2.5 DNS - The internet's Director Service Flashcards
What’s themain task of the Internet’s domain name system (DNS)?
The translation of hostnames to IP addresses.
What is technically a DNS?
i) a distributed database implemented in a hierarchy of DNS servers and
ii) an app-layer protocol that allows hosts to query the distributed database.
The DNS servers are often UNIX machines running the Berkeley Internet Name Domain (BIND) software [BIND 2012]. The DNS protocol runs over UDP and uses port 53.
How is the additional delay managed?
The desired IP address is often cached in a “nearby” DNS server, which helps to reduce DNS network traffic as well as the average DNS delay.
Other services provided by DNS.
- host aliasing: 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 alisaing: DNS can be invoked by a mail application to obtain the canonical hostname for a supplied alias hostname as well as the IP address of the host.
- Load distribution: For replicated Web servers, a set of IP addresses is thus associated with one canonical hostname. The DNS database contains this set of IP addresses. 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.
problems with a centralized design include:
- A single point of failure. If the DNS server crashes, so does the entire Internet!
- Traffic volume. A single DNS server would have to handle all DNS queries (for all the HTTP requests and e-mail messages generated from hundreds of millions of hosts).
- Distant centralized database. A single DNS server cannot be “close to” all the querying clients. If we put the single DNS server in New York City, then all queries from Australia must travel to the other side of the globe, perhaps over slow and congested links. This can lead to significant delays.
- Maintenance. The single DNS server would have to keep records for all Internet hosts. Not only would this centralized database be huge, but it would have to be updated frequently to account for every new host.
In summary, a centralized database in a single DNS server simply doesn’t scale. Consequently, the DNS is distributed by design. In fact, the DNS is a wonderful example of how a distributed database can be implemented in the Internet.
Suppose a DNS client wants to determine the IP address for the hostname www.amazon.com.
To a first approximation, the following events will take place. The client first contacts one of the root servers, which returns IP addresses for TLD 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.
3 classes of DNS servers.
- Root DNS servers. In the Internet there are 13 root DNS servers (labeled A through M), most of which are located in North America. An October 2006 map of the root DNS servers is shown in Figure 2.20; a list of the current root DNS servers is available via [Root-servers 2012]. Although we have referred to each of the 13 root DNS servers as if it were a single server, each “server” is actually a network of replicated servers, for both security and reliability purposes. All together, there are 247 root servers as of fall 2011.
- Top-level domain (TLD) servers. These servers are responsible for top-level domains such as com, org, net, edu, and gov, and all of the country top-level domains such as uk, fr, ca, and jp. The company Verisign Global Registry Services maintains the TLD servers for the com top-level domain, and the company Educause maintains the TLD servers for the edu top-level domain. See [IANA TLD 2012] for a list of all top-level domains.
- Authoritative DNS servers. Every organization with publicly accessible hosts (such as Web servers and mail servers) on the Internet must provide publicly accessible DNS records that map the names of those hosts to IP addresses. An organiza- tion’s authoritative DNS server houses these DNS records. An organization can choose to implement its own authoritative DNS server to hold these records; alter- natively, the organization can pay to have these records stored in an authoritative DNS server of some service provider. Most universities and large companies implement and maintain their own primary and secondary (backup) authoritative DNS server.
local DNS server
There is another important type of DNS server called the local DNS server. A local DNS server does not strictly belong to the hierarchy of servers but is nevertheless central to the DNS architecture. Each ISP—such as a university, an academic department, an employee’s company, or a residential ISP—has a local DNS server (also called a default name 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, which is discussed in Chapter 4).
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.
DNS caching
The idea behind DNS caching is very simple. In a query chain, when a DNS server receives a DNS reply (containing, for example, a mapping from a hostname to an IP address), it can cache the mapping in its local memory.
hostname/IP address pair is cached in a DNS server and another query arrives to the DNS server for the same hostname, the DNS server can provide the desired IP address, even if it is not authoritative for the hostname. Because hosts and mappings between hostnames and IP addresses are by no means permanent, DNS servers discard cached information after a period of time (often set to two days).
resource records (RRs). What are they?
They are records stored by the DNS servers that together implement the DNS distributed database. Some RRs provide hostname-to-IP address mappings.
Structure of RR.
(Name, Value, Type, TTL)
Name possible RRs.
Type A. Value: IP address for the hostname
Type NS. Value: hostname of an authorittaive DNS server that knows how to obtain the ip addresses for hosts in the domain.
Type CNAME. Value: canonical hostname for the alias hostname NAME.
Type MX. Value: canonical name of a mail server that has an alias hostname NAME.
