Domain Name System

• A hostname is a label assigned to a device connected to a computer network, nicknames for IP addresses.
• if a computer were a telephone, then the IP address is the phone number and the DNS methodology is the phone book, while a DNS record to find a computer might be similar to using your first and last name to look up your phone number
• DNS is hierarchical system, where .com, .org, .ca, are examples of TLDs (Top-Level Domains)
• there are different kinds of dns records
• name servers are designated for translating domain names to IP addresses
  • these are hierarchical much like FQDNs themselves
  • authoritative name servers give answers to queries about domains under their control, and otherwise redirect requests
  • the zone file lives on the name server and is used to identify which IP address should be contacted
• DNS primarily uses UDP because the client will know to retry with TCP if needed, and UDP is reliable enough while its simplicity is well suited for DNS

Resolution

1. /etc/hosts to see if it has that domain name locally
2. checks DNS cache (client-side)
3. contact ICANN root DNS server
4. redirects to TLD servers
5. redirects to Domain-level name servers
6. domain-level server can return the proper IP address

nslookup

Tool for making queries to name servers, which uses resolv.conf for its name servers

resolv.conf

File used to configure the system's DNS resolver

• resolving is the process of determining the IP address from a domain name
• a DNS resolver is responsible for initiating and sequencing queries that lead to the right resource (translation of domain name into IP address)
• an individual DNS query may be non-recursive, recursive, iterative, combinations of these
• when DHCP is used, it is usually updated with resolvconf utility
• nameserver directive determines the IP address of a name server
• search and domain directives help to resolve short hostnames by suffixing domains during resolution
• note that etc/hosts is used for local machine customization that acts as an override to this

Round Robin DNS

A DNS server can respond with a list of A records (host names) and then the client should use each with equal frequency, client side load balancing

Errors

• NXDOMAIN means that the domain name is not known by the DNS server
• SERVFAIL says the FQDN exists but the authoritative name servers are not responding