Security Configuration Guide: Zone-Based Policy Firewall Cisco IOS Release 15.1M&T
Application Inspection and Control for SMTP
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Application Inspection and Control for SMTP

Table Of Contents

Application Inspection and Control for SMTP

Finding Feature Information

Contents

Prerequisites for Application Inspection and Control for SMTP

Restrictions for Application Inspection and Control for SMTP

Information About Application Inspection and Control for SMTP

Benefits of Application Inspection and Control for SMTP

Cisco Common Classification Policy Language

Common Classification Engine SMTP Database and Action Module

How to Configure Application Inspection and Control for SMTP

Configuring a Default Policy for Application Inspection

Restricting Spam from a Suspicious E-Mail Sender Address or Domain

Identifying and Restricting Spammers Searching for User Accounts in a Domain

Restricting the Number of Invalid SMTP Recipients

Specifying a Recipient Pattern to Learn Spam Senders and Domain Information

Hiding Specified Private SMTP Commands on an SMTP Connection

Preventing a DoS Attack by Limiting the Length of the SMTP Header

Preventing a DoS Attack by Limiting the Length or TYPE of SMTP Command Line

Examples

Restricting Content File Types in the Body of the E-Mail

Restricting Unknown Content Encoding Types from Being Transmitted

Specifying a Text String to Be Matched and Restricted in the Body of an E-Mail

Configuring the Monitoring of Text Patterns in an SMTP E-Mail Subject Field

Configuring a Parameter to Be Identified and Masked in the EHLO Server Reply

Configuring a Logging Action for a Class Type in an SMTP Policy-Map

Configuration Examples for Application Inspection and Control for SMTP

Example: Creating a Pinhole for the SMTP Port

Example: Preventing ESMTP Inspection

Example: MIME E-Mail Format

Additional References

Related Documents

Standards

MIBs

RFCs

Technical Assistance

Feature Information for Application Inspection and Control for SMTP

Glossary


Application Inspection and Control for SMTP


First Published: July 11, 2008
Last Updated: July 11, 2008

The Application Inspection for SMTP feature provides an intense provisioning mechanism that can be configured to inspect packets on a granular level so that malicious network activity, related to the transfer of e-mail at the application level, can be identified and controlled. This feature qualifies the Cisco IOS firewall extended Simple Mail Transfer Protocol (ESMTP) module as an "SMTP application firewall," which protects in a similar way to that of an HTTP application firewall.

Finding Feature Information

Your software release may not support all the features documented in this module. For the latest feature information and caveats, see the release notes for your platform and software release. To find information about the features documented in this module, and to see a list of the releases in which each feature is supported, see the "Feature Information for Application Inspection and Control for SMTP" section.

Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.

Contents

Prerequisites for Application Inspection and Control for SMTP

Restrictions for Application Inspection and Control for SMTP

Information About Application Inspection and Control for SMTP

How to Configure Application Inspection and Control for SMTP

Configuration Examples for Application Inspection and Control for SMTP

Additional References

Feature Information for Application Inspection and Control for SMTP

Glossary

Prerequisites for Application Inspection and Control for SMTP

Follow the appropriate configuration tasks outlined in the Zone-Based Policy Firewall module before configuring the Application Inspection and Control for SMTP feature. This module contains important information about class-maps and policy-maps and their associated "match" statements necessary for configuring an SMTP policy.

SMTP Policy Requirements

Both SMTP and ESMTP inspection provide a basic method for exchanging e-mail messages between the client and server to negotiate capabilities and use these capabilities in an e-mail transaction. An ESMTP session is similar to an SMTP session, except for one difference—the Extended HELO (EHLO) command. The EHLO command is sent by a client to initiate the capability dialogue. After the client receives a successful response to the EHLO command, the client works the same way as SMTP, except that the client may issue new extended commands, and it may add a few parameters to the MAIL FROM and REPT TO commands.

Previously, if the Cisco IOS software was configured to inspect SMTP session only, inspection was configured by entering the match protocol smtp command. This action would "mask" the EHLO command to prevent capability negotiation and cause the client to go back to the HELO command and basic SMTP.

To have a workable policy for both ESMTP and SMTP inspection, the match protocol smtp command must be configured in the top-level policy before the Application Inspection and Control for SMTP features are implemented. See the "Configuring a Default Policy for Application Inspection" section for more information.

The SMTP policy (which specifies the particular SMTP configuration) is included as a child-policy in the top-level "inspect" policy-map. See the "Top-level Class Maps and Policy Maps" section in the Zone-Based Policy Firewall module for more information.

Restrictions for Application Inspection and Control for SMTP

The Application Inspection and Control for SMTP feature has the following restrictions:

The match cmd-line length gt command filter can co-exist only with a match cmd verb command filter in the SMTP match-all class -map (class-map type inspect smtp). Any attempt to pair the match cmd-line length gt command filter with any other filter is not allowed by the CLI.

The alternative data transfer SMTP command extension BDAT is not supported. This command is substituted for the DATA command while the SMTP body is transferred. The BDAT command extension is used by the Cisco IOS firewall to mask the CHUNKING keyword in the EHLO response to the Application Inspection and Control for SMTP feature, preventing a client from using it.

The "mask" action can be configured only with a class having either or both of the match cmd verb or match ehlo reply commands. This action cannot be configured with a class having any other filter.

Information About Application Inspection and Control for SMTP

The Application Inspection and Control for SMTP feature inspects SMTP in a granular way and is complemented by an intensive provisioning system to help filter e-mail.

Benefits of Application Inspection and Control for SMTP

Cisco Common Classification Policy Language

Common Classification Engine SMTP Database and Action Module

Benefits of Application Inspection and Control for SMTP

The Application Inspection and Control for SMTP feature provides the following benefits:

E-mail senders and user accounts are restricted to filter spam e-mail from suspected domains.

An action can be specified, which occurs when a number of invalid recipients appears on an SMTP connection. This action helps identify spammers who are looking for valid user accounts.

The number of invalid SMTP recipients can be restricted by specifying a maximum number for invalid recipients on an SMTP connection.

A pattern can be specified that identifies e-mail addressed to a particular recipient or domain in cases where a server is functioning as a relay.

A provisioning mechanism that provides masks specified verbs in an SMTP connection to block potentially dangerous SMTP commands.

The maximum length value for the SMTP e-mail header can be specified to prevent a Denial of Service (DoS) attack (also called a buffer overflow attack). A DoS attack occurs when the attacker continuously sends a large number of incomplete IP fragments, causing the firewall to lose time and memory while trying to reassemble the fake packets.

The maximum length of an SMTP command line can be specified to prevent a DoS attack.

Multipurpose Internet Mail Extension (MIME) content file-types (text, HTML, images, applications, documents,and so on) can be restricted in the body of the e-mail from being transmitted over SMTP.

Unknown content-encoding types can be restricted from being transmitted over SMTP.

Specified content-types and content encoding types can be restricted in the SMTP e-mail body.

Monitor arbitrary patterns (text strings) in the SMTP e-mail message header (subject field) or body.

A parameter in an EHLO server reply and mask can be specified to prevent a sender (client) from using the service extension in the server reply.

An SMTP connection can be dropped with an SMTP sender (client) if the SMTP connection violates the specified policy.

SMTP commands or the parameters returned by the server in response to an EHLO command can be explicitly masked by specifying these SMTP commands.

An action can be logged for a class type in an SMTP policy-map.

Cisco Common Classification Policy Language

The Cisco Common Classification Policy Language (C3PL) CLI structure is used to provision ESMTP inspection. ESMTP is provisioned by defining a match criterion on an SMTP class-map and associate actions to the match criterion defined in the SMTP policy-map. The Application Inspection and Control for SMTP feature adds new match criteria and actions to the existing SMTP policy maps that are discussed in the Zone-Based Policy Firewall module, which describes the Cisco IOS unidirectional firewall policy between groups of interfaces known as zones.

Figure 1 ESMTP Communication Between a Sender and Receiver

Common Classification Engine SMTP Database and Action Module

The Common Classification Engine (CCE) SMTP database is the site at which manually configured policy information is processed and converted into signatures. The information in these signatures is put into regular expression tables, which are then used to parse packets as they are switched by a router.

The SMTP database has two interfaces. One interface has the control plane, which is used to accept user configured policies, and the other interface has the CCE data-plane engine, which is used to classify a packet.

An action module is used as a part of the Context-Based Access Control (CBAC) SMTP inspection module to organize and trigger SMTP inspection. CBAC is used to detect and block SMTP attacks (illegal SMTP commands) and sends notifications when SMTP attacks occur.

How to Configure Application Inspection and Control for SMTP

Configuring a Default Policy for Application Inspection

Restricting Spam from a Suspicious E-Mail Sender Address or Domain

Identifying and Restricting Spammers Searching for User Accounts in a Domain

Restricting the Number of Invalid SMTP Recipients

Specifying a Recipient Pattern to Learn Spam Senders and Domain Information

Hiding Specified Private SMTP Commands on an SMTP Connection

Preventing a DoS Attack by Limiting the Length of the SMTP Header

Preventing a DoS Attack by Limiting the Length or TYPE of SMTP Command Line

Restricting Content File Types in the Body of the E-Mail

Restricting Unknown Content Encoding Types from Being Transmitted

Specifying a Text String to Be Matched and Restricted in the Body of an E-Mail

Configuring the Monitoring of Text Patterns in an SMTP E-Mail Subject Field

Configuring a Parameter to Be Identified and Masked in the EHLO Server Reply

Configuring a Logging Action for a Class Type in an SMTP Policy-Map

Configuring a Default Policy for Application Inspection

If no policy is configured for SMTP, then there is no application inspection for SMTP. The firewall creates a TCP session and only performs "pinholing," which allows an application to have access to the protected network. Having an open gap in a firewall can expose the protected system to malicious abuse. The steps below are used to provide minimum application inspection protections for SMTP by enforcing the EHLO and HELO SMTP commands.

SUMMARY STEPS

1. enable

2. configure terminal

3. class-map type inspect smtp class-map-name

4. match protocol smtp

DETAILED STEPS

 
Command or Action
Purpose

Step 1 

enable

Example:

Router> enable

Enables privileged EXEC mode.

Enter your password if prompted.

Step 2 

configure terminal

Example:

Router# configure terminal

Enters global configuration mode.

Step 3 

class-map type inspect smtp class-map-name

Example:

Router(config)# class-map type inspect smtp c1

Creates a class map for the SMTP protocol and enters class-map configuration mode.

Step 4 

match protocol smtp

Example:

Router(config-cmap)# match protocol smtp

Enables inspection for ESMTP and SMTP.

Restricting Spam from a Suspicious E-Mail Sender Address or Domain

An e-mail sender and user accounts can be restricted to filter spam e-mail from suspected domains. Spam is restricted by using the match sender address regex command to match the parameter-map name of a specific traffic pattern that specifies a sender domain or e-mail address in the SMTP traffic. The specified pattern is scanned in the parameter for the SMTP MAIL FROM: command.

SUMMARY STEPS

1. enable

2. configure terminal

3. parameter-map type regex parameter-map-name

4. pattern traffic-pattern

5. exit

6. class-map type inspect smtp match-any class-map-name

7. match sender address regex parameter-map-name

8. exit

9. policy-map type inspect smtp policy-map-name

10. class type inspect smtp class-map-name

11. log

12. reset (optional)

DETAILED STEPS

 
Command or Action
Purpose

Step 1 

enable

Example:

Router> enable

Enables privileged EXEC mode.

Enter your password if prompted.

Step 2 

configure terminal

Example:

Router# configure terminal

Enters global configuration mode.

Step 3 

parameter-map type regex parameter-map-name

Example:

Router(config)# parameter-map type regex bad-guys

Enter the parameter-map name of a specific traffic pattern. Once the parameter-map name is specified, parameter-map profile configuration mode is entered.

Step 4 

pattern traffic-pattern

Example:

Router(config-profile)# pattern "*deals\.com"

Router(config-profile)# pattern "*crazyperson*@wrdmail\.com"

Specifies the Cisco IOS regular expression (regex) pattern that matches the traffic pattern for the e-mail sender or user accounts from suspected domains that are causing the spam e-mail.

Step 5 

exit

Exits parameter-map profile configuration mode.

Step 6 

class-map type inspect smtp match-any class-map-name

Example:

Router(config)# class-map type inspect smtp match-any c1

Creates a class map for the SMTP protocol so the match criteria is set to match any criteria for this class map and enters class-map configuration mode.

Step 7 

match sender address regex parameter-map-name

Example:

Router(config-cmap)# match sender address regex bad-guys

Enters the parameter-map name class, which was defined in Step 3, to specify the Cisco IOS regular expression (regex) patterns for the class-map.

Step 8 

exit

Exits class-map configuration mode.

Step 9 

policy-map type inspect smtp policy-map-name

Example:

Router(config)# policy-map type inspect smtp p1

Creates a Layer 7 SMTP policy map and enters policy-map configuration mode.

Step 10 

class type inspect smtp class-map-name

Example:

Router(config-pmap)# class type inspect smtp c1

Configures SMTP inspection parameters for this class map.

Step 11 

log

Example:

Router(config-pmap)# log

Logs an action related to this class-type in the SMTP policy map.

Step 12 

reset

Example:

Router(config-pmap)# reset

(Optional) Drops an SMTP connection with an SMTP sender (client) if it violates the specified policy. This action sends an error code to the sender and closes the connection gracefully.

Identifying and Restricting Spammers Searching for User Accounts in a Domain

Spammers who search for a large number of user accounts in a domain typically send the same e-mail to all the user accounts they find in this domain. Spammers can be identified and restricted from searching for user accounts in a domain by using the match recipient count gt command to specify an action that occurs when a number of invalid recipients appear on an SMTP connection.


Note The match recipient count gt command does not count the number of recipients specified in the To or Cc fields in the e-mail header.


SUMMARY STEPS

1. enable

2. configure terminal

3. class-map type inspect smtp class-map-name

4. match recipient count gt value

5. exit

6. policy-map type inspect smtp policy-map-name

7. class type inspect smtp class-map-name

8. reset

DETAILED STEPS

 
Command or Action
Purpose

Step 1 

enable

Example:

Router> enable

Enables privileged EXEC mode.

Enter your password if prompted.

Step 2 

configure terminal

Example:

Router# configure terminal

Enters global configuration mode.

Step 3 

class-map type inspect smtp class-map-name

Example:

Router(config)# class-map type inspect smtp c1

Creates a class map for the SMTP protocol and enters class-map configuration mode.

Step 4 

match recipient count gt value

Example:

Router(config-cmap)# match recipient count gt 25

Sets a limit on the number of RCPT SMTP commands sent by the sender (client) to recipients who are specified in a single SMTP transaction.

This command determines the number of RCPT lines and invalid recipients (for which the server has replied "500 No such address") in the SMTP transaction.

Step 5 

exit

Exits class-map configuration mode.

Step 6 

policy-map type inspect smtp policy-map-name

Example:

Router(config)# policy-map type inspect smtp p1

Creates a Layer 7 SMTP policy map and enters policy-map configuration mode.

The policy-map-name argument is the name of the policy map.

Step 7 

class type inspect smtp class-map-name

Example:

Router(config-pmap)# class type inspect smtp c1

Configures SMTP inspection parameters for this class map.

Step 8 

reset

Example:

Router(config-pmap)# reset

(Optional) Drops an SMTP connection with an SMTP sender (client) if it violates the specified policy. This action sends an error code to the sender and closes the connection gracefully.

Restricting the Number of Invalid SMTP Recipients

If a sender specifies in an invalid e-mail recipient and SMTP encounters this invalid recipient on the SMTP connection, then SMTP sends an error code reply to the e-mail sender (client) to specify another recipient. In this case, the event did not violate the SMTP protocol or indicate that this particular SMTP connection is bad. However, if a pattern of invalid recipients appears, then a reasonable threshold can be set to restrict these nuisance SMTP connections. The match recipient invalid count gt command is used to help identify and restrict the number of invalid SMTP recipients that can appear in an e-mail from senders who try common names on a domain in the hope that they discover a valid username to whom they can send spam.

SUMMARY STEPS

1. enable

2. configure terminal

3. class-map type inspect smtp class-map-name

4. match recipient invalid count gt value

5. exit

6. policy-map type inspect smtp policy-map-name

7. class type inspect smtp class-map-name

8. reset

DETAILED STEPS

 
Command or Action
Purpose

Step 1 

enable

Example:

Router> enable

Enables privileged EXEC mode.

Enter your password if prompted.

Step 2 

configure terminal

Example:

Router# configure terminal

Enters global configuration mode.

Step 3 

class-map type inspect smtp class-map-name

Example:

Router(config)# class-map type inspect smtp c1

Creates a class map for the SMTP protocol and enters class-map configuration mode.

Step 4 

match recipient invalid count gt value

Example:

Router(config-cmap)# match recipient invalid count gt 5

Specifies a maximum number of invalid e-mail recipients on this SMTP connection.

Step 5 

exit

Exits class-map configuration mode.

Step 6 

policy-map type inspect smtp policy-map-name

Example:

Router(config)# policy-map type inspect smtp p1

Creates a Layer 7 SMTP policy map and enters policy-map configuration mode.

Step 7 

class type inspect smtp class-map-name

Example:

Router(config-pmap)# class type inspect smtp c1

Configures SMTP inspection parameters for this class map.

Step 8 

reset

Example:

Router(config-pmap)# reset

(Optional) Drops an SMTP connection with an SMTP sender (client) if it violates the specified policy. This action sends an error code to the sender and closes the connection gracefully.

Specifying a Recipient Pattern to Learn Spam Senders and Domain Information

A nonexistent e-mail recipient pattern can be specified to learn about spam senders and their domain information by luring them to use this nonexistent e-mail recipient pattern. This pattern is a regular-expression (regex) that can be specified to identify an e-mail addressed to a particular recipient or domain when a server is functioning as a relay. The specified pattern is checked in the SMTP RCPT command (SMTP envelope) parameter to identify if the recipient is either used as an argument or a source-list to forward mail in the route specified in the list.


Note The match recipient address regex command does not operate on the To or Cc fields in the e-mail header.


SUMMARY STEPS

1. enable

2. configure terminal

3. parameter-map type regex parameter-map-name

4. pattern traffic-pattern

5. exit

6. class-map type inspect smtp class-map-name

7. match recipient address regex parameter-map-name

8. exit

9. policy-map type inspect smtp policy-map-name

10. class type inspect smtp class-map-name

11. log

12. reset

DETAILED STEPS

 
Command or Action
Purpose

Step 1 

enable

Example:

Router> enable

Enables privileged EXEC mode.

Enter your password if prompted.

Step 2 

configure terminal

Example:

Router# configure terminal

Enters global configuration mode.

Step 3 

parameter-map type regex parameter-map-name

Example:

Router(config)# parameter-map type regex known-unknown-users

Enter the parameter-map name of a specific traffic pattern. Once the parameter-map name is specified, parameter-map profile configuration mode is entered.

Step 4 

pattern traffic-pattern

Example:

Router(config-profile)# pattern "username@mydomain.com"

Specifies a Cisco IOS regular expression (regex) pattern that matches the traffic pattern for the e-mail sender or user accounts from suspected domains that are causing the spam e-mail. In the example, "username" is configured as the name for a fake e-mail account used to discover senders (and their domain) when they try to send spam e-mail to this fake account.

Step 5 

exit

Exits parameter-map profile configuration mode.

Step 6 

class-map type inspect smtp class-map-name

Example:

Router(config)# class-map type inspect smtp c1

Creates a class map for the SMTP protocol and enters class-map configuration mode.

Step 7 

match recipient address regex parameter-map-name

Example:

Router(config-cmap)# match recipient address regex known-unknown-users

Specifies the nonexistent e-mail recipient pattern in order to learn spam senders and their domain information by luring them to use this contrived e-mail recipient.

Step 8 

exit

Exits class-map configuration mode.

Step 9 

policy-map type inspect smtp policy-map-name

Example:

Router(config)# policy-map type inspect smtp p1

Creates a Layer 7 SMTP policy map and enters policy-map configuration mode.

Step 10 

class type inspect smtp class-map-name

Example:

Router(config-pmap)# class type inspect smtp c1

Configures SMTP inspection parameters for this class map.

Step 11 

log

Example:

Router(config-pmap)# log

Logs an action related to this class-type in the SMTP policy map.

Step 12 

reset

Example:

Router(config-pmap)# reset

(Optional) Drops an SMTP connection with an SMTP sender (client) if it violates the specified policy. This action sends an error code to the sender and closes the connection gracefully.

Hiding Specified Private SMTP Commands on an SMTP Connection

Use this task to hide or "mask" commonly encountered SMTP verbs (SMTP commands) or specified private SMTP verbs used to provision an SMTP connection.

Specified verbs, such as the ATRN, ETRN, BDAT verbs may be considered vulnerable to exploitation if seen by a sender (client). The most commonly encountered SMTP verbs are listed along with the facility to specify a private verb as a string (using the WORD option).


Note The BDAT verb (used as an alternative to DATA) is not used, so in its place, the CHUNKING keyword is masked in the EHLO response. However, if the sender (client) continues to send the BDAT command, it is masked.



Note Using the mask command applies to certain match command filters like match cmd verb. Validations are performed to make this check and the configuration is not be accepted in case of invalid combinations.


SUMMARY STEPS

1. enable

2. configure terminal

3. class-map type inspect smtp class-map-name

4. match cmd verb {verb-name | WORD}

5. exit

6. policy-map type inspect smtp policy-map-name

7. class type inspect smtp class-map-name

8. mask

DETAILED STEPS

 
Command or Action
Purpose

Step 1 

enable

Example:

Router> enable

Enables privileged EXEC mode.

Enter your password if prompted.

Step 2 

configure terminal

Example:

Router# configure terminal

Enters global configuration mode.

Step 3 

class-map type inspect smtp class-map-name

Example:

Router(config)# class-map type inspect smtp c1

Creates a class map for the SMTP protocol and enters class-map configuration mode.

Step 4 

match cmd verb {verb-name | WORD}

Example:

Router(config-cmap)# match cmd verb ATRN

Specifies either the private verb name to "mask" that is used to provision an SMTP connection.

The verb-name argument is the name of an SNMP command verb.

The WORD argument is the name of a user-specified SMTP command verb, which is treated as an unknown verb and is masked regardless of whether the `mask action is configured for the class or not.

Step 5 

exit

Exits class-map configuration mode.

Step 6 

policy-map type inspect smtp policy-map-name

Example:

Router(config)# policy-map type inspect smtp p1

Creates a Layer 7 SMTP policy map and enters policy-map configuration mode.

Step 7 

class type inspect smtp class-map-name

Example:

Router(config-pmap)# class type inspect smtp c1

Configures SMTP inspection parameters for this class map.

Step 8 

mask

Example:

Router(config-pmap)# mask

Explicitly masks the specified SMTP commands or the parameters returned by the server in response to an EHLO command.

Preventing a DoS Attack by Limiting the Length of the SMTP Header

A DoS attack (also called a buffer overflow attack) by a malicious sender (client) can cause the SMTP application firewall to lose time and memory while trying to reassemble the fake packets (large e-mail headers) associated with the e-mail. In an SMTP transaction, the header portion of an e-mail is considered part of the DATA area, which contains fields like Subject, From, To, Cc, Date, and proprietary information, which is used by a recipient's e-mail agent to process the e-mail. A DoS attack can be prevented by using the match header length gt command to limit the length of the SMTP header that can be received. If a match is found, possible actions that can be specified within the policy are as follows: allow, reset, or log (the log action triggers a syslog message when a match is found).

SUMMARY STEPS

1. enable

2. configure terminal

3. class-map type inspect smtp class-map-name

4. match header length gt bytes

5. exit

6. policy-map type inspect smtp policy-map-name

7. class type inspect smtp class-map-name

8. reset

DETAILED STEPS

 
Command or Action
Purpose

Step 1 

enable

Example:

Router> enable

Enables privileged EXEC mode.

Enter your password if prompted.

Step 2 

configure terminal

Example:

Router# configure terminal

Enters global configuration mode.

Step 3 

class-map type inspect smtp class-map-name

Example:

Router(config)# class-map type inspect smtp c1

Creates a class map for the SMTP protocol and enters class-map configuration mode.

Step 4 

match header length gt bytes

Example:

Router(config-cmap)# match header length gt 16000

Specifies a value from 1 to 65535 that limits the maximum length of the SMTP header in bytes to thwart DoS attacks.

Step 5 

exit

Exits class-map configuration mode.

Step 6 

policy-map type inspect smtp policy-map-name

Example:

Router(config)# policy-map type inspect smtp p1

Creates a Layer 7 SMTP policy map and enters policy-map configuration mode.

Step 7 

class type inspect smtp class-map-name

Example:

Router(config-pmap)# class type inspect smtp c1

Configures SMTP inspection parameters for this class map.

Step 8 

reset

Example:

Router(config-pmap)# reset

(Optional) Drops an SMTP connection with an SMTP sender (client) if it violates the specified policy. This action sends an error code to the sender and closes the connection gracefully.

Preventing a DoS Attack by Limiting the Length or TYPE of SMTP Command Line

The following task is used to limit the length of an SMTP command line to prevent a DoS attack, which occurs when a malicious sender (client) specifies large command lines in an e-mail to perform DoS attacks on SMTP servers.

SUMMARY STEPS

1. enable

2. configure terminal

3. class-map type inspect smtp {class-map-name | match-all class-map-name | match-any class-map-name}

4. match cmd {line length gt length | verb {AUTH | DATA | EHLO | ETRN | EXPN | HELO | HELP | MAIL NOOP | QUIT | RCPT | RSET | SAML | SEND | SOML | STARTTLS | VERB | VRFY | WORD}}

5. exit

6. policy-map type inspect smtp policy-map-name

7. class type inspect smtp class-map-name

8. reset

DETAILED STEPS

 
Command or Action
Purpose

Step 1 

enable

Example:

Router> enable

Enables privileged EXEC mode.

Enter your password if prompted.

Step 2 

configure terminal

Example:

Router# configure terminal

Enters global configuration mode.

Step 3 

class-map type inspect smtp {class-map-name | match-all class-map-name | match-any class-map-name}

Example:

Router(config)# class-map type inspect smtp c1

Enters class-map configuration mode and creates a class map for the SMTP protocol.

The class-map-name argument by itself specifies a single class-map.

The match-all keyword and class-map-name argument places logical and all matching statements under this class map.

The match-any keyword and class-map-name argument places logical or all matching statements under this class map.

Note If no match cmd verb command statement is specified in a class-map type inspect smtp match-all command statement for a class-map, which contains the match cmd line length gt command statement, then the class-map applies to all SMTP commands.

Step 4 

match cmd {line length gt length | verb {AUTH | DATA | EHLO | ETRN | EXPN | HELO | HELP | MAIL NOOP | QUIT | RCPT | RSET | SAML | SEND | SOML | STARTTLS | VERB | VRFY | WORD}}

Example:

Router(config-cmap)# match header length gt 16000

Specifies a value that limits the length of the ESMTP command line or ESMTP command line verb used to thwart DoS attacks.

The length argument specifies the ESMTP command line greater than the length of a number of characters from 1 to 65535.

Step 5 

exit

Exits class-map configuration mode.

Step 6 

policy-map type inspect smtp policy-map-name

Example:

Router(config)# policy-map type inspect smtp p1

Creates a Layer 7 SMTP policy map and enters policy-map configuration mode.

Step 7 

class type inspect smtp class-map-name

Example:

Router(config-pmap)# class type inspect smtp c1

Configures an SMTP class-map firewall for SMTP inspection parameters.

Step 8 

reset

Example:

Router(config-pmap)# reset

(Optional) Drops an SMTP connection with an SMTP sender (client) if it violates the specified policy. This action sends an error code to the sender and closes the connection gracefully.

Examples

The following configuration has class-map c2 match when the length of the e-mail (MAIL) command exceeds 256 bytes.

When the class-map type inspect smtp match-all command statement is configured with the match cmd verb command statement, only the match cmd line length gt command statement can coexist.

class-map type inspect smtp match-all c2
  match cmd line length gt 256
  match cmd verb MAIL

There are no match restrictions in case of a class-map type inspect smtp match-any command statement for a class map because the class-map applies to all SMTP commands.

Restricting Content File Types in the Body of the E-Mail

The match mime content-type regex command is used to specify MIME content file types, which are restricted in attachments in the body of the e-mail being sent over SMTP. See the "Example: MIME E-Mail Format" section for more information.

SUMMARY STEPS

1. enable

2. configure terminal

3. parameter-map type regex parameter-map-name

4. pattern traffic-pattern

5. exit

6. class-map type inspect smtp {class-map-name | match-all class-map-name | match-any class-map-name}

7. match mime content-type regex content-type-regex

8. exit

9. policy-map type inspect smtp policy-map-name

10. class type inspect smtp class-map-name

11. log

DETAILED STEPS

 
Command or Action
Purpose

Step 1 

enable

Example:

Router> enable

Enables privileged EXEC mode.

Enter your password if prompted.

Step 2 

configure terminal

Example:

Router# configure terminal

Enters global configuration mode.

Step 3 

parameter-map type regex parameter-map-name

Example:

Router(config)# parameter-map type regex jpeg

Enter the parameter-map name of a specific traffic pattern. Once the parameter-map name is specified, parameter-map profile configuration mode is entered.

Step 4 

pattern traffic-pattern

Example:

Router(config-profile)# pattern "*image//*"

Specifies a Cisco IOS regular expression (regex) pattern that matches the traffic pattern for the e-mail sender or user accounts from suspected domains that are causing the spam e-mail.

Step 5 

exit

Exits parameter-map profile configuration mode.

Step 6 

class-map type inspect smtp {class-map-name | match-all class-map-name | match-any class-map-name}

Example:

Router(config)# class-map type inspect smtp c1

Enters class-map configuration mode and creates a class map for the SMTP protocol.

The class-map-name argument by itself specifies a single class-map.

The match-all keyword and class-map-name argument places logical and all matching statements under this class map.

The match-any keyword and class-map-name argument places logical or all matching statements under this class map.

Step 7 

match mime content-type regex content-type-regex

Example:

Router(config-cmap)# match mime content-type regex jpeg

Specifies the MIME content file type, which are restricted in attachments in the body of the e-mail being sent over SMTP.

The content-type-regex argument is the type of content in the MIME header in regular expression form.

This example lets the user specify any form of JPEG image content to be restricted.

Note The actual content of the MIME part is not checked to see if it matches with the declared content-type in the MIME header.

Step 8 

exit

Exits class-map configuration mode.

Step 9 

policy-map type inspect smtp policy-map-name

Example:

Router(config)# policy-map type inspect smtp p1

Creates a Layer 7 SMTP policy map and enters policy-map configuration mode.

Step 10 

class type inspect smtp class-map-name

Example:

Router(config-pmap)# class type inspect smtp c1

Configures an SMTP class-map firewall for SMTP inspection parameters.

Step 11 

log

Example:

Router(config-pmap)# log

Logs an action related to this class-type in the SMTP policy map.

Restricting Unknown Content Encoding Types from Being Transmitted

Unknown MIME content-encoding types or values can be restricted from being transmitted over SMTP by using one of the following parameters with the match mime encoding command.

These preconfigured content-transfer-encoding types act as a filter on the content-transfer-encoding field in the MIME header within the SMTP body. The uuencode encoding type is not recognized as a standard type by the MIME RFCs because many subtle differences exist in its various implementations. However, since it is used by some mail systems, the x-uuencode type is included in the preconfigured list.

SUMMARY STEPS

1. enable

2. configure terminal

3. class-map type inspect smtp {class-map-name | match-all class-map-name | match-any class-map-name}

4. match mime encoding {unknown | WORD | encoding-type}

5. exit

6. policy-map type inspect smtp policy-map-name

7. class type inspect smtp class-map-name

8. log

DETAILED STEPS

 
Command or Action
Purpose

Step 1 

enable

Example:

Router> enable

Enables privileged EXEC mode.

Enter your password if prompted.

Step 2 

configure terminal

Example:

Router# configure terminal

Enters global configuration mode.

Step 3 

class-map type inspect smtp {class-map-name | match-all class-map-name | match-any class-map-name}

Example:

Router(config)# class-map type inspect smtp c1

Enters class-map configuration mode and creates a class map for the SMTP protocol.

The class-map-name argument by itself specifies a single class-map.

The match-all keyword and class-map-name argument places logical and all matching statements under this class map.

The match-any keyword and class-map-name argument places logical or all matching statements under this class map.

Step 4 

match mime encoding {unknown | WORD | encoding-type}

Example:

Router (config-cmap)# match mime encoding quoted-printable

Restricts unknown MIME content-encoding types or values.

The unknown keyword is used if content-transfer-encoding value in the e-mail does not match any of the ones in the list to restrict unknown and potentially dangerous encodings.

The WORD argument is a user-defined content-transfer encoding type, which must begin with "X-" (for example, "X-myencoding-scheme").

The encoding-type argument specifies one of the following preconfigured content-transfer-encoding types:

7-bit-ASCII characters

8-bit-Facilitates the exchange of e-mail messages containing octets outside the 7-bit ASCII range.

base64-Any similar encoding scheme that encodes binary data by treating it numerically and translating it into a base 64 representation.

quoted-printable-Encoding using printable characters (that is alphanumeric and the equals sign "=") to transmit 8-bit data over a 7-bit data path. It is defined as a MIME content transfer encoding for use in Internet e-mail.

binary-Representation for numbers using only two digits (usually, 0 and 1).

x-uuencode-Nonstandard encoding.

Note The quoted-printable and base64 encoding types tell the e-mail client that a binary-to-text encoding scheme was used and that appropriate initial decoding is necessary before the message can be read with its original encoding.

Step 5 

exit

Exits class-map configuration mode.

Step 6 

policy-map type inspect smtp policy-map-name

Example:

Router(config)# policy-map type inspect smtp p1

Creates a Layer 7 SMTP policy map and enters policy-map configuration mode.

Step 7 

class type inspect smtp class-map-name

Example:

Router(config-pmap)# class type inspect smtp c1

Configures an SMTP class-map firewall for SMTP inspection parameters.

Step 8 

log

Example:

Router(config-pmap)# log

Logs an action related to this class-type in the SMTP policy map.

Specifying a Text String to Be Matched and Restricted in the Body of an E-Mail

The match body regex command can be used to specify an arbitrary text expression to restrict specified content-types and content encoding types for text and HTML in the body of the e-mail. The text or HTML pattern is scanned only if the encoding is 7-bit or 8-bit and the encoding is checked before attempting to match the pattern. If the pattern is of another encoding type (for example, base64, zip files, and so on), then the pattern cannot be scanned.


Note Using this command can impact performance because the complete SMTP connection has to be scanned.


SUMMARY STEPS

1. enable

2. configure terminal

3. parameter-map type regex parameter-map-name

4. pattern traffic-pattern

5. exit

6. class-map type inspect smtp {class-map-name | match-all class-map-name | match-any class-map-name}

7. match body regex parameter-map-name

8. exit

9. policy-map type inspect smtp policy-map-name

10. class type inspect smtp class-map-name

11. log

DETAILED STEPS

 
Command or Action
Purpose

Step 1 

enable

Example:

Router> enable

Enables privileged EXEC mode.

Enter your password if prompted.

Step 2 

configure terminal

Example:

Router# configure terminal

Enters global configuration mode.

Step 3 

parameter-map type regex parameter-map-name

Example:

Router(config)# parameter-map type regex doc-data

Enter the parameter-map name of a specific traffic pattern. Once the parameter-map name is specified, parameter-map profile configuration mode is entered.

Step 4 

pattern traffic-pattern

Example:

Router(config-profile)# pattern "*UD-421590*"

Specifies a Cisco IOS regular expression (regex) pattern that matches the traffic pattern for the e-mail sender or user accounts from suspected domains that are causing the spam e-mail.

Step 5 

exit

Exits parameter-map profile configuration mode.

Step 6 

class-map type inspect smtp {class-map-name | match-all class-map-name | match-any class-map-name}

Example:

Router(config)# class-map type inspect smtp c1

Enters class-map configuration mode and creates a class map for the SMTP protocol.

The class-map-name argument by itself specifies a single class-map.

The match-all keyword and class-map-name argument places logical and all matching statements under this class map.

The match-any keyword and class-map-name argument places logical or all matching statements under this class map.

Step 7 

match body regex parameter-map-name

Example:

Router(config-cmap)# match body regex doc-data

Specifies an arbitrary text expression to restrict specified content-types and content encoding types for text and HTML in the "body" of the e-mail.

Step 8 

exit

Exits class-map configuration mode.

Step 9 

policy-map type inspect smtp policy-map-name

Example:

Router(config)# policy-map type inspect smtp p1

Creates a Layer 7 SMTP policy map and enters policy-map configuration mode.

Step 10 

class type inspect smtp class-map-name

Example:

Router(config-pmap)# class type inspect smtp c1

Configures an SMTP class-map firewall for SMTP inspection parameters.

Step 11 

log

Example:

Router(config-pmap)# log

Logs an action related to this class-type in the SMTP policy map.

Configuring the Monitoring of Text Patterns in an SMTP E-Mail Subject Field

The match header regex command can be used specify an arbitrary text expression in the SMTP e-mail message header (Subject field) or e-mail body such as Subject, Received, To, or other private header fields to monitor text patterns.

SUMMARY STEPS

1. enable

2. configure terminal

3. parameter-map type regex parameter-map-name

4. pattern traffic-pattern

5. exit

6. class-map type inspect smtp {class-map-name | match-all class-map-name | match-any class-map-name}

7. match header regex parameter-map-name

8. exit

9. policy-map type inspect smtp policy-map-name

10. class type inspect smtp class-map-name

11. reset

DETAILED STEPS

 
Command or Action
Purpose

Step 1 

enable

Example:

Router> enable

Enables privileged EXEC mode.

Enter your password if prompted.

Step 2 

configure terminal

Example:

Router# configure terminal

Enters global configuration mode.

Step 3 

parameter-map type regex parameter-map-name

Example:

Router(config)# parameter-map type regex lottery-spam

Enter the parameter-map name of a specific traffic pattern. Once the parameter-map name is specified, parameter-map profile configuration mode is entered.

Step 4 

pattern traffic-pattern

Example:

Router(config-profile)# pattern "Subject:*lottery*"

Specifies a Cisco IOS regular expression (regex) pattern that matches the traffic pattern for the e-mail sender or user accounts from suspected domains that are causing the spam e-mail.

Step 5 

exit

Exits parameter-map profile configuration mode.

Step 6 

class-map type inspect smtp {class-map-name | match-all class-map-name | match-any class-map-name}

Example:

Router(config)# class-map type inspect smtp c1

Enters class-map configuration mode and creates a class map for the SMTP protocol.

The class-map-name argument by itself specifies a single class-map.

The match-all keyword and class-map-name argument places logical and all matching statements under this class map.

The match-any keyword and class-map-name argument places logical or all matching statements under this class map.

Step 7 

match header regex parameter-map-name

Example:

Router(config-cmap)# match header regex lottery-spam

Specifies an arbitrary text expression in the SMTP e-mail message header to monitor text patterns.

Step 8 

exit

Exits class-map configuration mode.

Step 9 

policy-map type inspect smtp policy-map-name

Example:

Router(config)# policy-map type inspect smtp p1

Creates a Layer 7 SMTP policy map and enters policy-map configuration mode.

Step 10 

class type inspect smtp class-map-name

Example:

Router(config-pmap)# class type inspect smtp c1

Configures an SMTP class-map firewall for SMTP inspection parameters.

Step 11 

reset

Example:

Router(config-pmap)# reset

(Optional) Drops an SMTP connection with an SMTP sender (client) if it violates the specified policy. This action sends an error code to the sender and closes the connection gracefully.

Configuring a Parameter to Be Identified and Masked in the EHLO Server Reply

The match reply ehlo command is used to identify and mask a service extension parameter in the EHLO server reply (for example, 8BITMIME and ETRN) to prevent a sender (client) from using that particular service extension.

SUMMARY STEPS

1. enable

2. configure terminal

3. class-map type inspect smtp {class-map-name | match-all class-map-name | match-any class-map-name}

4. match reply ehlo {parameter | WORD}

5. exit

6. policy-map type inspect smtp policy-map-name

7. class type inspect smtp class-map-name

8. log

9. mask

DETAILED STEPS

 
Command or Action
Purpose

Step 1 

enable

Example:

Router> enable

Enables privileged EXEC mode.

Enter your password if prompted.

Step 2 

configure terminal

Example:

Router# configure terminal

Enters global configuration mode.

Step 3 

class-map type inspect smtp {class-map-name | match-all class-map-name | match-any class-map-name}

Example:

Router(config)# class-map type inspect smtp c1

Enters class-map configuration mode and creates a class map for the SMTP protocol.

The class-map-name argument by itself specifies a single class-map.

The match-all keyword and class-map-name argument places logical and all matching statements under this class map.

The match-any keyword and class-map-name argument places logical or all matching statements under this class map.

Step 4 

match reply ehlo {parameter | WORD}

Example:

Router(config-cmap)# match reply ehlo ETRN

Identifies and masks a service extension parameter in the EHLO server reply.

The parameter argument specifies a parameter from the well-known EHLO keywords.

The WORD argument specifies an extension which is not on the EHLO list.

Step 5 

exit

Exits class-map configuration mode.

Step 6 

policy-map type inspect smtp policy-map-name

Example:

Router(config)# policy-map type inspect smtp p1

Creates a Layer 7 SMTP policy map and enters policy-map configuration mode.

Step 7 

class type inspect smtp class-map-name

Example:

Router(config-pmap)# class type inspect smtp c1

Configures an SMTP class-map firewall for SMTP inspection parameters.

Step 8 

log

Example:

Router(config-pmap)# log

Logs an action related to this class-type in the SMTP policy map.

Step 9 

mask

Example:

Router(config-pmap)# mask

Explicitly masks the specified SMTP commands or the parameters returned by the server in response to an EHLO command.

Configuring a Logging Action for a Class Type in an SMTP Policy-Map

A logging action can be configured for a class type in an SMTP policy-map when conditions specified by the traffic class are met. The logging action results in a LOG_WARNING syslog message followed by the specific log message. The log message format is similar to other application firewall modules (for example, HTTP, IM, Peer-to-Peer (P2P)); session initiator/responder information, and zone-pair and class names.


Note The log action currently exists for other types of policy-maps (http, pop3).


SUMMARY STEPS

1. enable

2. configure terminal

3. class-map type inspect smtp {class-map-name | match-all class-map-name | match-any class-map-name}

4. match cmd verb {parameter | WORD}

5. exit

6. policy-map type inspect smtp policy-map-name

7. class type inspect smtp class-map-name

8. log

DETAILED STEPS

 
Command or Action
Purpose

Step 1 

enable

Example:

Router> enable

Enables privileged EXEC mode.

Enter your password if prompted.

Step 2 

configure terminal

Example:

Router# configure terminal

Enters global configuration mode.

Step 3 

class-map type inspect smtp {class-map-name | match-all class-map-name | match-any class-map-name}

Example:

Router(config)# class-map type inspect smtp c1

Enters class-map configuration mode and creates a class map for the SMTP protocol.

The class-map-name argument by itself specifies a single class-map.

The match-all keyword and class-map-name argument places logical and all matching statements under this class map.

The match-any keyword and class-map-name argument places logical or all matching statements under this class map.

Step 4 

match cmd verb {parameter | WORD}

Example:

Router(config-cmap)# match cmd verb ATRN

Identifies and masks a service extension parameter in the EHLO server reply.

The parameter argument specifies a parameter from the well-known EHLO keywords.

The WORD argument specifies an extension which is not on the EHLO list.

Step 5 

exit

Exits class-map configuration mode.

Step 6 

policy-map type inspect smtp policy-map-name

Example:

Router(config)# policy-map type inspect smtp p1

Creates a Layer 7 SMTP policy map and enters policy-map configuration mode.

Step 7 

class type inspect smtp class-map-name

Example:

Router(config-pmap)# class type inspect smtp c1

Configures an SMTP class-map firewall for SMTP inspection parameters.

Step 8 

log

Example:

Router(config-pmap)# log

Logs an action related to this class-type in the SMTP policy map.

Configuration Examples for Application Inspection and Control for SMTP

Example: Creating a Pinhole for the SMTP Port

Example: Preventing ESMTP Inspection

Example: MIME E-Mail Format

Example: Creating a Pinhole for the SMTP Port

The following example shows a configuration without any Layer 7 SMTP policy that creates a pinhole only for the SMTP port. Any command sent to the server, including the EHLO command is accepted.

class-map type inspect smtp c1
match protocol smtp
policy-map type inspect smtp c1
	class type inspect smtp c1
		inspect

Note No SMTP policy is configured by default. If an SMTP policy is not configured, then no SMTP inspection is done by default.


Example: Preventing ESMTP Inspection

If a user decides to create a workable policy that is configured for SMTP inspection only, then it now needs to be explicitly specified in the policy.

The following example can be used to prevent ESMTP inspection:

class-map type inspect smtp c1
	match cmd verb EHLO

policy-map type inspect smtp c1
	class type inspect smtp c1
		mask

Example: MIME E-Mail Format

The format of data being transmitted through SMTP is specified by using the MIME standard, which uses headers to specify the content-type, encoding, and the filenames of data being sent (text, html, images, applications, documents and so on). The following is an example of an e-mail using the MIME format:


From: "username2" <username2@example.com>
To: username3 <username3@example.com>
Subject: testmail
Date: Sat, 7 Jan 2006 20:18:47 -0400
Message-ID: <000dadf7453e$bee1bb00$8a22f340@oemcomputer>
MIME-Version: 1.0
Content-Type: image/jpeg;
name='picture.jpg' 
Content-Transfer-Encoding: base64

<base64 encoded data for the picture.jpg image>

In the above example, the "name='picture.jpg'" is optional. Even without the definition, the image is sent to the recipient. The e-mail client of the recipient may display the image as "part-1" or "attach-1" or it may render the image in-line. Also, attachments are not `stripped' from the e-mail. If a content-type for which reset action was configured is detected, an 5XX error code is sent and the connection is closed, in order to prevent the whole e-mail from being delivered. However, the remainder of the e-mail message is sent.

Additional References

Related Documents

Related Topic
Document Title

Cisco IOS commands

Cisco IOS Master Commands List, All Releases

Firewall commands

Cisco IOS Security Command Reference

ESMTP firewall information.

ESMTP Support for Cisco IOS Firewall

Information for configuring an SMTP policy.

Zone-Based Policy Firewall


Standards

Standard
Title

No new or modified standards are supported by this feature, and support for existing standards has not been modified by this feature.


MIBs

MIB
MIBs Link

No new or modified MIBs are supported by this feature, and support for existing MIBs has not been modified by this feature.

To locate and download MIBs for selected platforms, Cisco IOS releases, and feature sets, use Cisco MIB Locator found at the following URL:

http://www.cisco.com/go/mibs


RFCs

RFC
Title

RFC 1869 and other SMTP RFC extensions apart from RFC 821.

SMTP Service Extensions


Technical Assistance

Description
Link

The Cisco Support and Documentation website provides online resources to download documentation, software, and tools. Use these resources to install and configure the software and to troubleshoot and resolve technical issues with Cisco products and technologies. Access to most tools on the Cisco Support and Documentation website requires a Cisco.com user ID and password.

http://www.cisco.com/cisco/web/support/index.html


Feature Information for Application Inspection and Control for SMTP

Table 1 lists the features in this module and provides links to specific configuration information.

Use Cisco Feature Navigator to find information about platform support and software image support. Cisco Feature Navigator enables you to determine which software images support a specific software release, feature set, or platform. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.


Note Table 1 lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature.


Table 1 Feature Information for Application Inspection and Control for SMTP

Feature Name
Releases
Feature Information

Application Inspection and Control for SMTP

12.4(20)T

The Application Inspection and Control for SMTP feature provides an intense provisioning mechanism that can be configured to inspect packets on a granular level so that malicious network activity, related to the transfer of e-mail at the application level, can be identified and controlled. This feature qualifies the Cisco IOS firewall extended SMTP (ESMTP) module as an "SMTP application firewall," which protects in a similar way to that of an HTTP application firewall.

The following commands were introduced or modified by this feature: log (policy-map and class-map), mask (policy-map), match body regex, match cmd, match header length gt, match header regex, match mime content-type regex, match mime encoding, match sender address regex, match recipient address regex, match recipient count gt, match recipient invalid count gt, match reply ehlo, reset (policy-map).


Glossary

C3PL—Cisco Common Classification Policy Language. Structured, feature-specific configuration commands that use policy maps and class maps to create traffic policies based on events, conditions, and actions.

EHLO—Extended HELO substitute command for starting the capability negotiation. This command identifies the sender (client) connecting to the remote SMTP server by using the ESMTP protocol.

ESMTP—Extended Simple Mail Transfer Protocol. Extended version of the Simple Mail Transfer Protocol (SMTP), which includes additional functionality, such as delivery notification and session delivery. ESMTP is described in RFC 1869, SMTP Service Extensions.

HELO—Command that starts the SMTP capability negotiation. This command identifies the sender (client) connecting to the remote SMTP server by its fully qualified DNS hostname.

MAIL FROM —Start of an e-mail message that identifies the sender e-mail address (and name, if used), which appears in the From: field of the message.

MIME—Multipurpose Internet Mail Extension. Standard for transmitting nontext data (or data that cannot be represented in plain ASCII code) in e-mail, such as binary, foreign language text (such as Russian or Chinese), audio, or video data. MIME is defined in RFC 2045.

RCPT TO—Recipient e-mail address (and name, if used) that can be repeated multiple times for a likely message to deliver a single message to multiple recipients.

SMTP—Simple Mail Transfer Protocol. Internet protocol providing e-mail services.