DNS 服务器响应和超时

Question

我们在 LAN 上遇到了令人沮丧的问题。定期对我们的 ISP 域名服务器进行 DNS 查询会导致 5 秒的延迟。即使我/etc/resolv.conf通过直接挖掘我们的 DNS 服务器来绕过，我仍然会遇到问题。下面是一个例子：

mv-m-dmouratis:~ dmourati$ time dig www.google.com @209.81.9.1 

; <<>> DiG 9.8.3-P1 <<>> www.google.com @209.81.9.1
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 14473
;; flags: qr rd ra; QUERY: 1, ANSWER: 5, AUTHORITY: 4, ADDITIONAL: 4

;; QUESTION SECTION:
;www.google.com.            IN  A

;; ANSWER SECTION:
www.google.com.     174 IN  A   74.125.239.148
www.google.com.     174 IN  A   74.125.239.147
www.google.com.     174 IN  A   74.125.239.146
www.google.com.     174 IN  A   74.125.239.144
www.google.com.     174 IN  A   74.125.239.145

;; AUTHORITY SECTION:
google.com.     34512   IN  NS  ns2.google.com.
google.com.     34512   IN  NS  ns1.google.com.
google.com.     34512   IN  NS  ns3.google.com.
google.com.     34512   IN  NS  ns4.google.com.

;; ADDITIONAL SECTION:
ns2.google.com.     212097  IN  A   216.239.34.10
ns3.google.com.     207312  IN  A   216.239.36.10
ns4.google.com.     212097  IN  A   216.239.38.10
ns1.google.com.     212096  IN  A   216.239.32.10

;; Query time: 8 msec
;; SERVER: 209.81.9.1#53(209.81.9.1)
;; WHEN: Fri Jul 26 14:44:25 2013
;; MSG SIZE  rcvd: 248


real    0m5.015s
user    0m0.004s
sys 0m0.002s

其他时候，查询会立即响应，大约在 20 毫秒以内。我进行了数据包跟踪并发现了一些有趣的事情。DNS 服务器正在响应，但客户端忽略初始响应，然后发送第二个相同的查询，该查询立即得到响应。

请参阅数据包跟踪。注意查询的相同源端口 (62076)。

问题：是什么导致第一个 DNS 查询失败？

更新

资源：

数据包跟踪：

http://www.cloudshark.org/captures/8b1c32d9d015

Dtruss (strace for mac):

https://gist.github.com/dmourati/6115180

Mountain Lion 防火墙随机延迟来自 apple.stackexchange.com 的 DNS 请求：

https://apple.stackexchange.com/questions/80678/mountain-lion-firewall-is-randomly-delaying-dns-requests

更新 2

System Software Overview:

  System Version:   OS X 10.8.4 (12E55)
  Kernel Version:   Darwin 12.4.0
  Boot Volume:  Macintosh HD
  Boot Mode:    Normal
  Computer Name:    mv-m-dmouratis
  User Name:    Demetri Mouratis (dmourati)
  Secure Virtual Memory:    Enabled
  Time since boot:  43 minutes

Hardware Overview:

  Model Name:   MacBook Pro
  Model Identifier: MacBookPro10,1
  Processor Name:   Intel Core i7
  Processor Speed:  2.7 GHz
  Number of Processors: 1
  Total Number of Cores:    4
  L2 Cache (per Core):  256 KB
  L3 Cache: 6 MB
  Memory:   16 GB

Firewall Settings:

  Mode: Limit incoming connections to specific services and applications
  Services:
  Apple Remote Desktop: Allow all connections
  Screen Sharing:   Allow all connections
  Applications:
  com.apple.java.VisualVM.launcher: Block all connections
  com.getdropbox.dropbox:   Allow all connections
  com.jetbrains.intellij.ce:    Allow all connections
  com.skype.skype:  Allow all connections
  com.yourcompany.Bitcoin-Qt:   Allow all connections
  org.m0k.transmission: Allow all connections
  org.python.python:    Allow all connections
  Firewall Logging: Yes
  Stealth Mode: No

Answer 1

这似乎是 Lion 防火墙中的一个错误。您的系统上启用了吗？

在此 MacRumors 线程（更新到 Mountain Lion (10.8) 后的 DNS 问题）中，讨论了可能的解决方法：

尝试减小 MTU 大小。

系统偏好设置 > 网络 > WiFi > 高级 > 硬件 > 手动 > MTU：自定义 > 1300

为我工作。

您能否检查一下减小 MTU 大小是否可以缓解您的问题？