The Domain Name System
Hostnames
• IP Addresses are great for computers
– IP address includes information used for
routing.
• IP addresses are tough for humans to
remember.
• IP addresses are impossible to guess.
– ever guessed at the name of a WWW site?
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The Domain Name System
• The domain name system is usually
used to translate a host name into an IP
address .
• Domain names comprise a hierarchy so
that names are unique, yet easy to
remember.
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DNS Hierarchy
edu
com
org
jp
rpi albany
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Host name structure
• Each host name is made up of a
sequence of labels separated by
periods.
– Each label can be up to 63 characters
– The total name can be at most 255
characters.
• Examples:
– whitehouse.gov
– barney.the.purple.dinosaur.com
– monica.cs.rpi.edu
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Domain Name
• The domain name for a host is the
sequence of labels that lead from the
host (leaf node in the naming tree) to
the top of the worldwide naming tree.
• A domain is a subtree of the worldwide
naming tree.
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Top level domains
• edu, gov, com, net, org, mil, …
• Countries each have a top level domain
(2 letter domain name).
• New top level domains include:
.aero .biz .coop .info .name .pro
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DNS Organization
• Distributed Database
– The organization that owns a domain name
is responsible for running a DNS server
that can provide the mapping between
hostnames within the domain to IP
addresses.
– So - some machine run by RPI is
responsible for everything within the
rpi.edu domain.
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DNS Distributed Database
• There is one primary server for a
domain, and typically a number of
secondary servers containing replicated
databases.
rpi.edu DNS server
rpi.edu
DNS DB
rpi.edu
rpi.edu
rpi.edu
DNS
DB
DNS
DNSDB
DB
Authoritative
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Replicas
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DNS Clients
• A DNS client is called a resolver.
• A call to gethostbyname()is handled by
a resolver (typically part of the client).
• Most Unix workstations have the file
/etc/resolv.conf that contains the
local domain and the addresses of DNS
servers for that domain.
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/etc/resolv.conf
domain rpi.edu
128.113.1.5
128.113.1.3
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nslookup
• nslookup is an interactive resolver that
allows the user to communicate directly
with a DNS server.
• nslookup is usually available on Unix
workstations.
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DNS Servers
• Servers handle requests for their
domain directly.
• Servers handle requests for other
domains by contacting remote DNS
server(s).
• Servers cache external mappings.
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Server - Server Communication
• If a server is asked to provide the
mapping for a host outside it’s domain
(and the mapping is not in the server
cache):
– The server finds a nameserver for the
target domain.
– The server asks the nameserver to provide
the host name to IP translation.
• To find the right nameserver, use DNS!
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DNS Data
• DNS databases contain more than just
hostname-to-address records:
– Name server records
– Hostname aliases
– Mail Exchangers
– Host Information
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NS
CNAME
MX
HINFO
15
The Root DNS Server
• The root server needs to know the
address of 1st (and many 2nd) level
domain nameservers.
edu
rpi
com
org
jp
albany
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Server Operation
• If a server has no clue about where to
find the address for a hostname, ask the
root server.
• The root server will tell you what
nameserver to contact.
• A request may get forwarded a few
times.
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DNS Message Format
HEADER
QUERIES
Response RESOURCE RECORDS
Response AUTHORITY RECORDS
Response ADDITIONAL INFORMATION
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Question Format
• Name: domain name (or IP address)
• Query type (A, NS, MX, …)
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Response Resource Record
•
•
•
•
•
•
Domain Name
Response type
Class (IP)
Time to live (in seconds)
Length of resource data
Resource data
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UDP & TCP
• Both UDP and TCP are used:
– TCP for transfers of entire database to
secondary servers (replication).
– UDP for lookups
– If more than 512 bytes in response requestor resubmits request using TCP.
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Name to Address Conversion
• There is a library of functions that act as
DNS client (resolver).
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DNS Data Flow
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Example suranga.com
This is named.conf in /etc/ folder
According to the following configuration I have
created a zone called suranga.com and in that
zone im3 is a host. Therefore the full name of
that host would be im3.suranga.com. like that
you can have any amount of PCs in your
domain. This will definitely works if you type
following according to your network.
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Example Network
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This is named.conf in /etc/
folder
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options {
directory "/var/named";
// query-source address * port 53;
};
zone "." IN {
type hint;
file "named.ca";
};
zone "localhost" IN {
type master;
file "localhost.zone";
allow-update { none; };
};
zone "0.0.127.in-addr.arpa" IN {
type master;
file "named.local";
allow-update { none; };
};
zone "suranga.com" IN {
type master;
file "suranga.com.zone";
allow-update { none; };
};
zone "200.168.192.in-addr.arpa" IN {
type master;
allow-update { none; };
};
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BIND Files
• Following files should be created in
/var/named/ folder we do not need to
customize the default file named.ca in the
/var/named/ folder.
• 1. 0.0.127.in-addr.arpa.zone
• 2. 200.168.192.in-addr.arpa.zone
• 3. suranga.com.zone
• 4. localhost.zone
• 5. named.local
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The contents of 0.0.127.inaddr.arpa.zone as follows
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$TTL 86400
@
IN
SOA localhost.
1 ; serial
28800 ; refresh
7200 ; retry
604800 ; expire
86400 ; ttk
)
@
IN
NS
localhost.
1
IN
PTR localhost.
root.localhost (
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200.168.192.in-addr.arpa.zone
file
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$TTL 86400
@
IN
SOA 192.168.200.8. root.localhost (
1 ; serial
28800 ; refresh
7200 ; retry
604800 ; expire
86400 ; ttk
)
@
IN
NS
localhost.
1
IN
PTR localhost.
2
IN
PTR im3.
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suranga.com.zone file
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$TTL 86400
@
IN
SOA @ root.suranga.com (
1 ; serial
28800 ; refresh
7200 ; retry
604800 ; expire
86400 ; ttl
)
IN
NS
suranga.com.
@
IN
A
192.168.200.8
im3 IN
A
192.168.200.250
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localhost.zone file
• $TTL 86400
• @
IN
SOA @ root.localhost (
•
1 ; serial
•
28800 ; refresh
•
7200 ; retry
•
604800 ; expire
•
86400 ; ttl
•
)
•
IN
NS
localhost.
• @
IN
A
127.0.0.1
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named.localhost file
• $TTL 86400
• @
IN
SOA localhost. root.localhost. (
•
1997022700 ; Serial
•
28800
; Refresh
•
14400
; Retry
•
3600000 ; Expire
•
86400 ) ; Minimum
•
IN
NS
localhost.
• 1
IN
PTR localhost.
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Results
• Now go to the prompt and type service named start and type
nslookup commands to verify your domain as follows.
• #nslookup
>suranga.com
• Server: 192.168.200.8
Address: 192.168.200.8#53
Name: suranga.com
Address: 192.168.200.8
• >im3.suranga.com
• Server: 192.168.200.8
Address: 192.168.200.8#53
Name: im3.suranga.com
Address: 192.168.200.250
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Configuration Files
• named configuration file (/etc/named.conf):
• It basically defines the parameters that point to the sources of
domain database information, which can be local files or on
remote servers.
• Hint file (cache file)(/var/named/named.ca):
• It actually provides the name of root server which gets activated
in case the machine name, which is to be searched, is not there
in user defined zone.
• localhost file (/var/named.local):
• All configuration have a local domain Database for resolving
address to the host name localhost.
• Zone:
• Basically a zone that keeps the information about the domain
database.
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More on zone files
• @: It means from the origin to the lastname object that is
suranga.com.
• IN: This stands for Internet servers
• SOA: This stands for `Start Of Authority’. It marks the beginning
of a zone’s data and defines the parameter that affects the
entire zone. Followed by the current machine name where the
DNS server is maintained.
• 20000011301;serial: This is the serial number--a numeric value
that tells or notifies the slave server, that the database has been
updated. So slave server should also update it.
• 3600;refresh: This is the refresh cycle in seconds. In every
refresh cycle the slave server comes to master server and
checks for the updated database.
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More on zone files
• 1800;retry: This particular line refers to the retry cycle which in
turn means that the slave server should wait before asking the
master server again in case master server doesn’t respond.
• 1209600;expire: This is the time for slave server to respond to
queries of client for the expiration time if master server fails and
has to be up and not getting up. After this period slave server
also fails to solve the queries of clients and sits idle.
• 432100;default_ttl: This refers to the default time to leave, for
this domain to work for, when named is once started.
Remember the user doesn’t have to play with this unless he
wants that the query time from the slave server should be
somewhat less or more. In case we want to change, we should
change only the refresh time in both master and slave. The best
way is to make it 2, which means after each 2 seconds slave
server will query to master server.
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