fix: ProxyAgent causes request.headers.host to be forcibly reset

lib/dispatcher/proxy-agent.js

@@ -64,19 +64,19 @@ class ProxyAgent extends DispatcherBase {
     this[kAgent] = new Agent({
       ...opts,
       connect: async (opts, callback) => {
-        let requestedHost = opts.host
+        let requestedPath = opts.host
         if (!opts.port) {
-          requestedHost += `:${defaultProtocolPort(opts.protocol)}`
+          requestedPath += `:${defaultProtocolPort(opts.protocol)}`
         }
         try {
           const { socket, statusCode } = await this[kClient].connect({
             origin,
             port,
-            path: requestedHost,
+            path: requestedPath,
             signal: opts.signal,
             headers: {
               ...this[kProxyHeaders],
-              host: requestedHost
+              host: opts.host
             },
             servername: this[kProxyTls]?.servername || proxyHostname
           })
@@ -108,16 +108,18 @@ class ProxyAgent extends DispatcherBase {
   }
 
   dispatch (opts, handler) {
-    const { host } = new URL(opts.origin)
     const headers = buildHeaders(opts.headers)
     throwIfProxyAuthIsSent(headers)
+
+    if (headers && !('host' in headers) && !('Host' in headers)) {
+      const { host } = new URL(opts.origin)
+      headers.host = host
+    }
+
     return this[kAgent].dispatch(
       {
         ...opts,
-        headers: {
-          ...headers,
-          host
-        }
+        headers
       },
       handler
     )

test/proxy-agent.js

@@ -842,6 +842,35 @@ test('Proxy via HTTPS to HTTP fails on wrong SNI', async (t) => {
   proxyAgent.close()
 })
 
+test('ProxyAgent keeps customized host in request headers - #3019', async (t) => {
+  t = tspl(t, { plan: 2 })
+  const server = await buildServer()
+  const proxy = await buildProxy()
+
+  const serverUrl = `http://localhost:${server.address().port}`
+  const proxyUrl = `http://localhost:${proxy.address().port}`
+  const proxyAgent = new ProxyAgent(proxyUrl)
+  const customHost = 'example.com'
+
+  proxy.on('connect', (req) => {
+    t.strictEqual(req.headers.host, `localhost:${server.address().port}`)
+  })
+
+  server.on('request', (req, res) => {
+    t.strictEqual(req.headers.host, customHost)
+    res.end()
+  })
+
+  await request(serverUrl, {
+    headers: { Host: customHost },
+    dispatcher: proxyAgent
+  })
+
+  server.close()
+  proxy.close()
+  proxyAgent.close()
+})
+
 function buildServer () {
   return new Promise((resolve) => {
     const server = createServer()