Lecture

1
Lecture DNS

• How to find things

2
Domain Name System

• Associate human-friendly names with
  machine-friendly IP addresses
• Resolution of a given hostname to an IP address
• Domain Names, as opposed to IP addresses have the
  top-most element on the right
• Each element can be up to 63 characters long, the
  full name can be no more than 255 characters
• Letters, numbers or dashes can be used in a name
  element

3
DNS

• Allows machines to be grouped logically, by
  domain name
• Right-most element is called the (TLD) Top Level
  Domain
• The full name is referred to as the (FQDN) Fully
  Qualified Domain Name
• lugh.student.comp.dit.ie or lugh
• Internet Assigned Numbers Authority (IANA)
  controls the top-level domains
• Host names map to IP addresses in a one-to-many
  relationship, each machine may have many IP
  addresses, and each IP address may be associated
  with many machines

4
Mail Routing using DNS

• DNS builds in some application specific
  information
• Hosts that are designed to perform email routing,
  mail exchangers, have special-purpose records in
  DNS, MX records
• A domain should have multiple mail exchangers.
• Mail that cannot sent to one mail exchanger, can
  instead be delivered to an alternative server,
  providing a failsafe redundancy.

5
Before DNS

• Before DNS, name resolution was accomplished
  solely by text file databases residing on each
  host (hosts files)
• The method is not scalable, and it requires
  centralised management of the text files

6
Internet DNS Hierarchy

• Root Name Servers
• Provide references to the appropriate zone
  authoritative name servers for the top-level
  domains
• Zone-Authoritative name servers
• Master and slave servers for zones

7
Root Name Servers

• There are 13 root-name servers
• Each has an associated letter name (a to m)
• No more names can be used because of protocol
  limitations
• UDP packet can only carry 512 bytes reliably
• A hint file with more than 13 servers would be
  larger than 512 bytes
• C, F, I, J, K and M servers now exist in multiple
  locations on different continents

8
Root name servers
Letter Old name Operator Location
A ns.internic.net VeriSign Dulles, Virginia, USA
B ns1.isi.edu ISI Marina Del Rey, California, USA
C c.psi.net Cogent Communications distributed using anycast
D terp.umd.edu University of Maryland College Park, Maryland, USA
E ns.nasa.gov NASA Mountain View, California, USA
F ns.isc.org ISC distributed using anycast
G ns.nic.ddn.mil U.S. DoD NIC Columbus, Ohio, USA
H aos.arl.army.mil U.S. Army Research Lab Aberdeen Proving Ground, Maryland, USA
I nic.nordu.net Autonomica distributed using anycast
J   VeriSign distributed using anycast
K   RIPE NCC distributed using anycast
L   ICANN Los Angeles, California, USA
M   WIDE Project distributed using anycast
9
A simplified domain-map
(Root)
.ie domain
.com
.org
dit
Wicklow
Taranaki
Aisling
hermes
Wicklow.dit.ie.
10
Domains

• Dividing domains into sub-domains is important in
  several regards
• Division of a namespace into sub-domains in an
  hierarchical manner
• Removes the requirement that the names of
  individual hosts be unique
• but the FQDNs must still be unique
• It allows for the decentralised management of the
  entire namespace
• Up to 127 levels deep(!)

11
cs.dit.ie domain-map
(Root)
.ie domain
Wicklow.cs.dit.ie.
dit
cs
hermes
Taranki
MyLaptop
Wicklow
12
Zones, Domains and Delegation

• A Domain is a complete sub-tree of the
  hierarchical namespace
• A zone is part of the domain managed by a
  particular server
• Sub domains may be delegated into additional
  zones
• A zone may directly manage some sub domains
• A zone represents the scope of administration for
  which one body is responsible

13
cs.dit.ie zones (?)
(Root)
.ie domain
dit
cs
Taranki
hermes
Wicklow
MyLaptop
?
Wicklow.cs.dit.ie.
Aisling
14
Relationship between domains, zones and DNS
Servers

• The DNS database is effectively spread across all
  servers
• DNS Servers are delegated to manage particular
  zones and the links to the rest of the database.
• Zone is not necessarily equivalent to domain
• A DNS server can manage one or more zones

15
Comp.dit.ie dns-servers
(Root)
.ie domain
hermes.dit.ie.
dit
147.252.224.67
cs
hermes
Taranki
MyLaptop
Aisling
Wicklow
16
The DNS Server

• Server receives request from client
• If the server does not have the answer it will
  either ask a root server or it forwards the
  request to another name server
• This may happen a number of times until a name
  server is found that knows the answer
• When the server gets a response it will place a
  copy in its local cache and return a copy to the
  requesting client

17
Name Server Hierarchy

• Master Name Server
• Contains the master copy of data for the zone
• Slave Name Server
• Provides an automatic backup to the master name
  server
• All slave servers maintain synchronisation with
  their master name server
• Both Master and Slave servers contain
  authoritative data
• Zone may have multiple slaves but only one master
• Slave may get its data from another slave

18
Authoritative ?

• If the name server responding to a query is
  authoritative with respect to the query
  performed, the data returned is said to be
  authoritative
• Alternatively, responses may come from a name
  server which has cached the information, in which
  case the response is said to be
  non-authoritative
• The client may choose not to accept
  non-authoritative information

19
Resolver

• The DNS client is called the resolver
• Resolver capability is built into any program
  that needs it by way of the resolver library
  calls
• Resolver functions implemented in libresolv.so
• DNS Clients and servers communicate using UDP
  packets in most cases
• UDP is fast, but packets can be no larger than
  512 bytes
• If query or response is larger than 512bytes, it
  must be sent by TCP

20
Resolution Configuration Files

• /etc/host.conf mainly used to indicate which
  source of information is to be used and in what
  orderorder hosts,bind

21
Resolution Configuration Files

• /etc/resolv.conf is used to configure which
  servers are to be used and whether any domains
  are assumed for non qualified host namessearch
  cs.dit.ienameserver 147.252.224.70nameserver
  147.252.224.73nameserver 147.252.1.37

22
How did I find out the name servers?

• C\gtnslookup
• Default Server WL.domain.name
• Address 192.168.1.1
• gt set typens
• gt cs.dit.ie
• Server WL.domain.name
• Address 192.168.1.1
• Non-authoritative answer
• cs.dit.ie nameserver cara.comp.dit.ie
• gt microsoft.com
• Server WL.domain.name
• Address 192.168.1.1
• Non-authoritative answer
• microsoft.com nameserver ns2.msft.net
• microsoft.com nameserver ns3.msft.net
• microsoft.com nameserver ns5.msft.net
• microsoft.com nameserver ns1.msft.net
• microsoft.com nameserver ns4.msft.net
• gt

23
nslookup on wicklow

• rbradley_at_wicklow nslookup
• gt set typens
• gt cs.dit.ie
• Server 147.252.1.37
• Address 147.252.1.3753
• cs.dit.ie nameserver cara.comp.dit.ie.
• gt microsoft.com
• Server 147.252.1.37
• Address 147.252.1.3753
• Non-authoritative answer
• microsoft.com nameserver ns2.msft.net.
• microsoft.com nameserver ns3.msft.net.
• microsoft.com nameserver ns4.msft.net.
• microsoft.com nameserver ns5.msft.net.
• microsoft.com nameserver ns1.msft.net.
• Authoritative answers can be found from

24
Nslookup on my laptop

• C\gtnslookup wicklow
• Server WL.domain.name
• Address 192.168.1.1
• WL.domain.name can't find wicklow
  Non-existent domain
• C\gtnslookup wicklow.cs.dit.ie
• Server WL.domain.name
• Address 192.168.1.1
• Non-authoritative answer
• Name wicklow.cs.dit.ie
• Address 147.252.224.108
• C\gt

25
Deeper into nslookup

• rbradley_at_wicklow nslookup
• gt set typemx
• gt dit.ie
• Server 147.252.1.37
• Address 147.252.1.3753
• dit.ie mail exchanger 5 smtp.dit.ie.
• dit.ie mail exchanger 10 staffmail.dit.ie.
• dit.ie mail exchanger 15 mymail.dit.ie.
• gt cs.dit.ie
• Server 147.252.1.37
• Address 147.252.1.3753
• Can't find cs.dit.ie No answer
• gt

26
Deeper into nslookup

• gt set typea
• gt hermes.dit.ie
• Server 147.252.1.37
• Address 147.252.1.3753
• Name hermes.dit.ie
• Address 147.252.1.43
• 147.252.1.43
• Server 147.252.1.37
• Address 147.252.1.3753
• 43.1.252.147.in-addr.arpa name
  hermes.dit.ie.
• gt