DNS (Domain Name System)

By
Shashidhara Vyakaranal & Shivaraj Kengond
School of Computing
KLE Technological University
Hubballi-580031
Karnataka, India
DNS (Domain Name System)
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Contents
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 Port/Protocol (DNS over UDP vs. TCP)
 Basic Flow (DNS hierarchy)
 Query types (Recursive vs. Iterative)
 Resource Records (A, SOA, MX, CNAME...)
 Caching (TTL)
 Zone file
 Zone Transfer: AXFR vs IXFR

Contents
3
 Different Zones: Stub/P/S
 Reverse Vs Forward Lookup
 DNS Forwarding
 Free Third party DNS providers (GoogleDNS,
OpenDNS)
 EDN0
 DNS tools on Windows/Unix (NSlookup, DIG, Hosts
file, DNS flush.)

What is DNS?
 The Domain Name System (DNS) is used to resolve
human-readable hostnames like www.google.com into
machine-readable IP addresses like 8.8.8.8 . DNS also
provides other information about domain names, such as
mail services
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Why is DNS important?
 DNS is like a phone book for the Internet. If we know a
person‘s name but don‘t know their telephone number, we
can simply look it up in a phone book. DNS provides this
same service to the Internet.
 When you visit http://www.google.com in a browser,
your computer uses DNS to retrieve the website‘s IP
address of 8.8.8.8. Without DNS, you would only be able
to visit the website (or any website) by visiting its IP
address directly, such as 8.8.8.8.
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How does DNS work?
 When you visit a domain such as google.com, your
computer follows a series of steps to turn the human-
readable web address into a machine-readable IP address.
This happens every time you use a domain name, whether
you are viewing websites, sending email
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Step 1: Request information
 The process begins when you ask your computer to
resolve a hostname, such as visiting google.com. The first
place your computer looks is its local DNS cache, which
stores information that your computer has recently
retrieved.
 If your computer doesn‘t already know the answer, it
needs to perform a DNS query to find out.
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Step 2: Ask the Recursive DNS Servers
 If the information is not stored locally, your computer
queries (contacts) your ISP‘s Recursive DNS servers.
 These specialized computers perform the legwork of a
DNS query on your behalf.
 Recursive servers have their own caches, so the process
usually ends here and the information is returned to the
user.
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Step 3: Ask the root Nameservers
 If the recursive servers don‘t have the answer, they query
the root nameservers.
 A nameserver is a computer that answers questions about
domain names, such as IP addresses.
 The twenty two root name servers act as a kind of
telephone switchboard for DNS.
 They don‘t know the answer, but they can direct our query
to someone that knows where to find it.
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Step 4: Ask the TLD nameservers
 The root nameservers will look at the first part of our
request, reading from right to left — www.google.com —
and direct our query to the Top-Level Domain (TLD)
nameservers for .com.
 Each TLD, such as .com, .org, and .org, have their own
set of nameservers, which act like a receptionist for each
TLD. These servers don‘t have the information we need,
but they can refer us directly to the servers that do have
the information.
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Step 5: Ask the Authoritative DNS servers
 The TLD nameservers review the next part of our request
— www.google.com — and direct our query to the
nameservers responsible for this specific domain.
 These Authoritative nameservers are responsible for
knowing all the information about a specific domain,
which are stored in DNS records.
 There are many types of records, which each contain a
different kind of information. In this example, we want to
know the IP address for www.google.com, so we ask the
authoritative nameserver for the Address Record (A).
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Step 6: Retrieve the record
 The recursive server retrieves the A record
for google.com from the authoritative nameservers and
stores the record in its local cache.
 If anyone else requests the host record for google.com, the
recursive servers will already have the answer and will not
need to go through the lookup process again.
 All records have a time-to-live value, which is like an
expiration date. After a while, the recursive server will need
to ask for a new copy of the record to make sure the
information doesn‘t become out-of-date.
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Step 7: Receive the Answer
 Armed with the answer, recursive server returns the A
record back to your computer.
 Your computer stores the record in its cache, reads the IP
address from the record, then passes this information to
your browser.
 The browser then opens a connection to the webserver and
receives the website.
 This entire process, from start to finish, takes only
milliseconds to complete.
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Cont‘d…
 Domain Name System (DNS) translates between domain
names and IP addresses, and is supported by nearly every
operating system.
 All Internet based name resolution utilizes DNS. DNS is
organized as a hierarchy. Consider the following
translation.
 Consider the following translation:
www.google.com = 8.8.8.8
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 The above domain name represents a Fully Qualified
Domain Name (FQDN):
 .com represents a top level domain.
 .google represents a secondary level domain
 www represents a host computer in the .google.com
domain
 Other top level domains include .org, .net, and .gov.
 Top level domains can also include country codes, such as
.in, .us, .ca, .cn, and .ru.
Cont‘d…
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DNS Record Types
There are a variety of DNS record types, including:
 NS (Name Server) – identifies a DNS server for the domain.
 SOA (Start of Authority) – identifies the primary (authoritative) DNS server for the
domain.
 A (Address) – identifies an individual host in the domain.
 CNAME (Canonical Name) – assigns an alias for another host name.
 MX (Mail Exchanger) - identifies a mail server in the domain.
 PTR (Pointer) - used for reverse DNS lookups.
The number defined in the MX record is a priority. A lower priority is more preferred.
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Nslookup
 Nslookup is a network administration tool for querying the
Domain Name System (DNS) to obtain domain name or IP
address mapping or any other specific DNS record.
 It is also used to troubleshoot DNS related problems.
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Cont‘d..
 nslookup can operate on both ―Interactive mode‖ and
―Non-Interactive mode‖.
 Interactive mode allows the user to query the DNS-
Server about various host, and domains.
 Non-Interactive mode allows the user to query the
information for a host or domain.
 All the commands explained are ―Non-Interactive
mode‖.
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 nslookup followed by the domain name will display the
―A Record‖ ( IP Address ) of the domain.
Server refers to the IP address of the DNS server. Then the
below section provides the ―A Record‖ ( IP Address ) of the
domain ―google.com‖
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1. Query the MX Record using -query=mx
 MX ( Mail Exchange ) record maps a domain name to a list
of mail exchange servers for that domain. The MX record
tells that all the mails sent to ―@google.com‖ should be
routed to the Mail server in that domain.
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In the above example, we have 2 MX records for the domain
―redhat.com‖. The number ( 5, 10 ), associated with the MX records tells
the preference of mail server. Lower the number, higher the preference.
So when a mail is sent to ―@redhat.com‖, first preference will be
―mx1.redhat.com‖, then ―mx2.redhat.com‖.21

Authoritative Answer vs. Non Authoritative Answer
 we may have also noticed the keyword ―Authoritative Answer‖ and
―Non-Authoritative Answer‖ in the above output.
 Any answer that originates from the DNS Server which has the
complete zone file information available for the domain is said to be
authoritative answer.
 In many cases, DNS servers will not have the complete zone file
information available for a given domain. Instead, it maintains a
cache file which has the results of all queries performed in the past
for which it has got authoritative response. When a DNS query is
given, it searches the cache file, and return the information available
as ―Non-Authoritative Answer‖.
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2. Query the NS Record using -query=ns
 NS ( Name Server ) record maps a domain name to a list of DNS
servers authoritative for that domain. It will output the name servers
which are associated with the given domain.
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3. Query the SOA Record using -query=soa
 SOA record (Start of Authority), provides the authoritative information about
the domain, the e-mail address of the domain admin, the domain serial
number, etc…
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Cont‘d..
 mail addr – specifies the mail address of the domain admin (
noc@redhat.com )
 serial – sort of revision numbering system. The standard convention
is to use ―YYYYMMYYNN‖ format. ( 2012-07-16. 01 will be
incremented, if more than one edit has taken place on a same day )
 refresh – specifies (in seconds), when the secondary DNS will poll
the primary to see if the serial number has been increased. If
increased, secondary will make a new request to copy the new zone
file.
 retry – specifies the interval to re-connect with the Primary DNS
 expire – specifies the time that the secondary DNS will keep the
cached zone file as valid
 minimum – specifies the time that the secondary DNS should cache
the zone file
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4. Reverse DNS lookup
 You can also do the reverse DNS look-up by
providing the IP Address as argument to nslookup.
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Host File
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 The computer file hosts is an operating system file that
maps hostnames to IP addresses. It is a plain text file.
Originally a file named HOSTS.txt.
 Assists in addressing network nodes in a computer network
 Serves the function of translating human-friendly hostnames
into numeric protocol addresses, called IP addresses, that
identify and locate a host in an IP network.

Applications
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 Redirecting local domains-accessing the company's
internal resources or to test local websites in development.
 Internet resource blocking-achieved by adding entries for
those sites to redirect requests to another address that does
not exist or to a harmless destination, such as the IP address
"0.0.0.0"

Dig
 Using dig command you can query DNS name servers for
your DNS lookup related tasks.
 When you pass a domain name to the dig command, by
default it displays the A record (the ip-address of the site
that is queried).
 In this example, it displays the A record of redhat.com in
the ―ANSWER SECTION‖ of the dig command output.
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The dig command output has the following sections:
 Header: This displays the dig command version number, the global options used by
the dig command, and few additional header information.
 QUESTION SECTION: This displays the question it asked the DNS. i.e This is
your input. Since we said ‗dig redhat.com‘, and the default type dig command uses
is A record, it indicates in this section that we asked for the A record of the
redhat.com website.
 ANSWER SECTION: This displays the answer it receives from the DNS. i.e This
is your output. This displays the A record of redhat.com.
 AUTHORITY SECTION: This displays the DNS name server that has the
authority to respond to this query. Basically this displays available name servers of
redhat.com.
 ADDITIONAL SECTION: This displays the ip address of the name servers listed
in the AUTHORITY SECTION.
 Stats section at the bottom displays few dig command statistics including how
much time it took to execute this query
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1. Query MX Records Using dig -t MX
 To query MX records, pass MX as an argument to the dig
command as shown below
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2.Query NS Records Using dig -t NS
 To query the NS record use the type NS as shown below
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3. View ALL DNS Records Types Using dig -t ANY
 To view all the record types (A, MX, NS, etc.), useANY as the
record type as shown below
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4. View Short Output Using dig +short
 To view just the ip-address of a web site (i.e the A record),
use the short form option as shown below
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Cont‘d..
 we can also specify a record type that you want to view with
the +short option.
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5. DNS Reverse Look-up Using dig -x
 To perform a DNS reverse look up using the ip-address using
dig -x as shown below
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Caching(TTL)
 TTL (Time to Live) is a setting for each DNS record that
specifies how long a resolver is supposed to cache (or
remember) the DNS query before the query expires and a new
one needs to be done.
 The benefits of caching are:
 It is a lot faster to check your local resolver‘s cache then
having to look up a DNS record that isn‘t already cached.
 This speeds up your Internet experience when visiting a site
which we go often (since less time is needed to complete DNS
lookups) and also helps lower the load on DNS servers around
the world
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Zone Files
 Zone files contain Resource Records that describe a domain or
sub-domain. Almost any sensible DNS software should be able
to read zone files. A zone file will consist of the following types
of data:
 Data that indicates the top of the zone and some of its general
properties (a SOA Record).
 Authoritative data for all nodes or hosts within the zone
(typically A (IPv4) or AAAA (IPv6) Records).
 Data that describes global information for the zone (including
mail MX Records and Name Server NS Records).
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Cont‘d..
 In the case of sub-domain delegation the name servers
responsible for this sub-domain (one or more NS
Records).
 In the case of sub-domain delegation one or
more glue records that allows a name server to reach
the sub-domain (typically one or more A or AAAA
Records) for the sub-domain name servers.
 The individual Resource Records are described and
numerous sample configuration files are provided and
documented.
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AXFR vs. IXFR
 AXFR is a mechanism for replicating DNS data across
DNS servers. If, for example, the bvb.edu administrator has
two DNS servers, a.ns.bvb.edu and b.ns.bvb.edu, he can
edit the bvb.edu data on a.ns.bvb.edu, and rely
on AXFR to pull the same data to b.ns.bvb.edu
 Incremental zone transfer, or IXFR for short, solves the
problem by allowing slave name servers to tell their master
servers which version of a zone they currently hold and to
request just the changes to the zone between that version
and the current one. This can reduce the size and duration of
a zone transfer dramatically.
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Cont’d..
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 When the master name server receives an incremental zone
transfer request, it looks for the record of the changes to
the zone between the slave's version of the zone and the
version the master holds.
 If that record is missing, the master sends a full zone
transfer. Otherwise, it sends just the differences between
the versions of the zone.

Different Zones: Primary/Secondary/Stub
 Primary Zone: The DNS server is the primary source for
information about this zone, and it stores the master copy of
zone data in a local file.
 Secondary Zone:
 The zone at this server must be obtained from another remote
DNS server computer that also hosts the zone.
 This DNS server must have network access to the remote DNS
server that supplies this server with updated information about
the zone.
 A secondary zone is merely a copy of a primary zone that is
hosted on another server
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Cont‘d..
 Stub: This DNS server is a source only for information
about the Authoritative Name Servers for this zone.
The zone at this server must be obtained from another
DNS server that hosts the zone.
This DNS server must have network access to the remote
DNS server to copy the authoritative name server
information about the zone.
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DNS Forwarding
 DNS forwarding is the process by which particular sets of
DNS queries are handled by a designated server, rather than
being handled by the initial server contacted by the client.
 All DNS servers that handle address resolution within the
network are configured to forward requests for addresses that
are outside the network to a dedicated forwarder.
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Cont‘d…
 Internal DNS information can be exposed on the open
Internet. It‘s far better to have a strict separation between
internal and external DNS. Exposing internal domains on
the open Internet creates a potential security and
privacy vulnerability.
 Without forwarding, all DNS servers will query external
DNS resolvers if they don‘t have the required addresses
cached. This can result in excessive network traffic
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Open DNS
 It‘s domain servers for you to use are 208.67.222.222, and
208.67.220.220.
 It has servers in Amsterdam, Chicago, Dallas, London, LA,
Miami, New York, Palo Alto, Seattle and Washington.
 OpenDNS is currently serving 35 Billion DNS requests per
day.
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Pros
48
 With servers all around the world it allows your DNS to resolve
faster, and if one goes down, another will work for you.
 It has phishing protection so you are protected from fake
websites.
 You can block websites individually or by one of the 40+
filters.Much more reliable then your ISP.
 If another OpenDNS user visited a website, but now its offline, if
you have smart cache enabled it will continue to resolve
(assuming it‘s a dns issue).
 Many high end company‘s, schools, and governments are using
the service daily.

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 Has plans (paid) for Enterprises.
 Its free basic service is only lacking faster customer support, and
larger website blocking features.
 Websites can be voted by the users with category‘s so that websites
can be blocked if they are 18+, have ad ware, are gaming, or are
high network load.
 Stats (Charts, requests, domains, blocked, etc).
 Has guides to help you set it up on your router or computer.
 Has an application to deal with dynamic IP‘s.
 Hasn‘t ever completely gone down in its 4 years.

Cons
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 Unless you purchase a plan if a url does not resolve you will
see a page like Google with ads.
 The website voting can be highly inaccurate.
 Its phishing service Phishtank which is run by its users can
take DAYS or even months to flag a url.

Google DNS
 It‘s domain servers are 8.8.8.8, and 8.8.4.4
 Is run on Google‘s servers.
 Extremely reliable
 Privacy is kept in tact
 Phishing protection
 Handles 100 billion DNS querys a day
 Set and go. No fuss no configuring.
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Pros
 It‘s free
 It‘s simple
 It‘s from Google
 It‘s reliable
Cons
 Business users may find it too simple (i.e. no domain
blocking)

Extension mechanisms for DNS (EDN0)
 Extension mechanisms for DNS (EDNS) is a specification for
expanding the size of several parameters of the Domain Name
System (DNS) protocol which had size restrictions that
the Internet Engineering community deemed too limited for
increasing functionality of the protocol.
 The restrictions in the size of several flags fields, return codes
and label types available in the basic DNS protocol prevented the
support of some desirable features.
 Moreover, DNS messages carried by UDP were restricted to 512
bytes, not considering the Internet Protocol (IP) and transport
layer headers
54

Cont‘d..
 Since no new flags could be added in the DNS header,
EDNS adds information to DNS messages in the form
of pseudo-resource-records included in the "additional
data" section of a DNS message.
55

Extra info…..
 The pseudo-RR EDNS introduces for this are of type OPT.
 As pseudo-RRs, OPT type RRs never appear in any zone file; they exist only in
messages, fabricated by the DNS participants.
 The mechanism is backward compatible, because older DNS responders ignore any
RR of the unknown OPT type in a request and a newer DNS responder never
includes an OPT in a response unless there was one in the request. The presence of
the OPT in the request signifies a newer requester that knows what to do with an
OPT in the response.
 The OPT pseudo-record provides space for up to 16 flags and it extends the space
for the response code. The overall size of the UDP packet and the version number
(at present 0) are contained in the OPT record. A variable length data field allows
further information to be registered in future versions of the protocol. The original
DNS protocol provided two label types, which are defined by the first two bits in
DNS packets (RFC 1035): 00 (standard label) and 11 (compressed label). EDNS
introduces the label type 01 as extended label. The lower 6 bits of the first byte may
be used to define up to 63 new extended labels.
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Extra info…..
 An example of an OPT pseudo-record, as displayed by
the Domain Information Groper (dig) utility tool:: OPT
PSEUDOSECTION: ; EDNS: version: 0, flags: do; udp:
4096.
 The result of "EDNS: version: 0" indicates full
conformance with EDNS0.The result "flags: do" indicates
that "DNSSEC OK" is set.
 EDNS is essential for the implementation of DNS Security
Extensions
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