COMS/CSEE 4140 Networking Laboratory

Lecture 09

Salman Abdul BasetSpring 2008

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Announcements Prelab 8 and Lab report 7 due next week

before your lab slot Weekly project reports due before Friday

5pm

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Agenda DNS history DNS concepts Recursive and iterative queries Caching Resource records mDNS King tool

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DNS History hosts.txt file

(<1983) Download a single file (hosts.txt) from a central server with FTP

Names in hosts.txt are not structured. The hosts.txt file still works on most operating systems.

It can be used to define local names. Paul Mockapetris

DNS RFC 882, 883 – updated by RFC 1032, 1033 DNS implementation

Written by four Berkeley students (client/server) Renamed BIND in 1985 (Berkeley Internet Name

Daemon)

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Domain Names and IP addresses People prefer to use easy-to-remember names

instead of IP addresses Domain names are alphanumeric names for IP

addresses e.g., neon.ece.utoronto.ca, www.google.com, ietf.org

The domain name system (DNS) is an Internet-wide distributed database that translates between domain names and IP addresses

How important is DNS? Imagine what happens when the local DNS server is down.

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Design Principles of DNS Name space (domain namespace)

Hierarchical and logical tree Distributed (Delegation)

Be maintained in a distributed manner Size of DNS database and frequency of updates Per organization – can additional layers of hierarchy

Names for hosts, email servers, and others Name space and protocols

Notion of address in various protocols 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 128.143/16 have domain

name suffix virginia.edu

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DNS Usage Assumptions Size of the total database

Proportional to number of hosts in the Internet ~110 million gTLDs (as of April 7, 2008)

Slow (days) and fast (min/sec) changing data

Access is critical than update guarantees! Each organization is responsible for

providing redundant DNS servers for its hosts.

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Elements of DNS Domain Name Space

Specification for tree structured name space Resource Records (RR)

Name Servers Hold information about part of domain’s tree

structure called Zone Resolvers

Programs that extract information from name servers in response to client requests

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Resolver and Name Server1. An application program on a

host accesses the domain system through a DNS client, called the resolver

2. Resolver contacts DNS server, called name server

3. DNS server returns IP address to resolver which passes the IP address to application

Reverse lookups are also possible, i.e., find the hostname given an IP address

HTTP Resolver

Hostname (neon.tcpip-lab.edu)

IP address (128.143.71.21)

Name server

Ho

stna

me

(n

eo

n.tcp

ip-la

b.e

du

)

IP a

dd

ress (1

28

.14

3.71

.21

)

10

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

Siblings should have different names

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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

.

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Domain Names Hosts and DNS domains are named based on their position

in the domain tree Every node in the DNS domain tree can be identified by a

unique Fully Qualified Domain Name (FQDN). The FQDN gives the position in the DNS tree.

A FQDN consists of labels (“cs”,“virginia”,”edu”) separated by a period (“.”)

There can be a period (“.”) at the end. Each label can be up to 63 characters long FQDN contains characters, numerals, and dash character

(“-”) FQDNs are not case-sensitive

cs.virginia.edu cs.virginia.edu.or

Note the dot

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Top-level domains (TLDs) IANA classifies TLD into three types

gTLD (generic): 3-character code indicates the function of the organization

Used primarily within the US .com, .net etc.– .mil and .gov reserved for use by US

ccTLD (country code): 2-character country or region code Examples: us, va, jp, de

iTLD (infrastructure): A special domain (in-addr.arpa) used for IP address-to-name mapping

Pseudo domains .local for the Zeroconf protocol

Reserved TLD .example, .invalid, .localhost, .test

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Top-level domains (TLDs) http://www.iana.org/domains/root/db/ iTLD

.arpa, .root gTLD

.aero, .asia, .biz, .cat, .com, .coop, .edu, .gov, .info, .int, .jobs, .mil, .mobi, .museum, .name, .net, .org, .pro, .tel, .travel

ccTLD >200 countries

Non-conventional usage del.icio.us, inter.net

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TLD Distribution

0

10,000,000

20,000,000

30,000,000

40,000,000

50,000,000

60,000,000

70,000,000

80,000,000

.com .net .org .info .biz .us

Total 110 million records Source: http://icannwiki.org/Domain_Statistics

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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

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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”.

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DNS Domain and Zones

columbia.edu

cs.columbia.eduwww.columbia.edu

edu

.

harvard.eduyale.edu

Domain .edu conceptually contains all data for columbia.edu, www.columbia.edu, cs.columbia.edu, yale.edu, etc.

Zone for .edu contains nameserver reference for yale.edu and columbia.edu and all data for harvard.edu

.edu domain

.edu zone

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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 (It could consists 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

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Primary & 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

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Root Name Servers The root name

servers know how to find the authoritative name servers for all top-level zones.

There are only 13 root name servers

Root servers are critical for the proper functioning of name resolution

A.ROOT-SERVERS.NET. 198.41.0.4B.ROOT-SERVERS.NET.

192.228.79.201C.ROOT-SERVERS.NET. 192.33.4.12D.ROOT-SERVERS.NET. 128.8.10.90E.ROOT-SERVERS.NET.

192.203.230.10F.ROOT-SERVERS.NET. 192.5.5.241G.ROOT-SERVERS.NET. 192.112.36.4H.ROOT-SERVERS.NET. 128.63.2.53I.ROOT-SERVERS.NET. 192.36.148.17J.ROOT-SERVERS.NET. 192.58.128.30K.ROOT-SERVERS.NET. 193.0.14.129 L.ROOT-SERVERS.NET. 197.7.83.42M.ROOT-SERVERS.NET. 202.12.27.33

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Agenda DNS history DNS concepts Recursive and iterative queries Caching Resource records mDNS King tool

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Domain Name Resolution1. User program issues a

request for the IP address of a hostname

2. Local resolver formulates a DNS query to the name server of the host

3. Name server checks if it is authorized to answer the query. a) If yes, it responds. b) Otherwise, it will query

other name servers, starting at the root tree

4. When the name server has the answer it sends it to the resolver.

HTTP Resolver

Hostname (neon.tcpip-lab.edu)

IP address (128.143.71.21)

Nameserver

Ho

stna

me

(ne

on

.tcpip

-lab

.ed

u)

IP a

dd

ress (1

28

.14

3.7

1.2

1)

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Recursive and Iterative Queries There are two types of queries:

Recursive queries Iterative (non-recursive) queries

The type of query is determined by a bit in the DNS query

Recursive query: When the name server of a host cannot resolve a query, the server issues a query to resolve the query

Iterative queries: When the name server of a host cannot resolve a query, it sends a referral to another server to the resolver.

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Recursive Queries In a recursive query, the

resolver expects the response from the name server

If the server cannot supply the answer, it will send the query to the “closest known” authoritative name server (here: In the worst case, the closest known server is the root server)

The root sever sends a referral to the “edu” server. Querying this server yields a referral to the server of “virginia.edu”

… and so on

root server

edu server

virginia.edu server

cs.virginia.edu server

Resolver

Nameserver

quer

y

resp

onse

Referral to edu name server

1st query: neon.cs.virginia.edu

2nd query: neon.cs.virginia.edu

Referral to virginia.edu nameserver

3rd query:neon.cs.virginia.edu

Referral tocs.virginia.eduname server

4th query:neon.cs.virginia.edu

IP address ofneon.cs.virginia.edu

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Iterative Queries In an iterative query,

the name server sends a closest known authoritative name server a referral to the root server.

This involves more work for the resolver

root server

edu server

virginia.edu server

cs.virginia.edu server

Resolver

Name server

quer

y

refe

rral

to r

oot s

erve

r

Referral to

edu name serve

r

1st query:

neon.cs.vi

rginia.edu

2nd query: neon.cs.virginia.edu

Referral to

virginia.edu name

server

3rd query: neon.cs.virginia.edu

Referral to cs.virginia.edu

name server

4th query: neon.cs.virginia.edu

IP address of neon.cs.virginia.edu

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Recursive vs. Iterative Queries Recursive

[Con] resource intensive – DNS server maintains state

[Pro] Cache namespace – helpful for other queries

Iterative [Pro] DNS server never maintains state [Con] Caching is local to a machine – others

cannot benefit from it

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Caching To reduce DNS traffic, name servers

caches information on domain name/IP address mappings

When an entry for a query is in the cache, the server does not contact other servers

Note: If an entry is sent from a cache, the reply from the server is marked as “unauthoritative”

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Resource Records The database records of

the distributed data base are called resource records (RR)

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

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

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Resource Record Types (Name, Value, Type, TTL) Type=A, name=hostname, value=IPv4 address Type=AAAA, name=hostname, value=IPv6 address Type=NS, name=domain value=hostname of authoritative

DNS server nslookup –query=ns google.com

Type=CNAME, name=hostname, value=canonical host name (columbia.edu, ns1.columbia.edu, CNAME)

Type=MX, name=mail server hostname, value=canonical name of mail server

Type=PTR, name=address (IP), value=hostname Type=SRV, name=service name, value=canonical

hostname

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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* hostmaster@mylab.com 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

Slave refresh timeSlave retry time

Slave expiration time

Cache time for RR

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DNS Summary Domain name space Domain and zone Zone and resource record

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DNS Example nslookup google.com

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DNS Example

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DNS Example

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DNS Example Query 1 >nslookup google.com ns.google.com Server: ns1.google.com Address: 216.239.32.10

Name: google.com Addresses: 64.233.167.99, 72.14.207.99, 64.233.187.99

Query 2 >nslookup google.com Server: disco.cs.columbia.edu Address: 128.59.16.7

Non-authoritative answer: Name: google.com Addresses: 64.233.187.99, 64.233.167.99, 72.14.207.99

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Agenda DNS history DNS concepts Recursive and iterative queries Caching Resource records mDNS King tool

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mDNS Multicast DNS (mDNS)

name-to-address translation on a local network Multicast address: 224.0.0.251

Self-assigned local name It only has significance on the local network. .local. Subdomain Name conflict resolution Default name: <host name>.local, SALMAN_PDA.local

MDNS:

Standard query ANY SALMAN_PDA.local

MDNS:

Standard query response A 169.254.18.87 PTR SALMAN_PDA.local

SALMAN_PDA.local

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King ToolHow to estimate latency between two

arbitrary hosts accurately?

AA BB

CC

C can measure latency between A and BC can measure latency between A and B

Source: King tool slides by Krishna Gummadi

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King: A Latency Measurement Tool Estimate latency between arbitrary end hosts Requires no additional infrastructure

leverages existing DNS infrastructure enabling a large fraction of Internet hosts to be measured

Provides highly accurate latency estimates Fast and light-weight

requires only a few DNS queries per estimate

We hope that King will be used in many unanticipated ways like in the case of Ping and Traceroute

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Name Servernear Host A

Name Servernear Host B

Host BHost AActual Latency Between End Hosts

Latency Estimated By King

How King Works: The Basic IdeaHow King Works: The Basic Idea

Challenge 1: How to find name servers that are close to end hosts

Challenge 2: How to estimate latency between two name servers

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Challenge 2: How do we estimate the latency between name servers?

Our Client C(King)

Name Server B foo.barName Server A

1. Request Q: Resolve xyz.foo.bar

4. Reply Q (Forwarded)

2. Request Q (Forwarded)

3. Reply Q: IP addr of xyz.foo.bar

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Success of Recursive DNS For King to work, name servers must

support recursive queries in a large random sample, > 75% of name

servers supported recursion translates to > 90% success rate given a pair

as we can measure from A->B, or B->A

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Example Estimating latency between MIT and

google.com salman@irtcluster05:~$ time nslookup mit.edu bitsy.mit.edu Server: bitsy.mit.edu Address: 18.72.0.3#53

Name: mit.edu Address: 18.7.22.69

real 0m0.013s user 0m0.000s sys 0m0.008s salman@irtcluster05:~$

salman@irtcluster05:~$ time nslookup x.google.com bitsy.mit.edu Server: bitsy.mit.edu Address: 18.72.0.3#53

** server can't find x.google.com.columbia.edu: NXDOMAIN

real 0m0.061s user 0m0.000s sys 0m0.008s