DNS
DNS stands for Domain Name System. The DNS is:
A distributed database implemented in a hierarchy of DNS servers
An application-layer protocol that allows hosts to query the distributed databases
The DNS protocol runs over UDP and uses port 53.
DNS for Resolving Domain Names
A domain name has a maximum length of 255 bytes. It's formed by a series of labels. Each label:
Has a maximum length of 63 bytes.
Can include alphanumeric characters and hyphens (-), but no other symbols.
Domain Name Servers (DNS servers) are responsible for managing zones. These servers are categorized into:
Root Domain Name Server
Top-Level Domain Server
Authoritative Domain Name Server
Local Domain Name Server
It's worth noting that a single server can function as both a Local and an Authoritative Domain Name Server simultaneously. For instance, a university's DNS server might store cached Resource Records (RRs) while also housing the actual address of its domain name.
DNS provides a few other important services in addition to translating hostnames to IP addresses:
Host aliasing
Canonical hostname vs Alias hostname The latter is more memonic. DNS can be invoked by an application to obtain the canonical hostname for a supplied alias hostname as well as the IP address of the host.
Mail server aliasing
Load distribution
When clients make a DNS query for a name mapped to a set of addresses, the server responds with the entire set of IP addresses, but rotates the ordering of the addresses within each reply. Because a client typically sends its HTTP request message to the IP address that is listed first in the set, DNS rotation distributes the traffic among the replicated servers.
How DNS Works
The application will invoke the client side of DNS, specifying the hostname that needs to be translated. (On many UNIX-based machines, gethostbyname() is the function call that an application calls in order to perform the translation.)
DNS in the user’s host then takes over, sending a query message into the network. All DNS query and reply messages are sent within UDP datagrams to port 53.
The client first contacts one of the root servers, which returns IP addresses for TLD(Top Level Domain) servers for the top-level domain com. The client then contacts one of these TLD servers, which returns the IP address of an authoritative server for amazon.com. Finally, the client contacts one of the authoritative servers for amazon.com, which returns the IP address for the hostname www.amazon.com.
An organization’s authoritative DNS server houses these DNS records. An organization can choose to implement its own authoritative DNS server to hold these records; alternatively, the organization can pay to have these records stored in an authoritative DNS server of some service provider.
Local DNS server When a host connects to an ISP, the ISP provides the host with the IP addresses of one or more of its local DNS servers (typically through DHCP). When a host makes a DNS query, the query is sent to the local DNS server, which acts a proxy, forwarding the query into the DNS server hierarchy.
The query sent from cse.nyu.edu to dns.nyu.edu is a recursive query, sicne the query asks dns.nyu.edu to obtain the mapping on its behalf. But the subsequent three queries are iterative since all of the replies are directly returned to dns.nyu.edu. In theory, any DNS query can be iterative or recursive.
The query from the requesting host to the local DNS server is recursive, and the remaining queries are iterative.
Each time the local DNS server dns.nyu.edu receives a reply from some DNS server, it can cache any of the information contained in the reply.
A local DNS server can also cache the IP addresses of TLD servers, thereby allowing the local DNS server to bypass the root DNS servers in a query chain. In fact, because of caching, root servers are bypassed for all but a very small fraction of DNS queries.
DNS Records
The DNS servers store resource records (RRs) in a zoom file, including RRs that provides hostname-to-IP address mappings. Each DNS reply message carries one or more resource records.
A five-tuple (Name, Value, Type, TTL, Class) where:
A
Hostname
IPv4 Address
AAAA
Hostname
IPv6 Address
CNAME
Alias
Canonical Name (Hostname)
MX
Domain
Mail Server Hostname, Priority
TXT
Domain
Text Information
NS
Domain
Name Server Hostname
SOA
Domain
Primary NS, Responsible Email, Serial, etc.
PTR
IP Address
Pointer to Hostname
SRV
Service
Priority, Weight, Port, Target (Hostname)
TTL is the time to live of the resource record.
Class is IN in Internet.
If a DNS server is authoritative for a particular hostname, then the DNS server will contain a Type A record for the hostname. (Even if the DNS server is not authoritative, it may contain a Type A record in its cache.)
If a server is not authoritative for a hostname, then the server will contain a Type NS record for the domain that includes the hostname; it will also contain a Type A record that provides the IP address of the DNS server in the Value field of the NS record.
DNS Messages
Both query and reply messages have the same format. This semantics of the various fields in a DNS message are as follows:
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