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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.

Latest Version



Go 1.11

Go Modules Support

Martian Proxy added support for Go modules since v3.0.0. If you use a Go version that does not support modules, this will break you. The latest version without Go modules support was tagged v2.1.0.

Getting Started


Martian Proxy can be installed using go install

go get && \
go install

Start the Proxy

Assuming you've installed Martian, running the proxy is as simple as


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

$GOPATH/bin/proxy -v=2

By default, Martian will be running on port 8080, and the Martian API will be running on 8181 . The port can be specified via flags:

$GOPATH/bin/proxy -addr=:9999 -api-addr=:9898


For logging of requests and responses a logging modifier is available or HAR logs are available if the -har flag is used.

HAR Logging

To enable HAR logging in Martian call the binary with the -har flag:

$GOPATH/bin/proxy -har

If the -har flag has been enabled two HAR related endpoints will be available:

GET http://martian.proxy/logs

Will retrieve the HAR log of all requests and responses seen by the proxy since the last reset.

DELETE http://martian.proxy/logs/reset

Will reset the in-memory HAR log. Note that the log will grow unbounded unless it is periodically reset.


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 name and value 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 http://martian.proxy/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 \

Intercepting HTTPS Requests and Responses

Martian supports modifying HTTPS requests and responses if configured to do so.

In order for Martian to intercept HTTPS traffic a custom CA certificate must be installed in the browser so that connection warnings are not shown.

The easiest way to install the CA certificate is to start the proxy with the necessary flags to use a custom CA certificate and private key using the -cert and -key flags, or to have the proxy generate one using the -generate-ca-cert flag.

After the proxy has started, visit http://martian.proxy/authority.cer in the browser configured to use the proxy and a prompt will be displayed to install the certificate.

Several flags are available in examples/main.go to help configure MITM functionality:

  PEM encoded private key file of the CA certificate provided in -cert; used
  to sign certificates that are generated on-the-fly
  PEM encoded CA certificate file used to generate certificates
  generates a CA certificate and private key to use for man-in-the-middle;
  most users choosing this option will immediately visit
  http://martian.proxy/authority.cer in the browser whose traffic is to be
  intercepted to install the newly generated CA certificate
-organization="Martian Proxy"
  organization name set on the dynamically-generated certificates during
  window of time around the time of request that the dynamically-generated
  certificate is valid for; the duration is set such that the total valid
  timeframe is double the value of validity (1h before & 1h after)

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 return responses with a 200 OK.

        "url.Filter": {
          "scope": ["request", "response"],
          "host" : "",
          "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 http://martian.proxy/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( status code verify failure: got 500, want 200"
              "message": "response( status code verify failure: got 500, want 200"

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

POST http://martian.proxy/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 RequestModifier, ResponseModifier or RequestResponseModifier (defined in martian.go).

ModifyRequest(req *http.Request) error

ModifyResponse(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.


For questions and comments on how to use Martian, features announcements, or design discussions check out our public Google Group at!forum/martianproxy-users.

For security related issues please send a detailed report to our private core group at


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