dns_mx_reverse_check

diego@cryptos:~$ # consultamos los mx de un dominio

diego@cryptos:~$ dig mx juncotic.com

; <<>> DiG 9.11.2 <<>> mx juncotic.com
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 713
;; flags: qr rd ra; QUERY: 1, ANSWER: 3, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 512
;; QUESTION SECTION:
;juncotic.com.          IN  MX

;; ANSWER SECTION:
juncotic.com.       3536    IN  MX  10 mx.zoho.com.
juncotic.com.       3536    IN  MX  20 mx2.zoho.com.
juncotic.com.       3536    IN  MX  50 mx3.zoho.com.

;; Query time: 73 msec
;; SERVER: 8.8.4.4#53(8.8.4.4)
;; WHEN: Fri Jan 05 20:49:16 -03 2018
;; MSG SIZE  rcvd: 105



diego@cryptos:~$ # ahora obtenemos la ip de uno cualquiera:

diego@cryptos:~$ host mx.zoho.com
mx.zoho.com has address 204.141.32.121


diego@cryptos:~$ # ahora preguntamos a qué nombre corresponde esa ip:

diego@cryptos:~$ dig -x 204.141.32.121

; <<>> DiG 9.11.2 <<>> -x 204.141.32.121
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 64834
;; flags: qr rd ra; QUERY: 1, ANSWER: 2, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 512
;; QUESTION SECTION:
;121.32.141.204.in-addr.arpa.   IN  PTR

;; ANSWER SECTION:
121.32.141.204.in-addr.arpa. 783 IN PTR mx.zoho.com.
121.32.141.204.in-addr.arpa. 783 IN PTR mx.zohomail.com.

;; Query time: 72 msec
;; SERVER: 8.8.4.4#53(8.8.4.4)
;; WHEN: Fri Jan 05 20:49:56 -03 2018
;; MSG SIZE  rcvd: 107

diego@cryptos:~$ # está bien configurado porque entre los nombres a los que apunta (PTR) la ip está el mismo que resolvimos directamente, y que nos dio la misma IP.