DNS and understanding root servers

Question

Firstly, is this explanation on how DNS works right?

When we visit a site, say (www.example.com),we are doing a lookup to convert the domain name to an i.p address. So, our computer first looks at its internal DNS to see if the host name is there. If the host name is not there, it goes to the root name servers(.). Now, the root name servers get the request and tells us that the request is at the TLD servers and gives us the i.p address of the TLD servers. Now, when we query the TLD servers, the TLD servers contain the extension of websites like .com,.org,.net etc.The TLD servers redirect us to the com servers,in this case, and we get the i.p address of the com servers.When we query the com servers, they have "example" in their list and redirect us to the example's DNS servers. When we query the example.com's server, we get an i.p address and access the internet.

My question is, for example.com, the authoritative name server would be the com server right? since it is the one giving us the information?

Answer 1

My question is, for example.com, the authoritative name server would be the com server right?

No. Let dig it:

# dig example.com SOA

 ;; ANSWER SECTION:
example.com.            3600    IN      SOA     sns.dns.icann.org. noc.dns.icann.org. 2018080109 7200 3600 1209600 3600

;; AUTHORITY SECTION:
example.com.            86400   IN      NS      a.iana-servers.net.
example.com.            86400   IN      NS      b.iana-servers.net.

Authoritative name server for example.com is:

a.iana-servers.net.
b.iana-servers.net. 

That's the servers who are holding DNS records for example.com

Now we can query them directly:

dig @a.iana-servers.net example.com A

;; ANSWER SECTION:
example.com.            86400   IN      A       93.184.216.34

DNS resolver disassemble FQDN (Fully qualified domain name) from right to left.
First query going to root DNS servers asking who is authoritative DNS server in TLD for .com, then resolver query particular TLD for example.com from those servers.

# dnstracer -4 -r1 -s. example.com

Tracing to example.com[a] via A.ROOT-SERVERS.NET, maximum of 1 retries
A.ROOT-SERVERS.NET [.] (198.41.0.4)
 |\___ d.gtld-servers.net [com] (192.31.80.30)
 |     |\___ b.iana-servers.net [example.com] (2001:0500:008d:0000:0000:0000:0000:0053) Not queried
 |     |\___ b.iana-servers.net [example.com] (199.43.133.53) Got authoritative answer
 |     |\___ a.iana-servers.net [example.com] (2001:0500:008f:0000:0000:0000:0000:0053) Not queried
 |      \___ a.iana-servers.net [example.com] (199.43.135.53) Got authoritative answer

Let's try now another domain in .com TLD:

 # dig google.com SOA 

;; AUTHORITY SECTION:
google.com.             345600  IN      NS      ns3.google.com.
google.com.             345600  IN      NS      ns4.google.com.
google.com.             345600  IN      NS      ns1.google.com.
google.com.             345600  IN      NS      ns2.google.com.

we will see, that authoritative name servers for SLD google.com is different now.

since it is the one giving us the information?

No, it is a chain of authoritative DNS servers.
Root DNS servers holding only top level zones also known as TLD, - such as .com, .net When resolver got authoritative DNS servers responsible for TLD, resolver query particular zone for SLD(Second-level domain, example in our case) and when it found authoritative DNS server for SLD it query that server for FQDN (Fully qualified domain name) such as www.example.com

Usually people using internet provider's DNS servers that hold cached resolved DNS records. Such DNS servers called forwarding DNS servers. If they have records in cache, they reply to client immediately without bothering all intermediate servers starting from root. If such forwarding DNS servers doesn't have records in cache (or DNS record expired ) then forwarder resolving again and cache result. Client's DNS queries sent as recursive, that's means client should receive from DNS provider either an error or resolved record. Client shouldn't query chain of intermediate DNS servers on its own, it is job of forwarding DNS server who serves clients requests and cached results. This way forwarders reduce loading to intermediate DNS servers and reply to clients as soon as possible since providers DNS servers are closer to clients.
^{(BTW, public Google's DNS server is also a forwarder.)}

DNS records has parameter TTL (time to live) that set in authoritative servers by owner of domain, so in case you expecting your IP address would change often, you can set TTL=5 minutes or if one don't want his DNS server be bothered too often then TTL can be set for a days.

Answer 2

Mostly correct, but you've left out caching which is critical.

The first time you set up a simple caching only server, the only records it knows about are the root hints - this points to the root servers. If you ask for example.com the root servers will tell which server to query for records in the .com - those are the authoritative servers. Your DNS server then caches this information. It will then ask the .com server(s) for the info on example.com and they will return a pointer to whatever name servers example.com is configured to use in the registrar database. This info is cached as well. Then your DNS server asks the name servers for example.com for the IP that maps to the name you asked for - www.example.com or whatever.

Now, lets say you then ask for example2.com - the .com server info is already cached, so instead of bothering the root server your DNS server goes straight to the .com servers, gets the name server info for the example2.com domain, and queries them.