fix: preserve original hostname on HTTP socket when using DNS interceptor

When the DNS interceptor resolves the hostname and replaces the
connection origin with the resolved IP address, the original hostname
was lost for HTTP connections. For HTTPS, the hostname was still
available through TLS SNI (servername), but for plain HTTP it was
nowhere accessible on the socket.

This fix:
1. Sets client[kServerName] for HTTP connections when request.servername
   differs (the DNS interceptor already sets servername in dispatch opts)
2. Overrides socket._host with the original hostname for HTTP connections
   when servername is provided

Fixes: #5573

Author: mcollina

lib/core/connect.js

@@ -115,6 +115,10 @@ function buildConnector ({ allowH2, preferH2, useH2c, maxCachedSessions, socketP
       if (useH2c === true) {
         socket.alpnProtocol = 'h2'
       }
+
+      if (servername) {
+        socket._host = servername
+      }
     }
 
     // Set TCP keep alive options on the socket here instead of in connect() for the case of assigning the socket

lib/dispatcher/client.js

@@ -665,6 +665,8 @@ function _resume (client, sync) {
         client[kHTTPContext] = null
         resume(client)
       })
+    } else if (client[kUrl].protocol !== 'https:' && client[kServerName] !== request.servername) {
+      client[kServerName] = request.servername
     }
 
     if (client[kConnecting]) {

test/interceptors/dns.js

@@ -11,7 +11,7 @@ const { once } = require('node:events')
 const { tspl } = require('@matteo.collina/tspl')
 const pem = require('@metcoder95/https-pem')
 
-const { interceptors, Agent, Client, Pool, request } = require('../..')
+const { interceptors, Agent, Client, Pool, request, fetch } = require('../..')
 const { dns } = interceptors
 
 // Helper to check if IPv6 is available for localhost
@@ -2252,3 +2252,64 @@ test('#4234 - Should pass Pool requests without origin through', async t => {
   t.equal(await response.body.text(), 'hello world!')
   t.equal(lookupCount, 0)
 })
+
+test('Should preserve original hostname on HTTP socket when using DNS interceptor', async t => {
+  t = tspl(t, { plan: 2 })
+
+  const server = createServer()
+  server.on('request', (req, res) => {
+    res.writeHead(200, { 'content-type': 'text/plain' })
+    res.end('hello world!')
+  })
+
+  server.listen(0)
+  await once(server, 'listening')
+
+  let hostFromSocket = null
+  const RESOLVED_ADDRESS = '127.0.0.1'
+  const diagnosticsChannel = require('node:diagnostics_channel')
+  const netClientSocketChannel = diagnosticsChannel.channel('net.client.socket')
+
+  after(async () => {
+    await dispatcher.close()
+    server.close()
+    await once(server, 'close')
+  })
+
+  // Subscribe to the net.client.socket channel BEFORE creating the socket
+  const connectPromise = new Promise((resolve) => {
+    netClientSocketChannel.subscribe(({ socket }) => {
+      socket.once('connect', () => {
+        hostFromSocket = socket._host
+        resolve()
+      })
+    })
+  })
+
+  const dispatcher = new Agent().compose(
+    dns({
+      maxTTL: 60_000,
+      lookup (_origin, _opts, cb) {
+        cb(null, [
+          {
+            address: RESOLVED_ADDRESS,
+            family: 4,
+            ttl: 60_000
+          }
+        ])
+      }
+    })
+  )
+
+  const url = `http://localhost:${server.address().port}`
+
+  // Perform a request with the DNS interceptor
+  const response = await fetch(url, { dispatcher })
+  t.equal(response.status, 200)
+
+  // Wait for the connect event to fire
+  await connectPromise
+
+  // The socket._host should be the original hostname, not the resolved IP
+  t.equal(hostFromSocket, 'localhost')
+})