optimize: 代理请求使用了DNS获取到的IP时，添加响应头 `DS-Lookup`，返回当前所使用的IP。

packages/mitmproxy/src/lib/proxy/mitmproxy/createConnectHandler.js

@@ -62,7 +62,7 @@ function connect (req, cltSocket, head, hostname, port, dnsConfig) {
     if (dnsConfig && dnsConfig.providers) {
       const dns = DnsUtil.hasDnsLookup(dnsConfig, hostname)
       if (dns) {
-        options.lookup = dnsLookup.createLookupFunc(dns, 'connect', hostport, isDnsIntercept)
+        options.lookup = dnsLookup.createLookupFunc(null, dns, 'connect', hostport, isDnsIntercept)
       }
     }
     const proxySocket = net.connect(options, () => {

packages/mitmproxy/src/lib/proxy/mitmproxy/createRequestHandler.js

@@ -122,7 +122,7 @@ module.exports = function createRequestHandler (createIntercepts, middlewares, e
               }
             }
             if (dns) {
-              rOptions.lookup = dnsLookup.createLookupFunc(dns, 'request url', url, isDnsIntercept)
+              rOptions.lookup = dnsLookup.createLookupFunc(res, dns, 'request url', url, isDnsIntercept)
             }
           }
 

packages/mitmproxy/src/lib/proxy/mitmproxy/dnsLookup.js

@@ -3,7 +3,7 @@ const log = require('../../../utils/util.log')
 const defaultDns = require('dns')
 
 module.exports = {
-  createLookupFunc: function (dns, action, target, isDnsIntercept) {
+  createLookupFunc: function (res, dns, action, target, isDnsIntercept) {
     target = target ? (', target: ' + target) : ''
 
     return (hostname, options, callback) => {
@@ -12,6 +12,7 @@ module.exports = {
         const aliveIpObj = tester.pickFastAliveIpObj()
         if (aliveIpObj) {
           log.info(`----- ${action}: ${hostname}, use alive ip from dns '${aliveIpObj.dns}': ${aliveIpObj.host}${target} -----`)
+          if (res) res.setHeader('DS-Lookup', `IpTester: ${aliveIpObj.host}(${aliveIpObj.dns})`)
           callback(null, aliveIpObj.host, 4)
           return
         } else {
@@ -41,6 +42,7 @@ module.exports = {
           }
           if (isTestFailedIp === false) {
             log.info(`----- ${action}: ${hostname}, use ip from dns '${dns.name}': ${ip}${target} -----`)
+            if (res) res.setHeader('DS-Lookup', `DNS: ${ip.host}(${dns.name})`)
             callback(null, ip, 4)
             return
           } else {