ITEC350 Networks I Lecture 3

DNS (Domain Name Service)

Address “translation” Domain Name Service (DNS)  Given a Domain Name (e.g., yahoo.com), lookup the IP address.  Command nslookup returns: DNS Server name & IP addr IP address(es) of the domain Microsoft Windows 2000 [Version ] (C) Copyright Microsoft Corp. H:\>nslookup yahoo.com Server: newriver.radford.edu Address: Non-authoritative answer: Name: yahoo.com Addresses: ,

U.S. Top Level Domains

Address Resolution Early Internet had no Domain Name System – Just HOSTS file  Win2K: C:\WINNT\system32\drivers\etc\HOSTS  Unix\Linux: /etc/hosts Ancient History: Before DNS,  The master HOSTS file was maintained by SRI International  Periodically, every computer in the internet reloaded HOSTS file ~1984 someone realized that millions of computers and domains needed a central database – DNS was born Still, when a host needs to translate yahoo.com,  First, HOSTS file is scanned  Then DNS is used.

Domain Name Service (DNS) Developed by Postel & Mockapetris is a good site to browse The phone book of the internet  But more restrictive … Each entry must be Unique Authentic  Universal Resolvability is ensured  Distributed Database

Figure 1.27: Domain Name System (DNS) DNS Request Message “The host name is Voyager.cba.hawaii.edu” Originating Host DNS Server

Figure 1.27: Domain Name System (DNS) DNS Table Host Name … Voyager.cba.hawaii.edu … IP Address … … DNS Response Message “The IP address is ” Originating Host DNS Server

DNS 13 Identical Root Servers  All Top Level Domain (TLD) Registries  Database for each TLD.com.org.edu.biz …. (gTLDs).fr.ca etc. country-specific TLDs, or ccTLDs Root Servers are  Authoritative  Maintained by ICANN, (International Corporation for Assigned Names and Numbers)

DNS Why does Radford have a DNS server called newriver.radford.edu ?  The 13 authoritative servers could not handle the load of billions of name resolutions  Faster to have a “non-authoritative” local server  If there is a domain name that newriver.radford.edu does not have Newriver can contact one of the authoritative root servers

DNS Names Hierarchical, right to left  nth level ……. Top Level Domain (Label)  yadda.yadda.yadda.yadda……….edu TLD, or Label may be up to 63 chars long Total length of name must be <= 255 chars Total length <= 127 labels DNS Names are either  Relative (newriver)  Fully qualified (newriver.radford.edu, an actual host or server)

Sidebar on Domain Names in USA ISO 3166 is a standard that defines a unique TLD for each country (i.e.,.fr = France) Why don’t we see radford.edu.us?  Because the USA, alone among all other countries, decided not to bother using the ISO’s TLD scheme Other variations adopted by some countries include “sub-domains” e.g.:  In the UK, ac.uk, co.uk, and sch.uk are reserved for academic, company and schools  In Australia, a blend of the US and ISO is used:.edu.au,.com.au

The root zone, AKA DNS Zones: Any collection of hosts.edu radford.edu Network lab in DA214.

DNS Servers Each DNS zone has  A domain name  At least a primary server  Probably a secondary server as well A computer that maintains a single master list of DNS Names and IP Addresses for a zone  Has Authority for that Zone  Is known as the primary server for the zone

Application (Host Process) Interfaces Given a domain Name there are several methods of resolution  Host table lookup (on Unix, /etc/hosts is a text file )  Local name server process (on Unix, named)  Send msg to a DNS primary or secondary server

NSlookup – DNS Translation Manual Page for nslookup can be found at It is for Unix shell, but most operation is same for WIN2K or up. The command nslookup allows DNS translation to a DOS cmd window.

H:\>nslookup Default Server: newriver.radford.edu Address: > ? Commands: (identifiers are shown in uppercase, [] means optional) NAME - print info about the host/domain NAME using default server NAME1 NAME2 - as above, but use NAME2 as server help or ? - print info on common commands set OPTION - set an option all - print options, current server and host [no]debug - print debugging information [no]d2 - print exhaustive debugging information [no]defname - append domain name to each query [no]recurse - ask for recursive answer to query [no]search - use domain search list [no]vc - always use a virtual circuit domain=NAME - set default domain name to NAME srchlist=N1[/N2/.../N6] - set domain to N1 and search list to N1,N2, etc. root=NAME - set root server to NAME retry=X - set number of retries to X timeout=X - set initial time-out interval to X seconds type=X - set query type (ex. A,ANY,CNAME,MX,NS,PTR,SOA,SRV) querytype=X - same as type class=X - set query class (ex. IN (Internet), ANY) [no]msxfr - use MS fast zone transfer ixfrver=X - current version to use in IXFR transfer request server NAME - set default server to NAME, using current default server lserver NAME - set default server to NAME, using initial server finger [USER] - finger the optional NAME at the current default host root - set current default server to the root ls [opt] DOMAIN [> FILE] - list addresses in DOMAIN (optional: output to FILE) -a - list canonical names and aliases -d - list all records -t TYPE - list records of the given type (e.g. A,CNAME,MX,NS,PTR etc.) view FILE - sort an 'ls' output file and view it with pg exit - exit the program

(Example) Nslookup Server newriver.radford.edu  Not authoritative for yahoo.com  Authoritative for neelix Relative form used C:\>nslookup yahoo.com Server: newriver.radford.edu Address: Non-authoritative answer: Name: yahoo.com Addresses: , C:\>nslookup neelix Server: newriver.radford.edu Address: Name: neelix.RADFORD.EDU Addresses: ,

DNS records Domain Name Servers get their information from a database maintained by the domain administrator. A client sends a message to the DNS using the UDP protocol. A server has different types of entries

DNS Query types A, CNAME, NS, MX, PTR, SOA, TXT  Each query type returns slightly different subset of the total information. Following slides were the output from  Using radford.edu as the domain  Each different query type selected in turn

DNS Resource Records (RR) Start of Authority (SOA) – denotes the primary DNS and time limits. Address (A) – supplies a host name's IP address Canonical Name (CNAME) – provides alias host names Mail Exchanger (MX) – defines a domain's mail systems Name Server (NS) – defines a domain's name servers

RR example acme.com. IN SOA origin = acme.com. mail addr = jef.acme.com. serial = (Date format) refresh = (3 Hours) retry = 1200 (20 Minutes) expire = (1 Week) minimum = 3600 (1 Hours) acme.com. IN NS dns.acme.com. acme.com. IN MX 20 mail.acme.com. dns.acme.com. IN A ftp.acme.com. IN CNAME

RR example acme.com. IN SOA dns.acme.com. dnsowner.acme.com. ( ; serial # (date format) ; refresh (3 hours) 3600 ; retry (1 hour) ; expire (1 week) 86400) ; TTL (1 day) acme.com. IN NS dns.acme.com. IN NS ns1.isp.net. acme.com. IN MX 20 mail.acme.com. IN MX 40 mail.isp.com. dns.acme.com. IN A mail.acme.com. IN A IN A ftp.acme.com. IN CNAME pc.acme.com. IN A

All DNS records for Radford.edu Server: www-coastland-49.highertech.net Address: Non-authoritative answer: radford.edu nameserver = GNS1.NOMINUM.COM radford.edu nameserver = GNS2.NOMINUM.COM radford.edu nameserver = RUACAD.radford.edu radford.edu nameserver = RUSERVE.radford.edu radford.edu nameserver = WEBWORK.radford.edu radford.edu origin = RUSERVE.radford.edu mail addr = bbuskill.radford.edu serial = refresh = (3H) retry = 3600 (1H) expire = (1W) minimum = 900 (15M) Authoritative answers can be found from: radford.edu nameserver = GNS1.NOMINUM.COM radford.edu nameserver = GNS2.NOMINUM.COM radford.edu nameserver = RUACAD.radford.edu radford.edu nameserver = RUSERVE.radford.edu radford.edu nameserver = WEBWORK.radford.edu GNS1.NOMINUM.COM internet address = GNS2.NOMINUM.COM internet address = RUACAD.radford.edu internet address = RUSERVE.radford.edu internet address = WEBWORK.radford.edu internet address =

DNS uses two request flavors A recursive request will respond with the answer or an error message if the host is not known. This is the type of request made by a client when the user program executes a “gethostbyname” function. An Interactive request will respond with the answer or the name of a DNS that may be able to answer the question. This type of request is usually used between Domain Name Servers.

DNS Search Example. (ROOT) Newriver.radford.edu Cist_hp_da.radford. edu.COM DNS zone Server DNS.yahoo.com yahoo.com Cist_hp_da.radford.edu needs the address of a web server on yahoo.com Radford University Yahoo One of the 13 root servers Each blue oval is a network operated by a distinct organization. (ISP networks not shown)

Second DNS Search Example. (ROOT) Newriver.radford.edu Cist_da_hp.radford. edu.COM DNS DNS.yahoo.com ftp.yahoo.com cist_da_hp.radford.edu needs the address of ftp.yahoo.com after finding yahoo.com

.arpa An explicit way to signal for reverse translation Reverse lookups