ACIT 2620

Domain Name System (DNS)

• allow users to locate services by domain name rather than having to remember IP addresses
• like an address book for IP addresses
• hierarchical name system
• distributed database
  • each domain responsible for maintaining its own DNS servers (nowadays most domain registrars provide DNS services for their customers)
• allow changing server's IP address without requiring users to know about it.

Top Level Domains (TLDs)

• assigned by ICANN
• 7 original gTLD: .com, .net, .org, .int, .edu, .mil, .gov
• country-code domains: .us, .ca, .it, ...
• new generic TLDS (gTLD): .biz, .info, ...
  • anyone can apply for a subdomain

Stakeholders

• registrant: takes ownership of a domain by claiming the domain through a registrar
• registrar: leasing domain names (on yearly basis) and inserting the names in global DNS (delegation).
  • Create NS record for the domain in the TLD of the new name with the relevant registry operator

Stakeholders

• registry: databases of DNS names under TLDs. Each registry is operated by a different organization (e.g. .com is operated by VeriSign)
• ICANN: oversees all the above. Also oversees the internet root zone ('.') (a..m.root-servers.net).
  • Grants accreditation to registrars (the latter pay fees to ICANN which go towards the operation of ICANN itself)

Zones

• independently managed subtree of the DNS tree
• has own root DNS that is the suffix of every DNS name in the zone.
• though multiple levels are possible, many zones tend to define only one additional level and create subzones for deeper levels
• each zone has its own authoritative nameservers charged with maintaining zone records
  • must have at least 2 nameservers for redundancy

Zones

• a single nameserver can manage multiple unrelated zones (e.g. domain registrars)
• root zone (root of the DNS tree, represented by DNS name that is the empty string) is managed by root nameservers
  • contain only NS records for the immediate subzones or TLDs (.com, .net, ...)
  • 13 root nameservers widely distributed
  • a.root-servers.net to m.root-servers.net (each corresponding to a cluster of servers)

Registration

• registrant choose a domain name and creates zone for it
• claim domain with a registrar
  • registrar inserts domain in global DNS through delegation
• domain added to TLD registry
• registrant create records under the zone

DNS records

• generically called Resource Records (RR)
  • A: maps domain name to IPv4 addresses
  • AAAA: maps domain name to IPv6 addresses
  • NS: name servers for sub zones

• CNAME: canonical name or alias.
  • Allows multiple domain names to point to the same physical machine, thus requiring only a single A record.
• PTR: reverse DNS
• MX: Mail eXchange
  • maps domain name to a mail server that accepts email on behalf of the domain.
  • Thus, one domain name is able to represent both a web server (with a different IP address) and an email server.
• TXT: arbitrary text strings
  • often used for verification (e.g. SPF: Sender Policy Framework which protects agains spam)

Nameserver

• answers queries for names defined under a zone
• authoritative
• each zone has one or more associated nameservers

DNS resolver a.k.a recursive nameserver

• often provided by service providers (your ISP or your company)
• recursively forwards DNS queries to nameservers on the user's behalf
• caches queries for quick subsequent retrieval

Stub resolver

• runs on the client
• forwards DNS queries to DNS resolvers configured for the client
• also caches queries for quick retrieval

Tools

• nslookup
• dig
• whois