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Internetworking II. Organizational Communications and Technologies Prithvi Rao H. John Heinz III School of Public Policy and Management Carnegie Mellon University. Objectives. Understand how DNS works Present a DNS scenario. Naming Hosts. - PowerPoint PPT Presentation
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Internetworking II
Organizational Communications and Technologies
Prithvi RaoH. John Heinz III School of Public Policy and Management
Carnegie Mellon University
Objectives Understand how DNS works
Present a DNS scenario
Naming Hosts Nameserver is vehicle for mapping a name to
a network telnet akasha.tic.com vs telnet 192.135.128.129
Network object is passed to transport protocol interface
Naming evolved with other protocols
History of Naming Predecessor of Internet was ARPANET
Most important resource was IP address Used naming authority to assign IP addresses Most hosts had single network interfaces; hostname were
synonymous to interface
Central registry to maintained names and corresponding IP addresses
Administrator received a host and IP address for each new machine to be added to network
IP address known when network was established Name collision avoided by searching a host file
History of Naming Host files were copied to each machine
Unix systems consisted of /etc/hosts file
Operating systems supported lookup using library functions
gethostbyname() and gethostbyaddr() Worked well for small number of hosts (100s) Other operating systems used similar mechanisms but
basically the same Worked well because relatively few requests and table size
relatively small
History of Naming Exponential growth of the internet made static
host table impractical Load on servers hosting registry introduced delays in
access Names had to be unique to avoid name clashes
Solution to support growing internet was Domain Name System (DNS)
Domain Name System Internet’s official naming system
Distributed naming system Database is scattered across many hosts Maintained by many organizations (each has a small part)
Defines resource named and protocols used to communicate between nameservers that maintain the database
Domain Name System Delegation
Naming is delegated leaving central registry to register only naming authorities
Every host is not named by central authority
Dynamic Distribution Name lookup is dynamically distributed Site administrators did not have to copy host files
Redundancy Lookup algorithms were redundant; no single server Reliability was improved
Domain Name System Extensibility
Not necessarily restricted to IP addresses
Delegation Defines a name space that is a tree structure
Each node owned by single authority Child nodes can be created Each child node must have a unique name
Domain is any node and its descendant nodes Domain name uniquely indentifies single node within
domain Node names are written with separated period
Delegation
root
edu com org nz
co ac
…….
cmu
andrew
tic kiwilabs
unix5
akasha
Delegation Children of root are “top-level domains”
Domain name that traverses from node to root is called a Fully Qualified Domain Name (FQDN)
Always ends with a period cs.edu(.) Practically the period is dropped cs.edu Some applications (mail) do not permit the appending of a
period
Domain name traversing part of node is called a Relative Domain Name
Dynamic Distribution Descendants of a domain called subdomains
kiwilabs.com has authority for all names under kiwilabs.com Grant of authority is given when new subdomain is registered
Naming authority can assign subdomain names arbitrarily
Child node must be unique ux4.sp.cs.cmu.edu?
Hierarchy is broader than deeper
Extensibility Name gives resource a convenient reference;
name is mapped to resource
Can map DNS name to other resources DNS uses a typed resource record to identify resource
being named
<domain-name ttl IN resource_type resource_value)
domain_name is the FQDN for the resource that is key to identifying resource
Extensibility ttl is the time to live value
Time that the resource record can be cached before being discarded
Field is decremented every second and resource is discarded when ttl reaches zero
IN identifies resource as belonging to TCP/IP or INternet protocol
Extensibility resource_type is a unique identifier for type of
resource named During lookup resource_type is used to distinguish
between resource records mqpped to the domain name
resource_value is value of resource. Can be single value (IP address) or record with multiple values
DNS has standard set of resource record types
Resource Types IP addresses
domain_name A ip_addresses
Exampleticmac.tic.com A 192.135.128.131 and A is the record type corresponding to IP addresses
maps domain name ticmac.tic.com to 192.135.128.131
IP Address Multi-homed host or router has an A record for
each network interface
router.tic.com A 192.135.128.1
router.tic.com A 193.1.1.1
This illustrates mapping of name router.tic.com totwo IP addresses.
Machine has two interface cards
Host Information HINFO record indentifies and operating system
of host with given domain name
domain_name HINFO hardware os
Example
akasha.tic.com HINFO Sun SunOs
Alias Alias is CNAME record associating domain name
with another domain name
domain_name CNAME canonical_name
Example
mac.tic.com CNAME ticmac.tic.com says that namemac.tic.com is alias for ticmac.tic.com
DNS Operational Architecture
Server
Resolverlibrary
Application
Query or reply
Query or replyTo/from another server
query reply
function call function return
DNS Query Format
header
question
answer
authority
additional
DNS Operational Architecture question contains the target domain name and
the type and class of query Can match resource record type or be wildcarded to ask
for any resource
answer is completed by nameserver that replies to query
authority can name other authority that can answer query
DNS Operational Architecture additional completed by nameserver and
assists client with needed information
DNS Operational Steps Application sends DNS query to nameserver
and waits for response from resolver
Resolver generates query and and transmits it to nameserver and handles response and retransmits a query request
Examples of API for DNSgethostbyname() and gethostbyservice()
DNS Zones
root
edu com org nz
co ac
…….
cmu
andrew
tic kiwilabs
unix5
akasha
DNS Zones Each DNS zone has its own zone database
Primary name-server exists for each zone and maintains an up-to-date copy of zone database
Copies maintained in secondary nameservers (reliability)
DNS Scenario
1) Query from machine able.widget.com is sent to nameserveron ns.widget.com for the IP address for the domain namebaker.austin.tic.com: step 1
2) ns.widget.com has no cached resource records forbaker.austin.tic.com so the nameserver tries to find anNS record for the parent domain austin.tic.com
3) Finding no cached records for that domain it attempts tofind an NS record for the tic.com domain. It looks for thecom domain without success. It forwards original query toa root nameserver: step 2
DNS Scenario
4) Root nameserver repeats step 3 and finds an NS record for the com server and passes the query to that server
5) Nameserver for com domain once again repeats above algorithm and finds NS record and associated A record for the domain tic.com and returns information to nameserver on ns.widget.com: step 4
6) Information is cached on ns.widget.com (NS and A records) and sends original query to server for tic.com. Second server for that domain is contacted if timeout occurs: step 5
DNS Scenario
7) Server for tic.com receiving query forwards it to server for austin.tic.com domain: step 68) Destination server has answer desired by original node (baker.austin.tic.com) and returns answer to tic.com (7) which then sends answer to ns.widget.com (8) which in turn returns answer to able.widget.com (9) and this machine caches answer for later use
Query Example
ns.austin.tic.com
ns.widget.com rootserver
akasha.tic.com comserver
able.widget.com
1
9
2
6
7
8 5 4 3
Summary Presented a brief history of domains and host
naming
Examined the use of resource records
Presented DNS query example