dns: domain name system

DNS: Domain Name System

CMPSCI 491G: Computer Networking Lab

V. Arun

Slides adapted from Liebeherr & Zarki, Kurose & Ross, Kermani

Domain Name System: distributed database implemented in hierarchy of many name servers

application-layer protocol: hosts, name servers communicate to resolve names addresses note: core Internet function, implemented as application-layer protocol

complexity at network’s “edge”

Application Layer 2-2

DNS: domain name system

people: many identifiers: SSN, name, passport #

Internet hosts, routers: IP address (32 bit) - used for addressing datagrams

“name”, e.g., www.yahoo.com - used by humans

Q: how to map between IP address and name, and vice versa ?


DNS: services, structure why not centralize DNS?

single point of failure traffic volume distant centralized database

maintenance

DNS services Resolution

hostname IP address

Aliasing canonical, alias names

mail server aliasing

Load balancing with replicated web servers: many addresses map to one name

doesn’t scale!

Before there was DNS ….

…. there was the HOSTS.TXT file

• Before DNS (until 1985), name resolution was done by FTP’ing a single file (hosts.txt) from a central server. – Names in hosts.txt are not structured.– hosts.txt still works on most operating systems. It can be

used to define local names.

Design principle of DNS

• DNS naming system based on a hierarchical and logical tree structure called domain namespace.

• An organization obtains authority for parts of the name space, and can add additional layers of the hierarchy

• Names of hosts can be assigned without regard of location on a link layer network, IP network or autonomous system

• In practice, allocation of the domain names generally follows the allocation of IP address, e.g., – All hosts with network prefix 128.119/16 have domain name suffix

umass.edu– All hosts on network 128.119.240/24 are in the School of Computer

Science at UMass Amherst.

Managed by UofT

DNS Name hierarchy

• DNS hierarchy can be represented by a tree

• Root and top-level domains are administered by an Internet central name registration authority (ICANN)

• Below top-level domain, administration of name space is delegated to organizations

• Each organization can delegate further Managed by

ECE Dept.

. (root)

com

toronto.edu

goveduorg

uci.edu

ece.toronto.edumath.toronto.edu

neon.ece.toronto.edu

Top-level Domains

Domain name system

• Each node in the DNS tree represents a DNS name

• Each branch below a node is a DNS domain.– DNS domain can contain

hosts or other domains (subdomains)

• Example: DNS domains are ., edu, virginia.edu, cs.virginia.edu

virginia.edu

cs.virginia.eduwww.virginia.edu

neon.cs.virginia.edu

edu

.

Top-level domains

• Three types of top-level domains:– Organizational: 3-character code indicates the function of

the organization• Used primarily within the US • Examples: gov, mil, edu, org, com, net

– Geographical: 2-character country or region code• Examples: us, va, jp, de

– Expanded top-level domains (gTLDs)• Essentially arbitrary TLDs

– Reverse domains: A special domain (in-addr.arpa) used for IP address-to-name mapping

Organizational top-level domains

com Commercial organizations

edu Educational institutions

gov Government institutions

int International organizations

mil U.S. military institutions

net Networking organizations

org Non-profit organizations

Hierarchy of name servers

• The resolution of the hierarchical name space is done by a hierarchy of name servers

• Each server is responsible (authoritative) for a contiguous portion of the DNS namespace, called a zone.

• Zone is a part of the subtree

• DNS server answers queries about hosts in its zone

root server

com servergov serveredu serverorg server

uci.eduserver

.virginia.edu server

cs.virginia.edu server

Authority and delegation

• Authority for the root domain is with the Internet Corporation for Assigned Numbers and Names (ICANN)

• ICANN delegates to accredited registrars (for gTLDs) and countries for country code top level domains (ccTLDs)

• Authority can be delegated further

• Chain of delegation can be obtained by reading domain name from right to left.

• Unit of delegation is a “zone”.

DNS domain and zones

• Each zone is anchored at a specific domain node, but zones are not domains.

• A DNS domain is a branch of the namespace

• A zone is a portion of the DNS namespace generally stored in a file (could consist of multiple nodes)

• A server can divide part of its zone and delegate it to other servers

. (root)

.virginia.edu

.edu

.uci.edu

cs.virginia.edumath.virginia.edu

DomainZone

anddomain

Zone

Primary and secondary name servers

• For each zone, there must be a primary name server and a secondary name server– The primary server (master server) maintains a zone file which has

information about the zone. Updates are made to the primary server– The secondary server copies data stored at the primary server.

Adding a host:• When a new host is added (“gold.cs.virginia.edu”) to a zone, the

administrator adds the IP information on the host (IP address and name) to a configuration file on the primary server


Root DNS Servers

com DNS servers org DNS servers edu DNS servers

poly.eduDNS servers

umass.eduDNS servers

yahoo.comDNS servers

amazon.comDNS servers

pbs.orgDNS servers

DNS resolution: distributed, hierarchical

client wants IP for www.amazon.com; 1st approx: client queries root server to find .com TLD DNS

server client queries .com TLD DNS server for amazon.com

auth server client queries amazon.com DNS auth server to get

IP address for www.amazon.com

… …Top-level domain servers

Authoritative name servers


DNS: root name servers contacted when no info about top-level or auth server

root name server can: return top-level or auth name server address or contact auth server and return final resolved address

13 root name “servers” worldwide

a. Verisign, Los Angeles CA (5 other sites)b. USC-ISI Marina del Rey, CAl. ICANN Los Angeles, CA (41 other sites)

e. NASA Mt View, CAf. Internet Software C.Palo Alto, CA (and 48 other sites)

i. Netnod, Stockholm (37 other sites)

k. RIPE London (17 other sites)

m. WIDE Tokyo(5 other sites)

c. Cogent, Herndon, VA (5 other sites)d. U Maryland College Park, MDh. ARL Aberdeen, MDj. Verisign, Dulles VA (69 other sites )

g. US DoD Columbus, OH (5 other sites)


TLD, authoritative serverstop-level domain (TLD) servers:

responsible for com, org, net, edu, aero, jobs, museums, and all top-level country domains, e.g.: uk, fr, ca, jp

Network Solutions maintains servers for .com TLD

Educause for .edu TLD

authoritative DNS servers: organization’s own DNS server(s), providing authoritative hostname to IP mappings for organization’s named hosts

can be maintained by organization or service provider


Local DNS name server

does not strictly belong to hierarchy

deployed by ISP (residential, company, university) also called “default name server”

acts as proxy between host and DNS hierarchy has local cache of recent name-to-address translation pairs (but may be out of date!)


requesting hostcis.poly.edu

gaia.cs.umass.edu

root DNS server

local DNS serverdns.poly.edu

1

23

4

5

6

authoritative DNS serverdns.cs.umass.edu

78

TLD DNS server

DNS name resolution example

host at cis.poly.edu wants IP address for gaia.cs.umass.eduiterated query:

contacted server replies with name of server to contact

“I don’t know this name, but ask this server”


45

6

3

recursive query:

puts burden of name resolution on contacted name server

heavy load at upper levels of hierarchy?

requesting hostcis.poly.edu

gaia.cs.umass.edu

root DNS server

local DNS serverdns.poly.edu

1

27

authoritative DNS serverdns.cs.umass.edu

8

DNS name resolution example

TLD DNS server


DNS: caching, updating records any name server can cache learned mappings cache entries timeout (disappear) after some time (TTL)

TLD servers typically cached in local name servers, so root name servers not often visited

cached entries may be out-of-date (best effort name-to-address translation!) if name host changes IP address, may not be known Internet-wide until all TTLs expire

update/notify mechanisms proposed IETF standard RFC 2136


DNS records

DNS: distributed db storing resource records (RR)

type=NS name is domain (e.g., foo.com)

value is hostname of authoritative name server for this domain

RR format: (name, value, type, ttl)

type=A name is hostname value is IP address

type=CNAME name is alias name for some “canonical” (the real) name

www.ibm.com is really servereast.backup2.ibm.com value is canonical name

type=MX value is name of mailserver associated with name


DNS protocol, messages query and reply messages, both with same message format

msg header identification: 16

bit # for query, reply to query uses same #

flags: query or reply recursion desired recursion available

reply is authoritative

identification flags

# questions

questions (variable # of questions)

# additional RRs# authority RRs

# answer RRs

answers (variable # of RRs)

authority (variable # of RRs)

additional info (variable # of RRs)

2 bytes 2 bytes


name, type fields for a query

RRs in responseto queryrecords for

authoritative servers

additional “helpful”info that may be used

identification flags

# questions

questions (variable # of questions)

# additional RRs# authority RRs

# answer RRs

answers (variable # of RRs)

authority (variable # of RRs)

additional info (variable # of RRs)

DNS protocol, messages

2 bytes 2 bytes


Inserting records into DNS example: new startup “Network Utopia” register name networkuptopia.com at DNS registrar (e.g., Network Solutions) provide names, IP addresses of authoritative name server (primary and secondary)

registrar inserts two RRs into .com TLD server:(networkutopia.com, dns1.networkutopia.com, NS)

(dns1.networkutopia.com, 212.212.212.1, A) create authoritative server type A record for www.networkuptopia.com; type MX record for networkutopia.com

Resource Records

• Resource records are stored in configuration files (zone files) at name servers.

• Example resource records for a zone:

db.mylab.com $TTL 86400 mylab.com. IN SOA PC4.mylab.com. hostmaster.mylab.com. ( 1 ; serial 28800 ; refresh 7200 ; retry 604800 ; expire 86400 ; ttl ) ; mylab.com. IN NS PC4.mylab.com. ; localhost A 127.0.0.1 PC4.mylab.com. A 10.0.1.41 PC3.mylab.com. A 10.0.1.31 PC2.mylab.com. A 10.0.1.21 PC1.mylab.com. A 10.0.1.11

Resource Records

db.mylab.com $TTL 86400 mylab.com. IN SOA PC4.mylab.com. hostmaster.mylab.com. ( 1 ; serial 28800 ; refresh 7200 ; retry 604800 ; expire 86400 ; ttl ) ; mylab.com. IN NS PC4.mylab.com. ; localhost A 127.0.0.1 PC4.mylab.com. A 10.0.1.41 PC3.mylab.com. A 10.0.1.31 PC2.mylab.com. A 10.0.1.21 PC1.mylab.com. A 10.0.1.11

Max. age of cached data in seconds

* Start of authority (SOA) record. Means: “This name server is authoritative for the zoneMylab.com” * PC4.mylab.com is the name server* [email protected] is the email address of the person in charge

Name server (NS) record. One entry for each authoritative name server

Address (A) records. One entry for each hostaddress

Lab 7 (DHCP/NAT) review

Exercise 1(B)

28

NAT Table on Router2Router2#show ip nat translationsPro Inside global Inside local Outside local Outside global--- 200.0.0.2 10.0.1.2 --- ---

Which ping works and why?PC3% ping –c3 10.0.1.3PC3% ping –c3 128.143.136.1

Router1% ping –c3 10.0.1.2Router1% ping –c3 128.143.136.1

PC4% ping –c3 10.0.1.2PC4% ping –c3 200.0.0.2

Configuration:Router2(config)#ip nat inside source static 10.0.1.2 200.0.0.2.. ..PC4% route add –net 200.0.0.0 netmask 255.255.255.0 gw 128.195.7.32

Exercise 1(B)

29

NAT Table on Router2Router2#show ip nat translationsPro Inside global Inside local Outside local Outside global--- 200.0.0.2 10.0.1.2 --- ---



PC4% ping –c3 10.0.1.2PC4% ping –c3 200.0.0.2

Configuration:Router2(config)#ip nat inside source static 10.0.1.2 200.0.0.2.. ..PC4% route add –net 200.0.0.0 netmask 255.255.255.0 gw 128.195.7.32

Exercise 1(B)

30

NAT Table on Router2Router2#show ip nat translationsPro Inside global Inside local Outside local Outside global--- 200.0.0.1 10.0.1.1 --- ------ 200.0.0.2 10.0.1.2 --- ------ 200.0.0.3 10.0.1.3 --- ---



PC4% ping –c3 10.0.1.2PC4% ping –c3 200.0.0.2

Configuration:Router2(config)#ip nat inside source static 10.0.1.2 200.0.0.2Router2(config)#ip nat inside source static 10.0.1.1 200.0.0.1Router2(config)#ip nat inside source static 10.0.1.3 200.0.0.3

Exercise 1(B)

31




PC4% ping –c3 10.0.1.2PC4% ping –c3 200.0.0.2

Configuration:Router2(config)#ip nat inside source static 10.0.1.2 200.0.0.2Router2(config)#ip nat inside source static 10.0.1.1 200.0.0.1Router2(config)#ip nat inside source static 10.0.1.3 200.0.0.3

Exercise 1(B)

32

Before Router2:Src: 10.0.1.2 (10.0.1.2), Dst: 128.143.136.1 (128.143.136.1)

Show IP source/destination addresses before/after Router2PC3% ping –c3 128.143.136.1

After Router2:Src: 200.0.0.2 (200.0.0.2), Dst: 128.143.136.1 (128.143.136.1)


Exercise 1(C)- NAT/PAT/Masquerade

33

telnet commands; which one successful?PC1% telnet 10.0.1.3 (Router1)PC1% telnet 128.143.136.1 (PC4)

Router1# telnet 10.0.1.2 (PC1)Router1# 128.143.136.1 (PC4)

PC4: telnet 10.0.1.2 (PC3)

Exercise 1(C)- NAT/PAT/Masquerade

34

telnet commands; which one successful?PC1% telnet 10.0.1.3 (Router1)PC1% telnet 128.143.136.1 (PC4)

Router1# telnet 10.0.1.2 (PC1)Router1# 128.143.136.1 (PC4)

PC4: telnet 10.0.1.2 (PC3)

Exercise 1(C)- NAT & telnet

35

PC1% telnet 128.143.136.1 (PC4)

Before translation (PC2)Internet Protocol Source: 10.0.1.2 Destination: 128.143.136.1Transmission Control Protocol Source port: 32774 Destination port: telnet (23) Sequence number: 1857633137

After translation (PC2)Internet Protocol Source: 128.143.136.22 Destination: 128.143.136.1Transmission Control Protocol Source port: 32774 Destination port: telnet (23) Sequence number: 1857633137

Exercise 1(C)- PAT & ICMP (ping)

36

PC1% ping 128.143.136.1 (PC4)

Internet Protocol, Src Addr: 10.0.1.2, Dst Addr: 128.143.136.1 Identification: 0x0000 Protocol: ICMP (0x01) Source: 10.0.1.2 Destination: 128.143.136.1

Internet Protocol, Src Addr: 128.143.136.22, Dst Addr: 128.143.136.1 Identification: 0x0000 Protocol: ICMP (0x01) Source: 128.143.136.22 Destination: 128.143.136.1

•Ping (ICMP) does not use port number•“Identification” is used to help with NAT

Exercise 1(D)- NAT & FTP

FTP uses 2 ports Control connection, port 21 Data connection port 20

No problem with NAT & control connection. For data connection, the server initiates a

connection from its port 20 to a (random) port on client Causes problem with NAT Only client can initiate connection

PASSIVE mode solves this problem

37

Exercise 1(D)- NAT & FTP

38

PC3% ftp 128.143.136.22 (PC2)

dns: domain name system

Documents

dns domains

dns tree

filebefore dns

ip network

special domain

domain namespace

internet hosts

allocation of ip address