What is an authoritative DNS server? What is a recursive DNS server?

A DNS name server is anything serving DNS responses to DNS requests. But there are two kinds of name servers out there: authoritative name servers and recursive name servers. Authoritative name servers don’t need to consult any higher authority to serve their responses; they are the ultimate authority on the domains they are serving responses about. Conversely, recursive name servers serve their responses by consulting higher authorities; the recursive name servers are useful because they cache those responses and reduce the load on authoritative servers. The recursive name servers are a sort of global CDN for the DNS.

Name servers can be authoritative and recursive, but they broadly fall into those two categories. For examples:

Your WiFi router (e.g. at 192.168.1.254) runs a name server. It is recursive, and you can often configure it via some web interface. My “PlusNet” router uses some PlusNet-operated name server at 212.159.6.10.
Google operates a name server at 8.8.8.8. It is (entirely?) recursive.
Amazon Route53 operates name servers at many IPs, e.g. 205.251.197.202. It is an authoritative name server; not recursive.
NASA operates one of the 13 root name servers at 192.203.230.10. It is an authoritative name server, not recursive.

A way to check whether a name server is authoritative is to query it for a common domain, e.g.

$ dig @192.203.230.10 google.com.

; <<>> DiG 9.8.3-P1 <<>> @192.203.230.10 google.com.
; (1 server found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 20137
;; flags: qr rd; QUERY: 1, ANSWER: 0, AUTHORITY: 13, ADDITIONAL: 14
;; WARNING: recursion requested but not available
...
...

Notice that dig says “recursion requested but not available”. The server at 192.203.230.10 is therefore an authoritative name server only.

Tagged #dns, #networking, #internet-protocol, #web.

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What is an authoritative DNS server? What is a recursive DNS server?

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