CS335 Networking & Network Administration Wednesday, May 26, 2010

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 Aliases using CNAME Lets point to hobbes.foobar.comwww.foobar.com Allows companies to move WWW servers without changing names or addresses or lets one server answer to 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 Find out what you can about the ARPANET and how it originally resolved IP addresses

NSLOOKUP Use NSLOOKUP to find information on domain servers lookup.html lookup.html

Master DNS example ORIGIN lgdsd. $TTL IN SOA ns1.lgdsd. hostmaster.lgdsd. ( ; serial number ; refresh 8 hours 7200 ; retry 2 hour ; expire 7 days ; def. ttl 1 day ) ; IN NS ns1.lgdsd. ; 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 ; Mac www server mail IN A ; Novell GroupWise POA ns1 IN A ; MAC OSX Server ns2 IN A ; Linux Redhat 8.0 MRTG Server opaclhs IN A ; LaGrande High School Follett opaclms IN A ; Middle School Follett opacce IN A ;Central Elementary Follett opacge IN A ; Greenwood elementary Follett opacice IN A ; Island City Follett opacwe IN A ; Willow Elementary Follett iv IN A ; Infinite Vision Server we4300 IN A ; Willow Novell Server ice4300 IN A ; Island City Novell Server ge4300 IN A ; Greenwood Novell server do4200 IN A ; DO Novell Server fs4400 IN A ; Student File Server ce4300 IN A ; Central Novell Server lms4300 IN A ; LMS Novell Server lhs6300 IN A ; LHS novell Server

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

addresses User portion and mail system host addressing formats Left up to sys admins

message format ASCII text Header body

MIME Multipurpose Internet Mail Extensions Original 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 interface Transfer program

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

Mail Gateways gateway or relay Forwards to all recipients of a list

POP Post Office Protocol Client access

SMTP and POP links SMTP POP htm htm