Pluggable Transport To Parent Process Communication

All Pluggable Transport Proxies communicate to the parent process via writing NL-terminated lines to stdout. The line metaformat is:

<Line> ::= <Keyword> <OptArgs> <NL>
<Keyword> ::= <KeywordChar> | <Keyword> <KeywordChar>
<KeywordChar> ::= <any US-ASCII alphanumeric, dash, and underscore>
<OptArgs> ::= <Args>*
<Args> ::= <SP> <ArgChar> | <Args> <ArgChar>
<ArgChar> ::= <any US-ASCII character but NUL or NL>
<SP> ::= <US-ASCII whitespace symbol (32)>
<NL> ::= <US-ASCII newline (line feed) character (10)>

The parent process MUST ignore lines received from PT proxies with unknown keywords.

Common Messages

When a PT proxy first starts up, it must determine which version of the Pluggable Transports Specification to use to configure itself.

It does this via the "TOR_PT_MANAGED_TRANSPORT_VER" (3.2.1) environment variable which contains all of the versions supported by the application.

Upon determining the version to use, or lack thereof, the PT proxy responds with one of two messages.

VERSION-ERROR <ErrorMessage>

The "VERSION-ERROR" message is used to signal that there was no compatible Pluggable Transport Specification version present in the "TOR_PT_MANAGED_TRANSPORT_VER" list.

The <ErrorMessage> SHOULD be set to "no-version" for historical reasons but MAY be set to a useful error message instead.

PT proxies MUST terminate after outputting a "VERSION-ERROR" message.

Example:

VERSION-ERROR no-version

VERSION <ProtocolVersion>

The "VERSION" message is used to signal the Pluggable Transport Specification version (as in "TOR_PT_MANAGED_TRANSPORT_VER") that the PT proxy will use to configure its transports and communicate with the parent process.

The version for the environment values and reply messages specified by this document is "1".

PT proxies MUST either report an error and terminate, or output a "VERSION" message before moving on to client/server proxy initialization and configuration.

Example:

VERSION 1

After version negotiation has been completed the PT proxy must then validate that all of the required environment variables are provided, and that all of the configuration values supplied are well formed.

At any point, if there is an error encountered related to configuration supplied via the environment variables, it MAY respond with an error message and terminate.

ENV-ERROR <ErrorMessage>

The "ENV-ERROR" message is used to signal the PT proxy's failure to parse the configuration environment variables (3.2).

The <ErrorMessage> SHOULD consist of a useful error message that can be used to diagnose and correct the root cause of the failure.

PT proxies MUST terminate after outputting a "ENV-ERROR" message.

Example:

ENV-ERROR No TOR_PT_AUTH_COOKIE_FILE when TOR_PT_EXTENDED_SERVER_PORT set

Pluggable Transport Client Messages

After negotiating the Pluggable Transport Specification version, PT client proxies MUST first validate "TOR_PT_PROXY" (3.2.2) if it is set, before initializing any transports.

Assuming that an upstream proxy is provided, PT client proxies MUST respond with a message indicating that the proxy is valid, supported, and will be used OR a failure message.

PROXY DONE

The "PROXY DONE" message is used to signal the PT proxy's acceptance of the upstream proxy specified by "TOR_PT_PROXY".

PROXY-ERROR <ErrorMessage>

The "PROXY-ERROR" message is used to signal that the upstream proxy is malformed/unsupported or otherwise unusable.

PT proxies MUST terminate immediately after outputting a "PROXY-ERROR" message.

Example:

PROXY-ERROR SOCKS 4 upstream proxies unsupported.

After the upstream proxy (if any) is configured, PT clients then iterate over the requested transports in "TOR_PT_CLIENT_TRANSPORTS" and initialize the listeners.

For each transport initialized, the PT proxy reports the listener status back to the parent via messages to stdout.

CMETHOD <transport> <'socks4','socks5'> <address:port>

The "CMETHOD" message is used to signal that a requested PT transport has been launched, the protocol which the parent should use to make outgoing connections, and the IP address and port that the PT transport's forward proxy is listening on.

Example:

CMETHOD trebuchet socks5 127.0.0.1:19999

CMETHOD-ERROR <transport> <ErrorMessage>

The "CMETHOD-ERROR" message is used to signal that requested PT transport was unable to be launched.

Example:

CMETHOD-ERROR trebuchet no rocks available

Once all PT transports have been initialized (or have failed), the PT proxy MUST send a final message indicating that it has finished initializing.

CMETHODS DONE

The "CMETHODS DONE" message signals that the PT proxy has finished initializing all of the transports that it is capable of handling.

Upon sending the "CMETHODS DONE" message, the PT proxy initialization is complete.

Notes:

    - Unknown transports in "TOR_PT_CLIENT_TRANSPORTS" are ignored
      entirely, and MUST NOT result in a "CMETHOD-ERROR" message.
      Thus it is entirely possible for a given PT proxy to
      immediately output "CMETHODS DONE".

    - Parent processes MUST handle "CMETHOD"/"CMETHOD-ERROR"
      messages in any order, regardless of ordering in
      "TOR_PT_CLIENT_TRANSPORTS".

Pluggable Transport Server Messages

PT server reverse proxies iterate over the requested transports in "TOR_PT_CLIENT_TRANSPORTS" and initialize the listeners.

For each transport initialized, the PT proxy reports the listener status back to the parent via messages to stdout.

SMETHOD <transport> <address:port> [options]

The "SMETHOD" message is used to signal that a requested PT transport has been launched, the protocol which will be used to handle incoming connections, and the IP address and port that clients should use to reach the reverse-proxy.

If there is a specific address:port provided for a given PT transport via "TOR_PT_SERVER_BINDADDR", the transport MUST be initialized using that as the server address.

The OPTIONAL 'options' field is used to pass additional per-transport information back to the parent process.

The currently recognized 'options' are:

ARGS:[<Key>=<Value>,]+[<Key>=<Value>]

           The "ARGS" option is used to pass additional key/value
           formatted information that clients will require to use
           the reverse proxy.

           Equal signs and commas MUST be escaped with a backslash.

           Tor: The ARGS are included in the transport line of the
           Bridge's extra-info document.

       Examples:

         SMETHOD trebuchet 198.51.100.1:19999
         SMETHOD rot_by_N 198.51.100.1:2323 ARGS:N=13

     SMETHOD-ERROR <transport> <ErrorMessage>

The "SMETHOD-ERROR" message is used to signal that requested PT transport reverse proxy was unable to be launched.

Example:

SMETHOD-ERROR trebuchet no cows available

Once all PT transports have been initialized (or have failed), the PT proxy MUST send a final message indicating that it has finished initializing.

SMETHODS DONE

The "SMETHODS DONE" message signals that the PT proxy has finished initializing all of the transports that it is capable of handling.

Upon sending the "SMETHODS DONE" message, the PT proxy initialization is complete.

Pluggable Transport Log Message

This message is for a client or server PT to be able to signal back to the parent process via stdout or stderr any log messages.

A log message can be any kind of messages (human readable) that the PT sends back so the parent process can gather information about what is going on in the child process. It is not intended for the parent process to parse and act accordingly but rather a message used for plain logging.

For example, the tor daemon logs those messages at the Severity level and sends them onto the control port using the PT_LOG (see control-spec.txt) event so any third party can pick them up for debugging.

The format of the message:

LOG SEVERITY=Severity MESSAGE=Message

The SEVERITY value indicate at which logging level the message applies. The accepted values for <Severity> are: error, warning, notice, info, debug

The MESSAGE value is a human readable string formatted by the PT. The <Message> contains the log message which can be a String or CString (see section 2 in control-spec.txt).

Example:

LOG SEVERITY=debug MESSAGE="Connected to bridge A"

Pluggable Transport Status Message

This message is for a client or server PT to be able to signal back to the parent process via stdout or stderr any status messages.

The format of the message:

STATUS [TRANSPORT=Transport] TYPE=Type [<K_1>=<V_1> [<K_2>=<V_2> ...]]

The TYPE value indicates the status message type. Each Type has its own set of associated <K_n>=<V_n> values. Type can be a String or CString.

Following the TYPE specification are zero or more key=value pairs separated by spaces. Each TYPE has its own set of known keys. Each <V_n> can be a String or CString. To avoid confusion with the TYPE=Type specification, no <K_n> may be "TYPE". TYPE and the different K_n values may appear in any order.

The parent process should ignore STATUS messages that have a TYPE it does not understand. The parent process should ignore STATUS messages that do not have a TYPE set, or that have two or more TYPE=Type specifications.

Compatibility note:

Versions of Tor and Arti released before August of 2024 treated the TRANSPORT key as required, and rejected any status message that did not contain it. If you need to write a pluggable transport to work with one of those versions, you need to provide a value in the TRANSPORT field.

Here are the recognized TYPEs, and the keys that are understood for them.

TYPE=version

Required keys: IMPLEMENTATION, VERSION

The VERSION type reports the name of the PT implementation and its version number. One of the uses of this message is to enable contacting bridge operators that are running an out-of-date pluggable transport implementation.

IMPLEMENTATION is the name of the software package implementing the pluggable transport (because different implementations may have different version numbering schemes).

VERSION is the version number of the software package implementing the pluggable transport.

A single STATUS message of the VERSION type may be sent any time before "CMETHODS DONE" or "SMETHODS DONE". If sent after that, the parent process should ignore the message. If a VERSION message is sent more than once, only the first one counts.

Examples:

STATUS TYPE=version VERSION=0.0.13 IMPLEMENTATION=obfs4proxy
STATUS IMPLEMENTATION=snowflake-client VERSION=2.1.0 TYPE=version