컴퓨터 네트워크 ch2(4)

DNS

 

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DNS : Domain Name System

• Name & Addres
  • Name
    • Character string for human use e.g. www.naver.com
    • Mnemonic
  • Address : Where you are
    • IP address (32 bit string) : used by a machine
  • www.naver.com → 32-bit string으로 변환

 

How to map between IP addresses and name?

Mapping a name to an address or an address to a name is called name-address resolution.

 

 

DNS

• Name resolution
  • Solution 1 : Static Mapping
    • Hostname을 file 또는 host file을 이용하여 주소를 mapping시킨다.
    • 그러나 파일의 양이 많아져서 더 이상 사용하지 않는다
  • Solution 2 : Dynamic Mapping(DNS)
    • The internet has too many objects for a single management center
    • uses Distributed Database system
      • Scalabitlity, maintenance
      • 현존하는 세계 최대 D-DB
        
        • Partition the name space into a hierarchicla tree
          • Domain hierarchy

 

ad ~ zw :: 나라 이름

 

• The tree can have only 128 levels
  • level 0 (root) to level 127 
  • www.example.com (.으로 연결해서 사실 127개까지 내려갈 수 있음)
• In the Internet, the domain name space(tree) is divided into three different sections:
  • generic domains [ .net .edu .org ]
  • country domains [ .kr ]
  • inverse domain : IP는 아는데 nemonic을 모를 때 사용되는 도메인
    • 203.252.97.22 → 22.97.252.203.in-addr.arpa

 

 

 

Overview of DNS

 

Client wants IP for www.amaz.on.com

1. client는 root server에게 com DNS server를 요청한다
2. client는 DNS 서버로부터 amazon.com DNS server를 묻는다
3. client는 amazon.com DNS 서버로부터 www.amazon.com의 IP주소를 얻는다

 

 

13 root servers (A-M) in the Internet

• www.root-servers.org
• Each server is actually a cluster of replicated servers (한 대가 아니라 server farm)

TOP-level Domain (TLD) servers

• Responsible for com, org, net, edu, etc, and all top-level country domains uk, fr, ca, jp.

Authoritative DNS servers

• 단체에 대한 DNS
• organization's DNS servers, providing authoritative hostname to IP mappings for organization's servers
• Can be maintained by organization or service provider

 

 

.kr DNS

• 6 .KR name servers
• Primary : operated by KRNIC
• Secondary : 5 servers by 4 organizers

 

 

 

Local Name Server

• 반드시 설정해줘야 함
• does not strictly belong to hierarchy
• Each ISP (residential ISP, company, university) has one "Local Name Server"
• Also called "default name server"
• When a host makes a DNS query, query is sent to its local DNS server
• acts as proxy, forwards query into hierarchy

 

 

Name Resolution : Iterative Queries

• Example:
  • Host at cis.poly.edu wants IP address for gaia.cs.umass.edu

 

• Iterative queries
  • contacted server replies with name of server to contact
  • "I don't know this name, but ask this server"
  • Typical method

Name Resolution : Recursive Queries

• recursive query :
  • puts burden of name resolution on contacted name server
  • heavy load at upper levels of hierarchy

root DNS server의 부담이 커져서 잘 안씀

 

DNS Caching and Update Recoreds

• once (any) name server learns mapping, it caches mapping
  • TTL만큼의 시간이 지나면, cache entries는 없어진다.
  • TLD 서버를 local name 서버에 캐시로 저장한다.
    • 그래서 root name server를 방문하지 않아도 될 수 있다.
• cached entries may be out-of-date (best effort name-to-address translation!)
  • if name host changes IP address, may not be known Internet-wide until all TTLs expire
• update/notify mechanisms propsosed IETF standard(변경되면 알려주는 기능 O)
  • RFC 2136

 

Services Provided by DNS

• DNS
  • DNS can use the services of UDP or TCP using the well-known port 53
  • UDP가 주로 사용되는데 overhead를 줄이기 위해서이다. 에러가 나면 IP에서 그냥 버리고 time out되면 다시 요청하면 된다.
  • TCP는 connection-oriented이며, connect와 release로 overhead가 발생한다
• DNS identify objects on the Internet
  • Host name
    • Canonical hostname (원래 자기이름)
    • Alias hostnames (별명)
  • Name server (해당하는 도메인의 A.S)
  • Mail server
  • Information

 

 

 

DNS records

DNS : distributed db storing resource records (RR)

 

RR format : (name, ttl, class, type, value)

 

• Type : Specifies the types of the value
  • A: name = host, value = IPv4 address
  • NS (Name Server) : name = domain name, value = IP address of authoritative name server for this domian
  • CNAME : name = alias name, value = Canonical name
  • MX (Mail Server) : name = alias hostname, value = name of mailserver associated with name
  • HINFO : host information (CPU and OS)
  • TXT : text (uninterpreted ascii text)
• TTL : [Time To Live] how long the resource reocord is valid. cache에서 언제 살아질 것인가

 

 

 

DNS recrods: Example

SOA : Start of Authority : 정상적인 기록의 시작

 

 

DNS Messages

Query and reply messages have the same format

 

message header

identifiaction : 16 bit numbers for query, reply to query users same numbers

 

flags :

• query or reply
• recursion desired
• recursion available
• reply is authoritative

 

 

 

Inserting records into DNS

• How are new domains added to DNS?
  • This done through a registrar, a commercial entity accredited by ICANN.
  • A registrar first verifies that the requested domain name is unique and then enters it into the DNS database.
    • Need to provide registrar with names and IP addresses of your authoritative name server (primary and secondary)
    • Registrar inserts two RRs into the com TLD server
• A fee is charged.

 

 

 

Accredited Registrar in KOREA