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The simplest example of a service is 'chargen', or character
generator (port 19). If you make a telnet connection on the chargen
port to a server running the chargen service, this program will
react to this connection by sending a string of characters which
you will see being repeated across your telnet screen. All
you need to do is connect to the service.
Another example of a service is finger (port 79). If
you run a finger
program to request information on a particular user from a specific
host,
and the finger service (or 'fingerd') is running, and if the
user has not
instructed the finger service to ignore requests about him or
her, you will
get back information on that user.
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What services are run from these ports, and how can we learn
more about
them? Ports numbered from 1 to 1024 are called the 'well-known'
ports.
These are listed in RFC 1700 (see http://www.internetnorth.com.au/keith/networking/rfc.html). Many
of the
well-known ports are also listed in a file on your computer called
'services'. On Win95, it's c:\windows\services; on NT, it's
c:\winnt\system32\drivers\etc\services; on many Unix type computers
(your shell account) it's /etc/services.
These ports are called 'well-known' because they are commonly
used by
certain services. For example, the well-known port for sending
email is the
SMTP port, or port 25. Because it is 'well-known', anyone
can send email to anyone else. Because port 110 is the well-known
port for checking email, all email clients know that they have
to connect to a POP server on port 110 in order to retrieve email.
An excellent FAQ (frequently asked questions) on TCP/IP ports
can be found at http://www.technotronic.com/tcpudp.html
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You can get punched in the nose warning: There are many
port scanning
tools, and wannabe hackers use them ... a lot. But for what
purpose? In
most cases all that happens is that a sysadmin or firewall administrator
goes through the logs that computer keeps of who has tried to
hack that
site. He or she then decides whether to ignore your scan or call
the
sysadmin of the site that your scan came from. Even though (in
the US at
least) port scanning is legal, it makes systems administrators
really mad at
you! To avoid getting kicked off your Internet provider, get
permission to
scan first!
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