martian

Martian Proxy

Martian Proxy is a programmable HTTP proxy designed to be used for testing.

Martian is a great tool to use if you want to:

• Verify that all (or some subset) of requests are secure
• Mock external services at the network layer
• Inject headers, modify cookies or perform other mutations of HTTP requests and responses
• Verify that pingbacks happen when you think they should
• Unwrap encrypted traffic (requires install of CA certificate in browser)

By taking advantage of Go cross-compilation, Martian can be deployed anywhere that Go can target.

Getting Started

Start the Proxy

Running an instance of Martian is as simple as

go run examples/main.go

If you want to see system logs as Martian is running, pass in the verbosity flag:

go run examples/main.go -- -v=0

By default, Martian will be running on port 8080. The port can be specified via a flag:

go run examples/main.go -- --port=9999

Configure

Once Martian is running, you need to configure its behavior. Without configuration, Martian is just proxying without doing anything to the requests or responses. If enabled, logging will take place without additional configuration.

Martian is configured by JSON messages sent over HTTP that take the general form of:

{
  "header.Modifier": {
    "scope": ["response"],
    "name": "Test-Header",
    "value": "true"
  }
}

The above configuration tells Martian to inject a header with the name "Test-Header" and the value "true" on all responses.

Let's break down the parts of this message.

• [package.Type]: The package.Type of the modifier that you want to use. In this case, it's "header.Modifier", which is the name of the modifier that sets headers (to learn more about the header.Modifier, please refer to the modifier reference.

• [package.Type].scope: Indicates whether to apply to the modifier to requests, responses or both. This can be an array containing "request", "response", or both.

• [package.Type].[key]: Modifier specific data. In the case of the header modifier, we need the nameandvalue` of the header.

This is a simple configuration, for more complex configurations, modifiers are combined with groups and filters to compose the desired behavior.

To configure Martian, POST the JSON to host:port/martian/modifiers. You'll want to use whatever mechanism your language of choice provides you to make HTTP requests, but for demo purposes, curl works (assuming your configuration is in a file called modifier.json).

    curl -x localhost:8080 \
         -X POST \
         -H "Content-Type: application/json" \
         -d @modifier.json \
            "http://localhost:8080/martian/modifiers"

Check Verifiers

Let's assume that you've configured Martian to verify the presence a specific header in responses to a specific URL.

Here's a configuration to verify that all requests to example.com return responses with a 200 OK.

      {
        "url.Filter": {
          "scope": ["request", "response"],
          "host" : "example.com",
          "modifier" : {
            "status.Verifier": {
              "scope" : ["response"],
              "statusCode": 200
            }
          }
        }
      }

Once Martian is running, configured and the requests and resultant responses you wish to verify have taken place, you can verify your expectation that you only got back 200 OK responses.

To check verifications, perform

GET host:port/martian/verify

Failed expectations are tracked as errors, and the list of errors are retrieved by making a GET request to host:port/martian/verify, which will return a list of errors:

  {
      "errors" : [
          {
              "message": "response(http://example.com) status code verify failure: got 500, want 200"
          },
          {
              "message": "response(http://example.com/foo) status code verify failure: got 500, want 200"
          }
      ]
  }

Verification errors are held in memory until they are explicitly cleared by

POST host:port/martian/verify/reset

Martian as a Library

Martian can also be included into any Go program and used as a library.

Modifiers All The Way Down

Martian's request and response modification system is designed to be general and extensible. The design objective is to provide individual modifier behaviors that can arranged to build out nearly any desired modification.

When working with Martian to compose behaviors, you'll need to be familiar with these different types of interactions:

• Modifiers: Changes the state of a request or a response
• Filters: Conditionally allows a contained Modifier to execute
• Groups: Bundles multiple modifiers to be executed in the order specified in the group
• Verifiers: Tracks network traffic against expectations

Modifiers, filters and groups all implement RequestModifer, ResponseModifier or RequestResponseModifier (defined in martian.go).

ModifyRequest(ctx *martian.Context, req *http.Request) error

ModifyResponse(ctx *martian.Context, res *http.Response) error

Throughout the code (and this documentation) you'll see the word "modifier" used as a term that encompasses modifiers, groups and filters. Even though a group does not modify a request or response, we still refer to it as a "modifier".

We refer to anything that implements the modifier interface as a Modifier.

Parser Registration

Each modifier must register its own parser with Martian. The parser is responsible for parsing a JSON message into a Go struct that implements a modifier interface.

Martian holds modifier parsers as a map of strings to functions that is built out at run-time. Each modifier is responsible for registering its parser with a call to parse.Register in init().

Signature of parse.Register:

Register(name string, parseFunc func(b []byte) (interface{}, error))

Register takes in the key as a string in the form package.Type. For instance, cookie_modifier registers itself with the key cookie.Modifier and query_string_filter registers itself as querystring.Filter. This string is the same as the value of name in the JSON configuration message.

In the following configuration message, header.Modifier is how the header modifier is registered in the init() of header_modifier.go.

{
  "header.Modifier": {
    "scope": ["response"],
    "name" : "Test-Header",
    "value" : "true"
  }
}

Example of parser registration from header_modifier.go:

func init() {
  parse.Register("header.Modifier", modifierFromJSON)
}

func modifierFromJSON(b []byte) (interface{}, error) {
  ...
}

Adding Your Own Modifier

If you have a use-case in mind that we have not developed modifiers, filters or verifiers for, you can easily extend Martian to your very specific needs.

There are 2 mandatory parts of a modifier:

• Implement the modifier interface
• Register the parser

Any Go struct that implements those interfaces can act as a modifier.

Contact

Google Group (General and Dev): https://groups.google.com/forum/#!forum/martianproxy

Disclaimer

This is not an official Google product (experimental or otherwise), it is just code that happens to be owned by Google.