CS 330 Architecture Class 3

1
CS 330 Architecture Class 3

• Architecture plan for 09/10/07
• Loshin Chapter 2, Network Addresses and Network
  Names
• Loshin Chapter 11, TCP/IP Host Name Resolution
• Customer "I want to download the Internet. Do I
  need a bigger hard disk?" Download the Internet
  here

2
Interesting Colloquium Thursday (detour)

• Tom Leighton, Professor of Applied Mathematics at
  MIT and
• co-founder and Chief Scientist of Akamai
  Technologies
• Does anyone remember when Victoria's Secret took
  down
• Dallas in 1999?
• Problem centralized model of content delivery
  doesn't work
• Akamai
• web application accelerator
• distributed servers enable users to access
  content near themselves
• challenge server assignment and load balancing
• interesting caching algorithms for content at a
  locale
• 200 million in revenue last year

3
Interesting Attacks

• Phishing
• send an email that looks as though it cam from
  PayPal
• ask the reader to click on a link to "verify"
  bank information
• Pharming
• claim an IP (e.g. a bank)
• when someone uses it, present a login
• capture name/password
• Set up a wireless server on your laptop in
  Starbucks
• Use (unchanged) default password on someone's
  wireless router

4
Problem

• How does a person in Germany get pages from "the
  computer in Mac 104"
• put cshilepsky.wells.edu in the URL (send an HTTP
  request)
• but cshilepsky.wells.edu is really 24.39.181.207
• but internally, cshilepsky.wells.edu is really
  10.3.4.198
• and 24.39.181.207 is assigned to an Ethernet card
  with MAC address 00-07-E9-EF-42-73
• How does one get from the URL to the IP to the
  MAC?
• Start looking at network names (the URL)

5
Network Names

• Original top-level domains .com, .edu, .gov,
  .net, .mil, .fr, .de, .us, etc.
• New top-level domains .info, .rec, .biz, .tv,
  ...
• A mix of division by organizational type and by
  geography .com addresses may be in France, .fr
  may be in US
• Domain namespace is a tree
• root
• com net gov edu fr
• wells cornell cccc
• aurora www cshilepsky
• 
  (how does one often
  process tree data?)

6
IP Addresses

• 4 bytes, usually represented in dotted notation
• aurora.wells.edu 24.39.181.204
• cshilepsky.wells.edu 24.39.181.207
• henry.wells.edu 24.39.181.210
• www.wells.edu 24.39.181.211
• Network classes
• A 16 million
• B 65,534
• C 254
• What class is Wells?
• do we have enough addresses?
• ephemeral ports (later)
• What is your computer IP? (start/run/cmd
  ipconfig, whatismyip.com)
• why doesn't it start with 24.39.181?

7
Subnet

• A portion of a network that shares a common
  address component
• on TCP/IP networks all devices whose IP
  addresses have the same prefix (webopedia)
• aurora.wells.edu 24.39.181.204
• 
  network address host address
• Wells subnet mask
• 255.255.255.0 gives digits used to identify the
  wells.edu subnet
• Mask using binary "and" to get 24.39.181.0
• 255.255.255.0 11111111.11111111.11111111.0000000
  0
• 24.39.181.204 00011000.00100101.10110101.1100110
  0
• and 00011000.00100101.10110101.00000000
  
• Masked digits are the same for all IP addresses
  on the local net

8
Approaches to Addressing

• Functions
• disconnect the name and the hw
• names are easier to remember than IP addresses
• modularity (wells.edu)
• Early telephone book model
• a central repository everyone downloads daily
• doesn't scale need to report every new machine
  or change
• Later query a central server
• every named computer must be in the database
• every change must be registered
• traffic to the central server would be
  unsupportable
• in eight years www.wells.edu has moved to four
  different physical machines
• Today DNS

9
DNS - Domain Name System/Service

• How does an outside request get to
  aurora.wells.edu?
• All network traffic is routed via IPs
• the Internet gets a packet to 24.39.181
• Henry handles it inside Wells using a local hosts
  file
• Athena handles academic computers using its local
  hosts file
• DNS maps host names to IP addresses reverse
• henry.wells.edu ? 24.39.181.210
• www.wells.edu ? 24.39.181.4
• aurora.wells.edu ? 24.39.181.200 or 10.3.4.196
• webmail.wells.edu mary.wells.edu (alias)
• wells.edu ? 24.39.181.207 (MX record mail to
  Wells)
• 24.39.181.6 ? mary.wells.edu (reverse DNS)

10
Division of Labor Domains and Zones

• Zones a matter of describing who is responsible
  for what
• The zone does not necessarily coincide with
  physical location
• a company with offices all over the world might
  maintain a single zone, for which a single DNS
  server is authoritative
• Partition authority to the lowest posible level
  (zone)
• a distributed databaseno one location has all
  information
• goals scalable and reliable
• Distinction between domain and zone
• domain wells.edu, cornell.edu
• zone the addresses managed by a DNS server
• Often a domain is divided into many zones with
  separate DNS servers (e.g. Cornell)

11
Domains and Zones

• The domain wells.edu uses one zone
• Specific servers authoritative for specific
  zones
• wells.edu zone has two authoritative servers
• Henry is authoritative for requests from outside
  Wells
• Athena is authoritative for requests from inside
  Wells
• Resource records store DNS information for each
  zone
• like a phone book
• used to translate host names and IP addresses
• stored in the server that handles queries for the
  zone
• format
• sample name server records for Athena

12
(Wikipedia)

13
MX (Mail Exchange) Records

• DNS also provides the address of the mail server
  at a domain
• Thus we address mail to klarson_at_wells.edu without
  knowing the exact server that handles mail for
  Wells
• previously henry, now bulkmail
• not on the Wells diagram
• A machine sending mail to Wells queries DNS and
  learns that MX bulkmail.wells.edu

14
How Does DNS Work? (1)

• DNS carried out by
• name servers (software running on machines like
  Henry and Athena
• name resolvers (software running on client
  computers).
• Name servers have resource records for one or
  more zones
• Client resolver built into TCP/IP (more later)
• Look at the network configuration of your PC
• run/cmd /ipconfig /all

15
How Does DNS Work? (2)

• Resolve www.wikepedia.org
• local nameserver queries a root name server
• root name server queries the .org server
• an edu server queries the wikepedia server
• the wells server gives IP of www.wikepedia.org
• Root name server sends requests to the
  appropriate top-level domain.
• Named A.root-server.net through
  M.root-server.net which contain information in
  the root zone file A good ref.

16
How Does DNS Work? (3)

• More detail, find math.cornell.edu
• client checks its cache
• math.cornell.edu?
• if not there, it queries a name server it knows
• .cornell.edu?
• .edu?
• root nameserver
• the name server checks its cache to see if it has
  the answer if not
• it queries the name servers it knows
• if a domain has multiple zones, (e.g.
  marketing.microsoft.com), the domain DNS server
  sends back the name of the DNS server responsible
  for that particular zone (e.g. marketing)...
• eventually send an answer back to the client (and
  store it in own cache)
• records kept in cache a default period usually 4
  days or as specified by the particular host,
  then deleted.

17
DNS Servers at Wells

• Henry handles DNS queries from outside Wells
• e.g. aurora.wells.edu 24.39.181.204 NAT, PAT
• Athena and Zeus handle DNS queries from academic
  clients
• e.g. aurora.wells.edu 10.3.4.196
• Field trip to Professor Larson's office
• how Athena handles DNS queries