Amend Proxy Server

A little proxy server to patch HTTP requests before they reach their destination. Expected use case is for JSON payload modifications, though can be used for other purposes. Environment variables are used to define routing and transformation rules.

Rule syntax

Environment variable names should begin with AMEND_ and follow the syntax:

AMEND_<name>="IN_PORT OUT_HOST PATH_REGEX JS_EXPRESSION"

For example, invoking via the shell might look something like:

AMEND_0_DEFAULTROUTE="8080 http://example.com:80 .*" \
AMEND_1_FOOBAR="8080 http://example.com:80 foo/bar _.foo = 'bar'; console.log(JSON.stringify(_));" \
node ./amend.js
curl -X POST http://localhost:8080/foo/bar \
    -H "Content-Type: application/json" \
    -d '{"name": "John", "age": 30 }'

Components

• IN_PORT Specifies the port number the proxy server will listen to for incoming requests.
• OUT_HOST Hostname or IP address of the upstream server where requests should be forwarded.
• PATH_REGEX Regular expression to match against incoming request URLs to determine if the rule applies.
• JS_EXPRESSION JavaScript code, executed in the context of each matching request, allowing custom modifications to requests before forwarding.

Evaluation context for JS_EXPRESSION

• $ represents the parsed JSON body of the request, if available (if the body is not valid JSON or is absent, $ is undefined).
• _ represents the modifiable output body to be sent to the destination server, initialised to a deep copy of $ when available.

There are also analogues for the header map (_headers and $headers), HTTP method (_method and $method) and URL (_url and $url). Refer to the source for details.

Matching and precedence

Matching rules for a request are evaluated in order, meaning that rule occurring later will override the effects of earlier rules.

Rule order is defined by a lexicographic sort of the AMEND_ environment variable keys, with a special handling for numbers such that e.g. 1 < 10.

If no rule matches an incoming request, an error is logged to the console and the server responds with an HTTP 500.