hostname cmd

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UNIX ToolsG22.2245-001, Fall 2000

• Danielle S. Lahmani
• email lahmani_at_cs.nyu.edu

2
UNIX Networking

• UNIS was one of the first OS to provide access
  to widely distributed local networks as well as
  Internet networks.
• 1990's huge explosion in the use of UNIX
  networks, because of UNIX networking capabilities
  using a client-server paradigm.

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UNIX Networking

• Designed to allow different computers to
  communicate easily.
• provided ranges of communication and networking
  capabilities
• electronic mail
• file transfer
• logging in on remote system
• remote execution of cmds
• file sharing

4
Networking Terminology

• A computer network is a communication system for
  connecting end-systems.
• A host is a name used to refer to an end-system
• can range in size
• dedicated such as a file or print servers
• or general purpose time-sharing systems

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Types of networks

• LAN Local Area Network connects computer systems
  that are close together (single-building or a few
  kilometers apart)
• WAN Wide Area Network connects computers in
  different cities or countries (referred sometimes
  as Long Haul Network)
• Internet or Internetworking is the connection of
  two or more networks so that computers on one
  network are able to communicate with computers on
  another network.

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Local Area Networks

• popular technologies token ring or Ethernet
• Ethernet standard sending a message using a
  broadcast mechanism with possibility of a
  collision.
• LANs operate at high speeds
• 10Mbps (million bits per second) using Ethernet
• 100 Mbps using FDDI (Fiber Distributed Data
  Interface)

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Network Definition

• Different entities in an internet must agree on
• Protocols to communicate, (e.g., TCP/IP protocol
  suite.)
• Addressing scheme
• Naming scheme
• Routine scheme

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OSI Model

• protocols are often complex and are designed in
  layers
• The OSI Reference Model
• Application
• Presentation
• Session
• Transport
• Network
• Data Link
• Physical

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UNIX UUCP

• serial-line based or dial-up networking
• older than TCP/IP
• point to point communication store and forward
  type networking
• for many years, primary means of networking in
  UNIX
• provides basic networking utilities

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UUCP Usage Today

• viewed as second class alternative to the
  Internetbatch mode usage
• not adequate for high-speed networking
• UUCP connections are non-interactive
• not available on many OS
• therefore, cannot be used in heterogeneous
  environment

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Unix TCP/IP Internet Package

• is ethernet-based, supports high-speed
  connections
• bundled with BSD UNIX since 1982
• TCP is the Transmission Control Protocol and IP
  is the Internet Protocol

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TCP/IP Protocol Suite

• IP networks are packet-switched
• high-speed IP networking offers interactive
  connections between remote hosts
• implemented on different platforms and OS used
  in heterogenous environments
• networking based on TCP/IP is the basis for the
  Internet

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TCP/IP

• TCP is a connection-oriented protocol that
  provides a reliable, full-duplexed byte-stream
  for a user process.
• IP is the internet protocol that provides the
  packet delivery service for TCP, UDP or ICMP. It
  provides a connection-less and unreliable
  delivery system.
• IP datagrams contain source and destination
  addresses. Each can be routed and delivered
  independently.

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Internet Addresses using IP

• 32 bits IP addresses encode the network ID and
  the Host ID. The Host ID is relative to the
  Network ID.
• Every host on a network must have a unique ID
  assigned by a central authority
• internet addresses are written as four decimal
  numbers, separated by decimal points
• example spunky 128.122.80.31
• These addresses are being extended to 128 bits
  quantities in IPv6 standard.

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IP Naming

• correlation can be assigned between a host name
  and an IP address.
• Mapping is kept in a file called /etc/hosts on
  local host.

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UNIX TCP/IP Internet Package

• establishes TCP/IP networking
• provides set of user-level commands for
  networking tasks
• DARPA commands
• Berkeley Remote commands (aka as r commands)
• Unix System to Unix System commands

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The r commands

• rcp remote copy allows you to copy files
  between your local UNIX host and another remote
  UNIX host
• scp secure remote copy
• rlogin allows you to login on a remote UNIX host
• rsh allows you to execute a command on a remote
  UNIX host. Security issue
• ssh secure shell client (remote login program)
• provides secure encrypted communications between
  two untrusted hosts over an insecure network.

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Security Issues with r commands

• host equivalence based on the idea that if the
  user has been authenticated on one trusted
  computer (host), then there is no reason to
  reauthenticate the user on a second computer.
• host equivalence extended through use of .rhost,
  .netrc and /etc/hosts.equiv files.
• host level security /etc/host.equiv
• user level security .rhosts file

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Security Pitfalls of the r cmds

• An ordinary user can create a file .rhost in her
  HOME directory and extend host equivalence to
  herself when accessing the computer without any
  intervention from a system administrator.
• In Sun systems, the r commands have been
  modified to run on top of ssh (secure shell)
  which requires a password and ignores .rhosts
  equivalence.

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DARPA commands

• ftp (file transfer protocol) allows you to copy
  files between your local UNIX host and any other
  host (possibly non-UNIX) that support the ftp
  protocol.
• telnet allows you to execute on any remote host
  that has a telnet server.

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Obtaining Information about other users and hosts

• rwho
• finger get information on a particular user on
  any machine in your network
• ruptime to get status of all machines on the
  network
• ping to check if remote machine is up

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The Client/Server Model

• Server is a process that is waiting to be
  contacted by a client process so that server can
  do something for the client.
• Server is started, sleeps waiting for a service
  request from a client
• Client processes started on same system or
  another system within a network.
• Client process sends a request across the network
  to the server requesting service of some form.

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Unix Client/Server Model

• Implemented using sockets.
• One process, a server, creates a socket whose
  name is known by other client processes
• Client first creates an unnamed socket and then
  requests that it be connected to the server's
  named socket.
• Successful connection returns one file descriptor
  to the client and another one to the server, both
  of which may be used for reading and
  writing.