История изменений
Исправление Black_Shadow, (текущая версия) :
На самом деле, всё это условности и рекомендации. Ни кто не обязывает это соблюдать, в RFC это не написано. Пример:
$ nslookup -type=mx gmail.com
Server: 192.168.0.1
Address: 192.168.0.1#53
Non-authoritative answer:
gmail.com mail exchanger = 40 alt4.gmail-smtp-in.l.google.com.
gmail.com mail exchanger = 30 alt3.gmail-smtp-in.l.google.com.
gmail.com mail exchanger = 10 alt1.gmail-smtp-in.l.google.com.
gmail.com mail exchanger = 5 gmail-smtp-in.l.google.com.
gmail.com mail exchanger = 20 alt2.gmail-smtp-in.l.google.com.
Authoritative answers can be found from:
gmail.com nameserver = ns1.google.com.
gmail.com nameserver = ns2.google.com.
gmail.com nameserver = ns4.google.com.
gmail.com nameserver = ns3.google.com.
gmail-smtp-in.l.google.com internet address = 173.194.222.26
gmail-smtp-in.l.google.com has AAAA address 2a00:1450:4010:c0b::1a
$ nslookup alt4.gmail-smtp-in.l.google.com.
Server: 192.168.0.1
Address: 192.168.0.1#53
Non-authoritative answer:
Name: alt4.gmail-smtp-in.l.google.com
Address: 209.85.147.26
Name: alt4.gmail-smtp-in.l.google.com
Address: 2607:f8b0:4001:c20::1a
$ nslookup 209.85.147.26
26.147.85.209.in-addr.arpa name = jg-in-f26.1e100.net.
Authoritative answers can be found from:
147.85.209.in-addr.arpa nameserver = ns3.google.com.
147.85.209.in-addr.arpa nameserver = ns4.google.com.
147.85.209.in-addr.arpa nameserver = ns1.google.com.
147.85.209.in-addr.arpa nameserver = ns2.google.com.
$ telnet alt4.gmail-smtp-in.l.google.com. 25
Trying 209.85.147.26...
Connected to alt4.gmail-smtp-in.l.google.com.
Escape character is '^]'.
220 mx.google.com ESMTP p71-v6si5834019itc.69 - gsmtp
helo localhost
250 mx.google.com at your service
$ nslookup mx.google.com
Server: 192.168.0.1
Address: 192.168.0.1#53
** server can't find mx.google.com: NXDOMAIN
Исходная версия Black_Shadow, :
На самом деле, всё это условности и рекомендации. Ни кто не обязывает это соблюдать, в RFC это не написано. Пример:
$ nslookup -type=mx gmail.com
Server: 192.168.0.1
Address: 192.168.0.1#53
Non-authoritative answer:
gmail.com mail exchanger = 40 alt4.gmail-smtp-in.l.google.com.
gmail.com mail exchanger = 30 alt3.gmail-smtp-in.l.google.com.
gmail.com mail exchanger = 10 alt1.gmail-smtp-in.l.google.com.
gmail.com mail exchanger = 5 gmail-smtp-in.l.google.com.
gmail.com mail exchanger = 20 alt2.gmail-smtp-in.l.google.com.
Authoritative answers can be found from:
gmail.com nameserver = ns1.google.com.
gmail.com nameserver = ns2.google.com.
gmail.com nameserver = ns4.google.com.
gmail.com nameserver = ns3.google.com.
gmail-smtp-in.l.google.com internet address = 173.194.222.26
gmail-smtp-in.l.google.com has AAAA address 2a00:1450:4010:c0b::1a
$ nslookup alt4.gmail-smtp-in.l.google.com.
Server: 192.168.0.1
Address: 192.168.0.1#53
Non-authoritative answer:
Name: alt4.gmail-smtp-in.l.google.com
Address: 209.85.147.26
Name: alt4.gmail-smtp-in.l.google.com
Address: 2607:f8b0:4001:c20::1a
$ nslookup 209.85.147.26
26.147.85.209.in-addr.arpa name = jg-in-f26.1e100.net.
Authoritative answers can be found from:
147.85.209.in-addr.arpa nameserver = ns3.google.com.
147.85.209.in-addr.arpa nameserver = ns4.google.com.
147.85.209.in-addr.arpa nameserver = ns1.google.com.
147.85.209.in-addr.arpa nameserver = ns2.google.com.
$ telnet alt4.gmail-smtp-in.l.google.com. 25
Trying 209.85.147.26...
Connected to alt4.gmail-smtp-in.l.google.com.
Escape character is '^]'.
220 mx.google.com ESMTP p71-v6si5834019itc.69 - gsmtp
helo localhost
250 mx.google.com at your service