UoL CS Notes
Menu

DNS (Domain Name System)

COMP211 Lectures

DNS allows host-names to be mapped to IP addresses:

  • The DNS is implemented as a distributed database on a hierarchy of many name servers.
  • It is implemented in the application-layer to put complexity at the network edge.

DNS allows for the following:

  • Host-name to IP address translation.
  • Host aliasing.
  • Mail server aliasing.
  • Load distribution:
    • Many IP addresses can correspond to one name.

Against Centralised DNS

There are many reasons why you don’t want DNS to be centralised:

  • Single point of failure.
  • Traffic volume.
  • Can be physically distant from clients.
  • Maintenance will take down the whole internet.

In order to scale the internet DNS is implemented as a distributed database.

Hierarchy

flowchart
r[Root DNS Servers] --> com[.com DNS Servers] & org[.org DNS Servers] & edu[.edu DNS Servers]
com --> yahoo[yahoo.com DNS Servers] & amazon[amazon.com DNS Servers]
org --> pbs[pbs.org DNS Servers]
edu --> nyu[nyu.edu DNS Servers] & umass[umass.edu DNS Servers]
subgraph Root
r
end
subgraph Top Level Domain
com
org
edu
end
subgraph Authorative
yahoo
amazon
pbs
nyu
umass
end

Root Name Servers

There are 13 logical root name servers with each server being redundant across the world. They are:

  • Official last resort name servers.
  • Managed by ICANN (Internet Corporation for Assigned Names and Numbers)
  • Very important to the function of the internet.

Top Level Domains

TLD servers are responsive for all top level domains (.com, .org, .net, …) and all the country domains (.uk, .fr, .jp, …). There are companies responsible for all top level domains.

Authoritative Servers

There are organisations own DNS servers, providing authoritative host-name to IP mappings for an organisation named hosts.

These can be maintained by an organisation or the service provider.

Local DNS Name Servers

Each ISP has a default name server. When a host makes a DNS query it is sent to it’s local DNS server. This has:

  • A cache of recent name-to-address translation pairs.
  • Acts as a proxy, forwarding queries into the hierarchy.

Iterated Query

DNS also allows recursive queries.

In the worst case the following would happen when you search for a host-name via DNS:

  1. Client requests local DNS server.
  2. Local DNS server requests root DNS server for top level domain.
  3. Local DNS server requests TLD for authoritative domain.
  4. Authoritative responds with IP address.
  5. Local DNS server responds to client with IP address.

Caching Records

Once any name server learns a mapping it is cached:

  • Cache entries timeout after some time (TTL).
  • TLD servers are typically cache in the local name servers.
    • This means taht the root name servers are not often visited.
  • Cached entries may be out of date:
    • If name host changes IP address it may not be known until all TTLs expire.
  • Update and notify mechanisms are proposed in IETF standard RFC 2136.

DNS Record Types

DNS is a distributed databse that stores resource records (RR). The RR schema is:

RR(name, value, type, ttl)

There are several types:

  • type=A
    • name is the hostname
    • value is the IP address
  • type=CNAME
    • name is an alias name for some canonical name.
    • value is the canonical name.
  • type=NS
    • name is the domain.
    • value is the host-name of the authoritative name server for this domain.
  • type=MX
    • value is the name of the mail-server associated with name.