reverse-proxy-middleware

Project status

Use case

Easy and dynamically configurable reverse proxy. This tool can act as a middleware to check incoming requests via external bot filtering services such as ReCaptcha or friendlycaptcha. Adding state via session token with external authentication apis is possible as applying any external api given data to the incoming and conditionally forwarded requests.

Quick start

Simple forwarding

Simple reverse proxy request from http://localhost:8080 to https://www.google.com without modifying the entire request.

{
  "forwarders": {
    "google": {
      "host": "www.google.com"
    }
  }
}

Since the proxy starts at localhost on port 8080 as a default configuration you can check the configuration via a simple curl command:

curl --verbose http://localhost:8080

Behind there are a some commonly use defaults configured under key "configuration" in package.json. Please have a look.

Distributing requests to different backends

Here is how to distribute incoming requests randomly between google and bing:

{
  "forwarders": {
    "bing": {
      "host": "www.bing.com",
      "useExpression": "Math.random() < 0.5"
    },

    "google": {
      "host": "www.google.com"
    }
  }
}

Forwarders are iterated in alphabetical order of their name and useExpression are getting evaluated. Since "google" is always "used" it depends on the random outcome of bing's expression if it is beeing used or not.

Since this proxy just streams the whole request through it could be used as basic load balancer with this configuration.

Rewriting headers for underlying backend

Headers can be replaced in both directions. Client-Request to forward or retrieved responses given from configured backend:

{
  "forwarders": {
    "bing": {
      "host": "www.bing.com",

      "headerTransformations": {
        "send": {
          "source": "/X-Special-Client-Header-Name: (.+)/gi",
          "target": "'New-Header-Name: $1'"
        },

        "retrieve": {
          "source": "/set-cookie: (.+)/gi",
          "target": "'X-Prevented-Cookie: $1'"
        }
      }
    }
  }
}

Test via:

  curl \
    --header 'X-Special-Client-Header-Name: value' \
    --verbose \
    http://localhost:8080 \
      1>/dev/null

You should see a lot of cookie header ("set-cookie: ..." replaced by "X-Prevented-Cookie: ...". Note that muting the standart output ("1>/dev/null") enables you to focus on retrieved headers printed via secondary error output.

State-APIs

State-APIs enables you to conditionally trigger requests to third party endpoints and use responses for further decisions how to proceed. When a state api is used for a specified request that response can be used via expressions to transform subsequent api requests, decide which backend to use or transform the final backend request.

In the following example you can see how a state api is configured to catch specific request to deal with. The last running pre expressions which results in a boolean value will trigger if the referenced state api should be used for given request.

Here is an example:

{
  "forwarders": {
    "bing": {
      "host": "www.bing.com",

      "stateAPIs": {
        "name": "sense",
        "url": "https://www.google.com/search?q=${data.question}",

        "data": {
          "question": "What is the meaning of life?"
        },

        "expressions": {
          "pre": "request.headers['ask-for-sense-of-live'] ? true : null",
          "post": "response.statusCode >= 200 && response.statusCode < 300 ? null : response.statusCode"
        }
      }
    }
  }
}

What is going on here? We generally forward requests to "www.bing.com" but if there is a header called "ask-for-sense-of-live" present in client request we will first ask google for the meaning of life. If google answers with a "negative" response code not between 200 and 300 we will just do the final forwarding to bing. If google responses in a positive manner the resulting status code we will be used for answering client and no forwarding to "www.bing.com" happens.

Here es a test curl command:

  curl \
    --header 'ask-for-sense-of-live: value' \
    --verbose \
    http://localhost:8080 \
      1>/dev/null

This should result in a simple response but:

curl --verbose http://localhost:8080 1>/dev/null

will finally request "www.bing.com".

Pre and post evaluations can have various results. The meanings of them are described here:

Pre-Evaluation Results

Post-Evaluation Results

Smart configurations

Whenever you can configure list of items you can either use just one or a list of them. Consider this configuration example:

{
  "forwarders": {
    "bing": {
      "host": "www.bing.com",

      "headerTransformations": [{...}, {...}, ...],

      "stateAPIs": {
        "expressions": {
          "pre": ["...", "...", ...],
          "post": ["...", "...", ...]
        }
      }
    }
  }
}

If only one item is needed please consider that:

{
  "forwarders": {
    "bing": {
      "host": "www.bing.com",

      "headerTransformations": [{...}],

      "stateAPIs": {
        "expressions": {
          "pre": ["..."],
          "post": ["..."]
        }
      }
    }
  }
}

is equivalent to:

{
  "forwarders": {
    "bing": {
      "host": "www.bing.com",

      "headerTransformations": {...},

      "stateAPIs": {
        "expressions": {
          "pre": "...",
          "post": "..."
        }
      }
    }
  }
}

Use environment variables

While some configuration values are interpret as expression to be evalued at runtime e.g. to decide which endpoint to use:

{
  "forwarders": {
    "endpoint": {
      ...
      "useExpression": "..."
    }
  }
}

Every item can utilize expression to dynamically derive intial configurations:

{
  "port": {
    "__evaluate__": "environment.PORT ?? 8080"
  },
  ...
}

If an environment variabel "PORT" is set it will be used or "8080" as a fallback.

Use Base Forwarder

Base forwarder are inherited by every specific forwarder. This configuration:

{
  "forwarders": {
    "base: {
      "headerTransformations": {
        "send": {
          "source": "/(GET|POST) \\/.* (.*)/",
          "target": "'$1 /sub/path/to/forward/to $2'"
        }
      }
    },

    "bing": {
      "host": "www.bing.com",
      "useExpression": "Math.random() < 0.5"
    },

    "google": {
      "host": "www.google.com"
    }
  }
}

is equivalent to:

{
  "forwarders": {
    "bing": {
      "host": "www.bing.com",

      "useExpression": "Math.random() < 0.5",

      "headerTransformations": {
        "send": {
          "source": "/(GET|POST) \\/.* (.*)/",
          "target": "'$1 /sub/path/to/forward/to $2'"
        }
      }
    },

    "google": {
      "host": "www.google.com",

      "headerTransformations": {
        "send": {
          "source": "/(GET|POST) \\/.* (.*)/",
          "target": "'$1 /sub/path/to/forward/to $2'"
        }
      }
    }
  }
}

Use base State-APIs

As we support generic base forwarder confgurations there are also base state api configuraton sections. Consider the follwing configuration example:

{
  "forwarders": {
    "bing": {
      "host": "www.bing.com",

      "stateAPIs": [
        {
          "name": "base",
          "url": "https://www.google.com/search?q=${data.query}",

          "expressions": {
            "pre": "request.headers['ask-google'] ? true : null",
            "post": "response.statusCode >= 200 && response.statusCode < 300 ? null : response.statusCode"
          }
        }

        {
          "name": "sense",

          "data": {
            "query": "What is the meaning of life?"
          }
        },

        {
          "name": "nonesense",

          "data": {
            "query": "baby cats"
          }
        }
      ]
    }
  }
}

Note that url and expression are inherited to the state apis "sense" and "nonsense". The Data field can save various configuration items to be used in runtime expressions. Please also note that it is possible to access configuration, request or response informations from other state apis in every runtime expression.

Every expression can access the following environment: