dig

dig MX +trace gmail.com

首先寻找根服务器，有下面这些，82975代表可以缓存的时间（秒），IN 代表一种协议簇（约等于一系列的返回值），NS nameServer，表示这些是nameServer

;; global options: +cmd
.			82975	IN	NS	e.root-servers.net.
.			82975	IN	NS	f.root-servers.net.
.			82975	IN	NS	g.root-servers.net.
.			82975	IN	NS	h.root-servers.net.
.			82975	IN	NS	i.root-servers.net.
.			82975	IN	NS	j.root-servers.net.
.			82975	IN	NS	k.root-servers.net.
.			82975	IN	NS	l.root-servers.net.
.			82975	IN	NS	m.root-servers.net.
.			82975	IN	NS	a.root-servers.net.
.			82975	IN	NS	b.root-servers.net.
.			82975	IN	NS	c.root-servers.net.
.			82975	IN	NS	d.root-servers.net.

接着寻找gmail.com.,返回值内容同上，找到了gmail的name server了

com.			172800	IN	NS	l.gtld-servers.net.
com.			172800	IN	NS	g.gtld-servers.net.
com.			172800	IN	NS	m.gtld-servers.net.
com.			172800	IN	NS	d.gtld-servers.net.
com.			172800	IN	NS	k.gtld-servers.net.
com.			172800	IN	NS	e.gtld-servers.net.
com.			172800	IN	NS	a.gtld-servers.net.
com.			172800	IN	NS	c.gtld-servers.net.
com.			172800	IN	NS	f.gtld-servers.net.
com.			172800	IN	NS	i.gtld-servers.net.
com.			172800	IN	NS	h.gtld-servers.net.
com.			172800	IN	NS	b.gtld-servers.net.
com.			172800	IN	NS	j.gtld-servers.net.
;; Received 487 bytes from 199.7.91.13#53(d.root-servers.net) in 267 ms
gmail.com.		172800	IN	NS	ns2.google.com.
gmail.com.		172800	IN	NS	ns1.google.com.
gmail.com.		172800	IN	NS	ns3.google.com.
gmail.com.		172800	IN	NS	ns4.google.com.
;; Received 170 bytes from 192.26.92.30#53(c.gtld-servers.net) in 272 ms

接着要MX(mailExchange)的信息，mx后面的数字代表优先级，数字越低，优先级越高。接着是mail server

gmail.com.		3600	IN	MX	30 alt3.gmail-smtp-in.l.google.com.
gmail.com.		3600	IN	MX	5 gmail-smtp-in.l.google.com.
gmail.com.		3600	IN	MX	40 alt4.gmail-smtp-in.l.google.com.
gmail.com.		3600	IN	MX	20 alt2.gmail-smtp-in.l.google.com.
gmail.com.		3600	IN	MX	10 alt1.gmail-smtp-in.l.google.com.
;; Received 230 bytes from 216.239.36.10#53(ns3.google.com) in 61 ms