Inspection of Basic Internet Protocols

The following topics explain application inspection for basic Internet protocols. For information on why you need to use inspection for certain protocols, and the overall methods for applying inspection, see Getting Started with Application Layer Protocol Inspection.

DNS Inspection

DNS inspection is enabled by default. You need to configure it only if you want non-default processing. The following sections describe DNS application inspection.

Defaults for DNS Inspection

DNS inspection is enabled by default, using the preset_dns_map inspection class map:

  • The maximum DNS message length is 512 bytes.

  • The maximum client DNS message length is automatically set to match the Resource Record.

  • DNS Guard is enabled, so the ASA tears down the DNS session associated with a DNS query as soon as the DNS reply is forwarded by the ASA. The ASA also monitors the message exchange to ensure that the ID of the DNS reply matches the ID of the DNS query.

  • Translation of the DNS record based on the NAT configuration is enabled.

  • Protocol enforcement is enabled, which enables DNS message format check, including domain name length of no more than 255 characters, label length of 63 characters, compression, and looped pointer check.

Configure DNS Inspection Policy Map

You can create a DNS inspection policy map to customize DNS inspection actions if the default inspection behavior is not sufficient for your network.

You can optionally create a DNS inspection class map to define the traffic class for DNS inspection. The other option is to define the traffic classes directly in the DNS inspection policy map. The difference between creating a class map and defining the traffic match directly in the inspection map is that you can create more complex match criteria and you can reuse class maps. Although this procedure explains inspection maps, the matching criteria used in class maps are the same as those explained in the step relating to the Inspection tab. You can configure DNS class maps by selecting Configuration > Firewall > Objects > Class Maps > DNS, or by creating them while configuring the inspection map.


Tip

You can configure inspection maps while creating service policies, in addition to the procedure explained below. The contents of the map are the same regardless of how you create it.

Before you begin

Some traffic matching options use regular expressions for matching purposes. If you intend to use one of those techniques, first create the regular expression or regular expression class map.

Procedure

Step 1

Choose Configuration > Firewall > Objects > Inspect Maps > DNS.

Step 2

Do one of the following:

  • Click Add to add a new map.

  • Select a map to view its contents. You can change the security level directly, or click Customize to edit the map. The remainder of the procedure assumes you are customizing or adding a map.

Step 3

For new maps, enter a name (up to 40 characters) and description. When editing a map, you can change the description only.

Step 4

In the Security Level view of the DNS Inspect Map dialog box, select the level that best matches your desired configuration. The default level is Low.

If one of the preset levels matches your requirements, you are now done. Just click OK, skip the rest of this procedure, and use the map in a service policy rule for DNS inspection.

If you need to customize the settings further, click Details, and continue with the procedure.

Step 5

Click the Protocol Conformance tab and choose the desired options:

  • Enable DNS guard function—Using DNS Guard, the ASA tears down the DNS session associated with a DNS query as soon as the DNS reply is forwarded by the ASA. The ASA also monitors the message exchange to ensure that the ID of the DNS reply matches the ID of the DNS query.

  • Enable NAT re-write function—Translates the DNS record based on the NAT configuration.

  • Enable protocol enforcement—Enables DNS message format check, including domain name length of no more than 255 characters, label length of 63 characters, compression, and looped pointer check.

  • Randomize the DNS identifier for DNS query.

  • Enforce TSIG resource record to be present in DNS message—You can drop or log non-conforming packets, and optionally log dropped packets.

Step 6

Click the Filtering tab and choose the desired options.

  • Global Settings—Choose whether to drop packets that exceed the specified maximum length regardless of whether they are from the client or server, from 512 to 65535 bytes.

  • Server Settings—Drop packets that exceed specified maximum length and Drop packets sent to server that exceed length indicated by the RR—Sets the maximum server DNS message length, from 512 to 65535 bytes, or sets the maximum length to the value in the Resource Record. If you enable both settings, the lower value is used.

  • Client Settings—Drop packets that exceed specified maximum length and Drop packets sent to server that exceed length indicated by the RR—Sets the maximum client DNS message length, from 512 to 65535 bytes, or sets the maximum length to the value in the Resource Record. If you enable both settings, the lower value is used.

Step 7

Click the Mismatch Rate tab and choose whether to enable logging when the DNS ID mismatch rate exceeds the specified threshold. For example, you could set a threshold of 30 mismatches per 3 seconds.

Step 8

Click the Inspections tab and define the specific inspections you want to implement based on traffic characteristics.

You can define traffic matching criteria based on DNS class maps, by configuring matches directly in the inspection map, or both.

  1. Do any of the following:

    • Click Add to add a new criterion.

    • Select an existing criterion and click Edit.

  2. Choose Single Match to define the criterion directly, or Multiple Match, in which case you select the DNS class map that defines the criteria.

  3. If you are defining the criterion here, choose the match type for the criteria: Match (traffic must match the criterion) or No Match (traffic must not match the criterion). For example, if No Match is selected on the string “example.com,” then any traffic that contains “example.com” is excluded from the class map. Then, configure the criterion as follows:

    • Header Flag—Select whether the flag should equal or contain the specified value, then either select the header flag name, or enter the hex value of the header (0x0 to 0xfff). If you select multiple header values, “equals” requires that all flags are present, “contains” that any one of the flags is present, in the packet. Header flag names are AA (Authoritative Answer), QR (Query), RA (Recursion Available), RD (Recursion Desired), TC (Truncation).

    • Type—The DNS Type field name or value in the packet. Field names are A (IPv4 address), AXFR (full zone transfer), CNAME (canonical name), IXFR (incremental zone transfer), NS (authoritative name server), SOA (start of a zone of authority) or TSIG (transaction signature). Values are arbitrary numbers in the DNS Type field from 0 to 65535: either enter a specific value or a range of values.

    • Class—The DNS Class field name or value in the packet. Internet is the only possible field name. Values are arbitrary numbers in the DNS Class field from 0 to 65535: either enter a specific value or a range of values.

    • Question—The question portion of a DNS message.

    • Resource Record—The DNS resource record. Choose whether to match the additional, answer, or authority resource record section.

  4. Choose the primary action to take for matching traffic: drop packet, drop connection, mask (for Header Flag matches only) or none.

  5. Choose whether to enable or disable logging. You must disable logging if you want to enforce TSIG.

  6. Chose whether to enforce the presence of a TSIG resource record. You can drop the packet, log it, or drop and log it. Usually, you must select Primary Action: None and Log: Disable to enforce TSIG. However, for Header Flag matches, you can enforce TSIG along with the mask primary action.

  7. Click OK to add the inspection. Repeat the process as needed.

Step 9

Click OK in the DNS Inspect Map dialog box.

You can now use the inspection map in a DNS inspection service policy.

What to do next

You can now configure an inspection policy to use the map. See Configure Application Layer Protocol Inspection.

FTP Inspection

FTP inspection is enabled by default. You need to configure it only if you want non-default processing. The following sections describe the FTP inspection engine.

FTP Inspection Overview

The FTP application inspection inspects the FTP sessions and performs four tasks:

  • Prepares dynamic secondary data connection channels for FTP data transfer. Ports for these channels are negotiated through PORT or PASV commands. The channels are allocated in response to a file upload, a file download, or a directory listing event.

  • Tracks the FTP command-response sequence.

  • Generates an audit trail.

    • Audit record 303002 is generated for each file that is retrieved or uploaded.

    • Audit record 201005 is generated if the secondary dynamic channel preparation failed due to memory shortage.

  • Translates the embedded IP address.


Note

If you disable FTP inspection, outbound users can start connections only in passive mode, and all inbound FTP is disabled.

Strict FTP

Strict FTP increases the security of protected networks by preventing web browsers from sending embedded commands in FTP requests. To enable strict FTP, click the Configure button next to FTP on the Configuration > Firewall > Service Policy Rules > Edit Service Policy Rule > Rule Actions > Protocol Inspection tab.

When you use strict FTP, you can optionally specify an FTP inspection policy map to specify FTP commands that are not permitted to pass through the ASA.

Strict FTP inspection enforces the following behavior:

  • An FTP command must be acknowledged before the ASA allows a new command.

  • The ASA drops connections that send embedded commands.

  • The 227 and PORT commands are checked to ensure they do not appear in an error string.


Caution

Using strict FTP may cause the failure of FTP clients that are not strictly compliant with FTP RFCs.

With strict FTP inspection, each FTP command and response sequence is tracked for the following anomalous activity:

  • Truncated command—Number of commas in the PORT and PASV reply command is checked to see if it is five. If it is not five, then the PORT command is assumed to be truncated and the TCP connection is closed.

  • Incorrect command—Checks the FTP command to see if it ends with <CR><LF> characters, as required by the RFC. If it does not, the connection is closed.

  • Size of RETR and STOR commands—These are checked against a fixed constant. If the size is greater, then an error message is logged and the connection is closed.

  • Command spoofing—The PORT command should always be sent from the client. The TCP connection is denied if a PORT command is sent from the server.

  • Reply spoofing—PASV reply command (227) should always be sent from the server. The TCP connection is denied if a PASV reply command is sent from the client. This prevents the security hole when the user executes “227 xxxxx a1, a2, a3, a4, p1, p2.”

  • TCP stream editing—The ASA closes the connection if it detects TCP stream editing.

  • Invalid port negotiation—The negotiated dynamic port value is checked to see if it is less than 1024. As port numbers in the range from 1 to 1024 are reserved for well-known connections, if the negotiated port falls in this range, then the TCP connection is freed.

  • Command pipelining—The number of characters present after the port numbers in the PORT and PASV reply command is cross checked with a constant value of 8. If it is more than 8, then the TCP connection is closed.

  • The ASA replaces the FTP server response to the SYST command with a series of Xs to prevent the server from revealing its system type to FTP clients. To override this default behavior, use the no mask-syst-reply command in the FTP map.

Configure an FTP Inspection Policy Map

FTP command filtering and security checks are provided using strict FTP inspection for improved security and control. Protocol conformance includes packet length checks, delimiters and packet format checks, command terminator checks, and command validation.

Blocking FTP based on user values is also supported so that it is possible for FTP sites to post files for download, but restrict access to certain users. You can block FTP connections based on file type, server name, and other attributes. System message logs are generated if an FTP connection is denied after inspection.

If you want FTP inspection to allow FTP servers to reveal their system type to FTP clients, and limit the allowed FTP commands, then create and configure an FTP inspection policy map. You can then apply the map when you enable FTP inspection.

You can optionally create an FTP inspection class map to define the traffic class for FTP inspection. The other option is to define the traffic classes directly in the FTP inspection policy map. The difference between creating a class map and defining the traffic match directly in the inspection map is that you can create more complex match criteria and you can reuse class maps. Although this procedure explains inspection maps, the matching criteria used in class maps are the same as those explained in the step relating to the Inspection tab. You can configure DNS class maps by selecting Configuration > Firewall > Objects > Class Maps > FTP, or by creating them while configuring the inspection map.


Tip

You can configure inspection maps while creating service policies, in addition to the procedure explained below. The contents of the map are the same regardless of how you create it.

Before you begin

Some traffic matching options use regular expressions for matching purposes. If you intend to use one of those techniques, first create the regular expression or regular expression class map.

Procedure

Step 1

Choose Configuration > Firewall > Objects > Inspect Maps > FTP.

Step 2

Do one of the following:

  • Click Add to add a new map.

  • Select a map to view its contents. You can change the security level directly, or click Customize to edit the map. The remainder of the procedure assumes you are customizing or adding a map.

Step 3

For new maps, enter a name (up to 40 characters) and description. When editing a map, you can change the description only.

Step 4

In the Security Level view of the FTP Inspect Map dialog box, select the level that best matches your desired configuration. The default level is High.

If one of the preset levels matches your requirements, you are now done. Just click OK, skip the rest of this procedure, and use the map in a service policy rule for FTP inspection.

If you need to customize the settings further, click Details, and continue with the procedure.

Tip 

The File Type Filtering button is a shortcut to configure file media or MIME type inspection, which is explained later in this procedure.

Step 5

Click the Parameters tab and choose whether to mask the greeting banner from the server or mask the reply to the SYST command.

Masking these items prevents the client from discovering server information that might be helpful in an attack.

Step 6

Click the Inspections tab and define the specific inspections you want to implement based on traffic characteristics.

You can define traffic matching criteria based on FTP class maps, by configuring matches directly in the inspection map, or both.

  1. Do any of the following:

    • Click Add to add a new criterion.

    • Select an existing criterion and click Edit.

  2. Choose Single Match to define the criterion directly, or Multiple Match, in which case you select the FTP class map that defines the criteria.

  3. If you are defining the criterion here, choose the match type for the criteria: Match (traffic must match the criterion) or No Match (traffic must not match the criterion). For example, if No Match is selected on the string “example.com,” then any traffic that contains “example.com” is excluded from the class map. Then, configure the criterion as follows:

    • File Name—Match the name of the file being transferred against the selected regular expression or regular expression class.

    • File Type—Match the MIME or media type of the file being transferred against the selected regular expression or regular expression class.

    • Server—Match the FTP server name against the selected regular expression or regular expression class.

    • User—Match the name of the logged-in user against the selected regular expression or regular expression class.

    • Request Command—The FTP command used in the packet, any combination of the following:

      • APPE—Append to a file.

      • CDUP—Changes to the parent directory of the current working directory.

      • DELE—Delete a file on the server.

      • GET—Gets a file from the server.

      • HELP—Provides help information.

      • MKD—Makes a directory on the server.

      • PUT—Sends a file to the server.

      • RMD—Deletes a directory on the server.

      • RNFR—Specifies the “rename-from” filename.

      • RNTO—Specifies the “rename-to” filename.

      • SITE—Used to specify a server-specific command. This is usually used for remote administration.

      • STOU—Stores a file using a unique file name.

  4. Choose whether to enable or disable logging. The action is always to reset the connection, which drops the packet, closes the connection, and sends a TCP reset to the server or client.

  5. Click OK to add the inspection. Repeat the process as needed.

Step 7

Click OK in the FTP Inspect Map dialog box.

You can now use the inspection map in a FTP inspection service policy.

What to do next

You can now configure an inspection policy to use the map. See Configure Application Layer Protocol Inspection.

HTTP Inspection

If you are not using a purpose-built module for HTTP inspection and application filtering, such as ASA FirePOWER, you can manually configure HTTP inspection on the ASA.

HTTP inspection is not enabled in the default inspection policy, so you must enable it if you need this inspection. However, the default inspect class does include the default HTTP ports, so you can simply edit the default global inspection policy to add HTTP inspection. You can alternatively create a new service policy as desired, for example, an interface-specific policy.


Tip

Do not configure HTTP inspection in both a service module and on the ASA, as the inspections are not compatible.

The following sections describe the HTTP inspection engine.

HTTP Inspection Overview


Tip

You can install a service module that performs application and URL filtering, which includes HTTP inspection, such as ASA FirePOWER. The HTTP inspection running on the ASA is not compatible with these modules. Note that it is far easier to configure application filtering using a purpose-built module rather than trying to manually configure it on the ASA using an HTTP inspection policy map.

Use the HTTP inspection engine to protect against specific attacks and other threats that are associated with HTTP traffic.

HTTP application inspection scans HTTP headers and body, and performs various checks on the data. These checks prevent various HTTP constructs, content types, and tunneling and messaging protocols from traversing the security appliance.

The enhanced HTTP inspection feature, which is also known as an application firewall and is available when you configure an HTTP inspection policy map, can help prevent attackers from using HTTP messages for circumventing network security policy.

HTTP application inspection can block tunneled applications and non-ASCII characters in HTTP requests and responses, preventing malicious content from reaching the web server. Size limiting of various elements in HTTP request and response headers, URL blocking, and HTTP server header type spoofing are also supported.

Enhanced HTTP inspection verifies the following for all HTTP messages:

  • Conformance to RFC 2616

  • Use of RFC-defined methods only.

  • Compliance with the additional criteria.

Configure an HTTP Inspection Policy Map

To specify actions when a message violates a parameter, create an HTTP inspection policy map. You can then apply the inspection policy map when you enable HTTP inspection.

You can optionally create an HTTP inspection class map to define the traffic class for HTTP inspection. The other option is to define the traffic classes directly in the HTTP inspection policy map. The difference between creating a class map and defining the traffic match directly in the inspection map is that you can create more complex match criteria and you can reuse class maps. Although this procedure explains inspection maps, the matching criteria used in class maps are the same as those explained in the step relating to the Inspection tab. You can configure HTTP class maps by selecting Configuration > Firewall > Objects > Class Maps > HTTP, or by creating them while configuring the inspection map.


Tip

You can configure inspection maps while creating service policies, in addition to the procedure explained below. The contents of the map are the same regardless of how you create it.

Before you begin

Some traffic matching options use regular expressions for matching purposes. If you intend to use one of those techniques, first create the regular expression or regular expression class map.

Procedure

Step 1

Choose Configuration > Firewall > Objects > Inspect Maps > HTTP.

Step 2

Do one of the following:

  • Click Add to add a new map.

  • Select a map to view its contents. You can change the security level directly, or click Customize to edit the map. The remainder of the procedure assumes you are customizing or adding a map.

Step 3

For new maps, enter a name (up to 40 characters) and description. When editing a map, you can change the description only.

Step 4

In the Security Level view of the HTTP Inspect Map dialog box, select the level that best matches your desired configuration. The default level is Low.

If one of the preset levels matches your requirements, you are now done. Just click OK, skip the rest of this procedure, and use the map in a service policy rule for HTTP inspection.

If you need to customize the settings further, click Details, and continue with the procedure.

Tip 

The URI Filtering button is a shortcut to configure Request URI inspection, which is explained later in this procedure.

Step 5

Click the Parameters tab and configure the desired options.

  • Body Match Maximum—The maximum number of characters in the body of an HTTP message that should be searched in a body match. Default is 200 bytes. A large number will have a significant impact on performance.

  • Check for protocol violations—Whether to verify that packets conform to the HTTP protocol. For violations, you can drop the connection, reset it, or log it. When dropping or resetting, you can also enable logging.

  • Spoof server string—Replaces the server HTTP header value with the specified string, up to 82 characters.

Step 6

Click the Inspections tab and define the specific inspections you want to implement based on traffic characteristics.

You can define traffic matching criteria based on HTTP class maps, by configuring matches directly in the inspection map, or both.

  1. Do any of the following:

    • Click Add to add a new criterion.

    • Select an existing criterion and click Edit.

  2. Choose Single Match to define the criterion directly, or Multiple Match, in which case you select the HTTP class map that defines the criteria.

  3. If you are defining the criterion here, choose the match type for the criteria: Match (traffic must match the criterion) or No Match (traffic must not match the criterion). For example, if No Match is selected on the string “example.com,” then any traffic that contains “example.com” is excluded from the class map. Then, configure the criterion as follows:

    • Request/Response Content Type Mismatch—Match packets where the content type in the response does not match one of the MIME types in the accept field of the request.

    • Request Arguments—Match the arguments of the request against the selected regular expression or regular expression class.

    • Request Body Length—Match packets where the body of the request is greater than the specified number of bytes.

    • Request Body—Match the body of the request against the selected regular expression or regular expression class.

    • Request Header Field Count—Match packets where the number of header fields in the request is greater than the specified count. You can match the field header type to a regular expression or to a predefined type. The predefined types are: accept, accept-charset, accept-encoding, accept-language, allow, authorization, cache-control, connection, content-encoding, content-language, content-length, content-location, content-md5, content-range, content-type, cookie, date, expect, expires, from, host, if-match, if-modified-since, if-none-match, if-range, if-unmodified-since, last-modified, max-forwards, pragma, proxy-authorization, range, referer, te, trailer, transfer-encoding, upgrade, user-agent, via, warning.

    • Request Header Field Length—Match packets where the length of the header field in the request is greater than the specified bytes. You can match the field header type to a regular expression or to a predefined type. The predefined types are listed above for Request Header Field Count.

    • Request Header Field—Match the content of the selected header field in the request against the selected regular expression or regular expression class. You can specify a predefined header type or use a regular expression to select the headers.

    • Request Header Count—Match packets where the number of headers in the request is greater than the specified number.

    • Request Header Length—Match packets where the length of the header in the request is greater than the specified bytes.

    • Request Header Non-ASCII—Match packets where the header in the request contains non-ASCII characters.

    • Request Method—Match packets where the request method matches the predefined type or the selected regular expression or regular expression class. The predefined types are: bcopy, bdelete, bmove, bpropfind, bproppatch, connect, copy, delete, edit, get, getattribute, getattributenames, getproperties, head, index, lock, mkcol, mkdir, move, notify, options, poll, post, propfind, proppatch, put, revadd, revlabel, revlog, revnum, save, search, setattribute, startrev, stoprev, subscribe, trace, unedit, unlock, unsubscribe.

    • Request URI Length—Match packets where the length of the URI of the request is greater than the specified bytes.

    • Request URI—Match the content of the URI of the request against the selected regular expression or regular expression class.

    • Request Body—Match the body of the request against the selected regular expression or regular expression class, or to ActiveX or Java Applet content.

    • Response Body Length—Match packets where the length of the body of the response is greater than the specified bytes.

    • Response Header Field Count—Match packets where the number of header fields in the response is greater than the specified count. You can match the field header type to a regular expression or to a predefined type. The predefined types are: accept-ranges, age, allow, cache-control, connection, content-encoding, content-language, content-length, content-location, content-md5, content-range, content-type, date, etag, expires, last-modified, location, pragma, proxy-authenticate, retry-after, server, set-cookie, trailer, transfer-encoding, upgrade, vary, via, warning, www-authenticate.

    • Response Header Field Length—Match packets where the length of the header field in the response is greater than the specified bytes. You can match the field header type to a regular expression or to a predefined type. The predefined types are listed above for Response Header Field Count.

    • Response Header Field—Match the content of the selected header field in the response against the selected regular expression or regular expression class. You can specify a predefined header type or use a regular expression to select the headers.

    • Response Header Count—Match packets where the number of headers in the response is greater than the specified number.

    • Response Header Length—Match packets where the length of the header in the response is greater than the specified bytes.

    • Response Header Non-ASCII—Match packets where the header in the response contains non-ASCII characters.

    • Response Status Line—Match the content of the response status line against the selected regular expression or regular expression class.

  4. Choose whether to drop the connection, reset it, or log it. For drop connection and reset, you can enable or disable logging.

  5. Click OK to add the inspection. Repeat the process as needed.

Step 7

Click OK in the HTTP Inspect Map dialog box.

You can now use the inspection map in a HTTP inspection service policy.

What to do next

You can now configure an inspection policy to use the map. See Configure Application Layer Protocol Inspection.

ICMP Inspection

The ICMP inspection engine allows ICMP traffic to have a “session” so it can be inspected like TCP and UDP traffic. Without the ICMP inspection engine, we recommend that you do not allow ICMP through the ASA in an ACL. Without stateful inspection, ICMP can be used to attack your network. The ICMP inspection engine ensures that there is only one response for each request, and that the sequence number is correct.

However, ICMP traffic directed to an ASA interface is never inspected, even if you enable ICMP inspection. Thus, a ping (echo request) to an interface can fail under specific circumstances, such as when the echo request comes from a source that the ASA can reach through a backup default route.

For information on enabling ICMP inspection, see Configure Application Layer Protocol Inspection.

ICMP Error Inspection

When ICMP Error inspection is enabled, the ASA creates translation sessions for intermediate hops that send ICMP error messages, based on the NAT configuration. The ASA overwrites the packet with the translated IP addresses.

When disabled, the ASA does not create translation sessions for intermediate nodes that generate ICMP error messages. ICMP error messages generated by the intermediate nodes between the inside host and the ASA reach the outside host without consuming any additional NAT resource. This is undesirable when an outside host uses the traceroute command to trace the hops to the destination on the inside of the ASA. When the ASA does not translate the intermediate hops, all the intermediate hops appear with the mapped destination IP address.

For information on enabling ICMP Error inspection, see Configure Application Layer Protocol Inspection.

Instant Messaging Inspection

The Instant Messaging (IM) inspect engine lets you control the network usage of IM and stop leakage of confidential data, propagation of worms, and other threats to the corporate network.

IM inspection is not enabled in the default inspection policy, so you must enable it if you need this inspection. However, the default inspect class does include the default IM ports, so you can simply edit the default global inspection policy to add IM inspection. You can alternatively create a new service policy as desired, for example, an interface-specific policy.

If you decide to implement IM inspection, you can also configure an IM inspection policy map to specify actions when a message violates a parameter. The following procedure explains IM inspection policy maps.

You can optionally create an IM inspection class map to define the traffic class for IM inspection. The other option is to define the traffic classes directly in the IM inspection policy map. The difference between creating a class map and defining the traffic match directly in the inspection map is that you can create more complex match criteria and you can reuse class maps. This procedure explains inspection maps, but class maps are essentially the same, except that you do not specify the actions for matching traffic. You can configure IM class maps by selecting Configuration > Firewall > Objects > Class Maps > Instant Messaging (IM).


Tip

You can configure inspection maps while creating service policies, in addition to the procedure explained below. The contents of the map are the same regardless of how you create it.

Before you begin

Some traffic matching options use regular expressions for matching purposes. If you intend to use one of those techniques, first create the regular expression or regular expression class map.

Procedure

Step 1

Choose Configuration > Firewall > Objects > Inspect Maps > Instant Messaging (IM).

Step 2

Do one of the following:

  • Click Add to add a new map.

  • Select a map and click Edit.

Step 3

For new maps, enter a name (up to 40 characters) and description. When editing a map, you can change the description only.

Step 4

Define the specific inspections you want to implement based on traffic characteristics.

You can define traffic matching criteria based on IM class maps, by configuring matches directly in the inspection map, or both.

  1. Do any of the following:

    • Click Add to add a new criterion.

    • Select an existing criterion and click Edit.

  2. Choose Single Match to define the criterion directly, or Multiple Match, in which case you select the IM class map that defines the criteria. Click Manage to create new class maps.

  3. If you are defining the criterion here, choose the match type for the criteria: Match (traffic must match the criterion) or No Match (traffic must not match the criterion). For example, if No Match is selected on the string “example.com,” then any traffic that contains “example.com” is excluded from the class map. Then, configure the criterion.

    • Protocol—Match traffic of a specific IM protocol, such as Yahoo Messenger or MSN Messenger.

    • Service—Match a specific IM service, such as chat, file transfer, web cam, voice chat, conference, or games.

    • Version—Match the version of the IM message against the selected regular expression or regular expression class.

    • Client Login Name—Match the source client login name of the IM message against the selected regular expression or regular expression class.

    • Client Peer Login Name—Match the destination peer login name of the IM message against the selected regular expression or regular expression class.

    • Source IP Address—Match the source IP address and mask.

    • Destination IP Address—Match the destination IP address and mask.

    • Filename—Match the filename of the IM message against the selected regular expression or regular expression class.

  4. Choose whether to drop the connection, reset it, or log it. For drop connection and reset, you can enable or disable logging.

  5. Click OK to add the inspection. Repeat the process as needed.

Step 5

Click OK in the IM Inspect Map dialog box.

You can now use the inspection map in a IM inspection service policy.

What to do next

You can now configure an inspection policy to use the map. See Configure Application Layer Protocol Inspection.

IP Options Inspection

You can configure IP Options inspection to control which IP packets are allowed based on the contents of the IP Options field in the packet header. You can drop packets that have unwanted options, clear the options (and allow the packet), or allow the packet without change.

IP options provide control functions that are required in some situations but unnecessary for most common communications. In particular, IP options include provisions for time stamps, security, and special routing. Use of IP Options is optional, and the field can contain zero, one, or more options.

For a list of IP options, with references to the relevant RFCs, see the IANA page, http://www.iana.org/assignments/ip-parameters/ip-parameters.xhtml.

IP options inspection is enabled by default, but for RSVP traffic only. You need to configure it only if you want to allow additional options than the default map allows, or if you want to apply it to other types of traffic by using a non-default inspection traffic class map.


Note

IP options inspection does not work on fragmented packets. For example, options are not cleared from fragments.

The following sections describe IP Options inspection.

Defaults for IP Options Inspection

IP Options inspection is enabled by default for RSVP traffic only, using the _default_ip_options_map inspection policy map.

  • The Router Alert option is allowed.

    This option notifies transit routers to inspect the contents of the packet even when the packet is not destined for that router. This inspection is valuable when implementing RSVP and similar protocols that require relatively complex processing from the routers along the packet’s delivery path. Dropping RSVP packets containing the Router Alert option can cause problems in VoIP implementations.

  • Packets that contain any other options are dropped.

    This includes packets that contain unsupported options.

    Each time a packet is dropped due to inspection, syslog 106012 is issued. The message shows which option caused the drop. Use the show service-policy inspect ip-options command to view statistics for each option.

Configure an IP Options Inspection Policy Map

If you want to perform non-default IP options inspection, create an IP options inspection policy map to specify how you want to handle each option type.


Tip

You can configure inspection maps while creating service policies, in addition to the procedure explained below. The contents of the map are the same regardless of how you create it.

Procedure

Step 1

Choose Configuration > Firewall > Objects > Inspect Maps > IP Options.

Step 2

Do one of the following:

  • Click Add to add a new map.

  • Select a map and click Edit.

Step 3

For new maps, enter a name (up to 40 characters) and description. When editing a map, you can change the description only.

Step 4

Choose which options you want to allow.

You can allow the End of Options List, No Operation, and Router Alert options.

Packets that contain any option that you do not select are dropped. If a packet contains unsupported options, it is also dropped.

For a list of IP options, with references to the relevant RFCs, see the IANA page, http://www.iana.org/assignments/ip-parameters/ip-parameters.xhtml.

Step 5

For each option you allow, choose whether to clear the option before allowing the packet.

When you clear an option, inspection removes the option from the packet header before the packet is transmitted.

Step 6

Click OK.

You can now use the inspection map in an IP options inspection service policy.

What to do next

You can now configure an inspection policy to use the map. See Configure Application Layer Protocol Inspection.

IPsec Pass Through Inspection

IPsec Pass Through inspection is not enabled in the default inspection policy, so you must enable it if you need this inspection. However, the default inspect class does include the default IPsec ports, so you can simply edit the default global inspection policy to add IPsec inspection. You can alternatively create a new service policy as desired, for example, an interface-specific policy.

The following sections describe the IPsec Pass Through inspection engine.

IPsec Pass Through Inspection Overview

Internet Protocol Security (IPsec) is a protocol suite for securing IP communications by authenticating and encrypting each IP packet of a data stream. IPsec also includes protocols for establishing mutual authentication between agents at the beginning of the session and negotiation of cryptographic keys to be used during the session. IPsec can be used to protect data flows between a pair of hosts (for example, computer users or servers), between a pair of security gateways (such as routers or firewalls), or between a security gateway and a host.

IPsec Pass Through application inspection provides convenient traversal of ESP (IP protocol 50) and AH (IP protocol 51) traffic associated with an IKE UDP port 500 connection. It avoids lengthy ACL configuration to permit ESP and AH traffic and also provides security using timeout and max connections.

Configure a policy map for IPsec Pass Through to specify the restrictions for ESP or AH traffic. You can set the per client max connections and the idle timeout.

NAT and non-NAT traffic is permitted. However, PAT is not supported.

Configure an IPsec Pass Through Inspection Policy Map

An IPsec Pass Through map lets you change the default configuration values used for IPsec Pass Through application inspection. You can use an IPsec Pass Through map to permit certain flows without using an ACL.

The configuration includes a default map, _default_ipsec_passthru_map, that sets no maximum limit on ESP connections per client, and sets the ESP idle timeout at 10 minutes. You need to configure an inspection policy map only if you want different values, or if you want to set AH values.


Tip

You can configure inspection maps while creating service policies, in addition to the procedure explained below. The contents of the map are the same regardless of how you create it.

Procedure

Step 1

Choose Configuration > Firewall > Objects > Inspect Maps > IPsec Pass Through.

Step 2

Do one of the following:

  • Click Add to add a new map.

  • Select a map to view its contents. You can change the security level directly, or click Customize to edit the map. The remainder of the procedure assumes you are customizing or adding a map.

Step 3

For new maps, enter a name (up to 40 characters) and description. When editing a map, you can change the description only.

Step 4

In the Security Level view of the IPsec Pass Through Inspect Map dialog box, select the level that best matches your desired configuration.

If one of the preset levels matches your requirements, you are now done. Just click OK, skip the rest of this procedure, and use the map in a service policy rule for IPsec Pass Through inspection.

If you need to customize the settings further, click Details, and continue with the procedure.

Step 5

Choose whether to allow ESP and AH tunnels.

For each protocol, you can also set the maximum number of connections allowed per client, and the idle timeout.

Step 6

Click OK.

You can now use the inspection map in an IPsec Pass Through inspection service policy.

IPv6 Inspection

IPv6 inspection lets you selectively log or drop IPv6 traffic based on the extension header. In addition, IPv6 inspection can check conformance to RFC 2460 for type and order of extension headers in IPv6 packets.

IPv6 inspection is not enabled in the default inspection policy, so you must enable it if you need this inspection. You can simply edit the default global inspection policy to add IPv6 inspection. You can alternatively create a new service policy as desired, for example, an interface-specific policy.

Defaults for IPv6 Inspection

If you enable IPv6 inspection and do not specify an inspection policy map, then the default IPv6 inspection policy map is used, and the following actions are taken:

  • Allows only known IPv6 extension headers. Non-conforming packets are dropped and logged.

  • Enforces the order of IPv6 extension headers as defined in the RFC 2460 specification. Non-conforming packets are dropped and logged.

  • Drops any packet with a routing type header.

Configure an IPv6 Inspection Policy Map

To identify extension headers to drop or log, or to disable packet verification, create an IPv6 inspection policy map to be used by the service policy.

Procedure

Step 1

Choose Configuration > Firewall > Objects > Inspect Maps > IPv6.

Step 2

Do one of the following:

  • Click Add to add a new map.

  • Select a map and click Edit.

Step 3

For new maps, enter a name (up to 40 characters) and description. When editing a map, you can change the description only.

Step 4

Click the Enforcement tab and choose whether to permit only known IPv6 extension headers or to enforce the order of IPv6 extension headers as defined in RFC 2460. Non-conforming packets are dropped and logged.

Step 5

(Optional) Click the Header Matches tab to identify traffic to drop or log based on the headers in IPv6 messages.

  1. Do any of the following:

    • Click Add to add a new criterion.

    • Select an existing criterion and click Edit.

  2. Choose the IPv6 extension header to match:

    • Authentication (AH) header.

    • Destination Options header.

    • Encapsulating Security Payload (ESP) header.

    • Fragment header.

    • Hop-by-Hop Options header.

    • Routing header—Specify either a single header type number or a range of numbers.

    • Header count—Specify the maximum number of extension headers you will allow without dropping or logging the packet.

    • Routing header address count—Specify the maximum number of addresses in the type 0 routing header you will allow without dropping or logging the packet.

  3. Choose whether to drop or log the packet. If you drop the packet, you can also enable logging.

  4. Click OK to add the inspection. Repeat the process as needed.

Step 6

Click OK in the IPv6 Inspect Map dialog box.

You can now use the inspection map in an IPv6 inspection service policy.

What to do next

You can now configure an inspection policy to use the map. See Configure Application Layer Protocol Inspection.

NetBIOS Inspection

NetBIOS application inspection performs NAT for the embedded IP address in the NetBIOS name service (NBNS) packets and NetBIOS datagram services packets. It also enforces protocol conformance, checking the various count and length fields for consistency.

NetBIOS inspection is enabled by default. You can optionally create a policy map to drop or log NetBIOS protocol violations. The following procedure explains how to configure a NetBIOS inspection policy map.

Procedure

Step 1

Choose Configuration > Firewall > Objects > Inspect Maps > NetBIOS.

Step 2

Do one of the following:

  • Click Add to add a new map.

  • Select a map and click Edit.

Step 3

For new maps, enter a name (up to 40 characters) and description. When editing a map, you can change the description only.

Step 4

Select Check for Protocol Violations. There is no reason to create a map if you do not select this option.

Step 5

Select the action to take, either to drop the packet or log it. If you drop the packet, you can also enable logging.

Step 6

Click OK.

You can now use the inspection map in a NetBIOS inspection service policy.

What to do next

You can now configure an inspection policy to use the map. See Configure Application Layer Protocol Inspection.

PPTP Inspection

PPTP is a protocol for tunneling PPP traffic. A PPTP session is composed of one TCP channel and usually two PPTP GRE tunnels. The TCP channel is the control channel used for negotiating and managing the PPTP GRE tunnels. The GRE tunnels carry PPP sessions between the two hosts.

When enabled, PPTP application inspection inspects PPTP protocol packets and dynamically creates the GRE connections and xlates necessary to permit PPTP traffic.

Specifically, the ASA inspects the PPTP version announcements and the outgoing call request/response sequence. Only PPTP Version 1, as defined in RFC 2637, is inspected. Further inspection on the TCP control channel is disabled if the version announced by either side is not Version 1. In addition, the outgoing-call request and reply sequence are tracked. Connections and xlates are dynamically allocated as necessary to permit subsequent secondary GRE data traffic.

The PPTP inspection engine must be enabled for PPTP traffic to be translated by PAT. Additionally, PAT is only performed for a modified version of GRE (RFC2637) and only if it is negotiated over the PPTP TCP control channel. PAT is not performed for the unmodified version of GRE (RFC 1701 and RFC 1702).

For information on enabling PPTP inspection, see Configure Application Layer Protocol Inspection.

SMTP and Extended SMTP Inspection

ESMTP inspection detects attacks, including spam, phising, malformed message attacks, and buffer overflow/underflow attacks. It also provides support for application security and protocol conformance, which enforces the sanity of the ESMTP messages as well as block senders/receivers, and block mail relay.

ESMTP inspection is enabled by default. You need to configure it only if you want different processing than that provided by the default inspection map.

The following sections describe the ESMTP inspection engine.

SMTP and ESMTP Inspection Overview

Extended SMTP (ESMTP) application inspection provides improved protection against SMTP-based attacks by restricting the types of SMTP commands that can pass through the ASA and by adding monitoring capabilities. ESMTP is an enhancement to the SMTP protocol and is similar is most respects to SMTP.

ESMTP application inspection controls and reduces the commands that the user can use as well as the messages that the server returns. ESMTP inspection performs three primary tasks:

  • Restricts SMTP requests to seven basic SMTP commands and eight extended commands. Supported commands are the following:

    • Extended SMTP—AUTH, EHLO, ETRN, HELP, SAML, SEND, SOML, STARTTLS, and VRFY.

    • SMTP (RFC 821)—DATA, HELO, MAIL, NOOP, QUIT, RCPT, RSET.

  • Monitors the SMTP command-response sequence.

  • Generates an audit trail—Audit record 108002 is generated when an invalid character embedded in the mail address is replaced. For more information, see RFC 821.

ESMTP inspection monitors the command and response sequence for the following anomalous signatures:

  • Truncated commands.

  • Incorrect command termination (not terminated with <CR><LR>).

  • The MAIL and RCPT commands specify who are the sender and the receiver of the mail. Mail addresses are scanned for strange characters. The pipeline character (|) is deleted (changed to a blank space) and “<” ‚”>” are only allowed if they are used to define a mail address (“>” must be preceded by “<”).

  • Unexpected transition by the SMTP server.

  • For unknown or unsupported commands, the inspection engine changes all the characters in the packet to X, which are rejected by the internal server. This results in a message such as “500 Command unknown: 'XXX'.” Incomplete commands are discarded

    Unsupported ESMTP commands are ATRN, ONEX, VERB, CHUNKING, and private extensions..

  • TCP stream editing.

  • Command pipelining.


Note

With ESMTP inspection enabled, a Telnet session used for interactive SMTP may hang if the following rules are not observed: SMTP commands must be at least four characters in length; they must be terminated with carriage return and line feed; and you must wait for a response before issuing the next reply.

Defaults for ESMTP Inspection

ESMTP inspection is enabled by default, using the _default_esmtp_map inspection policy map.

  • The server banner is masked. The ESMTP inspection engine changes the characters in the server SMTP banner to asterisks except for the “2”, “0”, “0” characters. Carriage return (CR) and linefeed (LF) characters are ignored.

  • Encrypted connections are allowed but not inspected.

  • Special characters in sender and receiver address are not noticed, no action is taken.

  • Connections with command line length greater than 512 are dropped and logged.

  • Connections with more than 100 recipients are dropped and logged.

  • Messages with body length greater than 998 bytes are logged.

  • Connections with header line length greater than 998 are dropped and logged.

  • Messages with MIME filenames greater than 255 characters are dropped and logged.

  • EHLO reply parameters matching “others” are masked.

Configure an ESMTP Inspection Policy Map

To specify actions when a message violates a parameter, create an ESMTP inspection policy map. You can then apply the inspection policy map when you enable ESMTP inspection.

Before you begin

Some traffic matching options use regular expressions for matching purposes. If you intend to use one of those techniques, first create the regular expression or regular expression class map.

Procedure

Step 1

Choose Configuration > Firewall > Objects > Inspect Maps > ESMTP.

Step 2

Do one of the following:

  • Click Add to add a new map.

  • Select a map to view its contents. You can change the security level directly, or click Customize to edit the map. The remainder of the procedure assumes you are customizing or adding a map.

Step 3

For new maps, enter a name (up to 40 characters) and description. When editing a map, you can change the description only.

Step 4

In the Security Level view of the ESMTP Inspect Map dialog box, select the level that best matches your desired configuration.

If one of the preset levels matches your requirements, you are now done. Just click OK, skip the rest of this procedure, and use the map in a service policy rule for ESMTP inspection.

If you need to customize the settings further, click Details, and continue with the procedure.

Tip 

The MIME File Type Filtering button is a shortcut to configure file type inspection, which is explained later in this procedure.

Step 5

Click the Parameters tab and configure the desired options.

  • Mask Server Banner—Whether to mask the banner from the ESMTP server.

  • Encrypted Packet Inspection—Whether to allow ESMTP over TLS (encrypted connections) without inspection. You can optionally log encrypted connections. The default is to allow TLS sessions without inspection. If you deselect the option, the system strips the STARTTLS indication from any encrypted session connection attempt and forces a plain-text connection.

Step 6

Click the Filtering tab and configure the desired options.

  • Configure mail relay—Identifies a domain name for mail relay. You can either drop the connection and optionally log it, or log it.

  • Check for special characters—Identifies the action to take for messages that include the special characters pipe (|), back quote, and NUL in the sender or receiver email addresses. You can either drop the connection and optionally log it, or log it.

Step 7

Click the Inspections tab and define the specific inspections you want to implement based on traffic characteristics.

  1. Do any of the following:

    • Click Add to add a new criterion.

    • Select an existing criterion and click Edit.

  2. Choose the match type for the criteria: Match (traffic must match the criterion) or No Match (traffic must not match the criterion). For example, if No Match is selected on the string “example.com,” then any traffic that contains “example.com” is excluded from the class map. Then, configure the criterion:

    • Body Length—Matches messages where the length of an ESMTP body message is greater than the specified number of bytes.

    • Body Line Length—Matches messages where the length of a line in an ESMTP body message is greater than the specified number of bytes.

    • Commands—Matches the command verb in the message. You can specify one or more of the following commands: auth, data, ehlo, etrn, helo, help, mail, noop, quit, rcpt, rset, saml, soml, vrfy.

    • Command Recipient Count—Matches messages where the number of recipients is greater than the specified count.

    • Command Line Length—Matches messages where the length of a line in the command verb is greater than the specified number of bytes.

    • EHLO Reply Parameters—Matches ESMTP EHLO reply parameters. You can specify one or more of the following parameters: 8bitmime, auth, binaryname, checkpoint, dsn, etrn, others, pipelining, size, vrfy.

    • Header Length—Matches messages where the length of an ESMTP header is greater than the specified number of bytes.

    • Header Line Length—Matches messages where the length of a line in an ESMTP header is greater than the specified number of bytes.

    • Header To: Fields Count—Matches messages where the number of To fields in the header is greater than the specified number.

    • Invalid Recipients Count—Matches messages where the number of invalid recipients is greater than the specified count.

    • MIME File Type—Matches the MIME or media file type against the specified regular expression or regular expression class.

    • MIME Filename Length—Matches messages where a file name is longer than the specified number of bytes.

    • MIME Encoding—Matches the MIME encoding type. You can specify one or more of the following types: 7bit, 8bit, base64, binary, others, quoted-printable.

    • Sender Address—Matches the sender email address against the specified regular expression or regular expression class.

    • Sender Address Length—Matches messages where the sender address is greater than the specified number of bytes.

  3. Choose whether to drop the connection, reset it, or log it. For drop connection and reset, you can enable or disable logging. For command and EHLO reply parameter matching, you can also mask the command. For command matching, you can also apply a rate limit in packets per second.

  4. Click OK to add the inspection. Repeat the process as needed.

Step 8

Click OK in the ESMTP Inspect Map dialog box.

You can now use the inspection map in a ESMTP inspection service policy.

What to do next

You can now configure an inspection policy to use the map. See Configure Application Layer Protocol Inspection.

TFTP Inspection

TFTP inspection is enabled by default.

TFTP, described in RFC 1350, is a simple protocol to read and write files between a TFTP server and client.

The inspection engine inspects TFTP read request (RRQ), write request (WRQ), and error notification (ERROR), and dynamically creates connections and translations, if necessary, to permit file transfer between a TFTP client and server.

A dynamic secondary channel and a PAT translation, if necessary, are allocated on a reception of a valid read (RRQ) or write (WRQ) request. This secondary channel is subsequently used by TFTP for file transfer or error notification.

Only the TFTP server can initiate traffic over the secondary channel, and at most one incomplete secondary channel can exist between the TFTP client and server. An error notification from the server closes the secondary channel.

TFTP inspection must be enabled if static PAT is used to redirect TFTP traffic.

For information on enabling TFTP inspection, see Configure Application Layer Protocol Inspection.