Networking: UDP & DNS - University of the Pacific · 2018-06-27 · User Datagram Protocol (UDP) ì...

ìComputer Systems and NetworksECPE170– JeffShafer– UniversityofthePacific

Networking:UDP&DNS

Lab Schedule

Activitiesì ThisWeek

ì Lab9– NetworkProgramming

ì NextWeekì StartAssembly

Programming(lecturefor1+day)

AssignmentsDueì Lab9

ì DuebyApr4th 5:00am

Spring2017ComputerSystemsandNetworks

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ìUser Datagram Protocol (UDP)


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UDP versus TCP


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TCP UDPReliable? Yes

(Via acknowledgementsandretransmitting)

No

Connection-oriented?

Yes(Serverhasone socketperclient)

No(Serverhasonesocketandallmessagesfromallclientsarereceivedonit)

Programmingmodel?

Stream(continuousflow ofdata–maygetalittlebitatatime)

Datagram(data issentinitsentiretyornotatall.Sizeofeachdatagramissmall)

Applications HTTP(Lab8)Web,email,filetransfer

DNS(Lab9)StreamingAudio/Video, Gaming

User Datagram Protocol (UDP)

ì UDP:no“connection”betweenclientandserverì Nohandshakingì Senderexplicitly

attachesIPaddressandportofdestinationtoeachmessage

ì ReceivercanextractIPaddress,portofsenderfromreceiveddatagram


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applicationviewpoint

UDPprovidesunreliable transferofgroupsofbytes(“datagrams”)

betweenclientandserver

User Datagram Protocol (UDP)

ì EachUDPmessageisself-containedandcomplete

ì EachtimeyoureadfromaUDPsocket,yougetacompletemessageassentbythesenderì Thatis,assumingitwasn’tlostintransit!

ì ThinkofUDPsocketsasputtingastamponaletterandstickingitinthemailì Noneedtoestablishaconnectionfirstì Receiverhasnoidea“letter”isarrivinguntilthey

lookinthemailbox

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Python UDP Programming

ì Twonewfunctions:sendto() andrecvfrom()


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server_ip = 1.2.3.4port = 5678dest_addr = (server_ip, port)s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)......bytes_sent = s.sendto(raw_bytes, dest_addr)......max_bytes = 4096(raw_bytes, src_addr) = s.recvfrom(max_bytes)

ìDomain Name System (DNS)


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IP Addresses

ì IPversion4addressesare32bitslong

ì IPversion6addressare128bitslong

ì EverynetworkinterfacehasatleastoneIPaddressì Acomputermighthave2ormoreIPaddresses

ì IPv4addressesareusuallydisplayedindotteddecimalnotationì Eachbyterepresentedbydecimalvalueì Bytesareseparatedbyaperiodì IPaddress0x8002C2F2 = 128.2.194.242

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Motivation

ì IPaddressesarehardtorememberì 198.16.253.143?Orwasit.146?

ì Human-friendlynamesaremuchbetterì engineering.pacific.edu

ì Howcanwetranslatebetweenthetwo?

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Early Days (prior to 1983)

ì EachcomputerontheARPAnet (earlyInternet)hadasinglefileì hosts.txtmapsallknownhostnamestoIPaddress

ì MasterlistmaintainedbySRINetworkInformationCenterì Emailthemifyour

mappingchangesì Newlistproduced1-2

timesaweekì Allhostsdownloadthe

newlist

ì Problemswiththisapproach?

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Domain Name System (DNS)

ì Distributeddatabaseimplementedinhierarchyofmanynameservers

ì Application-layerprotocolì Hosts,routers,andnameserverscommunicateto

resolvenames(address/nametranslation)ì CoreInternetfunctionimplementedasapplication-

layerprotocol

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DNS is Decentralized

ì Nosinglepointoffailure

ì Nodistantcentralizeddatabase

ì Easiermaintenanceì Takeoneoradozenserversofflinewithoutissue

ì Supporthightrafficvolume

ì ***Scalability ***

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HowmanyDNSrequests/secondglobally?


DNS: Scalability

ì Challengingtofinddataonglobal DNSrequests/secì Noglobalinternet“dashboard”ì Internetisa“networkofnetworks”

ì WouldhavetoinquirewithAT&T,Comcast,TimeWarner,Pacific,etcì Theywouldhavetocheckstatsonalloftheirlocalservers

ì GooglePublicDNSì 400billionrequests/dayasofDec2014ì 70%internationalì http://googlewebmastercentral.blogspot.com/2014/12/google-public-dns-and-location.html

ì OpenDNSì 80billionrequests/dayasofSept2015ì http://system.opendns.com/

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What’s in a Name?

ì engineering.pacific.edu

ì .edu istop-leveldomainì “pacific”belongsto.eduì “engineering”belongsto“pacific”ì Hierarchical!Readfromrighttoleft

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Root DNS Servers

com DNS servers org DNS servers edu DNS servers

poly.eduDNS servers

umass.eduDNS serversyahoo.com

DNS serversamazon.comDNS servers

pbs.orgDNS servers

Distributed, Hierarchical Database


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ì ClientwantsIPforwww.amazon.com1. Clientqueriesarootservertofindcom DNSserver2. ClientqueriescomDNSservertogetamazon.com DNS

server3. Clientqueriesamazon.com DNSservertogetIPaddress

forwww.amazon.com

DNS: Root Name Serversì Contactedbylocalnameserverthatcannotresolvetop-leveldomain

ì Rootnameserver:ì ContactsauthoritativenameserverforTLDifnamemappingnotknownì Getsmappingì Returnsmappingtolocalnameserver

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13rootname“servers”worldwidelabeleda - m• Each“server”isreallya

cluster• Someclustersare

geographicallydistributed• 504totalinFall2014


DNS: Root Name Servers


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http://www.root-servers.org/

DNS and UDP

ì DNSusesUDPbydefaultì Itcan useTCP,butit’srareì Isn’tthisunreliable?

ì WhyuseUDPì Reliabilitynotneeded

ì DNSwilljustre-requestifnoresponsereceived(2-5seconds)

ì Faster(inthreeways!)ì Noneedtoestablishaconnection(RTT/latency

overhead)ì Lowerper-packetbyteoverheadinUDPheaderì Lesspacketprocessingbyhosts

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ìDemonstrations


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Demonstrations

1. DNSClient:dns.py

2. Wireshark packetcapture


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ìProgramming Tips


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The struct Module

ì ThedetailsofvariablesarehiddeninPythonì Forexample,howmanybytesisaninteger?

ì NeedamethodtodealwithbinarydataforfileI/OornetworkI/O:thestructmoduleì ModuleperformsconversionsbetweenbasicPython

datatypes andarraysofbytes


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The struct Module

ì Twomainfunctionsinthestructmoduleì pack:convertagroupofvariablesintoanarrayof

bytesì unpack:convertanarrayofbytesintoagroupof

variables

ì SimilartoC’sprintf andscanf

ì Eachfunctionrequiresaformatstringtodescribehowtopackorunpackthearguments


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The struct Module

ì Commonformatstringoptions:ì Seehttps://docs.python.org/3/library/struct.html

ì raw_bytes = struct.pack("BH", val1, val2)

ì (val1, val2) = struct.unpack("BH", raw_bytes)


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Format PythonType Size(bytes)

B Integer 1

H Integer 2

L Integer 4

Q Integer 8

The struct Module

ì Endianness mustbeconsideredwhendoingfileornetworkI/Owithfieldsgreaterthanonebyte

ì Thefirstcharacteroftheformatstringdeterminestheendianness


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Character Byteorder Size Alignment

@ Native Native Native

= Native Standard None

< Little Standard None

> Big Standard None

! Network (Big) standard None

DNS Endianness

ì Whatendianness isyourcomputer?ì Littleendian(x86)

ì Whatendianness istheDNSprotocol?(ormostnetworkprotocols)ì Bigendian

ì WhatfieldsintheDNSheaderdoesthismatterfor?ì Two-byteintegerfields

(questioncount,answercount,etc…)


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Bit Fields

ì Warning! struct onlydealswithbytes.Itcannothandlefieldswithdimensionslessthanonebyte

ì Problem– SomeoftheDNSfieldsareonly1bit,3bits,or4bitsinsize

ì Howcanwehandlethis?ì Manualbitshifting(ala C)orctypes


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QR | OPCODE | AA | TC | RD | RA | Resvd | RCODE(1) (4) (1) (1) (1) (1) (3) (4)

2bytes(16bits)

CTypes


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import ctypes

# Define a 2-byte structure (equivalent to a 'uint16' variable in C)class CustomStruct(ctypes.BigEndianStructure):

_fields_ = [("fieldA", ctypes.c_uint16, 1), # 1-bit field - Most Sig BIT("fieldB", ctypes.c_uint16, 6), # 6-bit field("fieldC", ctypes.c_uint16, 4), # 4-bit field("fieldD", ctypes.c_uint16, 5) # 5-bit field - Least SIG BIT

]

# Create new instance of the 'CustomStruct' data typespecial_variable = CustomStruct()

# Access the fields of the structurespecial_variable.fieldA = 1special_variable.fieldB = 18special_variable.fieldC = 5special_variable.fieldD = 17

CTypes


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# Print out individual fieldsprint("Field A = %i" % special_variable.fieldA)print("Field B = %i" % special_variable.fieldB)print("Field C = %i" % special_variable.fieldC)print("Field D = %i" % special_variable.fieldD)

# Convert the structure to a byte array and print it outprint(bytes(special_variable))

# Alternate printing method (won't decode bytes as ASCII)hex_string = "".join("%02x " % b for b in bytes(special_variable))print("0x%s" % hex_string)

Networking: UDP & DNS - University of the Pacific · 2018-06-27 · User Datagram Protocol (UDP) ì...

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