SMTP Preprocessor Options

You can enable or disable normalization, and you can configure options to control the types of anomalous traffic the SMTP decoder detects.

Note that decoding, or extraction when the MIME email attachment does not require decoding, includes multiple attachments when present, and large attachments that span multiple packets.

Note also that the highest value is used when the values for the Base64 Decoding Depth, 7-Bit/8-Bit/Binary Decoding Depth, Quoted-Printable Decoding Depth, or Unix-to-Unix Decoding Depth options are different in:

the default network analysis policy
any other custom network analysis policy invoked by network analysis rules in the same access control policy

If no preprocessor rule is mentioned in the following descriptions, the option is not associated with a preprocessor rule.

Ports

Specifies the ports whose SMTP traffic you want to normalize. You can specify a value greater than or equal to 0. Separate multiple ports with commas.

Stateful Inspection

When selected, causes SMTP decoder to save state and provide session context for individual packets and only inspects reassembled sessions. When cleared, analyzes each individual packet without session context.

Normalize

When set to All, normalizes all commands. Checks for more than one space character after a command.

When set to None, normalizes no commands.

When set to Cmds, normalizes the commands listed in Custom Commands.

Custom Commands

When Normalize is set to Cmds, normalizes the listed commands.

Specify commands which should be normalized in the text box. Checks for more than one space character after a command.

The space (ASCII 0x20) and tab (ASCII 0x09) characters count as space characters for normalization purposes.

Ignore Data

Does not process mail data; processes only MIME mail header data.

Ignore TLS Data

Does not process data encrypted under the Transport Layer Security protocol.

No Alerts

Disables intrusion events when accompanying preprocessor rules are enabled.

Detect Unknown Commands

Detects unknown commands in SMTP traffic.

You can enable rule 124:5 to generate events and, in an inline deployment, drop offending packets for this option.

Max Command Line Len

Detects when an SMTP command line is longer than this value. Specify 0 to never detect command line length.

RFC 2821, the Network Working Group specification on the Simple Mail Transfer Protocol, recommends 512 as a maximum command line length.

You can enable rule 124:1 to generate events and, in an inline deployment, drop offending packets for this option.

Max Header Line Len

Detects when an SMTP data header line is longer than this value. Specify 0 to never detect data header line length.

You can enable rules 124:2 and 124:7 to generate events and, in an inline deployment, drop offending packets for this option.

Max Response Line Len

Detects when an SMTP response line is longer than this value. Specify 0 to never detect response line length.

RFC 2821 recommends 512 as a maximum response line length.

You can enable rule 124:3 to generate events and, in an inline deployment, drop offending packets for this option and also for Alt Mac Command Line Len, when that option is enabled.

Alt Max Command Line Len

Detects when the SMTP command line for any of the specified commands is longer than this value. Specify 0 to never detect command line length for the specified commands. Different default line lengths are set for numerous commands.

This setting overrides the Max Command Line Len setting for the specified commands.

You can enable rule 124:3 to generate events and, in an inline deployment, drop offending packets for this option and also for Max Response Line Len when that option is enabled.

Invalid Commands

Detects if these commands are sent from the client side.

You can enable rule 124:6 to generate events and, in an inline deployment, drop offending packets for this option and also for Invalid Commands.

Valid Commands

Permits commands in this list.

Even if this list is empty, the preprocessor permits the following valid commands: ATRN AUTH BDAT DATA DEBUG EHLO EMAL ESAM ESND ESOM ETRN EVFY EXPN HELO HELP IDENT MAIL NOOP ONEX QUEU QUIT RCPT RSET SAML SEND SIZE SOML STARTTLS TICK TIME TURN TURNME VERB VRFY XADR XAUTH XCIR XEXCH50 X-EXPS XGEN XLICENSE X-LINK2STATE XQUE XSTA XTRN XUSR

Note	RCPT TO and MAIL FROM are SMTP commands. The preprocessor configuration uses command names of RCPT and MAIL, respectively. Within the code, the preprocessor maps RCPT and MAIL to the correct command name.

You can enable rule 124:4 to generate events and, in an inline deployment, drop offending packets for this option and also for Invalid Commands when that option is configured.

Data Commands

Lists commands that initiate sending data in the same way the SMTP DATA command sends data per RFC 5321. Separate multiple commands with spaces.

Binary Data Commands

Lists commands that initiate sending data in a way that is similar to how the BDAT command sends data per RFC 3030. Separate multiple commands with spaces.

Authentication Commands

Lists commands that initiate an authentication exchange between client and server. Separate multiple commands with spaces.

Detect xlink2state

Detects packets that are part of X-Link2State Microsoft Exchange buffer data overflow attacks. In inline deployments, the system can also drop those packets.

You can enable rule 124:8 to generate events and, in an inline deployment, drop offending packets for this option.

Base64 Decoding Depth

When Ignore Data is disabled, specifies the maximum number of bytes to extract and decode from each Base64 encoded MIME email attachment. You can specify from a positive value, or specify 0 to decode all the Base64 data. Specify -1 to ignore Base64 data. The preprocessor will not decode data when Ignore Data is selected.

Note that positive values not divisible by 4 are rounded up to the next multiple of 4 except for the values 65533, 65534, and 65535, which are rounded down to 65532.

When this option is enabled, you can enable rule 124:10 to generate events and, in an inline deployment, drop offending packets when decoding fails; decoding could fail, for example, because of incorrect encoding or corrupted data.

Note that this option replaces the deprecated options Enable MIME Decoding and Maximum MIME Decoding Depth, which are still supported in existing intrusion policies for backward compatibility.

7-Bit/8-Bit/Binary Decoding Depth

When Ignore Data is disabled, specifies the maximum bytes of data to extract from each MIME email attachment that does not require decoding. These attachment types include 7-bit, 8-bit, binary, and various multipart content types such as plain text, jpeg images, mp3 files, and so on. You can specify a positive value, or specify 0 to extract all data in the packet. Specify -1 to ignore non-decoded data. The preprocessor will not extract data when Ignore Data is selected.

Quoted-Printable Decoding Depth

When Ignore Data is disabled, specifies the maximum number of bytes to extract and decode from each quoted-printable (QP) encoded MIME email attachment.

You can specify from 1 to 65535 bytes, or specify 0 to decode all QP encoded data in the packet. Specify -1 to ignore QP encoded data. The preprocessor will not decode data when Ignore Data is selected.

When this option is enabled, you can enable rule 124:11 to generate events and, in an inline deployment, drop offending packets when decoding fails; decoding could fail, for example, because of incorrect encoding or corrupted data.

Unix-to-Unix Decoding Depth

When Ignore Data is disabled, specifies the maximum number of bytes to extract and decode from each Unix-to-Unix encoded (uuencoded) email attachment. You can specify from 1 to 65535 bytes, or specify 0 to decode all uuencoded data in the packet. Specify -1 to ignore uuencoded data. The preprocessor will not decode data when Ignore Data is selected.

When this option is enabled, you can enable rule 124:13 to generate events and, in an inline deployment, drop offending packets when decoding fails; decoding could fail, for example, because of incorrect encoding or corrupted data.

Log MIME Attachment Names

Enables extraction of MIME attachment file names from the MIME Content-Disposition header and associates the file names with all intrusion events generated for the session. Multiple file names are supported.

When this option is enabled, you can view file names associated with events in the Email Attachment column of the intrusion events table view.

Log To Addresses

Enables extraction of recipient email addresses from the SMTP RCPT TO command and associates the recipient addresses with all intrusion events generated for the session. Multiple recipients are supported.

When this option is enabled, you can view recipients associated with events in the Email Recipient column of the intrusion events table view.

Log From Addresses

Enables extraction of sender email addresses from the SMTP MAIL FROM command and associates the sender addresses with all intrusion events generated for the session. Multiple sender addresses are supported.

When this option is enabled, you can view senders associated with events in the Email Sender column of the intrusion events table view.

Log Headers

Enables extraction of email headers. The number of bytes to extract is determined by the value specified for Header Log Depth.

You can use the content or protected_content keyword to write intrusion rules that use email header data as a pattern. You can also view the extracted email header in the intrusion event packet view.

Header Log Depth

Specifies the number of bytes of the email header to extract when Log Headers is enabled. You can specify 0 to 20480 bytes. A value of 0 disables Log Headers.