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SCC 203 Computer Network > Domain Name System (DNS) > Flashcards

Domain Name System (DNS) Flashcards

1
Q

Naming & Addressing

A
  • Names & Addresses
    – To call someone, you need to ask for his/her phone number
    – To mail someone, you need to get their address
  • How does naming and addressing work in the
    Internet?
    – If you need to reach Google do you need their IP
    * Does anyone know Google’s IP?
    – Problem:
    * People can’t remember IP addresses
    * Need human-readable names that map to IP addresses
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2
Q

Internet Names & Addresses

A
  • IP Addresses, e.g. 148.88.2.80
    – Computer usable labels for machines
    – Conform to structure of the network
  • Names, e.g. www.lancaster.ac.uk
    – Human usable labels for machines
    – Conform to organizational structure
  • How do you map from one to the other?
    -> Domain Name System (DNS)
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3
Q

Internet Names & Addresses
(Indirection)

A
  • Indirection is the ability to reference objects (such as data) using a name (identity) instead of the value of the object (such as an address).
  • Quite simply, it means not direct.
    – If there is a direct connection between two things, indirection means that something is placed in the middle so that another level of indirection is created.
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4
Q

History

A
  • Before DNS, all mappings were in hosts.txt
    • /etc/hosts on Linux
    • C:\Windows\System32\drivers\etc\hosts on Windows
  • Process
    • Centralized, manual system
    • Changes were submitted
    • Machines periodically FTP new copies of hosts.txt
    • Administrators could pick names at their discretion
    • Any name was allowed
      • You could name your server as:
        • “best_server_in_the_world”
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5
Q

The Need for Something Better

A
  • System administrators had to update hosts file on every machine to include every host their users might access
  • Any machine not in hosts file could only be accessed using IP address
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6
Q

Hosts Files Today

A
  • Used mainly to bypass DNS
    • … not suited to Internet scale
  • Error prone
    • No trigger for updates
      • Name to IP mappings change
      • No guarantee of network wide consistency
  • Can ‘guarantee’ access to important local servers
    • Beware over use due to above problems
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7
Q

From host.txt to DNS

A
  • host.txt
    • Not scalable
      -> need for scalable system
      - SRI cannot handle load
    • Hard to enforce uniqueness of names
      -> need for unique naming system
      - e.g. UCL
      = University College London
      = Universite Catholique de Louvain
    • Many machines had inaccurate copies of hosts.txt
      -> need for real-time system
      - Stanford-Research-Insitute: Network-Information-Center (NIC) updated hosts.txt periodically
  • Hence, DNS was born
    • Paul Mockapteris released the first version in 1984
    • RFCs 882 and 883
      - Superseeded by 1034 and 1035
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8
Q

DNS: domain name system

A
  • People: many identifiers:
    – Name, passport #, National Insurance Number, etc.
  • 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?

  • Domain Name System:
    – Distributed database implemented as a hierarchy of many name servers
    – Application-layer protocol: hosts, name servers communicate to resolve names (address/name translation)
    • Note: core Internet function, implemented as application layer protocol
    • Complexity at network’s “edge
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9
Q

DNS: services, structure

A
  • DNS services
    – Hostname to IP address translation
    – Host aliasing
    * Alias namesècanonical
    – Mail server aliasing
    – Load distribution
    * Replicated Web servers: many IP addresses correspond to one name
  • Why not centralize DNS?
    – Single point of failure
    – Traffic volume
    – Distant centralized database
    – Maintenance

Answer: it wouldn’t scale! DNS handles billions of queries per day!

  • Host aliasing:
    • www.lancaster.ac.uk -> www.lancs.ac.uk
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10
Q

DNS: a distributed, hierarchical database

A
  • Client wants IP for www.amazon.com; 1st approximation:
    – Client queries root server to find com DNS server
    – Client queries .com DNS server to get amazon.com DNS server
    – Client queries amazon.com DNS server to get IP address for www.amazon.com
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11
Q

Internet Domain Names

A
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12
Q

Subdomains

A

One domain is a subdomain of another if
its domain name ends in the other’s domain name
– So comp.lancs.ac.uk is a subdomain of
* lancs.ac.uk
which is sub-domain of
* ac.uk
which is sub-domain of
* uk

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13
Q

Fully Qualified Domain Names (FQDN)

A
  • FQDNs end with a dot
    – Implies rooted at top of DNS hierarchy
    – No further resolution needed
    – cs-lab.co.uk.
  • Names without a dot can be extended toward root
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14
Q

DNS: root name servers

A
  • Contacted by local name server that can not resolve name
  • Root name server:
    – Contacts authoritative name server if name mapping not known
    – Gets mapping
    – Returns mapping to local name server
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15
Q

‘13’ Root Servers

A
  • Updated twice a day from non-public registry file server*
  • Each server has a redundant backup
  • They are also replicated across globe
    – More than 13 physical machines!
    – Get records for closest servers
    – Addresses for one of each server hard-coded into resolvers etc
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16
Q

‘13’ Root Servers (The magical few)

A
17
Q

TLD, authoritative servers

A
  • Top-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 .eduTLD
  • 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
18
Q

Local DNS name server

A
  • Does not strictly belong to hierarchy
  • Each ISP (residential ISP, company, university) has one
    – Also called “default name server”
  • When host makes DNS query, query is sent to its local DNS server
    – Has local cache of recent name-to-address translation pairs (but may be out of date!)
    – Acts as proxy, forwards query into hierarchy
19
Q

DNS name resolution example

A
  • Host as cis.poly.edu wants IP address for gaia.cs.umass.edu
  • Iterative query:
    • Contacted server replies with name of server to contact
    • “I don’t know this name, but ask this server”
20
Q

DNS name resolution example

A
  • Recursive query:
    – Puts burden of name resolution on contacted name server
    – Heavy load at upper levels of hierarchy?
21
Q

DNS: caching, updating records

A
  • Once (any) name server learns mapping, it caches mapping
    – Cache entries timeout (disappear) after some time (TTL)
    – TLD servers typically cached in local name servers
    * Thus 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
22
Q

DNS records

A

DNS: distributed database storing resource records (RR)

type=A
- name is hostname
- value is IP address

type=NS
– name is domain (e.g., foo.com)
– value is hostname of authoritative name server for this domain

type=CNAME
- name is alias name for some “canonical” (the real) name
- www.lancaster.ac.uk is really www.lancs.ac.uk
- value is canonical name

type=MX
- value is name of mailserver associated with name

23
Q

DNS protocol, messages

A
  • Query and reply messages, both with same message format
24
Q

Life of a DNS Registration

A
25
Q

Inserting records into DNS

A
  • 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 and insert:
    – type A record for www.networkuptopia.com
    – type MX record for networkutopia.com
26
Q

What happens if DNS is slow or
unreliable?

A
  • Really depends…
  • Can induce significantly delay to a request
    – Already 1 RTT
    – Usually ms (could be longer depending on resolution and local DNS state)
    – A blocking operation!
  • If the record is cached at the host, then we don’t need to look up
    – Valid until the cache expires
27
Q

Attacking DNS

A

DDoS attacks
– Bombard root servers with traffic
* Not successful to date
– October 2002: massive DDoS against the root name servers
– What was the effect?
» … users didn’t even notice
– Root zone file is cached almost everywhere
* Traffic filtering
* Local DNS servers cache IPs of TLD servers, allowing root server bypass

  • Bombard TLD servers
    – Potentially more dangerous
  • Redirect attacks
    – Man-in-middle
    * Intercept queries
    – DNS poisoning
    * Send bogus relies to DNS server, which caches
  • Exploit DNS for DDoS
    – Send queries with spoofed source address: target IP
    – Requires amplification
28
Q

DNS Hijacking

A
  • Infect OS or browser with a virus/trojan
    – e.g. Many trojans change entries in /etc/hosts
    – *.bankofamerica.com à evilbank.com
  • Response Spoofing
    – Eavesdrop on requests
    – Outrace the servers response
29
Q

DNS Hijacks: ID Prediction

A
  • Requester will believe first response it sees
    – Dangerous if query for server A record
    – Disaster if query for Name Server (use this for subsequent queries)
    – Response must
    * Have correct query ID
    * Be to requester’s port number
    – On many early systems these could be predicted
  • We now randomise
    – Query IDs
    – Source ports for queries
30
Q

Solution: DNSSEC

A
  • Cryptographically sign critical resource records
    – Resolver can verify the cryptographic signature
  • Two new resource types
    – Type = DNSKEY
    * Name = Zone domain name
    * Value = Public key for the zone
    – Type = RRSIG
    * Name = (type, name) tuple, i.e. the query itself
    * Value = Cryptographic signature of the query results
  • Deployment
    – On the roots since July 2010
    – Verisign enabled it on .com and .net in January 2011
    – Comcast is the first major ISP to support it (January 2012)

SCC 203 Computer Network (19 decks)

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