Lecture 10 Doman Name System

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Lecture 10Doman Name System

• CPE 401 / 601Computer Network Systems

slides are modified from Dave Hollinger and
Kurose Ross
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Hostnames

• People many identifiers
• SSN, name, passport
• Internet hosts, routers
• IP address (32/128 bit) - used for addressing
  datagrams
• name, e.g., ww.yahoo.com - used by humans
• http//www.youtube.com/watch?v9ilYrY9iBhk
• IP Addresses are great for computers
• IP address includes information used for routing.
• IP addresses are tough for humans to remember.
• IP addresses are impossible to guess.
• ever guessed at the name of a WWW site?

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Domain Name System

• The domain name system is usually used to
  translate a host name into an IP address .
• Domain names comprise a hierarchy so that names
  are unique, yet easy to remember.
• distributed database
• implemented in hierarchy of many name servers
• hosts, routers, communicate with name servers to
  resolve names (address/name translation)

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Host name structure

• Each host name is made up of a sequence of labels
  separated by periods.
• Each label can be up to 63 characters
• The total name can be at most 255 characters.
• Examples
• whitehouse.gov
• barney.the.purple.dinosaur.com
• cnl.cse.unr.edu

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Domain Name

• The domain name for a host is the sequence of
  labels that lead from the host (leaf node in the
  naming tree) to the top of the worldwide naming
  tree.
• A domain is a subtree of the worldwide naming
  tree.

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DNS services

• hostname to IP address translation
• host aliasing
• Canonical, alias names
• mail server aliasing
• load distribution
• replicated Web servers set of IP addresses for
  one canonical name
• Why not centralize DNS?
• single point of failure
• traffic volume
• distant centralized database
• maintenance

doesnt scale!
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Distributed, Hierarchical Database

• Client wants IP for www.unr.edu 1st approx
• client queries a root server to find edu DNS
  server
• client queries edu DNS server to get unr.edu DNS
  server
• client queries unr.edu DNS server to get IP
  address for www.unr.edu

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DNS Root name servers

• 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

a Verisign, Dulles, VA c Cogent, Herndon, VA
(also LA) d U Maryland College Park, MD g US DoD
Vienna, VA h ARL Aberdeen, MD j Verisign, ( 21
locations)
k RIPE London (also 16 other locations)
i Autonomica, Stockholm (plus 28 other
locations)
m WIDE Tokyo (also Seoul, Paris, SF)
e NASA Mt View, CA f Internet Software C. Palo
Alto, CA (and 36 other locations)
13 root name servers worldwide
b USC-ISI Marina del Rey, CA l ICANN Los
Angeles, CA
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Top-Level Domain Servers

• responsible for com, org, net, edu, gov,
• Network Solutions maintains servers for com TLD
• Educause for edu TLD
• Countries each have a top level domain
• 2 letter domain name
• uk, fr, ca, jp,
• New top level domains include
• .aero .biz .coop .info .name .pro

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Authoritative Servers

• organizations DNS servers,
• providing authoritative hostname to IP mappings
  for organizations servers (e.g., Web, mail).
• The organization that owns a domain name is
  responsible for running a DNS server that can
  provide the mapping between hostnames within the
  domain to IP addresses.
• So - some machine run by UNR is responsible for
  everything within the rpi.edu domain.
• can be maintained by organization or service
  provider

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DNS Distributed Database

• There is one primary server for a domain, and
  typically a number of secondary servers
  containing replicated databases.

unr.edu DNS server
unr.edu DNS DB
rpi.edu DNS DB
unr.edu DNS DB
rpi.edu DNS DB
Authoritative
Replicas
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Local Name Server

• 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
• acts as proxy, forwards query into hierarchy

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DNS caching and updating records

• once (any) name server learns mapping, it caches
  mapping
• cache entries timeout (disappear) after some time
• TLD servers typically cached in local name
  servers
• Thus root name servers not often visited
• DNS Cache poisoning attacks
• http//www.youtube.com/watch?v1d1tUefYn4U
• update/notify mechanisms under design by IETF
• RFC 2136
• http//www.ietf.org/html.charters/dnsind-charter.h
  tml

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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

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DNS Clients

• A DNS client is called a resolver.
• A call to gethostbyname()is handled by a resolver
  (typically part of the client).
• Most Unix workstations have the file
  /etc/resolv.conf that contains the local domain
  and the addresses of DNS servers for that domain.

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/etc/resolv.conf

• search cse.unr.edu me.unr.edu engr.unr.edu
  ce.unr.edu it.unr.edu unr.edu
• nameserver 134.197.5.1
• nameserver 134.197.6.1

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nslookup

• nslookup is an interactive resolver that allows
  the user to communicate directly with a DNS
  server.
• nslookup is usually available on Unix
  workstations.
• dig and host are also DNS clients

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DNS Servers

• Servers handle requests for their domain
  directly.
• Servers handle requests for other domains by
  contacting remote DNS server(s).
• Servers cache external mappings.

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Server - Server Communication

• If a server is asked to provide the mapping for a
  host outside its domain (and the mapping is not
  in the server cache)
• The server finds a nameserver for the target
  domain.
• The server asks the nameserver to provide the
  host name to IP translation.
• To find the right nameserver, use DNS!

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DNS Data

• DNS databases contain more than just
  hostname-to-address records
• Name server records NS
• Hostname aliases CNAME
• Mail Exchangers MX
• Host Information HINFO

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Server Operation

• If a server has no clue about where to find the
  address for a hostname, ask the root server.
• The root server will tell you what nameserver to
  contact.
• A request may get forwarded a few times.

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DNS name resolution example
root DNS server

• Host at cse.unr.edu wants
• IP address for rec.dri.edu

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TLD DNS server
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5

• iterated query
• contacted server replies with name of server to
  contact
• I dont know this name, but ask this server

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7
1
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authoritative DNS server dns.dri.edu
requesting host cse.unr.edu
rec.dri.edu
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Recursion

• A request can indicate that recursion is desired
  - this tells the server to find out the answer
  (possibly by contacting other servers).
• If recursion is not requested - the response may
  be a list of other name servers to contact.

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DNS name resolution example

• recursive query
• puts burden of name resolution on contacted name
  server
• heavy load?

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UDP TCP

• Both UDP and TCP are used
• TCP for transfers of entire database to secondary
  servers (replication)
• UDP for lookups
• If more than 512 bytes in response
• requestor resubmits request using TCP.

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DNS records

• DNS distributed db storing resource records (RR)

• TypeA
• name is hostname
• value is IP address

• TypeCNAME
• name is alias name for some canonical (the
  real) name
• www.ibm.com is really
• servereast.backup2.ibm.com
• value is canonical name

• TypeAAA

• TypeNS
• name is domain (e.g. foo.com)
• value is hostname of authoritative name server
  for this domain

• TypeMX
• value is name of mailserver associated with name

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DNS protocol, messages

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

• identification
• 16 bit for query,
• reply to query uses same
• flags
• QR Query0, Response1
• AA Authoritative Answer
• TC response truncated (gt 512 bytes)
• RD recursion desired
• RA recursion available
• rcode return code

Response
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DNS protocol, messages
Name, type fields for a query
RRs in response to query
records for authoritative servers
additional helpful info that may be used
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Name to Address Conversion

• There is a library of functions that act as DNS
  client (resolver).
• you dont need to write DNS client code to use
  DNS!
• With some OSs you need to explicitly link with
  the DNS resolver library
• -lnsl (nsl is Name Server Library)

Suns (Solaris) need this!
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DNS library functions

• gethostbyname
• gethostbyaddr
• gethostbyname2

IPV6!
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gethostbyname

• Return IP addresses of a host
• struct hostent gethostbyname( const char
  hostname)
• struct hostent is defined in netdb.h
• All the IP addresses returned via the hostent are
  in network byte order!

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hostent picture

• h_name
• h_aliases
• h_addrtype
• h_length
• h_addr_list

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Which Address?

• On success, gethostbyname returns the address of
  a hostent that has been created.
• has an array of ptrs to IP addresses
• Usually use the first one
• define h_addr h_addr_list0

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gethostbyname and errors

• On error gethostbyname return null.
• Gethostbyname sets the global variable h_errno to
  indicate the exact error
• HOST_NOT_FOUND
• TRY_AGAIN
• NO_RECOVERY
• NO_DATA
• NO_ADDRESS

All defined in netdb.h
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Getting at the address

• char h_addr_list
• h gethostbyname("joe.com")
• sockaddr.sin_addr.s_addr
• (h-gth_addr_list0)
• This won't work!!!!
• h_addr_list0 is a char !

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Using memcpy

• You can copy the 4 bytes (IPv4) directly
• h gethostbyname("joe.com")
• memcpy(sockaddr.sin_addr,
• h-gth_addr_list0,
• sizeof(struct in_addr))

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gethostbyaddr

• Return hostname corresponding to an IP.
• struct hostent gethostbyaddr( const char addr
• size_t len,
• int family)

sizeof(struct in_addr)
AF_INET (could be AF_INET6)
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Some other functions

• uname
• get hostname of local host
• getservbyname
• get port number for a named service
• getservbyaddr
• get name for service associated with a port
  number