Extended ORPort for pluggable transports

               George Kadianakis, Nick Mathewson


This document describes the "Extended ORPort" protocol, a wrapper around Tor's ordinary ORPort protocol for use by bridges that support pluggable transports. It provides a way for server-side PTs and bridges to exchange additional information before beginning the actual OR connection.

See tor-spec.txt for information on the regular OR protocol, and pt-spec.txt for information on pluggable transports.

This protocol was originally proposed in proposal 196, and extended with authentication in proposal 217.

Establishing a connection and authenticating

When a client (that is to say, a server-side pluggable transport) connects to an Extended ORPort, the server sends:

    AuthTypes                                   [variable]
    EndAuthTypes                                [1 octet]


  + AuthTypes are the authentication schemes that the server supports
    for this session. They are multiple concatenated 1-octet values that
    take values from 1 to 255.
  + EndAuthTypes is the special value 0.

The client reads the list of supported authentication schemes, chooses one, and sends it back:

AuthType [1 octet]


  + AuthType is the authentication scheme that the client wants to use
    for this session. A valid authentication type takes values from 1 to
    255. A value of 0 means that the client did not like the
    authentication types offered by the server.

If the client sent an AuthType of value 0, or an AuthType that the server does not support, the server MUST close the connection.

We define one authentication type: SAFE_COOKIE. Its AuthType value is 1. It is based on the client proving to the bridge that it can access a given "cookie" file on disk. The purpose of authentication is to defend against cross-protocol attacks.

If the Extended ORPort is enabled, Tor should regenerate the cookie file on startup and store it in $DataDirectory/extended_orport_auth_cookie.

The location of the cookie can be overridden by using the configuration file parameter ExtORPortCookieAuthFile, which is defined as:

ExtORPortCookieAuthFile <path>

where <path> is a filesystem path.

The format of the cookie-file is:

     StaticHeader                                [32 octets]
     Cookie                                      [32 octets]

  + StaticHeader is the following string:
    "! Extended ORPort Auth Cookie !\x0a"
  + Cookie is the shared-secret. During the SAFE_COOKIE protocol, the
    cookie is called CookieString.

Extended ORPort clients MUST make sure that the StaticHeader is present in the cookie file, before proceeding with the authentication protocol.

A client that performs the SAFE_COOKIE handshake begins by sending:

ClientNonce                                  [32 octets]


  • ClientNonce is 32 octets of random data.

Then, the server replies with:

     ServerHash                                  [32 octets]
     ServerNonce                                 [32 octets]

  + ServerHash is computed as:
        "ExtORPort authentication server-to-client hash" | ClientNonce | ServerNonce)
  + ServerNonce is 32 random octets.

Upon receiving that data, the client computes ServerHash, and validates it against the ServerHash provided by the server.

If the server-provided ServerHash is invalid, the client MUST terminate the connection.

Otherwise the client replies with:

ClientHash                                  [32 octets]

  + ClientHash is computed as:
        "ExtORPort authentication client-to-server hash" | ClientNonce | ServerNonce)

Upon receiving that data, the server computes ClientHash, and validates it against the ClientHash provided by the client.

Finally, the server replies with:

Status [1 octet]

  + Status is 1 if the authentication was successful. If the
    authentication failed, Status is 0.

The extended ORPort protocol

Once a connection is established and authenticated, the parties communicate with the protocol described here.


The extended server port protocol is as follows:

     COMMAND [2 bytes, big-endian]
     BODYLEN [2 bytes, big-endian]
     BODY [BODYLEN bytes]

     Commands sent from the transport proxy to the bridge are:

     [0x0000] DONE: There is no more information to give. The next
       bytes sent by the transport will be those tunneled over it.
       (body ignored)

     [0x0001] USERADDR: an address:port string that represents the
       client's address.

     [0x0002] TRANSPORT: a string of the name of the pluggable
       transport currently in effect on the connection.

     Replies sent from tor to the proxy are:

     [0x1000] OKAY: Send the user's traffic. (body ignored)

     [0x1001] DENY: Tor would prefer not to get more traffic from
       this address for a while. (body ignored)

     [0x1002] CONTROL: (Not used)

  Parties MUST ignore command codes that they do not understand.

If the server receives a recognized command that does not parse, it MUST close the connection to the client.

Command descriptions


  An ASCII string holding the TCP/IP address of the client of the
  pluggable transport proxy. A Tor bridge SHOULD use that address to
  collect statistics about its clients.  Recognized formats are:

(Current Tor versions may accept other formats, but this is a bug: transports MUST NOT send them.)

The string MUST not be NUL-terminated.


An ASCII string holding the name of the pluggable transport used by the client of the pluggable transport proxy. A Tor bridge that supports multiple transports SHOULD use that information to collect statistics about the popularity of individual pluggable transports.

The string MUST not be NUL-terminated.

Pluggable transport names are C-identifiers and Tor MUST check them for correctness.

Security Considerations

Extended ORPort or TransportControlPort do not provide link confidentiality, authentication or integrity. Sensitive data, like cryptographic material, should not be transferred through them.

An attacker with superuser access is able to sniff network traffic, and capture TransportControlPort identifiers and any data passed through those ports.

Tor SHOULD issue a warning if the bridge operator tries to bind Extended ORPort to a non-localhost address.

Pluggable transport proxies SHOULD issue a warning if they are instructed to connect to a non-localhost Extended ORPort.