Page 186 - DCAP516_COMPUTER_SECURITY

Computer Security

Notes (binaries, password files, capability/acl databases) and other host activities and state. An
example of a HIDS is OSSEC.
5. A hybrid intrusion detection system combines two or more approaches. Host agent data
is combined with network information to form a comprehensive view of the network. An
example of a Hybrid IDS is Prelude.

14.6 Electronic Mail (e-mail)

Email is the most widely used application service, which is used by computer users. It differs
from other uses of the networks as network protocols send packets directly to destinations using
timeout and retransmission for individual segments if no acknowledgement returns. However,
in the case of email the system must provide for instances when the remote machine or the
network connection has failed and take some special action. Email applications involve two
aspects:
1. User-agent (pine, elm, etc.)

2. Transfer agent (send mail daemon, etc.)
When an email is sent it is the mail transfer agent (MTA) of the source that contacts the MTA of
the destination. The protocol used by the MTA’s on the source and destination side is called
SMTP. SMTP stands for Simple Mail Transfer Protocol. The first e-mail systems: file transfer
protocols plus the convention that the first line of each file contained the recipient’s address.
Complaints:
1. Inconvenient for sending to a group of people.
2. Messages had no internal structure, making computer processing difficult.
3. The originator (sender) never knew if a message arrived or not.

4. Hard to redirect incoming mails to another address.
5. Poor integration of the user interface with the transmission system.
6. Impossible to create and send messages containing a mixture of text, drawing, facsimile,
and voice.
Various new proposals were made for more ambitious e-mail systems. Our discussion of email
will focus on the ARPANET email proposals published as RFC 821 (transmission protocol) and
RFC 822 (message format) as used in the Internet.

14.6.1 Functions of an e-mail

Five basic functions of an e-mail system:
1. Composition refers to the process of creating message and answers.

2. Transfer refers to moving messages from the originator to the recipient.
3. Reporting has to do with telling the originator what happened to the message.
4. Displaying incoming messages is needed so people can read their email.
(a) Sometimes, conversion may be necessary to make the message suitable for display
on recipient’s terminal or printer.
5. Disposition is the final step and concerns what the recipient does with the message after
receiving it.

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