本文介绍如何使用discoveryutil工具清理MAC OS（Yosemite）的DNS缓存，啥是DNS这里就不赘述了:)
你需要使用终端（Terminal）来清理DNS缓存，可以用Spotlight启动终端或者从/Applications/Utilities/里面找到，最终目标是清理MDNS和UDNS两种DNS缓存。

首先：打开MAC OS terminal
Control+Spase 输入：终端

然后在终端执行 discoveryutil 命令：
清理 MDNS (Multicast DNS) Cache

sudo discoveryutil mdnsflushcache

回车，需要输入管理员密码

清理 UDNS(Unicast DNS) Cache

sudo discoveryutil udnsflushcaches

回车，需要输入管理员密码

一步清理OS X Yosemite所有DNS缓存

sudo discoveryutil mdnsflushcache;sudo discoveryutil udnsflushcaches;say flushed

执行后没有返回结果输出：

MacBook-Pro:~ sudops.com$ sudo discoveryutil mdnsflushcache;sudo discoveryutil udnsflushcaches;say flushed
Password:
MacBook-Pro:~ sudops.com$ 

下面是查看现有的cache MDNS/UDNS情况：

sudo discoveryutil mdnscachestats
sudo discoveryutil udnscachestats

我的执行结果如下：

MacBook-Pro:~ sudops.com$ sudo discoveryutil udnscachestats

UDNS Cache Stats: Ext Resolver on en1
UDNS Cache Stats: Resolver domain [default], type Unicast, ifIndex 6, port 0, timeout 30, A yes, AAAA no, serviceID 0, scope None, order 0
  server(s) 8.8.8.8, 8.8.4.4
UDNS Cache Stats: Cached 115 of 5000, table size 256, hash coverage 0.22, max miss depth 8, avg miss depth 0.38, reclaims 0
Cached records after aging 89
UDNS Cache Stats: Ext Resolver on en1
UDNS Cache Stats: Resolver domain [default], type Unicast, ifIndex 6, port 0, timeout 30, A yes, AAAA no, serviceID 0, scope InterfaceID, order 0
  server(s) 8.8.8.8, 8.8.4.4
UDNS Cache Stats: Cached 0 of 5000, table size 256, hash coverage 0.00, max miss depth 0, avg miss depth 0.00, reclaims 0
Cached records after aging 0

看到我的DNS服务器是：8.8.8.8, 8.8.4.4，
UDNS缓存状态：UDNS Cache Stats: Cached 115 of 5000, table size 256, hash coverage 0.22 …

MacBook-Pro:~ sudops.com$ sudo discoveryutil mdnscachestats
MDNS Cache Stats: lo0: Cached 0 of 7500, table size 16, hash coverage 0.00, max miss depth 0, avg miss depth 0.00, reclaims 0
Cached records after aging 0
MDNS Cache Stats: vboxnet0: Cached 0 of 7500, table size 256, hash coverage 0.00, max miss depth 0, avg miss depth 0.00, reclaims 0
Cached records after aging 0
MDNS Cache Stats: awdl0: Cached 0 of 7500, table size 256, hash coverage 0.00, max miss depth 0, avg miss depth 0.00, reclaims 0
Cached records after aging 0
MDNS Cache Stats: en1: Cached 0 of 7500, table size 128, hash coverage 0.00, max miss depth 0, avg miss depth 0.00, reclaims 0
Cached records after aging 0

最后可以使用dig/nslookup命令查询域名的解析情况：
如：
dig google.com

MacBook-Pro:~ sudops.com$ dig google.com

; <<>> DiG 9.8.3-P1 <<>> google.com
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 46653
;; flags: qr rd ra; QUERY: 1, ANSWER: 6, AUTHORITY: 4, ADDITIONAL: 4

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

;; ANSWER SECTION:
google.com.		74	IN	A	74.125.23.100
google.com.		74	IN	A	74.125.23.101
google.com.		74	IN	A	74.125.23.102
google.com.		74	IN	A	74.125.23.113
google.com.		74	IN	A	74.125.23.138
google.com.		74	IN	A	74.125.23.139
...

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