Subjects -> Computer Networks -> Lectures -> Lecture #08

Lecture 8: The Domain Name System (DNS)

IP Addresses: A Sneak Preview

Until now, we have always referred to Internet-connected systems (or, to use the more formal term, hosts) by name. Most Internet users have a strong intuitive feel for the hierarchical domain-based naming system used in the Internet, and are very comfortable with the use of names like amazon.com and ironbark.bendigo.latrobe.edu.au.

We have not yet mentioned the fact that actual packet delivery in the Internet (what we called, in lecture 2, the network delivery service) is based on a separate, fixed-length numeric IP Address. IP addresses are used by the Internet's delivery service, the Internet Protocol, or IP, to route packets of data through the Internet to their destination.

We return to IP addresses in much greater detail later. However, for the moment we simply need to know that every system has a unique 4-byte address usually written in the form a.b.c.d, where each of the letters represents the decimal value of the appropriate byte in the address. For example, the IP address of ironbark is 149.144.21.60.

The DNS Hierarchy

The DNS Namespace is based on a "tree" structure, with a small number of generic Top Level Domains (eg, .com, .edu, .org) and a large number of country-based domains (eg .au, .my, .uk). Each TLD supports a group of "second-level" domains, and so on, all the way down to individual hosts.

A domain name is a dotted sequence describing a path through the name hierarchy from the root, maybe with a trailing dot, thus: bindi.bendigo.latrobe.edu.au.
An individual name component must be less than 63 characters, must begin with a letter, etc...
Upper and lowercase may be used, although name lookups are case insensitive by definition.

Resource Records

Each domain name has one or more resource records associated with it. Resource records are 5-tuples:

Domain_name  TTL  Type  Class  Value

Domain_name

the name of the domain to which this RR applies.

TTL

the Time To Live of this RR. When this RR is returned as a result of a DNS lookup, the remote host normally caches the information for efficiency. The TTL is the time, in seconds, after which the cached information should be regarded as potentially out of date.

Type

there are several types of RR, including:

SOA: Start Of Authority.
A: IP address of a host.
NS: Name Server, etc

Class

Always set to "IN", for Internet

Value

The actual value of this particular RR. Can be, for example, an IP address, a number, some ASCII text or a combination.

DNS Servers and Resolvers

A nameserver provides domain-name-to-IP-address mappings (and a few other functions, but "looking up" IP addresses is the most common) for one or more zones, which are sub-trees of the domain name space. For example, sheoak is a nameserver for the zone bendigo.latrobe.edu.au. This means that if I want to look up a particular IP address in that zone, I can ask sheoak.

Exactly which server is responsible for a particular zone is specified in start of authority (SOA) RRs. An SOA RR specifies, for the particular name server, the zones for which it has authority. It also has the email address of the site administrator, a unique serial number and various other bits and pieces.

The DNS system forms a distributed database of domain information.

A resolver is a library function^[1] which queries the nameserver when called from a user program. It can check the local cache of names and, if necessary, request a RR from a nameserver (privately caching the response). In other words, a resolver is software which asks a nameserver for information.

^[1] Such as is implemented in the Unix library function gethostbyname(3).

Nameserver Queries

The resolver sends a question to a name server, of the form:

{query domain name, type, class}

The server responds with one or more appropriate RRs. It also sends an ADDITIONAL INFORMATION section, which contains extra RRs which the resolver will probably find useful. For example, if a resolver queries for a particular NS RR, the server will return it, plus additional information giving the IP address of the name server specified in the main body of the reply.

The most common DNS query is of type A, where the resolver is required to map a domain name to an IP address - that is, "looking up" an IP address. Some typical type A RRs look like:

ironbark  86400  IN  A  149.144.21.60
redgum    86400  IN  A  149.144.21.3
bindi     86400  IN  A  149.144.20.82

Note that the "domain name" part of these RRs has been omitted (leaving only the hostnames) for clarity.

Recursive Queries

A host within a specified domain (eg a machine at Bendigo, in domain bendigo.latrobe.edu.au) is configured to "know" the IP address of its local nameserver. What happens when it sends a query for a non-local name, (eg amazon.com)? The sequence of events is something like:

The local nameserver will forward the query to its "parent" nameserver -- in our case, the nameserver for domain latrobe.edu.au.
This nameserver, in turn, (usually) forwards the query recursively up the "tree", to where a root nameserver will pass it to a nameserver for the .com domain, which will have the desired name-to-address mapping.
The result of the query is then passed back through the chain of nameservers (each of whom will normally cache the information), finally arriving at the originating host. This process is called a recursive query.

It's obvious that recursive queries could be quite slow. The DNS provides a way of "short-circuiting" the whole process. If (for example) the local nameserver already knows (due to caching) the IP address of a nameserver for the .com domain in the above example, it can contact it directly, thus avoiding many recursive stages. This is called an iterative query. In practice, most (all?) queries to root nameservers are iterative. Every nameserver is configured to know the IP address of at least one root nameserver.

Some DNS Subtleties

Mail eXchange

the DNS provides the MX type of RR to discover where email is to be delivered. An MX RR specifies a primary mailhost, and lesser preferential hosts where mail for a specified domain is be delivered. For For example, ironbark has:

ironbark  IN  MX  10  ironbark
          IN  MX  20  redgum
          IN  MX  40  sheoak

Reverse lookups

a special domain (in-addr.arpa) and address format is used to map IP addresses to domain names, thus:

60.21.144.149.in-addr.arpa

This is called a PTR RR. Performing reverse lookups is much more difficult than normal forward address lookups.

CNAME

Often a host may be known by several names: names other than the official host name are called aliases, and a CNAME RR maps an alias name to a host's "real" name.

HINFO

describes some basic information about the type of CPU and the OS it is running. Rarely kept up-to-date.

DNS Implementation Technicalities

The DNS works as a ditributed database because of two fundamental ideas: replication and caching. We have already seen how caching works -- at any point in a query, if a nameserver has a current copy of the desired information, it can supply it instead of contacting other nameservers.

The DNS requires that all nameservers be replicated at least once -- that is, for each zone of authority there must be at least two authoritative nameservers -- a primary and one or more secondaries. The rules for replication are quite stringent -- at least one backup server must be located "off-site", attached to a different network, and with an independent power supply. So, for example, a secondary for sheoak.bendigo.latrobe.edu.au is luga.latrobe.edu.au, located at the Bundoora campus of La Trobe.

DNS queries and responses are an excellent example of an application where the reliable, connection-oriented transport mechanism of TCP is not necessary, and simply has too much overhead. In fact, queries are encapsulated in unreliable UDP datagrams, see later. If a resolver does not receive a reply from a nameserver, it usually either tries again, or tries the next nameserver for the same domain.

Extra infomation

Here's the definitive source of information on DNS. Telstra BigPond Direct has a good explanation of the various RR types in the DNS. Here's the bare bones of another lecture on DNS, with something of a Linux emphasis.

The tutorial for this lecture is Tutorial #08.
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