Gmail（google）如何从NMAP隐藏其打开状态？

Question

For a networking project, I am trying to "hide the open ports of the email server running on linux".

In regard to this, whenever I run a NMAP port scan on "Gmail" (mail.google.com) only ports visible are ( 80 and 443 ) both HTTP and HTTPS Gmail NMAP result

However, Gmail is also accessible to people through email clients, which means that port 25 (SMTP), 993(imaps), 465 (smtps) etc. must also be open.

Now my question is that how can I do similar to my email server ? What concept / technique does Gmail uses to hide its open ports ?

ps: my email server shows port 25, 443, 465, 587 and 993 as open ports.

Answer 1

You have some incorrect assumptions about Google's mail infrastructure. The name mail.google.com points to Gmail's webmail servers : these are web servers with the Gmail web app that acts as a mail client . The mail servers are different machines. A Mail Transfer Agent determines where to send mail destined for @gmail.com addresses by querying for an MX record :

$ dig gmail.com MX
<snip>
;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;gmail.com.                     IN      MX

;; ANSWER SECTION:
gmail.com.              3543    IN      MX      30 alt3.gmail-smtp-in.l.google.com.
gmail.com.              3543    IN      MX      10 alt1.gmail-smtp-in.l.google.com.
gmail.com.              3543    IN      MX      5 gmail-smtp-in.l.google.com.
gmail.com.              3543    IN      MX      20 alt2.gmail-smtp-in.l.google.com.
gmail.com.              3543    IN      MX      40 alt4.gmail-smtp-in.l.google.com.

;; Query time: 0 msec
;; SERVER: 72.14.179.5#53(72.14.179.5)
;; WHEN: Mon Aug 08 13:47:39 UTC 2016
;; MSG SIZE  rcvd: 161

So those *.gmail-smtp-in.l.google.com. addresses will have port 25 open for SMTP .

In addition to the inbound mail servers, there are also mail access servers running IMAP or POP3. You can discover these by searching Gmail's help pages; they are, unsurprisingly, imap.gmail.com and pop.gmail.com .