Forusans DNS

Name space

A name space that maps each address to an uniqye name can be organised flat or hierarchical.

Flat Name

In a flat name space, a name is assigned to an address. A name in this space is a sequence of characters without structure. The names may or may not have a common that is cannot be used in large systerm such as the internet because it must be centrally controlled to avoid ambiguitive and duplication.

Hierarchical Name space

In a hierarchicaly name space, each name is made of several parts.

1) Nature of the organisation 2) Name of organisation 3) Define departments in the organisation etc.

Manager does not need to worry that the prefix chosen for a host is taken

Label

Each node in a name tree has a label:
- A string with max 63 chara
- ROot label is annull string
- DNS requires children of a node have a different label whcih guaranteese the uniquesness of the domain names

Domain

Fully qualified domain name

Label terminated by null string
a domain name that contains the full name of the host
Contain all labels from most specific to the most general
Label ends with dot (.)

Partially qualified domain name

Label not terminated by null string
Starts from node buyt does not reach a root
Used when the name to be resolved belongs to the same site as the client
Resolver can supply the missing part called suffix to create FQDN.

Domain is the subtree of domain name space. The name of the domain is the name of the node at the top of the subtree.

DNS server serves as a hierarchy of name server to divide the whole space into many domains based on the first level.

Zone

What server is responsible for is called a zone
Contigupous part of the entire tree
If server doesnt divide their zone, the domain and zone is the same thing
Server keeps every info about every node in its domain
If server creates subdomain, domain and zone are different
- The server only need to care about the top level nodes
Original server might keep some sort of references of these lower level servers but most details are kept by the lower level servers

Root server

Server whose zone consist of the whole tree
Does not store info about domains but keep referecnmes of other servers and delegates it authority to other server

Primary

Stores a file a about zone for which it has authority
Responsible of maintaining, creating and editing zone

Secondary

Transfer the info about a zone from another server
Store info in local disk
Neither creates or updates zonefile

Pri and secondary are both authroitative for the zones they serve - Create redundancy for data

A server can be a primary server for one zone and a secondary for another zone at the same time.

DNS in the internet

Generic domain

Define generic host accourding to their behavior

COuntry domains

Use two character country abbreviation
Second labels can be organisational and more specific
Inverse domain
Map address to name
Server ask resolver to send query to the DNS server to map an address to a name to determine if the client is on authorisation list
Pointer query: The inverse domain is added to the domain name space with the first level node called arpa
- Second level: A single node name in-addr which store the ipaddress in reverse order

Look up will be 121.45…

Registra

Handle new domains added to DNS
ICAAN
Verifies the domain is unique then enters it into the DNS database

Resolution

Mapping a name to address or vice versa

Resolver

DNS client that maps the address to name or the other way round
Access closest DNS server with mapping request
- Has info: Resolves
- No have info: Refers to others
Interpret the response to see if it real resolution or error
Delievers to the process that request

Mapping name to address

Gives domain name to server and ask from corresponding address
- Generic: chal.atc.fhda.edu
- Cannot resolve: Refers to other server
- Country: ch.fhada.cu.ca.us

Mapping address to name

Client sent ip address to server to map (PTR)
DNS use inverse domain
IP address is reverse with arpa and ipaddr appended
Resolver:
- Invert
- Send with in-addr.arpa appended

Recursive resolution

Resolver expects server to supply final answer
- Server not authority: ask other server and wait
- Server authority: Returns response

iterative

Same recursive but if cannot reply then it returns the ipaddress of the server that it thinks can reply
Client responsible for repeating the query till it gets the server
Iterative because the client repeatedly send the same query to different servers until it gets answers

Caching

Stores info into cache memory
Marks the response as unauthoritative if its from its cache
Speeds resolution but can be probamatic
Uses TTL to check if its current

DNS Messages

Query and response
Same format

Set to 0 for querys for some fields
12 bytes

Indetification: 16 bit use by client to match response with query. Use a different id number each time it send query.
Flags: 16 bit consisting of subfields

(4 bytes)

Num question record: Num of queries in the question section
Answer: Num of answer records in the answer section of the response message, value is 0 in query
Num of autho: Num of authroative records in response
Num of additional : Additional section of resposne message, value is 0 in query

Types of records

Question: Question section
Resouce: answer, authroitative, additional

Question records

Get info from server
COntains domain name in query name
COunt field: States the number of character

Types of query:

Query class:

Define the specific protocol using DNS

Resource records

Each domain name is associated with record called resource record
Return from server to client
Domain name: A pointer offset to the question record
Domain type: Same as query type field in question record
Domain class: Same as question records
TTL: 32 bits that determine when it is valid, 0 record means rr is used only in single transaction and is not chace
Resoucedata: Contain the answer to query or domain name of authrotitative server such as
- A number (ipv4 or ipv6)
- Domain name
- OFfset pointer: Domain names replaced by this
- Character string

CS3103 - Experiments 4