DNS Principles and Operations - Lecture 01

Mgr. Ondřej Surý, University of Ostrava
7DNSK: DNS Principles and Operations
Lecture 01
Domain Name System

Contents
• Dom
a
in n
a
mes
a
nd their roles
• Dom
a
in n
a
mes
a
nd d
a
t
a
• Dom
a
in n
a
mes
a
nd DNS
• Dom
a
in n
a
me tree
• DNS Records
a
nd their
components
• A, AAAA
a
nd TXT Types
• Dom
a
in N
a
me Summ
a
ry
• About the present
a
tion

Domain Names
• Dom
a
in n
a
mes
a
re
a
n obvious p
a
rt of the Internet. But wh
a
t role do
they pl
a
y?
• How do dom
a
in n
a
mes work?
is-stag.osu.cz
www.kagi.com
vlada.gov.cz
www.ietf.org
wanda.osu.cz
www.kernel.org
www.apple.com

Domain name as address
• As Internet users, we know th
a
t services c
a
n be found using
dom
a
ins.
• Are dom
a
ins
a
type of
a
ddress? – Yes, th
a
t is p
a
rt of the
a
nswer.

Communication on the Internet
• Internet communic
a
tion is b
a
sed on IP.
• The IP l
a
yer uses IP
a
ddresses for communic
a
tion.
• Routing directs p
a
ckets b
a
sed on IP
a
ddresses.
At the IP l
a
yer, there is no room for n
a
mes; only the IP
a
ddresses exists
there.

Internet Addressing
• At the IP level, the IP
a
ddress is used for
a
ddressing.
• How does the dom
a
in n
a
me come into pl
a
y?

Abstraction
• One c
a
n view the dom
a
in n
a
me
a
s
a
tr
a
nsl
a
tion of the IP
a
ddress.
• We begin with the dom
a
in n
a
me, which
a
cts
a
s
a
n
a
ddress,
a
nd
obt
a
in the IP
a
ddress necess
a
ry for communic
a
tion over TCP/IP.

Domain name instead of IP address
• Dom
a
in n
a
mes
a
re e
a
sier to remember th
a
n IP
a
ddresses.
• Dom
a
in n
a
mes c
a
n be words th
a
t h
a
ve me
a
ning or th
a
t we c
a
n
a
ssoci
a
te with.
• We c
a
n choose
a
suit
a
ble dom
a
in n
a
me.
• We often c
a
nnot do
a
nything
a
bout the IP
a
ddress.
• An IP
a
ddress is
a
string of numbers th
a
t is often di
ff
icult to remember.
• IPv6 (e.g., 2001:6b0:1:11c2::82ed:1c28) is worse th
a
n IPv4 (e.g.,
130.237.28.40)

Domain name as identi
fi
er
• For most people, it is e
a
sier to recognize dom
a
in n
a
mes th
a
n IP
a
ddresses.
• One c
a
n h
a
ve simil
a
r dom
a
in n
a
mes for di
ff
erent services to show
th
a
t they
a
re rel
a
ted.
www.nic.cz. – 217.31.205.50
m
a
il.nic.cz. – 217.31.204.67

Domain name for grouping
• Network devices (computers, routers, etc.) c
a
n be grouped under
a
prim
a
ry dom
a
in n
a
me, for ex
a
mple:
prf.osu.cz
• The grouped devices c
a
n be on completely di
ff
erent IP networks, for
both IPv4
a
nd IPv6.

Domain name to denote service
• The n
a
me c
a
n be used to indic
a
te the type of service.
www.x.y → prob
a
bly
a
web service (http)
m
a
il.x.y or mx.x.y → prob
a
bly
a
m
a
il server
ns.x.y or dns.x.y → prob
a
bly
a
DNS server
• However, one does not need to follow the convention
a
bove, but c
a
n
inste
a
d
f
ind one's own system.

Placing information in the domain name
We c
a
n use the n
a
me to tell other things. For ex
a
mple:
• Loc
a
tion. All routers in Pr
a
gue
a
re n
a
med "prg"
a
nd those in Ostr
a
v
a
a
re n
a
med "ov
a
", e.g., r2.prg.freef
a
rm.x
a
a
nd r1.ov
a
.freef
a
rm.x
a
respectively.
• Role. All servers used for production
a
re n
a
med "prod"
a
nd those for
pre-production veri
f
ic
a
tion
a
re n
a
med "st
a
ge", e.g.,
www.prod.freef
a
rm.x
a
a
nd www.st
a
ge.freef
a
rm.x
a
respectively.
• OS. All l
a
ptops with Windows
a
re n
a
med "win"
a
nd those with m
a
cOS
a
re n
a
med "m
a
c", e.g., 15.win.l
a
ptop.freef
a
rm.x
a
a
nd
23.m
a
c.l
a
ptop.freef
a
rm.x
a
respectively.

A domain name is more than just an address
When we s
a
y
a
ddress, we might think of something like ”Br
á
fov
a
7,
Pr
a
h
a
” or ”708 01 Ostr
a
v
a
”
• Dom
a
in n
a
mes
a
re more th
a
n th
a
t. Dom
a
in n
a
mes h
a
ve become the
n
a
me of
a
product, service, or comp
a
ny.

Domain name as a trademark
The dom
a
in n
a
me is
a
tr
a
dem
a
rk
• Dom
a
in n
a
mes
a
re registered for existing tr
a
dem
a
rks.
• New comp
a
nies look for
a
n
a
v
a
il
a
ble dom
a
in n
a
me before deciding
on
a
comp
a
ny n
a
me.
• Dom
a
in n
a
mes le
a
d to tr
a
dem
a
rk disputes.
• Most top-level dom
a
ins, e.g., .se, h
a
ve rules for how to h
a
ndle "b
a
d
f
a
ith registr
a
tions" th
a
t con
f
lict with existing tr
a
dem
a
rks.
• Good dom
a
in n
a
mes
a
re bought
a
nd sold for l
a
rge sums.

The domain name can signal a
ffi
liation
• The choice of top-level dom
a
in sign
a
ls
a
ff
ili
a
tion:
• .cz → Czech
• .fr → French
• .eu → Europe
a
n Union
• .com → intern
a
tion
a
l comp
a
ny
• .mil → belongs to the U.S. milit
a
ry
• .c
a
t → C
a
t
a
l
a
n!
• There
a
re m
a
ny, m
a
ny top-level dom
a
ins to choose from, ne
a
rly 2000.

Not just to identify the IP address
• The dom
a
in p
a
rt of
a
n em
a
il
a
ddress indic
a
tes the dom
a
in
a
ff
ili
a
tion.
• M
a
il is routed b
a
sed on the dom
a
in p
a
rt, not the IP
a
ddress.
ondrej.sury@ osu.cz
Domain part
that can be
looked up in
DNS

• IP
a
ddresses c
a
n be entered into DNS to identify the n
a
me th
a
t
points to the
a
ddress.
78.128.128.25 → w
a
nd
a
.osu.cz.

The domain name as an anchor for data
• It does not h
a
ve to be
a
n IP
a
ddress. It c
a
n be other technic
a
l d
a
t
a
.
• For ex
a
mple, v
a
rious kinds of cryptogr
a
phic keys c
a
n be pl
a
ced in
DNS.
• Or con
f
igur
a
tion d
a
t
a
.
• DNS c
a
n be used to distribute d
a
t
a
in
a
system
a
tic w
a
y.

• It is possible to insert text strings:
• For textu
a
l inform
a
tion
• Used by systems
a
nd progr
a
ms when con
f
igur
a
tion d
a
t
a
c
a
nnot
be
a
ccommod
a
ted otherwise.
• A "token" is inserted to demonstr
a
te control over the dom
a
in.

Roles of the domain name
1. Addressing
2. Abstr
a
ction of IP
a
ddress
3. Anchor for other technic
a
l d
a
t
a
4. Recognition
5. Br
a
nd
• This course will focus on the technic
a
l
a
spects of the dom
a
in n
a
me, points
1-3.
• However, we will keep points 4-5 in mind,
a
s they
a
re wh
a
t m
a
ke the choice of
a
dom
a
in n
a
me import
a
nt for the Internet
a
nd its users.

DNS
• DNS = "Dom
a
in N
a
me System”
• Dom
a
in N
a
me → “N
a
me” or “Dom
a
in”
• DNS is the function – protocol – st
a
nd
a
rd th
a
t m
a
n
a
ges Internet
dom
a
ins or dom
a
in n
a
mes.

Primary task
• The prim
a
ry function of DNS:
n
a
me (dom
a
in n
a
me) → IP
a
ddress
www.osu.cz → 78.128.128.25, 2001:718:1005:620::25

DNS
Answer:
78.128.128.25
2001:718:1005:620::25
Question:
What IP addresses does
www.osu.cz have?

DNS – facilitates address changes
• The
a
ddress c
a
n be ch
a
nged (ch
a
nge of
a
ddress) without ch
a
nging
the n
a
me.

192.0.2.2
192.0.2.213
www.example.com

DNS – both IPv4 and IPv6
• The n
a
me c
a
n point to both IPv4
a
nd IPv6 (or just one of them).

2001:718:1005:620::25
78.128.128.25
www.osu.cz

DNS – redundancy or load balancing
• The n
a
me c
a
n point to two or more servers (two or more IPv4
a
ddresses or likewise IPv6
a
ddresses).
• Why would one w
a
nt this?
• Redund
a
ncy with f
a
ilover
• Lo
a
d b
a
l
a
ncing

www.osu.cz
Tra+fic goes to both servers
• The client selects the server
• When both are up, double the
load is handled
2001:718:1005:620::25
78.128.128.25
2001:718:1005:620::26
78.128.128.26

www.osu.cz
All tra.fic goes to server
78.128.128.25
• A client that initially selects
78.128.128.26 will switch to
78.128.128.25 after a while
2001:718:1005:620::25
78.128.128.25
2001:718:1005:620::26
78.128.128.26

Domain Name Tree
How
a
re the di
ff
erent dom
a
in n
a
mes rel
a
ted?
• Di
ff
erent n
a
mes h
a
ve the s
a
me "ending", e.g., "cz".
• Some n
a
mes sh
a
re
a
longer "ending", e.g., "osu.cz".
is-stag.osu.cz
www.kagi.com
vlada.gov.cz
www.ietf.org
wanda.osu.cz
www.kernel.org
www.apple.com

Domain Name Tree
• Together, the dom
a
in n
a
mes form
a
hier
a
rchic
a
l n
a
me tree.

cz
(Root)
1. Starts with an unnamed node called the "root"
2. All other nodes in the tree have a name ("label")
3. The tree can branch downwards at the nodes
4. Strictly hierarchical (one path up)
5. Data can be placed in each node but not outside
the nodes
com
sk
net
aero
prg
airport-ostrava
osu
www
nic
www
prf
kip
ripe
wanda
ibm
telia
telia
www
www www
nix
nix
www

cz
(Root)
com
sk
net
aero
prg
airport-ostrava
osu
www
nic
www
prf
kip
ripe
wanda
ibm
telia
telia
www
www www
nix
nix
www
1
2
3
4
5
1 2 3 4 5
kip . prf . osu . cz .
Empty
l
a
bel

Unique node names at the same level
• Only one node "
a
pple" under "cz" – the node n
a
me must be unique
a
t
the s
a
me level.
• The s
a
me node n
a
me, e.g., "
a
pple", in sever
a
l di
ff
erent pl
a
ces – under
"cz", "com",
a
nd "net".
• The node n
a
me "www" exists (
a
lmost) everywhere, but in di
ff
erent
loc
a
tions.

Domain Name Structure
Ex
a
mple ”www.osu.se” (or ”www.osu.se.”)
This ex
a
mple consists of four p
a
rts (l
a
bels) where the l
a
st (topmost) p
a
rt is invisible
(
a
n empty string). E
a
ch p
a
rt (”l
a
bel”) corresponds to
a
node in the dom
a
in n
a
me tree
– it is the n
a
me of the node in the tree.
1. "www"
2. "osu"
3. "cz"
4. ""
The period ”.” m
a
rks the bound
a
ry between the p
a
rts (l
a
bels). Comp
a
re with the tree.

Domain Name Structure
www.osu.cz
• The le
a
st signi
f
ic
a
nt p
a
rt, ”www” in our ex
a
mple, is displ
a
yed
f
irst.
• The most signi
f
ic
a
nt p
a
rt is displ
a
yed l
a
st.
78.128.128.25
• In IPv4
a
ddresses, the most signi
f
ic
a
nt octet (p
a
rt) is shown
f
irst (78)
a
nd the le
a
st signi
f
ic
a
nt octet
is shown l
a
st (25).
2001:718:1005:620::25
• In the IPv6
a
ddress, the order is the s
a
me. 2001 is the most signi
f
ic
a
nt
a
nd 25 is the le
a
st
signi
f
ic
a
nt p
a
rt.
L
a
ter on, we will see th
a
t the di
ff
erence is signi
f
ic
a
nt.

Where is the Domain Name Tree?
• The Dom
a
in N
a
me Tree exists in the distributed d
a
t
a
b
a
se of DNS
d
a
t
a
th
a
t is sh
a
red
a
cross the Internet.
• There
a
re numerous n
a
me servers (DNS servers), e
a
ch holding
a
sm
a
ll portion of the dom
a
in n
a
me tree, but which together constitute
the entire dom
a
in n
a
me tree.

Where is the data?
• We c
a
n insert d
a
t
a
into e
a
ch n
a
me (node) in the dom
a
in n
a
me tree.
• In "www" under "osu" under "se", we insert the IP
a
ddresses for
University of Ostr
a
v
a
's web servers.
• No d
a
t
a
exists outside the nodes.

cz
(Root)
We will enter the IP addresses here:
78.128.128.25 and
2001:718:1005:620::25
com
sk
net
aero
prg
airport-ostrava
osu
www
nic
www
prf
kip
ripe
wanda
ibm
telia
telia
www
www www
nix
nix
www

Display format of the DNS data
• DNS d
a
t
a
is represented in di
ff
erent w
a
ys in di
ff
erent pl
a
ces. The
displ
a
y form
a
t is wh
a
t we norm
a
lly see, both in text
a
nd in d
a
t
a
f
iles
for n
a
me servers.
• It st
a
rts with the loc
a
tion (owner node) in the dom
a
in n
a
me tree
where the d
a
t
a
"resides".
w
a
nd
a
.osu.cz. 3600 IN A 78.128.128.25
w
a
nd
a
.osu.cz. 3600 IN AAAA 2001:718:1005:620::25
• Note the period
a
t the end of the dom
a
in n
a
me.

Where is the data?
• The d
a
t
a
is stored in the nodes
a
long with the dom
a
in n
a
me tree.
• If there
a
re no d
a
t
a
, then there is
a
lso no dom
a
in n
a
me tree.
• Without
a
dom
a
in n
a
me tree, we h
a
ve nowhere to put the d
a
t
a
.

The data entry forms the tree.

cz
(Root)
osu
wanda
The data entry forms the tree.
w
a
nd
a
.osu.cz. 3600 IN A 78.128.128.25
w
a
nd
a
.osu.cz. 3600 IN AAAA 2001:718:1005:620::25

DNS Data
• How is the DNS d
a
t
a
presented? E.g.
www.nic.cz. 1800 IN A 217.31.205.50
www.nic.cz. 1800 IN A 217.31.205.50

• "Owner n
a
me"
• The node (the n
a
me) in the DNS tree where the d
a
t
a
is loc
a
ted.
• Ends with
a
period "." which me
a
ns th
a
t it is rel
a
tive to the "root".
www.nic.cz. 1800 IN A 217.31.205.50

• "TTL", time-to-live.
• The number of seconds th
a
t we c
a
n s
a
ve
a
nd reuse the d
a
t
a
before we must retrieve it
a
g
a
in.
• Norm
a
lly between 60
a
nd 86,400 seconds (one d
a
y).
• C
a
n be
a
s low
a
s 0 seconds, i.e., use but do not s
a
ve.
• A 32-bit integer, i.e., up to 4,294,967,295 s ≈ 136 ye
a
rs; in pr
a
ctice,
a
m
a
ximum of 3-7 d
a
ys.
• C
a
n be omitted in m
a
ny contexts, but is import
a
nt in others.
www.nic.cz. 1800 IN A 217.31.205.50

• ”Cl
a
ss”.
• IN = Internet Cl
a
ss
• It is
a
lw
a
ys ”IN”; nothing else is used for Internet.
www.nic.cz. 1800 IN A 217.31.205.50

Class
• If the "cl
a
ss" is something other th
a
n IN, then it is not in the st
a
nd
a
rd
DNS tree.
• The BIND 9 (
a
nd other DNS server) uses the CH (Ch
a
os) cl
a
ss to
report,
a
mong other things, its softw
a
re version. Limited use.

• ”Resource Record Type”, record type
• A = ”Address” (IPv4
a
ddress)
• There
a
re de
f
ined record types with speci
f
ic
codes, ”A” in this c
a
se.
• E
a
ch record type determines wh
a
t d
a
t
a
c
a
n
a
nd
must be present
www.nic.cz. 1800 IN A 217.31.205.50

• ”Resource Record D
a
t
a
”, d
a
t
a
• C
a
n h
a
ve one or more sub-
f
ields (one sub-
f
ield in
this c
a
se)
• The sub-
f
ields
a
re determined by the record type.
For
a
n A record type, it must be
a
n IPv4
a
ddress.
www.nic.cz. 1800 IN A 217.31.205.50

DNS Data
www.nic.cz. 1800 IN A 217.31.205.50
• Often without TTL
a
nd ”Cl
a
ss”:
www.nic.cz. A 217.31.205.50
www.nic.cz. 1800 A 217.31.205.50
www.nic.cz. IN A 217.31.205.50

DNS Data
• TTL is relev
a
nt when we enter the DNS d
a
t
a
into
a
n
a
me server.
Otherwise, we c
a
n often omit the TTL when we
a
re just referencing
the d
a
t
a
.
• “Cl
a
ss” is only necess
a
ry when it is not IN, which me
a
ns
a
lmost never.
www.nic.cz. A 217.31.205.50

Relative names
• Rel
a
tive n
a
mes
• Sometimes, we c
a
n write DNS d
a
t
a
rel
a
tive to
a
speci
f
ic node in the
DNS tree. When referring to DNS d
a
t
a
within the nic.cz dom
a
in::
www A 217.31.205.50
• NOTE! No period "."
a
fter rel
a
tive n
a
mes.
Rel
a
tive n
a
mes should be
a
voided in direct DNS contexts except in the
con
f
igur
a
tion
f
ile (zone
f
ile) for the n
a
me server.

Absolute name
• If the
a
bsolute n
a
me is intended, there must be
a
period "."
a
t the end
of the n
a
me in direct DNS contexts.
• Correct:
www.nic.cz. A 217.31.205.50
• Incorrect (not
a
bsolute):
www.nic.cz A 217.31.205.50

Absolute or relative names
• Outside of direct DNS contexts,
a
bsolute n
a
mes
a
re most often used
without the tr
a
iling period.
• For ex
a
mple:
• em
a
il
a
ddress: ondrej@sury.org
• web URL: https://www.osu.cz/
• This
a
lso
a
pplies to m
a
ny progr
a
m con
f
igur
a
tions.

Relative names
• Sometimes rel
a
tive n
a
mes
a
re used.
• How "/etc/resolv.conf" works will be covered when we discuss the
loc
a
l resolver libr
a
ry.

Record Type A
• A = “Address”
• IPv4-
a
dress
www.nic.cz. A 217.31.205.50
• A sub
f
ield for
a
n A record.
• An IPv4
a
ddress c
a
n be written in v
a
rious w
a
ys depending on
the context, but here it must be form
a
tted
a
s four decim
a
l
octets sep
a
r
a
ted by periods. No period
a
t the end.

Record Type AAAA
• Like
a
n A record, but for
a
n IPv6
a
ddress
• A sub
f
ield in this c
a
se.
• With the AAAA record type, it must be
a
n IPv6
a
ddress.
• The IPv6
a
ddress must be written in st
a
nd
a
rd IPv6 form
a
t
(with or without
a
bbrevi
a
tion).
www.nic.cz. AAAA 2001:1488:0:3::2

Record Type AAAA
• Identic
a
l:
www.ietf.org. AAAA 2606:4700::6810:2d63
www.ietf.org. AAAA 2606:4700::6810:2D63
www.ietf.org. AAAA 2606:4700:0000:0000:0000:0000:6810:2d63
• Norm
a
lly, IPv6 is written in the
a
bbrevi
a
ted form
a
t, both in DNS
a
nd
in other contexts, but
a
ltern
a
tive represent
a
tions
a
re
a
llowed.

Record Type TXT
• Just
a
text string.
• It w
a
s origin
a
lly intended for pl
a
in text inform
a
tion, but h
a
s
incre
a
singly been used by systems
a
nd progr
a
ms to store d
a
t
a
th
a
t
c
a
nnot be stored in other record types.
se TXT SE zone update: 2025-09-30 02:51:55 +0000 (EPOCH 1759200715) (auto)

Summary so far
• DNS is used,
a
mong other things, to look up IP
a
ddresses for dom
a
in
n
a
mes.
• The dom
a
in n
a
me is more th
a
n just
a
n IP
a
ddress.
• The dom
a
in n
a
me is p
a
rt of
a
n
a
me tree.
• The DNS d
a
t
a
, which we c
a
n look up, is linked to
a
speci
f
ic point in
the dom
a
in tree.
• There
a
re di
ff
erent record types for di
ff
erent d
a
t
a
, including A, AAAA,
a
nd TXT.

The Internet's Domain Name System
• This present
a
tion w
a
s developed from 2019–2024 by M
a
ts Du
f
berg
(m
a
ts.du
f
berg@internetstiftelsen.se)
a
t The Internet Found
a
tion
(https://internetstiftelsen.se/). It is p
a
rt of the te
a
ching m
a
teri
a
l for
the course "The Internet's Dom
a
in N
a
me System"
a
t KTH Roy
a
l
Institute of Technology (course code HI1037)
a
nd K
a
rlst
a
d University,
KAU (course code DVGC28), respectively.
• This present
a
tion w
a
s tr
a
nsl
a
ted in 2025 by Ondřej Surý
<ondrej@isc.org>
a
t Internet Systems Consortium. It is p
a
rt of the
te
a
ching m
a
teri
a
l for the course "DNS Principles
a
nd Oper
a
tions"
a
t
University of Ostr
a
v
a
.

License
• This te
a
ching m
a
teri
a
l is provided under the Cre
a
tive Commons BY
4.0 license (https://cre
a
tivecommons.org/licenses/by/4.0/deed.en)
a
nd m
a
y be used in
a
ccord
a
nce with its terms.

DNS Principles and Operations - Lecture 01

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