; <<>> DiG 9.7.3 <<>> @8.8.8.8 www.serverfault.com
; (1 server found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY,status: NOERROR,id: 3937
;; flags: qr rd ra; QUERY: 1,ANSWER: 2,AUTHORITY: 0,ADDITIONAL: 0

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

;; ANSWER SECTION:
www.serverfault.com.    1800    IN  CNAME   serverfault.com.
serverfault.com.    1800    IN  A   64.34.119.12

;; Query time: 88 msec
;; SERVER: 8.8.8.8#53(8.8.8.8)
;; WHEN: Thu Feb 23 23:15:23 2012
;; MSG SIZE  rcvd: 67

具体来说,你需要注意答案中的值(在这种情况下为1800)

;; ANSWER SECTION:
www.serverfault.com.    1800    IN  CNAME   serverfault.com.
serverfault.com.    1800    IN  A   64.34.119.12

1800是TTL到期之前的秒数,并且在名称服务器上刷新记录(8.8.8.8).

如果您运行该命令几次,您将看到TTL编号下降,因此截至编写本文时,TTL现在是这样(1659秒)

;; ANSWER SECTION:
www.serverfault.com.    1659    IN  CNAME   serverfault.com.
serverfault.com.    1659    IN  A   64.34.119.12