Transcript CSCE 790: Computer Network Security

CSCE 515:
Computer Network Programming
Chin-Tser Huang
[email protected]
University of South Carolina
Simple Mail Transfer Protocol (SMTP)

Email is one of most popular applications



In 1991 about ½ TCP connections are for SMTP
Mail exchange is performed by a message
transfer agent (MTA), for example Sendmail
Users don’t interact with MTA but can choose
their user agent
4/19/2005
2
TCP/IP E-mail Exchange Architecture
user at a
terminal
user
agent
queue of mail
to be sent
message
transfer
agent
client
user
agent
user
mailboxes
message
transfer
agent
server
sender
user at a
terminal
receiver
4/19/2005
3
SMTP Protocol




Specified in RFC 821
Use NVT ASCII for communication between
two MTAs
Client sends commands to server
Server responds with numeric reply codes
with optional strings
4/19/2005
4
SMTP Commands

Basic implementation includes eight commands









HELO
MAIL
RCPT
DATA
QUIT
RSET
VRFY
NOOP
Additional commands include EXPN and TURN
4/19/2005
5
Envelopes, Headers, and Body

Envelopes are used by MTAs for delivery


Headers are used by user agents



Specified by MAIL and RCPT commands
RFC 822 specifies header format
Each header field contains a name, a colon, and field value
Body is content of message from sending user to
receiving user

When transferred using DATA command, headers are sent
first, followed by a blank line, followed by body
4/19/2005
6
Transmission of a Mail


User agent takes body, adds some headers,
and passes to MTA
MTA adds some headers, adds envelope, and
sends to another MTA
4/19/2005
7
Relay Agents


All individual systems send their mail to a relay
system
Two reasons of using relay system



Simplify configuration of all MTAs other than relay system’s
MTA
Allow one system at an organization to act as mail hub
Most organizations use relay system for Internet mail
4/19/2005
8
Improvements on SMTP



Envelope changes: Extended SMTP
Header changes: Non-ASCII characters
Body changes: Multipurpose Internet
Mail Extensions (MIME)
4/19/2005
9
Extended SMTP



Framework for adding extensions to SMTP
Client wishing to use new features can use
EHLO, instead of HELO, to initiate session
Server lists extended commands in the 250
reply to EHLO command
4/19/2005
10
Non-ASCII Characters


Allow additional characters in sender and
receiver names and in subject
Header can contain encoded words
=? charset ? encoding ? encoded-text ?=
 Charset can be us-ascii or iso-8859-1
 Encoding can be quoted-printable or base-64

4/19/2005
Using base-64, three consecutive bytes are encoded as
four 6-bit values
11
Multipurpose Internet Mail Extensions


Allow structures in body
Five new header fields
Mime-Version:
Content-Type:
Content-Transfer-Encoding:
Content-ID:
Content-Description:


Seven content types: text, multipart, message,
application, image, audio, video
Five encoding formats: 7bit, quoted-printable,
base64, 8bit, binary
4/19/2005
12
Simple Network Management Protocol

Need to manage systems within a coherent
framework



Network management consists of communication
between managers and agents



Number of networks within organization grows
Heterogeneity of systems in network
Managers are network management stations
Agents are management software running on network
elements, e.g. hosts, routers, X terminals
Current version is SNMPv3 (RFC 2570)
4/19/2005
13
Components of Network Management

Three components of SNMP



Management Information Base (MIB): specifies what
variables network elements maintain
Structure of Management Information (SMI): a set of
common structures and an identification scheme used to
reference variables in MIB
Simple Network Management Protocol (SNMP):
protocol between manager and element
4/19/2005
14
SNMP Protocol


Use UDP to send messages
Seven types of messages are exchanged between managers and
agents

get-request allows manager to fetch value of variables

get-next-request allows manager to fetch next variable after





specified variables
set-request allows manager to set value of variables
get-response allows agent to return value of variables in response
to get-request, get-next-request, and set-request
trap allows agent to notify manager when something happens on
agent
get-bulk-request allows manager to retrieve large blocks of data
inform-request allows one manager to send info to another
manager
4/19/2005
15
SNMP Messages
SNMP agent
SNMP manager
get-request
UDP port 161
get-response
get-next-request
UDP port 161
get-response
set-request
UDP port 161
get-response
UDP port 162
4/19/2005
trap
16
Structure of Management Info

Data types used by SNMP












INTEGER
OCTET STRING
DisplayString
OBJECT IDENTIFIER
NULL
IpAddress
PhysAddress
Counter
Gauge
TimeTicks
SEQUENCE
SEQUENCE OF
4/19/2005
17
Object Identifiers




A sequence of integers separated by decimal
points
These integers traverse a tree structure,
similar to DNS
Each node in the tree also has a textual name
All variables in MIB start with the object
identifier 1.3.6.1.2.1
4/19/2005
18
Instance Identification



Every variable in MIB must be identified
Simple variables are referenced by appending
“.0” to variable’s object identifier
Table entries are identified by combination of
variables in table
4/19/2005
19
Traps



Agent can send traps to manager to indicate that
something has happened on agent
Traps are sent to UDP port 162 on manager
Seven types of traps







coldStart
warmStart
linkDown
linkUp
authenticationFailure
egpNeighborLoss
enterpriseSpecific
4/19/2005
20
Next Class


Peer-to-peer applications
Read JNP Sec. 24.4
4/19/2005
21