DNS: Domain Name System

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DNS Domain Name System

• People many identifiers
• SSN, name, Passport
• Internet hosts, routers
• IP address (32 bit) - used for addressing
  datagrams
• name, e.g., gaia.cs.umass.edu - used by humans
• Q map between IP addresses and name ?

• Domain Name System
• distributed database implemented in hierarchy of
  many name servers
• application-layer protocol host, routers, name
  servers to communicate to resolve names
  (address/name translation)
• note core Internet function implemented as
  application-layer protocol
• complexity at networks edge

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DNS name servers

• no server has all name-to-IP address mappings
• local name servers
• each ISP, company has local (default) name server
• host DNS query first goes to local name server
• authoritative name server
• for a host stores that hosts IP address, name
• can perform name/address translation for that
  hosts name

• Why not centralize DNS?
• single point of failure
• traffic volume
• distant centralized database
• Maintenance
• DoS attacks?
• doesnt scale!

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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
• dozen root name servers worldwide
• 13 root DNS servers replication for security and
  reliability
• Top-level DNS server org, edu, com, jp,cn, fr, uk

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Simple DNS example
root name server

• host surf.eurecom.fr wants IP address of
  gaia.cs.umass.edu
• 1. Contacts its local DNS server, dns.eurecom.fr
• 2. dns.eurecom.fr contacts root name server, if
  necessary
• 3. root name server contacts authoritative name
  server, dns.umass.edu, if necessary

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4
3
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authorititive name server dns.umass.edu
1
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requesting host surf.eurecom.fr
gaia.cs.umass.edu
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DNS example
root name server

• Root name server
• may not know authoratiative name server
• may know intermediate name server who to contact
  to find authoritative name server

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3
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5
4
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authoritative name server dns.cs.umass.edu
requesting host surf.eurecom.fr
gaia.cs.umass.edu
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DNS iterated queries
root name server

• recursive query
• puts burden of name resolution on contacted name
  server
• heavy load?
• iterated query
• contacted server replies with name of server to
  contact
• I dont know this name, but ask this server

iterated query
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authoritative name server dns.cs.umass.edu
requesting host surf.eurecom.fr
gaia.cs.umass.edu
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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
• update/notify mechanisms under design by IETF
• RFC 2136
• http//www.ietf.org/html.charters/dnsind-charter.h
  tml

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DNS records

• DNS distributed db storing resource records (RR)

• TypeCNAME
• name is an alias name for some cannonical (the
  real) name
• value is cannonical name

• TypeA
• name is hostname
• value is IP address

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

• TypeMX
• value is hostname of mailserver associated with
  name

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DNS records

• For a particular hostname
• If a DNS server is authoritative, it contains
• a Type A record for the hostname
• Otherwise
• Maybe a Type A record for the hostname in cache
• a Type NS record for the domain of the hostname
• a Type A record for the DNS server for that
  domain
• Host gaia.cs.umass.edu
• (umass.edu, dns.umass.edu, NS)
• (dns.umass.edu, 128.119.40.111, A)

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DNS protocol, messages

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

• msg header
• identification 16 bit for query, repy to query
  uses same
• flags
• query or reply
• recursion desired
• recursion available
• reply is authoritative

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DNS protocol, messages
Name, type fields for a query
RRs in reponse to query
records for authoritative servers
additional helpful info that may be used
Try nslookup?
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Mystery How to set up your DNS server?

• You setup a company mynet.com
• Step 1 register your domain name with a
  registrar
• Provide name and IP address mapping
• Primary authoritative DNS server dns1.mynet.com,
  212.212.212.1
• Optional secondary DNS server dns.mynet.com,
  212.212.212.2
• Registrar will insert type NS and A records for
  you
• (mynet.com, dns1.mynet.com, NS)
• (dn1.mynet.com, 212.212.212.1, A)
• Step 2 insert records into your DNS server
• For web server (www.mynet.com, 212.212.212.3,A)
• For mail sever (mail.mynet.com, 212.212.212.4,
  MX)
• Then, others can access your web server and send
  emails

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Socket programming
Goal learn how to build client/server
application that communicate using sockets

• Socket API
• introduced in BSD4.1 UNIX, 1981
• explicitly created, used, released by apps
• client/server paradigm
• two types of transport service via socket API
• unreliable datagram
• reliable, byte stream-oriented

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Socket-programming using TCP

• Socket a door between application process and
  end-end-transport protocol (UCP or TCP)
• TCP service reliable transfer of bytes from one
  process to another

controlled by application developer
controlled by application developer
controlled by operating system
controlled by operating system
internet
host or server
host or server
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Socket programming with TCP

• Client must contact server
• server process must first be running
• server must have created socket (door) that
  welcomes clients contact
• Client contacts server by
• creating client-local TCP socket
• specifying IP address, port number of server
  process

• When client creates socket client TCP
  establishes connection to server TCP
• When contacted by client, server TCP creates new
  socket for server process to communicate with
  client
• allows server to talk with multiple clients

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Socket programming with TCP

• Example client-server app
• client reads line from standard input, sends to
  server via socket server reads line from socket
• server converts line to uppercase, sends back to
  client
• client reads, prints modified line from socket

• Input stream sequence of bytes into process
• Output stream sequence of bytes out of process

outToServer
iinFromServer
inFromUser
client socket
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Client/server socket interaction TCP
Server (running on hostid)
Client
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Example C client (TCP)
include ltstdio.hgt / Basic I/O routines
/ include ltsys/types.hgt / standard system
types / include ltnetinet/in.hgt / Internet
address structures / include ltsys/socket.hgt /
socket interface functions / include ltnetdb.hgt
/ host to IP resolution / int main(int argc,
char argv) / Address resolution
stage / struct hostent hen
gethostbyname(argv1) if (!hen)
perror("couldn't resolve host
name") struct sockaddr_in
sa memset(sa, 0, sizeof(sa)
sa.sin_family AF_INET sa.sin_port
htons(PORT) //server port number
memcpy(sa.sin_addr.s_addr, hen-gth_addr_list0,
hen-gth_length) int cli_socket
socket(AF_INET, SOCK_STREAM, 0)
assert(cli_socket gt 0) //I am just lazy here!!
connect(s, (struct sockaddr )sa,
sizeof(sa)) write(s,
argv2, strlen(argv2)) //send it to server
char bufBUFLEN int rc
memset(buf, 0, BUFLEN) char pc
buf while(rc read(cli_socket, pc,
BUFLEN (pc - buf))) pc rc
write(1, buf, strlen(buf))
close(cli_socket)
Create client socket, connect to server
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Example C server (TCP)
//include header files define PORT 6789 int
main(int argc, char argv) struct
sockaddr_in sa, csa memset(sa, 0,
sizeof(sa) sa.sin_family AF_INET
sa.sin_port htons(PORT) sa.sin_addr.s_addr
INADDR_ANY //any IP addr. Is accepted int
s socket(AF_INET,SOCK_STREAM, 0) assert(
sgt0) int rc bind(s, (struct sockaddr )
sa, sizeof(sa)) //hook s with port rc
listen(s, 5) int cs_socket accept(s,
(struct sockaddr)csa, sizeof(csa)) char
bufBUFLEN memset(buf, 0, BUFLEN) char
pc buf while(rc read(cs_socket, pc,
BUFLEN (pc - buf))) pc rc
upper_case(buf) // covert it into upper case
write(cs_socket, buf, strlen(buf))
close(cs_socket) close(s)
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Multi-Clients Servers

• Two main approaches to designing such servers.
• Approach 1.
• The first approach is using one process that
  awaits new connections, and one more process (or
  thread) for each Client already connected. This
  approach makes design quite easy, cause then the
  main process does not need to differ between
  servers, and the sub-processes are each a
  single-Client server process, hence, easier to
  implement.
• However, this approach wastes too many system
  resources (if child processes are used), and
  complicates inter-Client communication If one
  Client wants to send a message to another through
  the server, this will require communication
  between two processes on the server, or locking
  mechanisms, if using multiple threads.
• See tutor for details!

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Socket programming with UDP

• UDP no connection between client and server
• no handshaking
• sender explicitly attaches IP address and port of
  destination
• server must extract IP address, port of sender
  from received datagram
• UDP transmitted data may be received out of
  order, or lost

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Summary

• Our study of network apps now complete!

• application service requirements
• reliability, bandwidth, delay
• client-server paradigm
• Internet transport service model
• connection-oriented, reliable TCP
• unreliable, datagrams UDP

• specific protocols
• http
• ftp
• smtp, pop3
• dns
• socket programming
• client/server implementation
• using tcp, udp sockets

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Summary

• Most importantly learned about protocols

• typical request/reply message exchange
• client requests info or service
• server responds with data, status code
• message formats
• headers fields giving info about data
• data info being communicated

• control vs. data msgs
• in-based, out-of-band
• centralized vs. decentralized
• stateless vs. stateful
• reliable vs. unreliable msg transfer
• complexity at network edge
• security authentication