How to Build a Super Simple HTTP Proxy in Kotlin in just 20 lines of code

Oct 1, 2023 ยท 3 min read

Kotlin makes server-side development concise yet powerful. Here is a basic HTTP proxy server in Kotlin in less than 20 lines of code.

First we import the required packages:

import io.ktor.server.engine.*
import io.ktor.server.netty.*
import io.ktor.http.*
import io.ktor.client.*
import io.ktor.client.request.*

The ktor server package provides the API for the proxy server, ktor client to make requests, and ktor http for dealing with HTTP connections.

Next we create an embedded server:

embeddedServer(Netty, port = 8080) {
  // routing code
}.start(wait = true)

Inside the routing configuration we handle GET requests:

get("/") {
  // proxy logic

To proxy the request we:

  • Extract the target URL from the request path
  • val url = call.request.path().substring(1)
  • Make a GET request to the URL
  • val response = client.get<String>(url)
  • Return the proxied response
  • call.respond(response)

    Here is the full HTTP proxy in Kotlin:

    import io.ktor.server.engine.*
    import io.ktor.server.netty.*
    // other imports
    fun main() {
      embeddedServer(Netty, port = 8080) {
        get("/") {
          val url = call.request.path().substring(1)
          val response = client.get<String>(url)
      }.start(wait = true)

    This shows how concise and expressive an HTTP proxy can be in Kotlin using its coroutines and ktor library.

    This is great as a learning exercise but it is easy to see that even the proxy server itself is prone to get blocked as it uses a single IP. In this scenario where you may want a proxy that handles thousands of fetches every day using a professional rotating proxy service to rotate IPs is almost a must.

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