Proxy HTTP requests

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A reverse proxy receives requests and forwards them to another server. Because fetch, Request, and Response are the same objects on both sides, a proxy in Deno is a few lines: rewrite the URL, forward, return.

The server we forward every request to.

const UPSTREAM = "https://example.com";

async function handler(req: Request): Promise<Response> {

Build the upstream URL from the incoming request's path and query.

  const url = new URL(req.url);
  const target = new URL(url.pathname + url.search, UPSTREAM);

Forward the method, headers, and body as they came in. The host header must match the upstream, so let fetch regenerate it.

  const headers = new Headers(req.headers);
  headers.delete("host");

  const response = await fetch(target, {
    method: req.method,
    headers,
    body: req.body,
    redirect: "manual",
  });

Responses stream straight through; nothing is buffered in memory.

  return response;
}

Deno.serve(handler);

Requests to the proxy now answer with the upstream's content: curl -s http://localhost:8000/ | head -c 15 <!doctype html>

A real deployment usually adds forwarding headers so the upstream knows the original client, e.g. headers.set("x-forwarded-for", clientIp) using the info argument of the handler.

The same proxy works on a node:http server; fetch bridges to the web Response, whose body is then streamed out through the Node response.

import { createServer } from "node:http";

createServer(async (req, res) => {
  const target = new URL(req.url ?? "/", UPSTREAM);
  const response = await fetch(target);
  res.writeHead(
    response.status,
    Object.fromEntries(response.headers.entries()),
  );
  for await (const chunk of response.body ?? []) {
    res.write(chunk);
  }
  res.end();
}).listen(8001);

Run this example locally using the Deno CLI:

deno run -N https://docs.deno.com/examples/scripts/http_proxy.ts

Proxy HTTP requests

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