Chapter 26
Remote Logging,
Electronic Mail,
and File Transfer
26.1
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26-1 REMOTE LOGGING
It would be impossible to write a specific client/server
program for each demand. The better solution is a
general-purpose client/server program that lets a user
access any application program on a remote computer.
Topics discussed in this section:
TELNET
SSH
26.2
Note
TELNET is a general-purpose
client/server application program.
Remote virtual terminal
One server supports multiple VTs.
26.3
Example of Telnet (Character mode)
26.4
Image from Dr. Wang’s course at:
http://www.cse.scu.edu/~mwang2/tcpip/
Some Common Features of Telnet
• Telnet is on a single TCP connection to port 23
• Telnet connection is unencrypted
• So easy for eavesdropper!
• In most cases, telnet is a remote terminal. Each
character is transmitted in a separated packet
• The same is true for SSH login stage
• A feature that could be exploited for “timing
attack”.
26.5
Telnet Commands
• telnet longwood.eecs.ucf.edu
• setup a remote shell to the server
• telnet longwood.eecs.ucf.edu 25
• Setup a TCP connection to port 25 (email
service)
• We will show this usage in Email lecture
26.6
SSH – Secure Shell
• Replacement of old unsecure Telnet program
• Both ends authenticate with each other
• Rely on public key cryptography
• Will introduce PK in last chapter
• All communication messages are encrypted
• SSH is used also as a secure tunneling channel
for other applications
• File transfer
• Port forwarding (such as X window)
26.7 • Virtual private network (VPN)
SSH – Secure Shell
• Under Unix machine:
• Hidden directory .ssh saves all configuration
files
• “known_hosts” saves public keys of all
previous connected SSH servers
• Attacker cannot impersonates as a known
SSH server anymore
• Have to trust what you get in the first
connection attempt
26.8
SSH Programs and Usages
• Many open source ssh client software
•
http://en.wikipedia.org/wiki/Comparison_of_SSH_clients
• PuTTY
• SSH uses TCP connection on port 22
• Login methods:
• Password
• private key kept on client, public key stored
on server (come to this in later security chapter)
26.9
26-2 ELECTRONIC MAIL
One of the most popular Internet services is electronic
mail (e-mail). The designers of the Internet probably
never imagined the popularity of this application
program. Its architecture consists of several
components that we discuss in this chapter.
Topics discussed in this section:
Architecture
User Agent
Message Transfer Agent: SMTP
Message Access Agent: POP and IMAP
Web-Based Mail
26.10
Figure 26.6 First scenario in electronic mail
MUA: mail user agent
26.11
Note
When the sender and the receiver of an
e-mail are on the same system,
we need only two user agents.
MUA:Outlook Express, Netscape
Messenger, Mozilla Thunderbird,
Eudora, Foxmail,…….
26.12
Note
When the sender and the receiver of an
e-mail are on different systems,
Wikipidia shows a good example of
email operation (next page)
26.13
Typical Email Operation (from Wikipidia)
Could also be
IMAP4
Email server’s DNS
name and IP
26.14
DNS query for MX
RR
Client’s Email Operation Mode
• Remote terminal using SSH to email server
directly
• Then uses MUA on email server
• Such as Pine, Elm on Unix machines
• Use MUA on a client’s own computer
• The MUA formats and sends email to
sender’s email server
• Web-based email systems using web browser
• Gmail, yahoo mail, hotmail,….
• Spammer: program a simple code sending
email to email servers via SMTP protocol
26.15
Figure 26.11 Services of user agent
26.16
Note
Some examples of command-driven
user agents are mail, pine,
and elm (under Unix).
26.17
Note
Some examples of GUI-based user
agents are Eudora, Outlook, and
Netscape.
26.18
Figure 26.12 Format of an e-mail
26.19
Figure 26.13 E-mail address
• Sender uses DNS query for MX record on the
domain name in order to find the name and IP
of the target email server
26.20
Figure 26.14 MIME (Multipurpose Internet Mail Extensions)
NVT: network virtual terminal
(telnet)
SMTP protocol only this data
26.21
MIME
• User agents makes the translation between
non-ASCII data and ASCII email message
• If you check the source file of an email that has
attachment, you will find the MIME usage
• This is the main reason why your email with an
attachment becomes bigger?
26.22
Figure 26.15 MIME header
26.23
Table 26.5 Data types and subtypes in MIME
26.24
Table 26.6 Content-transfer-encoding
26.25
Figure 26.16 SMTP (Simple Mail Transfer Protocol) range
POP3 or IMAP4
26.26
Figure 26.17 Commands and responses
26.27
Figure 26.18 Command format
26.28
Table 26.7 Commands
26.29
Table 26.8 Responses
26.30
Table 26.8 Responses (continued)
26.31
Example 26.3
Let us see how we can directly use SMTP to send an
e-mail and simulate the commands and responses we
described in this section. We use TELNET to log into port
25 (the service port for SMTP). We then use the
commands directly to send an e-mail.
Through this process, you can see why you cannot trust
the “sender” or any part of a received email, why email
spam is so easy to be generated.
26.32
Sample email sending using telnet
“telnet longwood.cs.ucf.edu 25”
C:
S:
C:
S:
C:
S:
C:
S:
C:
C:
C:
C:
C:
C:
S:
C:
S:
HELO fake.domain
250 Hello crepes.fr, pleased to meet you
MAIL FROM: <alice@crepes.fr>
250 alice@crepes.fr... Sender ok
RCPT TO: <bob@gmail.com>
250 bob@gmail.com ... Recipient ok
DATA
354 Enter mail, end with "." on a line by itself
from: “fake man” <fake@fake.fake.fake>
to: “dr. who” <who@who>
subject: who am I?
Do you like ketchup?
How about pickles?
.
250 Message accepted for delivery
QUIT
221 longwood.cs.ucf.edu closing connection
Try SMTP interaction for yourself:



telnet servername 25
see 220 reply from server
enter HELO, MAIL FROM, RCPT TO, DATA, QUIT
commands



“mail from” the domain may need to be
existed
“rcpt to” the user needs to be existed
A mail server may or may not support “relay”

CS email server supports relay from inside campus
Enable you send email without using email user agent
NOTES: Ethnical Issue on Manual Sending Email
• Do not try to use telnet on department email
server except for the homework!
• Do not try to use it to spam your friends
without upfront notice!
26.35
Figure 26.19 POP3 (Post Office Protocol) and IMAP4 (Internet Mail Access
Protocol)
26.36
Figure 26.20 The exchange of commands and responses in POP3
26.37
POP3 and IMAP4
• IMAP4 provides more functionalities
• Both POP3 and IMAP4 have corresponding
encrypted protocols
26.38
Web-based Email
• Email clients use web browser for all
interactions
• A user can access his/her email at any where,
via any computer that has Internet access
• Email provider may provide more functions
• Gmail has very good search capability
• Gmail has the message grouping function
• Many provide bundled services with their
other products
• Voice/video, photo sharing, ….
26.39
26-3 FILE TRANSFER
Transferring files from one computer to another is one
of the most common tasks expected from a networking
or internetworking environment. As a matter of fact,
the greatest volume of data exchange in the Internet
today is due to file transfer.
Topics discussed in this section:
File Transfer Protocol (FTP)
Anonymous FTP
26.40
Note
FTP uses the services of TCP. It needs
two TCP connections.
The well-known port 21 is used for the
control connection and the well-known
port 20 for the data connection.
26.41
Figure 26.21 FTP
26.42
Figure 26.22 Using the control connection
26.43
Figure 26.23 Using the data connection
26.44
Example 26.4 (continued)
26.45
Example 26.5
We show an example of anonymous FTP. We assume that
some public data are available at internic.net.
continued on next slide
26.46
Example 26.5 (continued)
26.47
Dying of FTP
• People seldom use FTP nowadays
• Replaced with two ways:
• Web-based file upload/download
• Browser shows a directory content if the
directory does not have a default html file
• SSH-based file transfer
26.48