1. Domain Name System IP assigns 32-bit addresses to hosts (interfaces)
Binary addresses easy for computers to manage
All applications use IP addresses through the TCP/IP protocol software
Difficult for humans to remember:
% telnet
The Domain Name System (DNS) provides translation between symbolic names and IP addresses
DNS serves listen on UDP port 53 and TCP port 53. For short requests and replies UDP is used. TCP is used for ZONE transfer.

2. Domain names and IP addresses
People prefer to use easy-to-remember names instead of IP addresses
Domain names are alphanumeric names for IP addresses e.g., neon.ece.utoronto.ca, ietf.org
The domain name system (DNS) is an Internet-wide distributed database that translates between domain names and IP addresses
How important is DNS?
Imagine what happens when the local DNS server is down.

3. Before there was DNS …. …. there was the HOSTS.TXT file
Before DNS (until 1985), the name-to-IP address was done by downloading a single file (hosts.txt) from a central server with FTP.
Names in hosts.txt are not structured.
The hosts.txt file still works on most operating systems. It can be used to define local names.

4. DNS Services Hostname to IP Address mapping
Inverse Mapping: IP Address to Hostname mapping
Mail Domain to Mail Server mapping; needed for mail delivery by mail servers
Host aliasing; Mail Domain aliasing
Canonical (real) and alias names
Load distribution
Replicate a heavily accessed web site (e.g. “ to multiple web servers (i.e. different hosts with different IP addresses)
Associate “ with the IP addresses of web servers

5. Structure of DNS names
Each name consists of a sequence of alphanumeric components separated by periods
Examples:
charcoal.eg.bucknell.edu
Names are hierarchical, with most-significant component on the right
Left-most component is computer name
Local name server, root name server, authoritative name server

6. Domain Name Tree
Root Server
b USC-ISI Marina del Rey, CA
l ICANN Marina del Rey, CA
e NASA Mt View, CA
f Internet Software C. Palo Alto, CA
i NORDUnet Stockholm
k RIPE London
m WIDE Tokyo
a NSI Herndon, VA
c PSInet Herndon, VA
d U Maryland College Park, MD
g DISA Vienna, VA
h ARL Aberdeen, MD
j NSI (TBD) Herndon, VA
13 root name servers worldwide, according to

7. Design principle of DNS
The naming system on which DNS is based is a hierarchical and logical tree structure called the domain namespace.
An organization obtains authority for parts of the name space, and can add additional layers of the hierarchy
Domain Names are assigned independent of geographic locations or IP addresses. However, in practice, allocation of domain names generally follows the allocation of IP address, e.g.,
All hosts with network prefix /16 have domain name suffix virginia.edu
All hosts on network /24 are in the Computer Science Department of the University of Virginia

8. Top-level domains Three types of top-level domains:
Organizational: 3-character code indicates the function of the organization
Used primarily within the US
Examples: gov, mil, edu, org, com, net
Geographical: 2-character country or region code
Examples: us, va, jp, de
Reverse domains: A special domain (in-addr.arpa) used for IP address-to-name mapping

9. gTLDs versus ccTLDs
gTLDS (generic TLDs) were originally meant to be US-centric. Nowadays, only mil and gov TLDs are for US.
ccTLDs (countrycode TLDs) used for organizations in other countries:
Countries define their own internal hierarchy: ac.uk and .edu.au are used for academic organizations in the United Kingdom and Australia. In SA, it is edu.sa.

10. DNS naming structure
Top level domains (rightmost components; also known as TLDs) defined by global authority
com Commercial organization
edu Educational institution
gov Government organization
mil Military organization
Organizations apply for names under a top-level domain:
kfupm.edu
macdonalds.com
Organizations determine their own internal structure
ccse.kfupm.edu
cs.purdue.edu

11. Domain names within an organization
Organizations can create any internal DNS hierarchy
Uniqueness of TLD and organization name guarantee uniqueness of any internal name (much like file names in your directories)
All but the left-most component of a domain name is called the domain for that name:
Authority for creating new subdomains is delegated to each domain
Administrator of kfupm.edu has authority to create ri.kfupm.edu and need not contact any central naming authority
DNS domains are logical concepts and need not correspond to physical location of organizations
DNS domain for an organization can span multiple networks

12. DNS and client-server computing
DNS names are managed by a hierarchy of DNS servers
There could be multiple ways of dividing up DNS name space
Root server at top of tree knows about next level servers.
Next level servers, in turn, know about lower level servers
Each DNS server is the authoritative server for the names it manages
What a server is responsible for or has authority over is called a zone. A domain can span multiple servers.
Primary server is also called authoritative server
Second server has a copy

13. Distributed, Hierarchical Database
Root DNS Servers
com DNS servers
ca DNS servers
edu DNS servers
poly.edu
DNS servers
umass.edu
yahoo.com
amazon.com
ucalgary.ca
TLD
Servers
usask.ca
Root servers and TLD servers typically do not contain hostname to IP mappings; they contain mappings for locating authoritative servers.

14. Root name servers
The root name servers know how to find the authoritative name servers for all top-level zones.
There are only 13 root name servers
Root servers are critical for the proper functioning of name resolution

15. Addresses of root servers
A.ROOT-SERVERS.EDU. (formerly NS.INTERNIC.NET)
A.ROOT-SERVERS.NET (formerly NS1.ISI.EDU)
B.ROOT-SERVERS.NET. (formerly C.PSI.NET)
C.ROOT-SERVERS.NET. (TERP.UMD.EDU)
D.ROOT-SERVERS.NET. (NS.NASA.GOV)
E.ROOT-SERVERS.NET. (NS.ISC.ORG)
F.ROOT-SERVERS.NET. (NS.NIC.DDN.MIL)
G.ROOT-SERVERS.NET. (AOS.ARL.ARMY.MIL)
H.ROOT-SERVERS.NET. (NIC.NORDU.NET)
I.ROOT-SERVERS.NET. (at NSI (InterNIC))
J.ROOT-SERVERS.NET. (operated by RIPE NCC)
K.ROOT-SERVERS.NET. (at ISI (IANA))
L.ROOT-SERVERS.NET. (operated by WIDE, Japan)
M.ROOT-SERVERS.NET

16. TLD and Authoritative Servers
Top-level domain (TLD) servers: responsible for com, org, net, edu, etc, and all top-level country domains like: uk, fr, ca, jp.
Network solutions maintains servers for COM TLD
Educause for EDU TLD
Authoritative DNS servers: organization’s DNS servers, providing authoritative hostname to IP mappings for organization’s servers (e.g., Web and mail).
Can be maintained by organization or service provider

17. Local Name Server
Each ISP (residential ISP, company, university) has one.
Also called “default name server”
When a host makes a DNS query, query is sent to its local DNS server
Acts as a proxy, forwards query into hierarchy.
Reduces lookup latency for commonly searched hostnames

18. Resolver and name server
An application program on a host accesses the domain system through a DNS client, called the resolver
Resolver contacts DNS server, called name server
DNS server returns IP address to resolver which passes the IP address to application
Reverse lookups are also possible, i.e., find the hostname given an IP address

19. DNS Name hierarchy DNS hierarchy can be represented by a tree
Root and top-level domains are administered by an Internet central name registration authority (ICANN)
Below top-level domain, administration of name space is delegated to organizations
Each organization can delegate further
Managed by UofT
Managed by ECE Dept.

20. Domain name system
Each node in the DNS tree represents a domain (or a domain name)
Each branch below a node is a DNS domain.
DNS domain can contain hosts or other domains (subdomains)
Example: DNS domains are . (root domain), edu, virginia.edu, cs.virginia.edu

21. Domain names
Hosts and DNS domains are named based on their position in the domain tree
Every node in the DNS domain tree can be identified by a unique Fully Qualified Domain Name (FQDN). The FQDN gives the position in the DNS tree.
A FQDN consists of labels (“cs”,“virginia”,”edu”) separated by a period (“.”)
There can be a period (“.”) at the end.
Each label can be up to 63 characters long
FQDN may contain letters, numerals, and dash character (“-”)
FQDNs are not case-sensitive

22. Hierarchy of name servers
The resolution of the hierarchical name space is done by a hierarchy of name servers
Each server is responsible (authoritative) for a contiguous portion of the DNS namespace, called a zone.
Zone is a part of the subtree
DNS server answers queries about hosts in its zone

23. Authority and delegation
Authority for the root domain is with the Internet Corporation for Assigned Numbers and Names (ICANN)
ICANN delegates to accredited registrars (for gTLDs) and countries for country code top level domains (ccTLDs)
Authority can be delegated further
Chain of delegation can be obtained by reading domain name from right to left.
Unit of delegation is a “zone”.

24. DNS domain and zones
Each zone is anchored at a specific domain node, but zones are not domains.
A DNS domain is a branch of the namespace
A zone is a portion of the DNS namespace generally stored in a file (It could consists of multiple nodes)
A server can divide part of its zone and delegate it to other servers

25. Primary and secondary name servers
For each zone, there must be a primary name server and a secondary name server
The primary server (master server) maintains a zone file which has information about the zone. Updates are made to the primary server
The secondary server copies data stored at the primary server.
Adding a host:
When a new host is added (“gold.cs.virginia.edu”) to a zone, the administrator adds the IP information on the host (IP address and name) to a configuration file on the primary server

26. Domain name resolution
User program issues a request for the IP address of a hostname
Local resolver formulates a DNS query to the name server of the host
Name server checks if it is authorized to answer the query.
If yes, it responds.
Otherwise, it will query other name servers, starting at the root tree
When the name server has the answer it sends it to the resolver.

27. DNS server architecture
All DNS servers are linked together to form a unified system. Each server keeps a set of NS (Name Server) Resource Records that identify root servers and servers for child domains.
Small organizations can use a single “primary” server
Easy to administer
Inexpensive
Large organizations often use multiple servers
Reliability through redundancy
Improved response time through load-sharing
Delegation of naming authority
Locality of reference applies - users will most often look up names of computers within same organization

28. Recursive and Iterative Queries
There are two types of queries:
Recursive queries
Iterative (non-recursive) queries
The type of query is determined by a bit in the DNS query
Recursive query: When the contacted Name Server cannot resolve a query, the server becomes a DNS client, issues a query, wait for reply and pass reply back to requestor
Iterative queries: When the contacted Name Server cannot resolve a query, it replies to the requestor with the IP address/name of another Name Server

29. authoritative name server
Name Resolution
root name server
A.ROOT-SERVERS.NET.
host surf.eurecom.fr wants IP address of
1. contacts its local DNS server, dns.eurecom.fr
2. dns.eurecom.fr contacts root name server, if necessary
3. root name server eventually contacts authoritative name server, dns.cs.umass.edu, if necessary
This is called “Recursive Resolution” 2 6 7 3
edu name server
A.GTLD-SERVERS.NET.
local name server
dns.eurecom.fr 4 5 1 8
authoritative name server
dns.umass.edu
requesting host
surf.eurecom.fr

30. authoritative name server
Types of Queries
root name server
recursive query:
puts burden of name resolution on contacted name server
heavy load?
iterated query:
contacted server replies with name of server to contact
“I don’t know this name, but ask the following server(s)”
Gives more control to client
iterated query 2 3 4
edu name server
A.GTLD-SERVERS.NET. 7
local name server
dns.eurecom.fr 5 6 1 8
authoritative name server
dns.cs.umass.edu
requesting host
surf.eurecom.fr
gaia.cs.umass.edu

31. DNS: Root Name Servers
contacted by local name servers that can not resolve query (mapping)
Root Name Server:
contacts authoritative name server if mapping not known
gets mapping
returns mapping to local name server

32. DNS Resource Records (RRs)
A DNS server stores DNS info. in a DNS database (act. a simple txt file):
A set of records known as Resource Records (RRs)
RR format: (class, name, type, value, TTL); class value set to “IN” for Internet
Type=A
name is hostname
value is IP address
Type=CNAME
name is alias name for some “canonical” (the real) name
is really
servereast.backup2.ibm.com
value is canonical name
Type=NS*
name is some domain name (e.g. foo.com)
value is hostname of authoritative name server for the domain
Type=MX*
name is a mail domain
value is hostname of mail server responsible for mail domain
*Note: IP addresses associated with the relevant hostnames are entered as A records

33. DNS Resource Records (RRs): continued
Type=SOA
Start of Authority Record. The SOA is the first record in entered in the zone’s file
name is domain name for this zone
value (MNAME, RNAME, SERIAL, REFRESH , RETRY, Min. TTL); the Serial field acts as a time stamp that is cahnged whenever the zone’s file is modified. This filed is regularly checked by secondary to tell if a zone transfer is needed.
Type= PTR
used to resolve IP-address to hostname queries
name is IP address (e.g in-addr.arpa.)
value is hostname

34. DNS protocol, messages
DNS protocol : query and reply messages, both with same message format
msg header
identification: 16 bit # for query, reply to query uses same #
flags:
query or reply
recursion desired
recursion available
reply is authoritative

35. DNS protocol, messages Name, type fields for a query RRs in response
to query
NS records for
authoritative servers
additional “helpful”
info that may be used

36. Becoming Part of DNS Example: just created startup “Network Utopia”
Register name networkutopia.com at a Registrar (e.g., Network Solutions)
Need to provide Registrar with names and IP addresses of your authoritative name servers (primary and secondary)
Registrar inserts two RRs into the COM TLD server:
(networkutopia.com, NS, dns1.networkutopia.com)
(dns1.networkutopia.com, A, )
Network Utopia adds it own RRs as part of the configuration of its DNS server (dns1.networkutopia.com), for example:
Type A record for
Type MX record for networkutopia.com

37. Measures to Improve Performance of DNS
DNS resolution can be very inefficient
Every host referenced by name triggers a DNS request
Every DNS request for the address of a host in a different organization goes through the root server
Caching: Name Servers and hosts use caching of DNS replies to reduce the need to issue further DNS requests
Cache is a list of recently resolved names and IP addresses
A DNS reply (i.e. RR) is kept in Cache for no more than the time set by the TTL (time-to-live) field
Replication of Name Servers
There are 13 root name servers
Every zone maintains at least two name servers (primary and secondary)

38. (DDNS) Dynamic Domain name System
When a new node is added/deleted authoritative DNS server is updated accordingly.
When a new domain is added, update message is sent to upper servers
Servers query other servers periodically for latest changes
update/notify mechanisms under design by IETF
RFC 2136