TCP proxy that also duplicates traffic to a secondary host

npm install duplicator
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TCP proxy that also duplicates traffic to a secondary host

I built this because I needed a way to "tap" production traffic and shoot it at a new system to see how it handles load.

usage (cli)

npm install -g duplicator
duplicator -f localhost:80 -d localhost:3000 -p 8080
  • forward all traffic to localhost:80
  • duplicate all traffic to localhost:3000, ignoring responses
  • listen on port 8080

usage (code)

The equivalent to the call above, in node.js:

var duplicator = require('duplicator')

var server = duplicator(function(connection, forward, duplicate) {



Creates a new net.Server. When cb is called on a successful connection, it will receive the connection to the client and two special callback functions, forward and duplicate.

duplicator(function(connection, forward, duplicate) {
  duplicate(Math.random() < 0.5 ? 'host1:80' : 'host2:80')

In this example, we always use origin:80 as the primary server to forward requests to, but we send half the duplicated rquests to host1, and the other half to host2.

forward(host, [stream])

Forward stream to host. stream defaults to the current connection unless otherwise specified.

duplicator(function(connection, forward, duplicate) {
  // forward every connection to localhost:80

Note that it is not safe to forward the same connection multiple times, as the responses from the different servers may interfere with one another. Therefore if you do call forward multiple times, only the first call will succeed, and subsequent calls will merely duplicate the connection.

duplicate(host, [stream], [rate])

Duplicate on average rate copies of stream to host. stream defaults to the current connection unless otherwise specified, rate defaults to 1.

Rate is the expected number of copies sent per connection, and is interpreted as follows:

  • 0.1 -> send once with 10% probability
  • 1 -> send exactly one copy
  • 5 -> send out 5 copies
  • 2.5 -> send out 2 copies, with a 50% chance of a third

Note that both stream and rate are optional, but have sensible defaults. If only two parameters are specified, the second will be interpreted as rate if it's a number, stream otherwise.

duplicator(function(connection, forward, duplicate) {
  // duplicate 50% of requests to localhost:3000
  duplicate('localhost:3000', 0.5)
  // duplicate all connections to stdout
  duplicate('localhost:3000', process.stdout)

specifying hosts

A host can be any one of:

  • an object as net.connect would expect it ({ host: 'google.com', port: 80 })
  • a number representing a port (80, 3000), host is implied to be localhost
  • a string representing a host (localhost:3000, google.com:80)
  • a function that behaves like net.connect (function(cb) { net.connect({ port:80 }, cb) })

mit license

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