我们的局域网遇到了令人沮丧的问题.对我们的ISP名称服务器的DNS查询会定期超时,强制延迟5秒.即使我通过直接挖掘到我们的一个DNS服务器来绕过/etc/resolv.conf,我仍然会遇到问题.这是一个例子：
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,查询立即响应,大约在20毫秒左右.我做了一个数据包跟踪并发现了一些有趣的东西. 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,该查询立即响应.
请参见packet trace.请注意查询的相同源端口(62076).
问题：导致第一个DNS查询失败的原因是什么？
UPDATE
资源：
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：
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
解决方法
这似乎是Lion防火墙中的一个错误.它是否在您的系统上启用？
在这个MacRumors线程(DNS problems after updating to Mountain Lion (10.8))中,讨论了一种可能的解决方法：
Try reducing MTU size.
System Preferences > Network > WiFi > Advanced > Hardware > Manually > MTU: Custom > 1300
Worked for me.
你能检查一下降低MTU大小是否可以缓解你的问题？
