CS 497C – Introduction to UNIXLecture 35: - TCP/IP Networking

Tools

Chin-Chih Changchang@cs.twsu.edu

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 packet-switched 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 charlietalk 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.plrcp 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.