Transcript CS 497C - Lecture 12

CS 497C – Introduction to UNIX
Lecture 35: - TCP/IP Networking
Tools
Chin-Chih Chang
[email protected]
TCP/IP Basics
• TCP/IP is a set of networking protocols.
• These protocols define a set of rules that
each machine must comply with to
communicate with another machine in the
network.
• The term expands to Transmission
Control/Internet Protocol. The features of
TCP/IP include:
– Independence of vendor, type of machine and
operating system.
TCP/IP Basics
– Independence of vendor, type of machine and
operating system.
– The delivery of data in multiple packets
– Ability to divert data immediately through other
routines if one or more parts of the network
went down.
– One hundred percent reliability of transmission
with facilities for full error control.
• Unlike the telephone system, TCP/IP is a
packet-switching system. In a packetswitched network, there is no dedicated
connection.
TCP/IP Basics
• The data is broken into packets, and each
packet is provided with a header (envelop).
• As the packets travel along a vast network
like the Internet, they encounter routers.
• Routers are special computers or devices
that look at the envelope addresses and then
determine the most efficient route.
• In a network, a computer is known as a
host, and every such host has a hostname.
TCP/IP Basics
$ hostname
• Every host in the network has an address
called the IP address.
• This address is a series of four dot-delimited
members which could typically look like
this:
156.26.10.41
• On a small network, the name-address
mappings are placed in the file /etc/hosts
in every host of the network.
TCP/IP Basics
• The Domain Name System (DNS) is a
service available in a TCP/IP network
which uses the concept of domains and
zones to describe uniquely the name of a
host in a network.
• TCP/IP works in the client-server model.
The client application (like ftp)
communicates with its server counterpart at
the other end to achieve its task.
TCP/IP Basics
• The server programs are known as
daemons, which run in the background and
listen for requests.
• The http daemon listens for a Web page
request. sendmail is the daemon which
handles your mail.
• A specific port number is associated with
the ftp service, so the packet reaches the ftp
server.
TCP/IP Basics
• Daemons listen for requests at certain
specific port numbers assigned to them.
• sendmail listens on port 25, ftp on 21 and
telenet on 23.
• The port numbers used by the server
programs are listed in /etc/services.
• Every packet includes a set of four numbers
– the IP addresses and TCP port numbers at
each end.
Networking Tools
• talk is a popular network communications
program.
talk charlie
talk charlie@kirk
• Weather you can write or talk depends on
the setting of mesg.
• The command mesg n prevents other
people from writing to a terminal. mesg y
enables receipt of such messages.
finger: Details of Users
• finger (from Berkeley) is a useful command
that reveals details of users.
$ finger @kirk
$ finger romeo@kirk
• A finger enquiry of a user displays the
contents of two files, .plan and .project, in
the user’s home directory.
• telnet lets you log on to a remote machine
by supplying a username and password.
telnet: Remote Login
• When telnet is used without the address, the
system displays the telnet> prompt.
• You can now invoke a login session from
here with open, close a session with close,
log out with logout.
• You can do a telnet connection from the
Web browser: telnet://kirk.cs.twsu.edu.
• rlogin is Berkeley’s implementation of the
remote login facility, but doesn’t require a
password.
rlogin, ftp
$ rlogin kira
• rlogin can also be used with the –l option to
access other accounts.
$ rlogin -l franklin sisko
• ftp is used to upload (put and mput) and
download (get and mget) files between two
hosts.
• For the purpose of transfer, files can be
seen as belonging to two types – ascii (text)
and binary.
ftp: File Transfer Protocol
• The name “anonymous” and the email
address are used to access an anonymous ftp
site.
• You can specify a ftp session in a Web
browser: ftp://ftp.cs.twsu.edu
• rcp can also transfer files but without have
to log in.
rcp kira:/home/henry/count.pl calculate.pl
rcp henry@kira:count.pl calculate.pl
rcp and rsh
rcp kira:/home/henry/*
rcp –r kira:/home/henry/cgi-bin .
• rsh is used in executing a command on a
remote machine.
rsh kirk ls -l
• The r-utilities relogin, rcp, and rsh can only
be used if proper authorization is provided
at the server end.
Enforcing Security for the
Berkeley r-Utilities
• System level authorization is controlled by
/etc/hosts.equiv.
• Authorization can also be enforced at the
user level with .rhosts.