 4 -- DESTROY (Stop using a circuit) (See Sec 5.4)
280 5 -- CREATE_FAST (Create a circuit, no PK) (See Sec 5.1)
281 6 -- CREATED_FAST (Circuit created, no PK) (See Sec 5.1)
282 7 -- VERSIONS (Negotiate proto version) (See Sec 4)
283 8 -- NETINFO (Time and address info) (See Sec 4)
285 The interpretation of 'Payload' depends on the type of the cell.
286 PADDING: Payload is unused.
287 CREATE: Payload contains the handshake challenge.
288 CREATED: Payload contains the handshake response.
289 RELAY: Payload contains the relay header and relay body.
290 DESTROY: Payload contains a reason for closing the circuit.
292 Upon receiving any other value for the command field, an OR must
293 drop the cell. [XXXX Versions prior to 0.1.0.?? logged a warning
294 when dropping the cell; this is bad behavior. -NM]
296 The payload is padded with 0 bytes.
298 PADDING cells are currently used to implement connection keepalive.
299 If there is no other traffic, ORs and OPs send one another a PADDING
300 cell every few minutes.
302 CREATE, CREATED, and DESTROY cells are used to manage circuits;
305 RELAY cells are used to send commands and data along a circuit; see
308 VERSIONS and NETINFO cells are used to set up connections. See section 4
311 4. Negotiating and initializing connections
313 4.1. Negotiating versions with VERSIONS cells
315 There are multiple instances of the Tor link connection protocol. Any
316 connection negotiated using the "certificates up front" handshake (see
317 section 2 above) is "version 1". In any connection where both parties
318 have behaved as in the "renegotiation" handshake, the link protocol
319 version is 2 or higher.
321 To determine the version, in any connection where the "renegotiation"
322 handshake was used (that is, where the server sent only one certificate
323 at first and where the client did not send any certificates until
324 renegotiation), both parties MUST send a VERSIONS cell immediately after
325 the renegotiation is finished, before any other cells are sent. Parties
326 MUST NOT send any other cells on a connection until they have received a
329 The payload in a VERSIONS cell is a series of big-endian two-byte
330 integers. Both parties MUST select as the link protocol version the
331 highest number contained both in the VERSIONS cell they sent and in the
332 versions cell they received. If they have no such version in common,
333 they cannot communicate and MUST close the connection.
335 Since the version 1 link protocol does not use the "renegotiation"
336 handshake, implementations MUST NOT list version 1 in their VERSIONS
341 If version 2 or higher is negotiated, each party sends the other a
342 NETINFO cell. The cell's payload is:
345 Other OR's address [variable]
346 Number of addresses [1 byte]
347 This OR's addresses [variable]
349 The address format is a type/length/value sequence as given in section
350 6.4 below. The timestamp is a big-endian unsigned integer number of
351 seconds since the unix epoch.
353 Implementations MAY use the timestamp value to help decide if their
354 clocks are skewed. Initiators MAY use "other OR's address" to help
355 learn which address their connections are originating from, if they do
356 not know it. Initiators SHOULD use "this OR's address" to make sure
357 that they have connected to another OR at its canonical address.
359 [As of 0.2.0.23-rc, implementations use none of the above values.]
362 5. Circuit management
364 5.1. CREATE and CREATED cells
366 Users set up circuits incrementally, one hop at a time. To create a
367 new circuit, OPs send a CREATE cell to the first node, with the
368 first half of the DH handshake; that node responds with a CREATED
369 cell with the second half of the DH handshake plus the first 20 bytes
370 of derivative key data (see section 5.2). To extend a circuit past
371 the first hop, the OP sends an EXTEND relay cell (see section 5)
372 which instructs the last node in the circuit to send a CREATE cell
373 to extend the circuit.
375 The payload for a CREATE cell is an 'onion skin', which consists
376 of the first step of the DH handshake data (also known as g^x).
377 This value is hybrid-encrypted (see 0.3) to Bob's onion key, giving
380 Padding padding [PK_PAD_LEN bytes]
381 Symmetric key [KEY_LEN bytes]
382 First part of g^x [PK_ENC_LEN-PK_PAD_LEN-KEY_LEN bytes]
383 Symmetrically encrypted:
384 Second part of g^x [DH_LEN-(PK_ENC_LEN-PK_PAD_LEN-KEY_LEN)
387 The relay payload for an EXTEND relay cell consists of:
390 Onion skin [DH_LEN+KEY_LEN+PK_PAD_LEN bytes]
391 Identity fingerprint [HASH_LEN bytes]
393 The port and address field denote the IPV4 address and port of the next
394 onion router in the circuit; the public key hash is the hash of the PKCS#1
395 ASN1 encoding of the next onion router's identity (signing) key. (See 0.3
396 above.) (Including this hash allows the extending OR verify that it is
397 indeed connected to the correct target OR, and prevents certain
398 man-in-the-middle attacks.)
400 The payload for a CREATED cell, or the relay payload for an
401 EXTENDED cell, contains:
402 DH data (g^y) [DH_LEN bytes]
403 Derivative key data (KH) [HASH_LEN bytes] <see 5.2 below>
405 The CircID for a CREATE cell is an arbitrarily chosen 2-byte integer,
406 selected by the node (OP or OR) that sends the CREATE cell. To prevent
407 CircID collisions, when one node sends a CREATE cell to another, it chooses
408 from only one half of the possible values based on the ORs' public
409 identity keys: if the sending node has a lower key, it chooses a CircID with
410 an MSB of 0; otherwise, it chooses a CircID with an MSB of 1.
412 (An OP with no public key MAY choose any CircID it wishes, since an OP
413 never needs to process a CREATE cell.)
415 Public keys are compared numerically by modulus.
417 As usual with DH, x and y MUST be generated randomly.
420 To implement backward-compatible version negotiation, parties MUST
421 drop CREATE cells with all-[00] onion-skins.
424 5.1.1. CREATE_FAST/CREATED_FAST cells
426 When initializing the first hop of a circuit, the OP has already
427 established the OR's identity and negotiated a secret key using TLS.
428 Because of this, it is not always necessary for the OP to perform the
429 public key operations to create a circuit. In this case, the
430 OP MAY send a CREATE_FAST cell instead of a CREATE cell for the first
431 hop only. The OR responds with a CREATED_FAST cell, and the circuit is
434 A CREATE_FAST cell contains:
436 Key material (X) [HASH_LEN bytes]
438 A CREATED_FAST cell contains:
440 Key material (Y) [HASH_LEN bytes]
441 Derivative key data [HASH_LEN bytes] (See 5.2 below)
443 The values of X and Y must be generated randomly.
445 [Versions of Tor before 0.1.0.6-rc did not support these cell types;
446 clients should not send CREATE_FAST cells to older Tor servers.]
448 If an OR sees a circuit created with CREATE_FAST, the OR is sure to be the
449 first hop of a circuit. ORs SHOULD reject attempts to create streams with
450 RELAY_BEGIN exiting the circuit at the first hop: letting Tor be used as a
451 single hop proxy makes exit nodes a more attractive target for compromise.
453 5.2. Setting circuit keys
455 Once the handshake between the OP and an OR is completed, both can
456 now calculate g^xy with ordinary DH. Before computing g^xy, both client
457 and server MUST verify that the received g^x or g^y value is not degenerate;
458 that is, it must be strictly greater than 1 and strictly less than p-1
459 where p is the DH modulus. Implementations MUST NOT complete a handshake
460 with degenerate keys. Implementations MUST NOT discard other "weak"
463 (Discarding degenerate keys is critical for security; if bad keys
464 are not discarded, an attacker can substitute the server's CREATED
465 cell's g^y with 0 or 1, thus creating a known g^xy and impersonating
466 the server. Discarding other keys may allow attacks to learn bits of
469 (The mainline Tor implementation, in the 0.1.1.x-alpha series, discarded
470 all g^x values less than 2^24, greater than p-2^24, or having more than
471 1024-16 identical bits. This served no useful purpose, and we stopped.)
473 If CREATE or EXTEND is used to extend a circuit, the client and server
474 base their key material on K0=g^xy, represented as a big-endian unsigned
477 If CREATE_FAST is used, the client and server base their key material on
480 From the base key material K0, they compute KEY_LEN*2+HASH_LEN*3 bytes of
481 derivative key data as
482 K = H(K0 | [00]) | H(K0 | [01]) | H(K0 | [02]) | ...
484 The first HASH_LEN bytes of K form KH; the next HASH_LEN form the forward
485 digest Df; the next HASH_LEN 41-60 form the backward digest Db; the next
486 KEY_LEN 61-76 form Kf, and the final KEY_LEN form Kb. Excess bytes from K
489 KH is used in the handshake response to demonstrate knowledge of the
490 computed shared key. Df is used to seed the integrity-checking hash
491 for the stream of data going from the OP to the OR, and Db seeds the
492 integrity-checking hash for the data stream from the OR to the OP. Kf
493 is used to encrypt the stream of data going from the OP to the OR, and
494 Kb is used to encrypt the stream of data going from the OR to the OP.
496 5.3. Creating circuits
498 When creating a circuit through the network, the circuit creator
499 (OP) performs the following steps:
501 1. Choose an onion router as an exit node (R_N), such that the onion
502 router's exit policy includes at least one pending stream that
503 needs a circuit (if there are any).
505 2. Choose a chain of (N-1) onion routers
506 (R_1...R_N-1) to constitute the path, such that no router
507 appears in the path twice.
509 3. If not already connected to the first router in the chain,
510 open a new connection to that router.
512 4. Choose a circID not already in use on the connection with the
513 first router in the chain; send a CREATE cell along the
514 connection, to be received by the first onion router.
516 5. Wait until a CREATED cell is received; finish the handshake
517 and extract the forward key Kf_1 and the backward key Kb_1.
519 6. For each subsequent onion router R (R_2 through R_N), extend
522 To extend the circuit by a single onion router R_M, the OP performs
525 1. Create an onion skin, encrypted to R_M's public onion key.
527 2. Send the onion skin in a relay EXTEND cell along
528 the circuit (see section 5).
530 3. When a relay EXTENDED cell is received, verify KH, and
531 calculate the shared keys. The circuit is now extended.
533 When an onion router receives an EXTEND relay cell, it sends a CREATE
534 cell to the next onion router, with the enclosed onion skin as its
535 payload. The initiating onion router chooses some circID not yet
536 used on the connection between the two onion routers. (But see
537 section 5.1. above, concerning choosing circIDs based on
538 lexicographic order of nicknames.)
540 When an onion router receives a CREATE cell, if it already has a
541 circuit on the given connection with the given circID, it drops the
542 cell. Otherwise, after receiving the CREATE cell, it completes the
543 DH handshake, and replies with a CREATED cell. Upon receiving a
544 CREATED cell, an onion router packs it payload into an EXTENDED relay
545 cell (see section 5), and sends that cell up the circuit. Upon
546 receiving the EXTENDED relay cell, the OP can retrieve g^y.
548 (As an optimization, OR implementations may delay processing onions
549 until a break in traffic allows time to do so without harming
550 network latency too greatly.)
552 5.3.1. Canonical connections
554 It is possible for an attacker to launch a man-in-the-middle attack
555 against a connection by telling OR Alice to extend to OR Bob at some
556 address X controlled by the attacker. The attacker cannot read the
557 encrypted traffic, but the attacker is now in a position to count all
558 bytes sent between Alice and Bob (assuming Alice was not already
561 To prevent this, when an OR we gets an extend request, it SHOULD use an
562 existing OR connection if the ID matches, and ANY of the following
564 - The IP matches the requested IP.
565 - The OR knows that the IP of the connection it's using is canonical
566 because it was listed in the NETINFO cell.
567 - The OR knows that the IP of the connection it's using is canonical
568 because it was listed in the server descriptor.
570 [This is not implemented in Tor 0.2.0.23-rc.]
572 5.4. Tearing down circuits
574 Circuits are torn down when an unrecoverable error occurs along
575 the circuit, or when all streams on a circuit are closed and the
576 circuit's intended lifetime is over. Circuits may be torn down
577 either completely or hop-by-hop.
579 To tear down a circuit completely, an OR or OP sends a DESTROY
580 cell to the adjacent nodes on that circuit, using the appropriate
583 Upon receiving an outgoing DESTROY cell, an OR frees resources
584 associated with the corresponding circuit. If it's not the end of
585 the circuit, it sends a DESTROY cell for that circuit to the next OR
586 in the circuit. If the node is the end of the circuit, then it tears
587 down any associated edge connections (see section 6.1).
589 After a DESTROY cell has been processed, an OR ignores all data or
590 destroy cells for the corresponding circuit.
592 To tear down part of a circuit, the OP may send a RELAY_TRUNCATE cell
593 signaling a given OR (Stream ID zero). That OR sends a DESTROY
594 cell to the next node in the circuit, and replies to the OP with a
595 RELAY_TRUNCATED cell.
597 When an unrecoverable error occurs along one connection in a
598 circuit, the nodes on either side of the connection should, if they
599 are able, act as follows: the node closer to the OP should send a
600 RELAY_TRUNCATED cell towards the OP; the node farther from the OP
601 should send a DESTROY cell down the circuit.
603 The payload of a RELAY_TRUNCATED or DESTROY cell contains a single octet,
604 describing why the circuit is being closed or truncated. When sending a
605 TRUNCATED or DESTROY cell because of another TRUNCATED or DESTROY cell,
606 the error code should be propagated. The origin of a circuit always sets
607 this error code to 0, to avoid leaking its version.
610 0 -- NONE (No reason given.)
611 1 -- PROTOCOL (Tor protocol violation.)
612 2 -- INTERNAL (Internal error.)
613 3 -- REQUESTED (A client sent a TRUNCATE command.)
614 4 -- HIBERNATING (Not currently operating; trying to save bandwidth.)
615 5 -- RESOURCELIMIT (Out of memory, sockets, or circuit IDs.)
616 6 -- CONNECTFAILED (Unable to reach server.)
617 7 -- OR_IDENTITY (Connected to server, but its OR identity was not
619 8 -- OR_CONN_CLOSED (The OR connection that was carrying this circuit
621 9 -- FINISHED (The circuit has expired for being dirty or old.)
622 10 -- TIMEOUT (Circuit construction took too long)
623 11 -- DESTROYED (The circuit was destroyed w/o client TRUNCATE)
624 12 -- NOSUCHSERVICE (Request for unknown hidden service)
626 [Versions of Tor prior to 0.1.0.11 didn't send reasons; implementations
627 MUST accept empty TRUNCATED and DESTROY cells.]
629 5.5. Routing relay cells
631 When an OR receives a RELAY cell, it checks the cell's circID and
632 determines whether it has a corresponding circuit along that
633 connection. If not, the OR drops the RELAY cell.
635 Otherwise, if the OR is not at the OP edge of the circuit (that is,
636 either an 'exit node' or a non-edge node), it de/encrypts the payload
637 with the stream cipher, as follows:
638 'Forward' relay cell (same direction as CREATE):
639 Use Kf as key; decrypt.
640 'Back' relay cell (opposite direction from CREATE):
641 Use Kb as key; encrypt.
642 Note that in counter mode, decrypt and encrypt are the same operation.
644 The OR then decides whether it recognizes the relay cell, by
645 inspecting the payload as described in section 6.1 below. If the OR
646 recognizes the cell, it processes the contents of the relay cell.
647 Otherwise, it passes the decrypted relay cell along the circuit if
648 the circuit continues. If the OR at the end of the circuit
649 encounters an unrecognized relay cell, an error has occurred: the OR
650 sends a DESTROY cell to tear down the circuit.
652 When a relay cell arrives at an OP, the OP decrypts the payload
653 with the stream cipher as follows:
654 OP receives data cell:
656 Decrypt with Kb_I. If the payload is recognized (see
657 section 6..1), then stop and process the payload.
659 For more information, see section 6 below.
661 6. Application connections and stream management
665 Within a circuit, the OP and the exit node use the contents of
666 RELAY packets to tunnel end-to-end commands and TCP connections
667 ("Streams") across circuits. End-to-end commands can be initiated
668 by either edge; streams are initiated by the OP.
670 The payload of each unencrypted RELAY cell consists of:
671 Relay command [1 byte]
672 'Recognized' [2 bytes]
676 Data [CELL_LEN-14 bytes]
678 The relay commands are:
679 1 -- RELAY_BEGIN [forward]
680 2 -- RELAY_DATA [forward or backward]
681 3 -- RELAY_END [forward or backward]
682 4 -- RELAY_CONNECTED [backward]
683 5 -- RELAY_SENDME [forward or backward] [sometimes control]
684 6 -- RELAY_EXTEND [forward] [control]
685 7 -- RELAY_EXTENDED [backward] [control]
686 8 -- RELAY_TRUNCATE [forward] [control]
687 9 -- RELAY_TRUNCATED [backward] [control]
688 10 -- RELAY_DROP [forward or backward] [control]
689 11 -- RELAY_RESOLVE [forward]
690 12 -- RELAY_RESOLVED [backward]
691 13 -- RELAY_BEGIN_DIR [forward]
693 32..40 -- Used for hidden services; see rend-spec.txt.
695 Commands labelled as "forward" must only be sent by the originator
696 of the circuit. Commands labelled as "backward" must only be sent by
697 other nodes in the circuit back to the originator. Commands marked
698 as either can be sent either by the originator or other nodes.
700 The 'recognized' field in any unencrypted relay payload is always set
701 to zero; the 'digest' field is computed as the first four bytes of
702 the running digest of all the bytes that have been destined for
703 this hop of the circuit or originated from this hop of the circuit,
704 seeded from Df or Db respectively (obtained in section 5.2 above),
705 and including this RELAY cell's entire payload (taken with the digest
708 When the 'recognized' field of a RELAY cell is zero, and the digest
709 is correct, the cell is considered "recognized" for the purposes of
710 decryption (see section 5.5 above).
712 (The digest does not include any bytes from relay cells that do
713 not start or end at this hop of the circuit. That is, it does not
714 include forwarded data. Therefore if 'recognized' is zero but the
715 digest does not match, the running digest at that node should
716 not be updated, and the cell should be forwarded on.)
718 All RELAY cells pertaining to the same tunneled stream have the
719 same stream ID. StreamIDs are chosen arbitrarily by the OP. RELAY
720 cells that affect the entire circuit rather than a particular
721 stream use a StreamID of zero -- they are marked in the table above
722 as "[control]" style cells. (Sendme cells are marked as "sometimes
723 control" because they can take include a StreamID or not depending
724 on their purpose -- see Section 7.)
726 The 'Length' field of a relay cell contains the number of bytes in
727 the relay payload which contain real payload data. The remainder of
728 the payload is padded with NUL bytes.
730 If the RELAY cell is recognized but the relay command is not
731 understood, the cell must be dropped and ignored. Its contents
732 still count with respect to the digests, though. [Before
733 0.1.1.10, Tor closed circuits when it received an unknown relay
734 command. Perhaps this will be more forward-compatible. -RD]
736 6.2. Opening streams and transferring data
738 To open a new anonymized TCP connection, the OP chooses an open
739 circuit to an exit that may be able to connect to the destination
740 address, selects an arbitrary StreamID not yet used on that circuit,
741 and constructs a RELAY_BEGIN cell with a payload encoding the address
742 and port of the destination host. The payload format is:
744 ADDRESS | ':' | PORT | [00]
746 where ADDRESS can be a DNS hostname, or an IPv4 address in
747 dotted-quad format, or an IPv6 address surrounded by square brackets;
748 and where PORT is a decimal integer between 1 and 65535, inclusive.
750 [What is the [00] for? -NM]
751 [It's so the payload is easy to parse out with string funcs -RD]
753 Upon receiving this cell, the exit node resolves the address as
754 necessary, and opens a new TCP connection to the target port. If the
755 address cannot be resolved, or a connection can't be established, the
756 exit node replies with a RELAY_END cell. (See 6.4 below.)
757 Otherwise, the exit node replies with a RELAY_CONNECTED cell, whose
758 payload is in one of the following formats:
759 The IPv4 address to which the connection was made [4 octets]
760 A number of seconds (TTL) for which the address may be cached [4 octets]
762 Four zero-valued octets [4 octets]
763 An address type (6) [1 octet]
764 The IPv6 address to which the connection was made [16 octets]
765 A number of seconds (TTL) for which the address may be cached [4 octets]
766 [XXXX Versions of Tor before 0.1.1.6 ignore and do not generate the TTL
767 field. No version of Tor currently generates the IPv6 format.
769 Tor servers before 0.1.2.0 set the TTL field to a fixed value. Later
770 versions set the TTL to the last value seen from a DNS server, and expire
771 their own cached entries after a fixed interval. This prevents certain
774 The OP waits for a RELAY_CONNECTED cell before sending any data.
775 Once a connection has been established, the OP and exit node
776 package stream data in RELAY_DATA cells, and upon receiving such
777 cells, echo their contents to the corresponding TCP stream.
778 RELAY_DATA cells sent to unrecognized streams are dropped.
780 Relay RELAY_DROP cells are long-range dummies; upon receiving such
781 a cell, the OR or OP must drop it.
783 6.2.1. Opening a directory stream
785 If a Tor server is a directory server, it should respond to a
786 RELAY_BEGIN_DIR cell as if it had received a BEGIN cell requesting a
787 connection to its directory port. RELAY_BEGIN_DIR cells ignore exit
788 policy, since the stream is local to the Tor process.
790 If the Tor server is not running a directory service, it should respond
791 with a REASON_NOTDIRECTORY RELAY_END cell.
793 Clients MUST generate an all-zero payload for RELAY_BEGIN_DIR cells,
794 and servers MUST ignore the payload.
796 [RELAY_BEGIN_DIR was not supported before Tor 0.1.2.2-alpha; clients
797 SHOULD NOT send it to routers running earlier versions of Tor.]
801 When an anonymized TCP connection is closed, or an edge node
802 encounters error on any stream, it sends a 'RELAY_END' cell along the
803 circuit (if possible) and closes the TCP connection immediately. If
804 an edge node receives a 'RELAY_END' cell for any stream, it closes
805 the TCP connection completely, and sends nothing more along the
806 circuit for that stream.
808 The payload of a RELAY_END cell begins with a single 'reason' byte to
809 describe why the stream is closing, plus optional data (depending on
810 the reason.) The values are:
812 1 -- REASON_MISC (catch-all for unlisted reasons)
813 2 -- REASON_RESOLVEFAILED (couldn't look up hostname)
814 3 -- REASON_CONNECTREFUSED (remote host refused connection) [*]
815 4 -- REASON_EXITPOLICY (OR refuses to connect to host or port)
816 5 -- REASON_DESTROY (Circuit is being destroyed)
817 6 -- REASON_DONE (Anonymized TCP connection was closed)
818 7 -- REASON_TIMEOUT (Connection timed out, or OR timed out
820 8 -- (unallocated) [**]
821 9 -- REASON_HIBERNATING (OR is temporarily hibernating)
822 10 -- REASON_INTERNAL (Internal error at the OR)
823 11 -- REASON_RESOURCELIMIT (OR has no resources to fulfill request)
824 12 -- REASON_CONNRESET (Connection was unexpectedly reset)
825 13 -- REASON_TORPROTOCOL (Sent when closing connection because of
826 Tor protocol violations.)
827 14 -- REASON_NOTDIRECTORY (Client sent RELAY_BEGIN_DIR to a
828 non-directory server.)
830 (With REASON_EXITPOLICY, the 4-byte IPv4 address or 16-byte IPv6 address
831 forms the optional data; no other reason currently has extra data.
832 As of 0.1.1.6, the body also contains a 4-byte TTL.)
834 OPs and ORs MUST accept reasons not on the above list, since future
835 versions of Tor may provide more fine-grained reasons.
837 [*] Older versions of Tor also send this reason when connections are
839 [**] Due to a bug in versions of Tor through 0095, error reason 8 must
840 remain allocated until that version is obsolete.
842 --- [The rest of this section describes unimplemented functionality.]
844 Because TCP connections can be half-open, we follow an equivalent
845 to TCP's FIN/FIN-ACK/ACK protocol to close streams.
847 An exit connection can have a TCP stream in one of three states:
848 'OPEN', 'DONE_PACKAGING', and 'DONE_DELIVERING'. For the purposes
849 of modeling transitions, we treat 'CLOSED' as a fourth state,
850 although connections in this state are not, in fact, tracked by the
853 A stream begins in the 'OPEN' state. Upon receiving a 'FIN' from
854 the corresponding TCP connection, the edge node sends a 'RELAY_FIN'
855 cell along the circuit and changes its state to 'DONE_PACKAGING'.
856 Upon receiving a 'RELAY_FIN' cell, an edge node sends a 'FIN' to
857 the corresponding TCP connection (e.g., by calling
858 shutdown(SHUT_WR)) and changing its state to 'DONE_DELIVERING'.
860 When a stream in already in 'DONE_DELIVERING' receives a 'FIN', it
861 also sends a 'RELAY_FIN' along the circuit, and changes its state
862 to 'CLOSED'. When a stream already in 'DONE_PACKAGING' receives a
863 'RELAY_FIN' cell, it sends a 'FIN' and changes its state to
866 If an edge node encounters an error on any stream, it sends a
867 'RELAY_END' cell (if possible) and closes the stream immediately.
869 6.4. Remote hostname lookup
871 To find the address associated with a hostname, the OP sends a
872 RELAY_RESOLVE cell containing the hostname to be resolved. (For a reverse
873 lookup, the OP sends a RELAY_RESOLVE cell containing an in-addr.arpa
874 address.) The OR replies with a RELAY_RESOLVED cell containing a status
875 byte, and any number of answers. Each answer is of the form:
878 Value (variable-width)
880 "Length" is the length of the Value field.
885 0xF0 -- Error, transient
886 0xF1 -- Error, nontransient
888 If any answer has a type of 'Error', then no other answer may be given.
890 The RELAY_RESOLVE cell must use a nonzero, distinct streamID; the
891 corresponding RELAY_RESOLVED cell must use the same streamID. No stream
892 is actually created by the OR when resolving the name.
898 Each node should do appropriate bandwidth throttling to keep its
901 Communicants rely on TCP's default flow control to push back when they
906 Link padding can be created by sending PADDING cells along the
907 connection; relay cells of type "DROP" can be used for long-range
910 Currently nodes are not required to do any sort of link padding or
911 dummy traffic. Because strong attacks exist even with link padding,
912 and because link padding greatly increases the bandwidth requirements
913 for running a node, we plan to leave out link padding until this
914 tradeoff is better understood.
916 7.3. Circuit-level flow control
918 To control a circuit's bandwidth usage, each OR keeps track of two
919 'windows', consisting of how many RELAY_DATA cells it is allowed to
920 originate (package for transmission), and how many RELAY_DATA cells
921 it is willing to consume (receive for local streams). These limits
922 do not apply to cells that the OR receives from one host and relays
925 Each 'window' value is initially set to 1000 data cells
926 in each direction (cells that are not data cells do not affect
927 the window). When an OR is willing to deliver more cells, it sends a
928 RELAY_SENDME cell towards the OP, with Stream ID zero. When an OR
929 receives a RELAY_SENDME cell with stream ID zero, it increments its
932 Each of these cells increments the corresponding window by 100.
934 The OP behaves identically, except that it must track a packaging
935 window and a delivery window for every OR in the circuit.
937 An OR or OP sends cells to increment its delivery window when the
938 corresponding window value falls under some threshold (900).
940 If a packaging window reaches 0, the OR or OP stops reading from
941 TCP connections for all streams on the corresponding circuit, and
942 sends no more RELAY_DATA cells until receiving a RELAY_SENDME cell.
943 [this stuff is badly worded; copy in the tor-design section -RD]
945 7.4. Stream-level flow control
947 Edge nodes use RELAY_SENDME cells to implement end-to-end flow
948 control for individual connections across circuits. Similarly to
949 circuit-level flow control, edge nodes begin with a window of cells
950 (500) per stream, and increment the window by a fixed value (50)
951 upon receiving a RELAY_SENDME cell. Edge nodes initiate RELAY_SENDME
952 cells when both a) the window is <= 450, and b) there are less than
953 ten cell payloads remaining to be flushed at that edge.
956 A.1. Differences between spec and implementation
958 - The current specification requires all ORs to have IPv4 addresses, but
959 allows servers to exit and resolve to IPv6 addresses, and to declare IPv6
960 addresses in their exit policies. The current codebase has no IPv6