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Pluggable Transport Specification (Version 1)

Abstract

Pluggable Transports (PTs) are a generic mechanism for the rapid development and deployment of censorship circumvention, based around the idea of modular sub-processes that transform traffic to defeat censors.

This document specifies the sub-process startup, shutdown, and inter-process communication mechanisms required to utilize PTs.

Table of Contents

1. Introduction
  1.1. Requirements Notation
2. Architecture Overview
3. Specification
  3.1. Pluggable Transport Naming
  3.2. Pluggable Transport Configuration Environment Variables
     3.2.1. Common Environment Variables
     3.2.2. Pluggable Transport Client Environment Variables
     3.2.3. Pluggable Transport Server Environment Variables
  3.3. Pluggable Transport To Parent Process Communication
     3.3.1. Common Messages
     3.3.2. Pluggable Transport Client Messages
     3.3.3. Pluggable Transport Server Messages
  3.4. Pluggable Transport Shutdown
  3.5. Pluggable Transport Client Per-Connection Arguments
4. Anonymity Considerations
5. References
6. Acknowledgments
Appendix A. Example Client Pluggable Transport Session
Appendix B. Example Server Pluggable Transport Session

1. Introduction

This specification describes a way to decouple protocol-level obfuscation from an application's client/server code, in a manne that promotes rapid development of obfuscation/circumvention tools and promotes reuse beyond the scope of the Tor Project's efforts in that area.

This is accomplished by utilizing helper sub-processes that implement the necessary forward/reverse proxy servers that handle the censorship circumvention, with a well defined and standardized configuration and management interface.

Any application code that implements the interfaces as specified in this document will be able to use all spec compliant Pluggable Transports.

1.1. Requirements Notation

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].

    +------------+                    +---------------------------+
    | Client App +-- Local Loopback --+ PT Client (SOCKS Proxy)   +--+
    +------------+                    +---------------------------+  |
                                                                     |
                Public Internet (Obfuscated/Transformed traffic) ==> |
                                                                     |
    +------------+                    +---------------------------+  |
    | Server App +-- Local Loopback --+ PT Server (Reverse Proxy) +--+
    +------------+                    +---------------------------+

On the client's host, the PT Client software exposes a SOCKS proxy [RFC1928] to the client application, and obfuscates or otherwise transforms traffic before forwarding it to the server's host.

On the server's host, the PT Server software exposes a reverse proxy that accepts connections from PT Clients, and handles reversing the obfuscation/transformation applied to traffic, before forwarding it to the actual server software. An optional lightweight protocol exists to facilitate communicating connection meta-data that would otherwise be lost such as the source IP address and port [EXTORPORT].

All PT instances are configured by the respective parent process via a set of standardized environment variables (3.2) that are set at launch time, and report status information back to the parent via writing output in a standardized format to stdout (3.3).

Each invocation of a PT MUST be either a client OR a server.

All PT client forward proxies MUST support either SOCKS 4 or SOCKS 5, and SHOULD prefer SOCKS 5 over SOCKS 4.

3. Specification

Pluggable Transport proxies follow the following workflow throughout their lifespan.
(1) Parent process sets the required environment values (3.2) and launches the PT proxy as a sub-process (fork()/exec()).
(2) The PT Proxy determines the versions of the PT specification supported by the parent"TOR_PT_MANAGED_TRANSPORT_VER" (3.2.1)
(2.1) If there are no compatible versions, the PT proxy writes a "VERSION-ERROR" message (3.3.1) to stdout and terminates.
(2.2) If there is a compatible version, the PT proxy writes a "VERSION" message (3.3.1) to stdout.
(3) The PT Proxy parses the rest of the environment values.
(3.1) If the environment values are malformed, or otherwise invalid, the PT proxy writes a "ENV-ERROR" message (3.3.1) to stdout and terminates.
(3.2) Determining if it is a client side forward proxy or a server side reverse proxy can be done via examining the "TOR_PT_CLIENT_TRANSPORTS" and "TOR_PT_SERVER_TRANSPORTS" environment variables.
(4) (Client only) If there is an upstream proxy specified via "TOR_PT_PROXY" (3.2.2), the PT proxy validates the URI provided.
(4.1) If the upstream proxy is unusable, the PT proxy writes a "PROXY-ERROR" message (3.3.2) to stdout and terminates.
(4.2) If there is a supported and well-formed upstream proxy the PT proxy writes a "PROXY DONE" message (3.3.2) to stdout.
(5) The PT Proxy initializes the transports and reports the status via stdout (3.3.2, 3.3.3)
(6) The PT Proxy forwards and transforms traffic as appropriate.
(7) Upon being signaled to terminate by the parent process (3.4), the PT Proxy gracefully shuts down.

3.1. Pluggable Transport Naming

Pluggable Transport names serve as unique identifiers, and every PT MUST have a unique name.

PT names MUST be valid C identifiers. PT names MUST begin with a letter or underscore, and the remaining characters MUST be ASCII letters, numbers or underscores. No length limit is imposted.

PT names MUST satisfy the regular expression "[a-zA-Z_][a-zA-Z0-9_]*".

3.2. Pluggable Transport Configuration Environment Variables

All Pluggable Transport proxy instances are configured by their parent process at launch time via a set of well defined environment variables.

The "TOR_PT_" prefix is used for namespacing reasons and does not indicate any relations to Tor, except for the origins of this specification.

3.2.1. Common Environment Variables

When launching either a client or server Pluggable Transport proxy, the following common environment variables MUST be set.

"TOR_PT_MANAGED_TRANSPORT_VER"

Specifies the versions of the Pluggable Transport specification the parent process supports, delimited by commas. All PTs MUST accept any well-formed list, as long as a compatible version is present.
Valid versions MUST consist entirely of non-whitespace, non-comma printable ASCII characters.
The version of the Pluggable Transport specification as of this document is "1".

Example: TOR_PT_MANAGED_TRANSPORT_VER=1,1a,2b,this_is_a_valid_ver

"TOR_PT_STATE_LOCATION"

Specifies an absolute path to a directory where the PT is allowed to store state that will be persisted across invocations. The directory is not required to exist when the PT is launched, however PT implementations SHOULD be able to create it as required.
PTs MUST only store files in the path provided, and MUST NOT create or modify files elsewhere on the system.

Example:TOR_PT_STATE_LOCATION=/var/lib/tor/pt_state/

"TOR_PT_EXIT_ON_STDIN_CLOSE"

Specifies that the parent process will close the PT proxy's standard input (stdin) stream to indicate that the PT proxy should gracefully exit.
PTs MUST NOT treat a closed stdin as a signal to terminate unless this environment variable is set to "1".
PTs SHOULD treat stdin being closed as a signal to gracefully terminate if this environment variable is set to "1".

Example:TOR_PT_EXIT_ON_STDIN_CLOSE=1

"TOR_PT_OUTBOUND_BIND_ADDRESS_V4"

Specifies an IPv4 IP address that the PT proxy SHOULD use as source address for outgoing IPv4 IP packets. This feature allows people with multiple network interfaces to specify explicitly which interface they prefer the PT proxy to use.
If this value is unset or empty, the PT proxy MUST use the default source address for outgoing connections.
This setting MUST be ignored for connections to loopback addresses (127.0.0.0/8).

Example:TOR_PT_OUTBOUND_BIND_ADDRESS_V4=203.0.113.4

"TOR_PT_OUTBOUND_BIND_ADDRESS_V6"

Specifies an IPv6 IP address that the PT proxy SHOULD use as source address for outgoing IPv6 IP packets. This feature allows people with multiple network interfaces to specify explicitly which interface they prefer the PT proxy to use.
If this value is unset or empty, the PT proxy MUST use the default source address for outgoing connections.
This setting MUST be ignored for connections to the loopback address ([::1]).
IPv6 addresses MUST always be wrapped in square brackets.

Example:TOR_PT_OUTBOUND_BIND_ADDRESS_V6=[2001:db8::4]

3.2.2. Pluggable Transport Client Environment Variables

Client-side Pluggable Transport forward proxies are configured via the following environment variables.

"TOR_PT_CLIENT_TRANSPORTS"

Specifies the PT protocols the client proxy should initialize, as a comma separated list of PT names.
PTs SHOULD ignore PT names that it does not recognize.
Parent processes MUST set this environment variable when launching a client-side PT proxy instance.

Example: TOR_PT_CLIENT_TRANSPORTS=obfs2,obfs3,obfs4

"TOR_PT_PROXY"

Specifies an upstream proxy that the PT MUST use when making outgoing network connections. It is a URI [RFC3986] of the format:
<proxy_type>://[<user_name>[:][@]:.
The "TOR_PT_PROXY" environment variable is OPTIONAL and MUST be omitted if there is no need to connect via an upstream proxy.

Examples:
TOR_PT_PROXY=socks5://tor:test1234@198.51.100.1:8000
TOR_PT_PROXY=socks4a://198.51.100.2:8001
TOR_PT_PROXY=http://198.51.100.3:443

3.2.3. Pluggable Transport Server Environment Variables

Server-side Pluggable Transport reverse proxies are configured via the following environment variables.

"TOR_PT_SERVER_TRANSPORTS"

Specifies the PT protocols the server proxy should initialize, as a comma separated list of PT names.
PTs SHOULD ignore PT names that it does not recognize.
Parent processes MUST set this environment variable when launching a server-side PT reverse proxy instance.

Example: TOR_PT_SERVER_TRANSPORTS=obfs3,scramblesuit

"TOR_PT_SERVER_TRANSPORT_OPTIONS"

Specifies per-PT protocol configuration directives, as a semicolon-separated list of : pairs, where is a PT name and is a k=v string value with options that are to be passed to the transport.
Colons, semicolons, and backslashes MUST be escaped with a backslash.
If there are no arguments that need to be passed to any of PT transport protocols, "TOR_PT_SERVER_TRANSPORT_OPTIONS" MAY be omitted.

Example: TOR_PT_SERVER_TRANSPORT_OPTIONS=scramblesuit:key=banana;automata:rule=110;automata:depth=3

Will pass to 'scramblesuit' the parameter 'key=banana' and to 'automata' the arguments 'rule=110' and 'depth=3'.

"TOR_PT_SERVER_BINDADDR"

A comma separated list of <key>-<value> pairs, where <key> is a PT name and <value> is the <address>:<port> on which it should listen for incoming client connections.
The keys holding transport names MUST be in the same order as they appear in TOR_PT_SERVER_TRANSPORTS".
The <address> MAY be a locally scoped address as long as port forwarding is done externally.
The <address>:<port> combination MUST be an IP address supported by bind(), and MUST NOT be a host name.
Applications MUST NOT set more than one <address>:<port> pair per PT name.
If there is no specific <address>:<port> combination to be configured for any transports, "TOR_PT_SERVER_BINDADDR" MAY be omitted.

Example: TOR_PT_SERVER_BINDADDR=obfs3-198.51.100.1:1984,scramblesuit-127.0.0.1:4891

"TOR_PT_ORPORT"

Specifies the destination that the PT reverse proxy should forward traffic to after transforming it as appropriate, as an <address>:<port>.
Connections to the destination specified via "TOR_PT_ORPORT" MUST only contain application payload. If the parent process requires the actual source IP address of client connections (or other metadata), it should set "TOR_PT_EXTENDED_SERVER_PORT" instead.

Example: TOR_PT_ORPORT=127.0.0.1:9001

"TOR_PT_EXTENDED_SERVER_PORT"

Specifies the destination that the PT reverse proxy should forward traffic to, via the Extended ORPort protocol [EXTORPORT] as an <address>:<port>.
The Extended ORPort protocol allows the PT reverse proxy to communicate per-connection metadata such as the PT name and client IP address/port to the parent process.
If the parent process does not support the ExtORPort protocol, it MUST set "TOR_PT_EXTENDED_SERVER_PORT" to an empty string.

Example: TOR_PT_EXTENDED_SERVER_PORT=127.0.0.1:4200

"TOR_PT_AUTH_COOKIE_FILE"

Specifies an absolute filesystem path to the Extended ORPort authentication cookie, required to communicate with the Extended ORPort specified via "TOR_PT_EXTENDED_SERVER_PORT".
If the parent process is not using the ExtORPort protocol for incoming traffic, "TOR_PT_AUTH_COOKIE_FILE" MUST be omitted.

Example: TOR_PT_AUTH_COOKIE_FILE=/var/lib/tor/extended_orport_auth_cookie

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.

3.3.1. 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 OR_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 it's 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

3.3.2. 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".

3.3.3. 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.

3.3.4. 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 part 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"

3.3.5. 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 <K_1>=<V_1> [<K_2>=<V_2>, ...]

The TRANSPORT value indicate a hint on what the PT is such has the name or the protocol used for instance. As an example, obfs4proxy would use "obfs4". Thus, the Transport value can be anything the PT itself defines and it can be a String or CString (see section 2 in control-spec.txt).

The <K_n>=<V_n> values are specific to the PT and there has to be at least one. They are messages that reflects the status that the PT wants to report. <V_n> can be a String or CString.

Examples (fictional):]
STATUS TRANSPORT=obfs4 ADDRESS=198.51.100.123:1234 CONNECT=Success
STATUS TRANSPORT=obfs4 ADDRESS=198.51.100.222:2222 CONNECT=Failed FINGERPRINT= ERRSTR="Connection refused"
STATUS TRANSPORT=trebuchet ADDRESS=198.51.100.15:443 PERCENT=42

3.4. Pluggable Transport Shutdown

The recommended way for Pluggable Transport using applications and Pluggable Transports to handle graceful shutdown is as follows.

• (Parent) Set "TOR_PT_EXIT_ON_STDIN_CLOSE" (3.2.1) when launching the PT proxy, to indicate that stdin will be used for graceful shutdown notification.
• (Parent) When the time comes to terminate the PT proxy:
  1. Close the PT proxy's stdin.
  2. Wait for a "reasonable" amount of time for the PT to exit.
  3. Attempt to use OS specific mechanisms to cause graceful PT shutdown (eg: 'SIGTERM')
  4. Use OS specific mechanisms to force terminate the PT (eg: 'SIGKILL', 'ProccessTerminate()').
• PT proxies SHOULD monitor stdin, and exit gracefully when it is closed, if the parent supports that behavior.
• PT proxies SHOULD handle OS specific mechanisms to gracefully terminate (eg: Install a signal handler on 'SIGTERM' that causes cleanup and a graceful shutdown if able).
• PT proxies SHOULD attempt to detect when the parent has terminated (eg: via detecting that it's parent process ID haso changed on U*IX systems), and gracefully terminate.

3.5. Pluggable Transport Client Per-Connection Arguments

Certain PT transport protocols require that the client provides per-connection arguments when making outgoing connections. On the server side, this is handled by the "ARGS" optional argument as part of the "SMETHOD" message.
On the client side, arguments are passed via the authentication fields that are part of the SOCKS protocol.

First the "=" formatted arguments MUST be escaped, such that all backslash, equal sign, and semicolon characters are escaped with a backslash.
Second, all of the escaped are concatenated together.

Example: shared-secret=rahasia;secrets-file=/tmp/blob

Lastly the arguments are transmitted when making the outgoing connection using the authentication mechanism specific to the SOCKS protocol version.

• In the case of SOCKS 4, the concatenated argument list is transmitted in the "USERID" field of the "CONNECT" request.
• In the case of SOCKS 5, the parent process must negotiate "Username/Password" authentication [RFC1929], and transmit the arguments encoded in the "UNAME" and "PASSWD" fields.

If the encoded argument list is less than 255 bytes in length, the "PLEN" field must be set to "1" and the "PASSWD" field must contain a single NUL character.

4. Anonymity Considerations

When designing and implementing a Pluggable Transport, care should be taken to preserve the privacy of clients and to avoid leaking personally identifying information.

Examples of client related considerations are:

• Not logging client IP addresses to disk.
• Not leaking DNS addresses except when necessary.
• Ensuring that "TOR_PT_PROXY"'s "fail closed" behavior is implemented correctly.

Additionally, certain obfuscation mechanisms rely on information such as the server IP address/port being confidential, so clients also need to take care to preserve server side information confidential when applicable.

5. References

• [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
• [RFC1928] Leech, M., Ganis, M., Lee, Y., Kuris, R., Koblas, D., Jones, L., "SOCKS Protocol Version 5", RFC 1928, March 1996.
• [EXTORPORT] Kadianakis, G., Mathewson, N., "Extended ORPort and TransportControlPort", Tor Proposal 196, March 2012.
• [RFC3986] Berners-Lee, T., Fielding, R., Masinter, L., "Uniform Resource Identifier (URI): Generic Syntax", RFC 3986, January 2005.
• [RFC1929] Leech, M., "Username/Password Authentication for SOCKS V5", RFC 1929, March 1996.

6. Acknowledgments

This specification draws heavily from prior versions done by Jacob Appelbaum, Nick Mathewson, and George Kadianakis.

Appendix A. Example Client Pluggable Transport Session

Environment variables:

TOR_PT_MANAGED_TRANSPORT_VER=1
TOR_PT_STATE_LOCATION=/var/lib/tor/pt_state/
TOR_PT_EXIT_ON_STDIN_CLOSE=1
TOR_PT_PROXY=socks5://127.0.0.1:8001
TOR_PT_CLIENT_TRANSPORTS=obfs3,obfs4

Messages the PT Proxy writes to stdin:

VERSION 1
PROXY DONE
CMETHOD obfs3 socks5 127.0.0.1:32525
CMETHOD obfs4 socks5 127.0.0.1:37347
CMETHODS DONE

Appendix B. Example Server Pluggable Transport Session

Environment variables:

TOR_PT_MANAGED_TRANSPORT_VER=1
TOR_PT_STATE_LOCATION=/var/lib/tor/pt_state
TOR_PT_EXIT_ON_STDIN_CLOSE=1
TOR_PT_SERVER_TRANSPORTS=obfs3,obfs4
TOR_PT_SERVER_BINDADDR=obfs3-198.51.100.1:1984

Messages the PT Proxy writes to stdin:

VERSION 1
SMETHOD obfs3 198.51.100.1:1984
SMETHOD obfs4 198.51.100.1:43734 ARGS:cert=HszPy3vWfjsESCEOo9ZBkRv6zQ/1mGHzc8arF0y2SpwFr3WhsMu8rK0zyaoyERfbz3ddFw,iat-mode=0
SMETHODS DONE