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DNS

What is DNS?​

Each internet-connected device has a unique IP address that other machines use to locae it. Domain Name System (DNS) is the internet's phone book. DNS converts domain names to IP addresses, allowing browsers to access resources on the Internet.

Hierarchy of names​

There's a hierarchy of names in the DNS structure.

The system's root, or highest level, is unnamed.

TLDs nameservers, the Top-Level Domains, might be compared to a single shelf of books in a library. The last portion of a hostname is hosted by the nameserver, which is the following stage in the search for a specific IP address (in www.facebook.com, the TLD server is com). Most TLDs have been delegated to individual country managers, who are issued codes from the ISO-3166-1 table (i.e., country code Top-Level Domains or ccTLDs).

There are also a small number of "generic" Top-Level Domains (gTLDs) that are not associated with a specific country or region.

TLD managers have been granted responsibility for procedures and policies for the assignment of Second Level Domain Names (SLDs) and lower level hierarchies of names, according to the policy advice specified in ISO-3166-1.

Resource records​

Resource records are the results of DNS queries and have the following structure:

  • Resource record: Domain name, usually a FQDN.
  • TTL: Time-To-Live in seconds, defaults to the minimum value specified in the SOA record.
  • Record Class: Internet, Hesiod, or Chaos.
  • Start Of Authority (SOA): Each zone can only have one SOA record, and it contains the zone's values, such as a serial number and multiple expiration timeouts.
  • Name Servers (NS): The distributed database is bound together by NS record which are in charge of a zone's authoritative name server and the authority for a child zone to a name server.
  • IPv4 Addresses (A): It's only a mapping between an IP address and a hostname. 'Forward' zones are those with A records.
  • Poitner (PTR): It's also only a mapping between an IP address and a hostname. 'Reverse' zones are those that have PTR records.
  • Canonical Name (CNAME): An alias hostname is mapped to an A record hostname using the CNAME record.
  • Mail Exchange (MX): It identifies a host that will accept emails for a specific host. Multiple MX records can exist on the same host, and a prioritized list is made consisting of the records for a specific host.

Nslookup & dig​

With nslookup we can search for domain name servers on the Internet and ask them for information about hosts and domains.

Querying A records​

We can query A records by just submitting a domain name:

$ export TARGET="facebook.com"
$ nslookup $TARGET
Server:		1.1.1.1
Address:	1.1.1.1#53

Non-authoritative answer:
Name:	facebook.com
Address: 31.13.92.36
Name:	facebook.com
Address: 2a03:2880:f11c:8083:face:b00c:0:25de

Unlike nslookup, dig shows us some more information that can be of importance:

$ dig facebook.com @1.1.1.1

; <<>> DiG 9.16.1-Ubuntu <<>> facebook.com @1.1.1.1
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 58899
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 1232
;; QUESTION SECTION:
;facebook.com.                  IN      A

;; ANSWER SECTION:
facebook.com.           169     IN      A       31.13.92.36

;; Query time: 20 msec
;; SERVER: 1.1.1.1#53(1.1.1.1)
;; WHEN: Mo Okt 18 16:03:17 CEST 2021
;; MSG SIZE  rcvd: 57

Querying A records for a subdomain​

With nslookup:

$ export TARGET=www.facebook.com
$ nslookup -query=A $TARGET

Server:		1.1.1.1
Address:	1.1.1.1#53

Non-authoritative answer:
www.facebook.com	canonical name = star-mini.c10r.facebook.com.
Name:	star-mini.c10r.facebook.com
Address: 31.13.92.36

With dig:

$ dig a www.facebook.com @1.1.1.1

; <<>> DiG 9.16.1-Ubuntu <<>> a www.facebook.com @1.1.1.1
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 15596
;; flags: qr rd ra; QUERY: 1, ANSWER: 2, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 1232
;; QUESTION SECTION:
;www.facebook.com.              IN      A

;; ANSWER SECTION:
www.facebook.com.       3585    IN      CNAME   star-mini.c10r.facebook.com.
star-mini.c10r.facebook.com. 45 IN      A       31.13.92.36

;; Query time: 16 msec
;; SERVER: 1.1.1.1#53(1.1.1.1)
;; WHEN: Mo Okt 18 16:11:48 CEST 2021
;; MSG SIZE  rcvd: 90

Querying PTR records for an IP address​

With nslookup:

$ nslookup -query=PTR 31.13.92.36

Server:		1.1.1.1
Address:	1.1.1.1#53

Non-authoritative answer:
36.92.13.31.in-addr.arpa	name = edge-star-mini-shv-01-frt3.facebook.com.

With dig:

$ dig -x 31.13.92.36 @1.1.1.1

; <<>> DiG 9.16.1-Ubuntu <<>> -x 31.13.92.36 @1.1.1.1
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 51730
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 1232
;; QUESTION SECTION:
;36.92.13.31.in-addr.arpa.      IN      PTR

;; ANSWER SECTION:
36.92.13.31.in-addr.arpa. 1028  IN      PTR     edge-star-mini-shv-01-frt3.facebook.com.

;; Query time: 16 msec
;; SERVER: 1.1.1.1#53(1.1.1.1)
;; WHEN: Mo Okt 18 16:14:20 CEST 2021
;; MSG SIZE  rcvd: 106

Querying ANY existing records​

With nslookup:

export TARGET="google.com"
nslookup -query=ANY $TARGET

With dig:

dig any google.com @8.8.8.8
warning

The more recent RFC8482 specified that ANY DNS requests to be abolished. Therefore, we may not receive a response to our ANY request from the DNS server or get a reference to the said RFC8482.

Querying TXT records​

With nslookup:

$ export TARGET="facebook.com"
$ nslookup -query=TXT $TARGET

Server:		1.1.1.1
Address:	1.1.1.1#53

Non-authoritative answer:
facebook.com	text = "v=spf1 redirect=_spf.facebook.com"
facebook.com	text = "google-site-verification=A2WZWCNQHrGV_TWwKh6KHY90tY0SHZo_RnyMJoDaG0s"
facebook.com	text = "google-site-verification=wdH5DTJTc9AYNwVunSVFeK0hYDGUIEOGb-RReU6pJlY"

Authoritative answers can be found from:

With dig:

dig txt facebook.com @1.1.1.1

Querying MX records​

With nslookup:

$ export TARGET="facebook.com"
$ nslookup -query=MX $TARGET

Server:		1.1.1.1
Address:	1.1.1.1#53

Non-authoritative answer:
facebook.com	mail exchanger = 10 smtpin.vvv.facebook.com.

Authoritative answers can be found from:

With dig:

dig mx facebook.com @1.1.1.1