CS335 Networking &

Network Administration

Tuesday, November 25, 2008

Domain Name System DNS

The DNS system consists of three components: DNS data (called resource records) servers (called name servers) Internet protocols for fetching data from the

servers

Domain Name System

Top-level domains Maps to IP number Registration

DNS

Geographic structure .or.us ac.uk

DNS

Domain names within organizations computer.foobar.com computer.location.foobar.com computer.division.location.foobar computer.subdivision.candy.foobar.com

Organization DNS

No universal standard Each organization can choose how to

structure names How does eastern do it?

www or ftp

The first label in a domain name is done for humans, not computers

www is not necessary for a web server, but is common

DNS client-server model

Provides autonomy for organizations Can assign names as they see fit within their suffix

without informing a central authority Entire system operates as a large, distributed

database Each server contains information that links it to other

domain name servers When an application needs to translate a name to

an IP address the app becomes a client of the naming system

DNS server hierarchy

Root server at the top Is the authority for the top-level domain Doesn’t contain all possible domain names,

but contains information to reach other servers

Server hierarchy

DNS

Server architecture

Single server is simplest Depends on system size Large organizations might need more than

one to handle requests at high speed Administration is done by humans Each group can make changes as necessary

without centralized coordination

Locality of reference principle

Users tend to look up names of local machines

Users tend to look up the same domains repeatedly

How it works

Client computer generates a resolve request Application calls library routine gethostbyname

Directs it to the local DNS server If it is not an authority for that domain Then to the ISP’s DNS server Then up the tree to the root server if

necessary Waits for an authoritative answer

Authoritative servers

The billions of resource records in the DNS are split into millions of files called zones. Zones are kept on authoritative servers distributed all over the Internet, which answer queries based on the resource records stored in the zones they have copies of. Caching servers ask other servers for information and cache any replies. Most name servers are authoritative for some zones and perform a caching function for all other DNS information. Large name servers are often authoritative for tens of thousands of zones, but most name servers are authoritative for just a few zones.

Types of DNS entries

Domain name Record type Value Type A – address type FTP, ping, WWW MX – Mail eXchanger used by email Aliases using CNAME

Lets www.foobar.com point to hobbes.foobar.com Allows companies to move WWW servers without changing

names or addresses or lets one server answer to www.foobar.com and ftp.foobar.com with domain records

Abbreviations

Ex. Mail refers to mail.lagrande.k12.or.us Simplifies typing in full paths Put in a DNS record instead

DNS resources

http://www.ripe.net/ripe/docs/ripe-192.html http://www.dns.net/dnsrd/docs/whatis.html http://www.dns.net/dnsrd/rfc/ http://web.syr.edu/~djmolta/ist452/ch_07.ppt Find out what you can about the ARPANET

and how it originally resolved IP addresses

NSLOOKUP

Use NSLOOKUP to find information on domain servers

http://www.stopspam.org/usenet/mmf/man/nslookup.html

Master DNS example ORIGIN lgdsd. $TTL 86400 ; <name_of_this_server.> <your_e-mail_address.> @ IN SOA ns1.lgdsd. hostmaster.lgdsd. ( 2004073000 ; serial number 28800 ; refresh 8 hours 7200 ; retry 2 hour 604800 ; expire 7 days 86400 ; def. ttl 1 day ) ; <Primary DNS> IN NS ns1.lgdsd. ; <Secondary DNS> IN NS ns2.lgdsd. ; Aliases www IN CNAME lgdsd. mrtg IN CNAME ns2.lgdsd. xserve IN CNAME ns1.lgdsd. viruswall IN CNAME ns2.lgdsd.

;Fixed IPs lgdsd. IN A 10.10.6.8 ; Mac www server mail IN A 10.10.6.2 ; Novell GroupWise POA ns1 IN A 10.10.7.2 ; MAC OSX Server ns2 IN A 10.10.6.47 ; Linux Redhat 8.0 MRTG Server opaclhs IN A 10.10.32.2 ; LaGrande High School Follett opaclms IN A 10.10.16.2 ; Middle School Follett opacce IN A 10.10.32.2 ;Central Elementary Follett opacge IN A 10.10.64.3 ; Greenwood elementary Follett opacice IN A 10.10.80.3 ; Island City Follett opacwe IN A 10.10.48.3 ; Willow Elementary Follett iv IN A 10.10.96.3 ; Infinite Vision Server we4300 IN A 10.10.48.2 ; Willow Novell Server ice4300 IN A 10.10.80.2 ; Island City Novell Server ge4300 IN A 10.10.64.2 ; Greenwood Novell server do4200 IN A 10.10.96.2 ; DO Novell Server fs4400 IN A 10.10.6.5 ; Student File Server ce4300 IN A 10.10.6.4 ; Central Novell Server lms4300 IN A 10.10.7.5 ; LMS Novell Server lhs6300 IN A 10.10.6.7 ; LHS novell Server

Electronic mail

Originally designed to act like office memos Evolved to today’s sophisticated uses Automated responses

Email addresses

mailbox@computer User portion and mail system host Email addressing formats

Left up to sys admins

Email message format

ASCII text Header body

MIME

Multipurpose Internet Mail Extensions Original email system designed for text only To transfer binary data or graphics data

needed to be encoded, sent, decoded MIME is a set of standards for encoding data

allowing for new encodings to be invented at any time

MIME includes information so receiving app can decode message

Mail transfer

User email interface Transfer program

SMTP

Simple Mail Transfer Protocol TCP connection Runs on port 25 Server protocol

Mail Gateways

Email gateway or email relay Forwards email to all recipients of a list

POP

Post Office Protocol Client access

SMTP and POP links

SMTP http://www.faqs.org/rfcs/rfc821.html http://www.freesoft.org/CIE/Topics/94.htm

POP http://www.faqs.org/rfcs/rfc1939.html http://www.networksorcery.com/enp/protocol/

pop.htm