Outline SDK (Beta)

Note

This code is under active development and not guaranteed to be stable. If you are interested in integrating with it, we'd love your feedback.

The Outline SDK allows you to:

• Create tools to protect against network-level interference.
• Add network-level interference protection to existing apps, such as content or communication apps.
• Troubleshoot connectivity and measure interference with a collection of command-line tools.

Advantages

Multi-Platform	Proven Technology	Composable
Supports Android, iOS, Windows, macOS and Linux.	Field-tested in the Outline Client and Server, helping millions to access the internet under harsh conditions.	Designed for modularity and reuse, allowing you to craft custom transports.

Interoperable and Reusable

The Outline SDK is built upon a simple basic concepts, defined as interoperable interfaces that allow for composition and easy reuse.

Connections enable communication between two endpoints over an abstract transport. There are two types of connections:

• transport.StreamConn: stream-based connection, like TCP and the SOCK_STREAM Posix socket type.
• transport.PacketConn: datagram-based connection, like UDP and the SOCK_DGRAM Posix socket type. We use "Packet" instead of "Datagram" because that is the convention in the Go standard library.

Connections can be wrapped to create nested connections over a new transport. For example, a StreamConn could be over TCP, over TLS over TCP, over HTTP over TLS over TCP, over QUIC, among other options.

Dialers enable the creation of connections given a host:port address while encapsulating the underlying transport or proxy protocol. The StreamDialer and PacketDialer types create StreamConn and PacketConn connections, respectively, given an address. Dialers can also be nested. For example, a TLS Stream Dialer can use a TCP dialer to create a StreamConn backed by a TCP connection, then create a TLS StreamConn backed by the TCP StreamConn. A SOCKS5-over-TLS Dialer could use the TLS Dialer to create the TLS StreamConn to the proxy before doing the SOCKS5 connection to the target address.

Resolvers (dns.Resolver) enable the answering of DNS questions while encapsulating the underlying algorithm or protocol. Resolvers are primarily used to map domain names to IP addresses.

Bypass DNS-based Blocking

The Outline SDK offers two types of strategies for evading DNS-based blocking: resilient DNS or address override.

• The dns package can replace the resolution based on the system resolver with more resillient options:
  • Encrypted DNS over HTTPS (DoH) or TLS (DoT)
  • Alternative hosts and ports for UDP and TCP resolvers, making it possible to use resolvers that are not blocked.
• The override config from x/config with a host option can be used to force a specific address, or you can implement your own Dialer that can map addresses.

Bypass SNI-based Blocking

The Outline SDK offers several strategies for evading SNI-based blocking:

At the TCP layer:

• TCP stream fragmentation with transport/split
• TLS record fragmentation with transport/tlsfrag

At the application layer:

• Domain-fronting and SNI hiding with transport/tls

Tunnel Connections over a Proxy

The Outline SDK offers two protocols to create connections over proxies:

• Shadowsocks, available in transport/shadowsocks. Easily create servers in the cloud using the Outline Manager.
• SOCKS5, available in transport/socks5. You can leverage a local SOCKS5 proxy that tunnels connections over SSH.

Build a VPN

Use the network package to create TUN-based VPNs using transport-layer proxies (often called "tun2socks").

Add the SDK to Your App

Choose from one of the following methods to integrate the Outline SDK into your project:

• Generated Mobile Library: For Android, iOS, and macOS apps. Uses gomobile bind to generate Java and Objective-C bindings.
• Side Service: For desktop and Android apps. Runs a standalone Go binary that your application communicates with (not available on iOS due to subprocess limitations).
• Go Library: Directly import the SDK into your Go application. API Reference.
• Generated C Library: Generate C bindings using go build.

The Outline Client uses a generated mobile library on Android, iOS and macOS (based on Cordova) and a side service on Windows and Linux (based on Electron).

Below we provide more details on each integration approach. For more details about setting up and using Outline SDK features, see the Discussions tab.

Generated Mobile Library

See our MobileProxy page to learn about the easiest way to integrate the Outline SDK into a mobile app. It runs a local forward proxy that implements resillience strategies that you can use to configure your app's networking libraries.

For advanced users, it is possible to generate your own mobile library, following these steps:

1. Create a Go library: Create a Go package that wraps the SDK functionalities you need.
2. Generate mobile library: Use gomobile bind to generate Android Archives (AAR) and Apple Frameworks with Java and Objective-C bindings.
   • Android examples: Outline Android Archive, Intra Android Archive.
   • Apple examples: Outline iOS Framework, Outline macOS Framework.
3. Integrate into your app: Add the generated library to your app. For more details, see Go Mobile's SDK applications and generating bindings.

Note: You must use gomobile bind on the package you create, not directly on the SDK packages.

Side Service

To integrate the SDK as a side service, follow these steps:

1. Define IPC mechanism: Choose an inter-process communication (IPC) mechanism (for example, sockets, standard I/O).
2. Build the service: Create a Go binary that includes the server-side of the IPC and used the SDK.
   • Examples: Outline Electron backend code, Outline Windows Client backend build, Outline Linux Client backend build.
3. Bundle the service: Include the Go binary in your application bundle.
   • Examples: Outline Windows Client, Outline Linux Client
4. Start the service: Launch the Go binary as a subprocess from your application.
   • Example: Outline Electron Clients
5. Service Calls: Add code to your app for communication with the service.

Go Library

To integrate the Outline SDK as a Go library, you can directly import it into your Go application. See the API Reference for what's available.

This approach is suitable for both command-line and GUI-based applications. You can build GUI-based applications in Go with frameworks like Wails, Fyne.io, Qt for Go, or Go Mobile app.

For examples, see x/examples.

Generated C Library

This approach is suited for applications that require C bindings. It is similar to the Generated Mobile Library approach, where you need to first create a Go library to generate bindings for.

Steps:

1. Create a Go library: Create a Go package that wraps the SDK functionalities you need. Functions to be exported must be marked with //export, as described in the cgo documentation.
2. Generate C library: Use go build with the appropriate -buildmode flag. Anternatively, you can use SWIG.
3. Integrate into your app: Add the generated C library to your application, according to your build system.

You can find detailed steps at the tutorial Go for beginners: Getting started.

Command-line Tools

The Outline SDK has several command-line utilities that illustrate the usage of the SDK, but are also valuable for debugging and trying the different strategies without having to build an app.

They all take a -transport flag with a config that specifies what transport should be used to establish connections. The config format can be found in x/config.

Resolve a Domain Name

The resolve tool resolves a domain name, similar to dig:

$ go run github.com/Jigsaw-Code/outline-sdk/x/examples/resolve@latest -type A -transport "tls" -resolver 8.8.8.8:853 -tcp getoutline.org.
216.239.34.21
216.239.32.21
216.239.38.21
216.239.36.21

Fetch a Web Page

The fetch tool fetches a URL, similar to curl. The example below would bypass blocking of meduza.io in Russia:

$ go run github.com/Jigsaw-Code/outline-sdk/x/examples/fetch@latest -transport "override:host=cloudflare.net|tlsfrag:1" -method HEAD -v https://meduza.io/
[DEBUG] 2023/12/28 18:44:56.490836 main.go:105: Cf-Ray: [83cdac8ecdccc40e-EWR]
[DEBUG] 2023/12/28 18:44:56.491231 main.go:105: Alt-Svc: [h3=":443"; ma=86400]
[DEBUG] 2023/12/28 18:44:56.491237 main.go:105: Date: [Thu, 28 Dec 2023 23:44:56 GMT]
[DEBUG] 2023/12/28 18:44:56.491241 main.go:105: Connection: [keep-alive]
[DEBUG] 2023/12/28 18:44:56.491247 main.go:105: Strict-Transport-Security: [max-age=31536000; includeSubDomains; preload]
[DEBUG] 2023/12/28 18:44:56.491251 main.go:105: Cache-Control: [max-age=0 no-cache, no-store]
[DEBUG] 2023/12/28 18:44:56.491257 main.go:105: X-Content-Type-Options: [nosniff]
[DEBUG] 2023/12/28 18:44:56.491262 main.go:105: Server: [cloudflare]
[DEBUG] 2023/12/28 18:44:56.491266 main.go:105: Content-Type: [text/html; charset=utf-8]
[DEBUG] 2023/12/28 18:44:56.491270 main.go:105: Expires: [Thu, 28 Dec 2023 23:44:56 GMT]
[DEBUG] 2023/12/28 18:44:56.491273 main.go:105: Cf-Cache-Status: [DYNAMIC]

Run a Local Forward Proxy

The http2transport tool runs a local proxy that creates connections according to the transport. It's effectively a circumvention tool.

The example below is analogous to the previous fetch example.

Start the local proxy:

$ go run github.com/Jigsaw-Code/outline-sdk/x/examples/http2transport@latest -transport "override:host=cloudflare.net|tlsfrag:1" -localAddr localhost:8080
2023/12/28 18:50:48 Proxy listening on 127.0.0.1:8080

Using the proxy with curl:

$ curl -p -x http://localhost:8080 https://meduza.io --head
HTTP/1.1 200 Connection established

HTTP/2 200
date: Thu, 28 Dec 2023 23:51:01 GMT
content-type: text/html; charset=utf-8
strict-transport-security: max-age=31536000; includeSubDomains; preload
expires: Thu, 28 Dec 2023 23:51:01 GMT
cache-control: max-age=0
cache-control: no-cache, no-store
cf-cache-status: DYNAMIC
x-content-type-options: nosniff
server: cloudflare
cf-ray: 83cdb579bbec4376-EWR
alt-svc: h3=":443"; ma=86400

Test Proxy Connectivity

The test-connectivity tool is useful to test connectivity to a proxy. It uses DNS resolutions over TCP and UDP using the transport to test if there is stream and datagram connectivity.

$ go run github.com/Jigsaw-Code/outline-sdk/x/examples/test-connectivity@latest -transport "$OUTLINE_KEY" && echo success || echo failure
{"resolver":"8.8.8.8:53","proto":"tcp","time":"2023-12-28T23:57:45Z","duration_ms":39,"error":null}
{"resolver":"8.8.8.8:53","proto":"udp","time":"2023-12-28T23:57:45Z","duration_ms":17,"error":null}
{"resolver":"[2001:4860:4860::8888]:53","proto":"tcp","time":"2023-12-28T23:57:45Z","duration_ms":31,"error":null}
{"resolver":"[2001:4860:4860::8888]:53","proto":"udp","time":"2023-12-28T23:57:45Z","duration_ms":16,"error":null}
success

Test Download Speed

The fetch-speed tool fetches a URL, similar to curl and calculates the download speed. It could be used for troubleshooting.

$ go run github.com/Jigsaw-Code/outline-sdk/x/examples/fetch@latest -transport ss://[redacted]@[redacted]:80 http://speedtest.ftp.otenet.gr/files/test10Mb.db

Downloaded 10.00 MB in 1.78s

Downloaded Speed: 5.61 MB/s