CLI Book 2: Cisco ASA Series Firewall CLI Configuration Guide, 9.1

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Book Title

CLI Book 2: Cisco ASA Series Firewall CLI Configuration Guide, 9.1

Chapter Title

Configuring Filtering Services

Results

Updated:
August 14, 2014

Chapter: Configuring Filtering Services

Configuring Filtering Services

This chapter describes how to use filtering services to provide greater control over traffic passing through the ASA and includes the following sections:

Information About Web Traffic Filtering

You can use web traffic filtering in two distinct ways:

  • Filtering ActiveX objects or Java applets
  • Filtering with an external filtering server

Instead of blocking access altogether, you can remove specific undesirable objects from web traffic, such as ActiveX objects or Java applets, that may pose a security threat in certain situations.

You can use web traffic filtering to direct specific traffic to an external filtering server, such an Secure Computing SmartFilter (formerly N2H2) or the Websense filtering server. You can enable long URL, HTTPS, and FTP filtering using either Websense or Secure Computing SmartFilter for web traffic filtering. Filtering servers can block traffic to specific sites or types of sites, as specified by the security policy.

Note URL caching will only work if the version of the URL server software from the URL server vendor supports it.

Because web traffic filtering is CPU-intensive, using an external filtering server ensures that the throughput of other traffic is not affected. However, depending on the speed of your network and the capacity of your web traffic filtering server, the time required for the initial connection may be noticeably slower when filtering traffic with an external filtering server.

Configuring ActiveX Filtering

This section includes the following topics:

Information About ActiveX Filtering

ActiveX objects may pose security risks because they can contain code intended to attack hosts and servers on a protected network. You can disable ActiveX objects with ActiveX filtering.

ActiveX controls, formerly known as OLE or OCX controls, are components that you can insert in a web page or another application. These controls include custom forms, calendars, or any of the extensive third-party forms for gathering or displaying information. As a technology, ActiveX creates many potential problems for network clients including causing workstations to fail, introducing network security problems, or being used to attack servers.

The filter activex command blocks the HTML object commands by commenting them out within the HTML web page. ActiveX filtering of HTML files is performed by selectively replacing the <APPLET> and </APPLET>, and <OBJECT CLASSID> and </OBJECT> tags with comments. Filtering of nested tags is supported by converting top-level tags to comments.

Caution The filter activex command also blocks any Java applets, image files, or multimedia objects that are embedded in object tags.

If the <object> or </object> HTML tags split across network packets or if the code in the tags is longer than the number of bytes in the MTU, the ASA cannot block the tag.

ActiveX blocking does not occur when users access an IP address referenced by the alias command or for clientless SSL VPN traffic.

Licensing Requirements for ActiveX Filtering

The following table shows the licensing requirements for this feature:

 

Model
License Requirement

All models

Base License.

Guidelines and Limitations for ActiveX Filtering

This section includes the guidelines and limitations for this feature.

Context Mode Guidelines

Supported in single and multiple context mode.

Firewall Mode Guidelines

Supported in routed and transparent firewall mode.

IPv6 Guidelines

Does not support IPv6.

Configuring ActiveX Filtering

To remove ActiveX objects in HTTP traffic that is passing through the ASA, enter the following command:

Command
Purpose
filter activex port[-port] local_ip local_mask foreign_ip foreign_mask
 

ciscoasa# filter activex 80 0 0 0 0

Removes ActiveX objects. To use this command, replace port[-port] with the TCP port to which filtering is applied. Typically, this is port 80, but other values are accepted. The http or url literal can be used for port 80. You can specify a range of ports by using a hyphen between the starting port number and the ending port number. The local IP address and mask identify one or more internal hosts that are the source of the traffic to be filtered. The foreign address and mask specify the external destination of the traffic to be filtered.

Configuration Examples for ActiveX Filtering

You can set either address to 0.0.0.0 (or in shortened form, 0) to specify all hosts. You can use 0.0.0.0 for either mask (or in shortened form, 0) to specify all masks. This command specifies that the ActiveX object blocking applies to HTTP traffic on port 80 from any local host and for connections to any foreign host.

The following example shows how to configure ActiveX filtering to block all outbound connections:

ciscoasa(config)# filter activex 80 0 0 0 0
 

The following example shows how to remove ActiveX filtering:

ciscoasa(config)# no filter activex 80 0 0 0 0
 

Feature History for ActiveX Filtering

Table 29-1 lists the release history for ActiveX Filtering. ASDM is backwards-compatible with multiple platform releases, so the specific ASDM release in which support was added is not listed.

 

Table 29-1 Feature History for ActiveX Filtering
Feature Name
Platform Releases
Feature Information

ActiveX filtering

7.0(1)

Filters specific undesirable objects from HTTP traffic, such as ActiveX objects, which may pose a security threat in certain situations.

Configuring Java Applet Filtering

This section includes the following topics:

Information About Java Applet Filtering

Java applets may pose security risks because they can contain code intended to attack hosts and servers on a protected network. You can remove Java applets with the filter java command.

Note Use the filter activex command to remove Java applets that are embedded in <object> tags.

The filter java command filters out Java applets that return to the ASA from an outbound connection. You still receive the HTML page, but the web page source for the applet is commented out so that the applet cannot execute. The filter java command does not filter clientless SSL VPN traffic.

Licensing Requirements for Java Applet Filtering

The following table shows the licensing requirements for Java applet filtering:

 

Table 29-2 Licensing Requirements
Model
License Requirement

All models

Base License.

Guidelines and Limitations for Java Applet Filtering

This section includes the guidelines and limitations for this feature.

Context Mode Guidelines

Supported in single and multiple context mode.

Firewall Mode Guidelines

Supported in routed and transparent firewall mode.

Does not support IPv6.

Configuring Java Applet Filtering

To apply filtering to remove Java applets from HTTP traffic passing through the ASA, enter the following command:

 

Command
Purpose
filter java port[-port] local_ip local_mask foreign_ip foreign_mask
 

ciscoasa# filter java 80 0 0 0 0

Removes Java applets in HTTP traffic passing through the ASA.

To use this command, replace port[-port] with the TCP port to which filtering is applied. Typically, this is port 80, but other values are accepted. The http or url literal can be used for port 80. You can specify a range of ports by using a hyphen between the starting port number and the ending port number.

The local IP address and mask identify one or more internal hosts that are the source of the traffic to be filtered. The foreign address and mask specify the external destination of the traffic to be filtered.

You can set either address to 0.0.0.0 (or in shortened form, 0) to specify all hosts. You can use 0.0.0.0 for either mask (or in shortened form, 0) to specify all hosts.

You can set either address to 0.0.0.0 (or in shortened form, 0) to specify all hosts. You can use 0.0.0.0 for either mask (or in shortened form, 0) to specify all hosts.

Configuration Examples for Java Applet Filtering

The following example specifies that Java applets are blocked on all outbound connections:

ciscoasa(config)# filter java 80 0 0 0 0
 

This command specifies that the Java applet blocking applies to web traffic on port 80 from any local host and for connections to any foreign host.

The following example blocks downloading of Java applets to a host on a protected network:

ciscoasa(config)# filter java http 192.168.3.3 255.255.255.255 0 0
 

This command prevents host 192.168.3.3 from downloading Java applets.

The following example removes the configuration for downloading Java applets to a host on a protected network:

ciscoasa(config)# no filter java http 192.168.3.3 255.255.255.255 0 0
 

This command allows host 192.168.3.3 to download Java applets.

Feature History for Java Applet Filtering

Table 29-1 lists the release history for Java applet filtering. ASDM is backwards-compatible with multiple platform releases, so the specific ASDM release in which support was added is not listed.

 

Table 29-3 Feature History for Java Applet Filtering
Feature Name
Platform Releases
Feature Information

Java applet filtering

7.0(1)

Filters specific undesirable objects from HTTP traffic, such as Java applets, which may pose a security threat in certain situations.

Filtering URLs and FTP Requests with an External Server

This section describes how to filter URLs and FTP requests with an external server and includes the following topics:

Information About URL Filtering

You can apply filtering to connection requests originating from a more secure network to a less secure network. Although you can use ACLs to prevent outbound access to specific content servers, managing usage this way is difficult because of the size and dynamic nature of the Internet. You can simplify configuration and improve ASA performance by using a separate server running one of the following Internet filtering products:

  • Websense Enterprise for filtering HTTP, HTTPS, and FTP.
  • McAfee SmartFilter (formerly N2H2) for filtering HTTP, HTTPS, FTP, and long URL filtering.

In long URLs, the URL in the Referer field might contain a “host:” text string, which could cause the HTTP GET header to be incorrectly parsed as containing the HTTP Host parameter. The ASA, however, correctly parses the Referer field even when it contains a “host:” text string and forwards the header to the McAfee SmartFilter server with the correct Referer URL.

Note URL caching will only work if the version of the URL server software from the URL server vendor supports it.

Although ASA performance is less affected when using an external server, you might notice longer access times to websites or FTP servers when the filtering server is remote from the ASA.

When filtering is enabled and a request for content is directed through the ASA, the request is sent to the content server and to the filtering server at the same time. If the filtering server allows the connection, the ASA forwards the response from the content server to the originating client. If the filtering server denies the connection, the ASA drops the response and sends a message or return code indicating that the connection was not successful.

If user authentication is enabled on the ASA, then the ASA also sends the username to the filtering server. The filtering server can use user-specific filtering settings or provide enhanced reporting about usage.

Licensing Requirements for URL Filtering

The following table shows the licensing requirements for URL filtering:

 

Table 29-4 Licensing Requirements
Model
License Requirement

All models

Base License.

Guidelines and Limitations for URL Filtering

This section includes the guidelines and limitations for this feature.

Context Mode Guidelines

Supported in single and multiple context mode.

Firewall Mode Guidelines

Supported in routed and transparent firewall mode.

IPv6 Guidelines

Does not support IPv6.

Identifying the Filtering Server

You can identify up to four filtering servers per context. The ASA uses the servers in order until a server responds. In single mode, a maximum of 16 of the same type of filtering servers are allowed. You can only configure a single type of server (Websense or Secure Computing SmartFilter) in your configuration.

Note You must add the filtering server before you can configure filtering for HTTP or HTTPS with the filter command. If you remove the filtering servers from the configuration, then all filter commands are also removed.

To specify the external filtering server, enter the following command:

 

Command
Purpose

 

Choose from the following options:

 

For Websense:

hostname(config)# url-server (if_name) host local_ip [timeout seconds] [protocol TCP | UDP
version [1|4] [connections num_conns]]
 
ciscoasa(config)# url-server (perimeter) host 10.0.1.1 protocol TCP version 4

Identifies the address of the filtering server. if_name is the name of the ASA interface connected to the filtering server (the default is inside). For the vendor { secure-computing | n2h2 } option, use secure-computing as the vendor string; however, n2h2 is acceptable for backward compatibility. When the configuration entries are generated, secure-computing is saved as the vendor string. The host local_ip option is the IP address of the URL filtering server. The port number option is the Secure Computing SmartFilter server port number of the filtering server; the ASA also listens for UDP replies on this port.

Note The default port is 4005, which is used by the Secure Computing SmartFilter server to communicate to the ASA via TCP or UDP. For information about changing the default port, see the Filtering by N2H2 Administrator's Guide.

The timeout seconds option is the number of seconds that the ASA should keep trying to connect to the filtering server. The connections number option is the number of tries to make a connection between the host and server.

The example identifies a Websense filtering server with the IP address 10.0.1.1 on a perimeter interface of the ASA.Version 4, which is enabled in this example, is recommended by Websense because it supports caching.

 

For Secure Computing SmartFilter (formerly N2H2):

hostname(config)# url-server (if_name) vendor { secure-computing | n2h2 } host
local_ip [port number] [timeout seconds] [protocol {TCP [ connections number]} | UDP]
 
ciscoasa(config)# url-server (perimeter) vendor n2h2 host 10.0.1.1
ciscoasa(config)# url-server (perimeter) vendor n2h2 host 10.0.1.2

The example identifies redundant Secure Computing SmartFilter servers that are both on a perimeter interface of the ASA.

Configuring Additional URL Filtering Settings

After you have accessed a website, the filtering server can allow the ASA to cache the server address for a certain period of time, as long as each website hosted at the address is in a category that is permitted at all times. When you access the server again, or if another user accesses the server, the ASA does not need to consult the filtering server again to obtain the server address.

Note Requests for cached IP addresses are not passed to the filtering server and are not logged. As a result, this activity does not appear in any reports.

This section describes how to configure additional URL filtering settings and includes the following topics:

Buffering the Content Server Response

When you issue a request to connect to a content server, the ASA sends the request to the content server and to the filtering server at the same time. If the filtering server does not respond before the content server, the server response is dropped. This behavior delays the web server response for the web client, because the web client must reissue the request.

By enabling the HTTP response buffer, replies from web content servers are buffered, and the responses are forwarded to the requesting client if the filtering server allows the connection. This behavior prevents the delay that might otherwise occur.

To configure buffering for responses to HTTP or FTP requests, enter the following command:

 

 
Command
Purpose

Step 1
url-block block block-buffer-limit
 

ciscoasa# url-block 3000

Enables buffering of responses for HTTP or FTP requests that are pending a response from the filtering server.

Replaces block-buffer with the maximum number of HTTP responses that can be buffered while awaiting responses from the URL server.

Note Buffering of URLs longer than 3072 bytes is not supported.

Step 2
url-block mempool-size memory-pool-size
 

ciscoasa# url-block mempool-size 5000

Configures the maximum memory available for buffering pending URLs (and for buffering long URLs).

Replaces memory-pool-size with a value from 2 to 10240 for a maximum memory allocation of 2 KB to 10 MB.

Caching Server Addresses

After you access a website, the filtering server can allow the ASA to cache the server address for a certain period of time, as long as each website hosted at the address is in a category that is permitted at all times. When you access the server again, or if another user accesses the server, the ASA does not need to consult the filtering server again.

Note Requests for cached IP addresses are not passed to the filtering server and are not logged. As a result, this activity does not appear in any reports. You can accumulate Websense run logs before using the url-cache command.

To improve throughput, enter the following command:

 

Command
Purpose
url-cache dst | src_dst size
 

ciscoasa## url-cache src_dst 100

Replaces size with a value for the cache size within the range from 1 to 128 (KB).

Uses the dst keyword to cache entries based on the URL destination address. Choose this option if all users share the same URL filtering policy on the Websense server.

Uses the src_dst keyword to cache entries based on both the source address initiating the URL request as well as the URL destination address. Choose this option if users do not share the same URL filtering policy on the Websense server.

Filtering HTTP URLs

This section describes how to configure HTTP filtering with an external filtering server and includes the following topics:

Enabling HTTP Filtering

You must identify and enable the URL filtering server before enabling HTTP filtering. When the filtering server approves an HTTP connection request, the ASA allows the reply from the web server to reach the originating client. If the filtering server denies the request, the ASA redirects you to a block page, indicating that access was denied.

To enable HTTP filtering, enter the following command:

 

Command
Purpose
filter url [ http | port [- port ] local_ip local_mask foreign_ip foreign_mask ] [ allow ] [ proxy-block ]
 

ciscoasa# filter url http 80 allow proxy-block

Replaces port [ -port ] with one or more port numbers if a different port than the default port for HTTP (80) is used.

Replaces local_ip and local_mask with the IP address and subnet mask of a user or subnetwork making requests.

Replaces foreign_ip and foreign_mask with the IP address and subnet mask of a server or subnetwork responding to requests.

The allow option causes the ASA to forward HTTP traffic without filtering when the primary filtering server is unavailable. Use the proxy-block command to drop all requests to proxy servers.

Enabling Filtering of Long HTTP URLs

By default, the ASA considers an HTTP URL to be a long URL if it is greater than 1159 characters. You can increase the maximum length allowed.

To configure the maximum size of a single URL, enter the following command:

 

Command
Purpose

url-block url-size long-url-size

 

ciscoasa# url-block url-size 3

Replaces the long-url-size with the maximum size in KB for each long URL being buffered. For Websense servers, this is a value from 2 to 4 for a maximum URL size from 2 KB to 4 KB; for Secure Computing SmartFilter servers, this is a value between 2 and 3 for a maximum URL size from 2 KB to 3 KB. The default value is 2.

Truncating Long HTTP URLs

By default, if a URL exceeds the maximum permitted size, then it is dropped. To avoid this occurrence, truncate a long URL by entering the following command:

 

Command
Purpose
filter url [ longurl-truncate | longurl-deny | cgi-truncate ]
 

ciscoasa# filter url longurl-truncate

The longurl-truncate option causes the ASA to send only the hostname or IP address portion of the URL for evaluation to the filtering server when the URL is longer than the maximum length permitted. Use the longurl-deny option to deny outbound URL traffic if the URL is longer than the maximum permitted.

Use the cgi-truncate option to truncate CGI URLs to include only the CGI script location and the script name without any parameters. Many long HTTP requests are CGI requests. If the parameters list is very long, waiting and sending the complete CGI request, including the parameter list, can use up memory resources and affect ASA performance.

Exempting Traffic from Filtering

To exempt traffic from filtering, enter following command:

 

Command
Purpose
filter url except source_ip source_mask dest_ip dest_mask
 
ciscoasa(config)# filter url http 0 0 0 0
ciscoasa(config)# filter url except 10.0.2.54 255.255.255.255 0 0

Exempts specific traffic from filtering.

The example shows how to cause all HTTP requests to be forwarded to the filtering server, except for those from 10.0.2.54.

Filtering HTTPS URLs

You must identify and enable the URL filtering server before enabling HTTPS filtering.

Note Websense and Secure Computing Smartfilter currently support HTTPS; older versions of the Secure Computing SmartFilter (formerly N2H2) do not support HTTPS filtering.

Because HTTPS content is encrypted, the ASA sends the URL lookup without directory and filename information. When the filtering server approves an HTTPS connection request, the ASA allows the completion of SSL connection negotiation and allows the reply from the web server to reach the originating client. If the filtering server denies the request, the ASA prevents the completion of SSL connection negotiation. The browser displays an error message, such as “The Page or the content cannot be displayed.”

Note The ASA does not provide an authentication prompt for HTTPS, so you must authenticate with the ASA using HTTP or FTP before accessing HTTPS servers.

To enable HTTPS filtering, enter the following command:

 

Command
Purpose
filter https port[-port] localIP local_mask foreign_IP foreign_mask [allow]
 

ciscoasa# filter https 443 0 0 0 0 0 0 0 0 allow

Enables HTTPS filtering.

Replaces port[-port] with a range of port numbers if a different port than the default port for HTTPS (443) is used.

Replaces local_ip and local_mask with the IP address and subnet mask of a user or subnetwork making requests.

Replaces foreign_ip and foreign_mask with the IP address and subnet mask of a server or subnetwork responding to requests.

The allow option causes the ASA to forward HTTPS traffic without filtering when the primary filtering server is unavailable.

Filtering FTP Requests

You must identify and enable the URL filtering server before enabling FTP filtering.

Note Websense and Secure Computing Smartfilter currently support FTP; older versions of Secure Computing SmartFilter (formerly known as N2H2) did not support FTP filtering.

When the filtering server approves an FTP connection request, the ASA allows the successful FTP return code to reach the originating client. For example, a successful return code is “250: CWD command successful.” If the filtering server denies the request, the FTP return code is changed to show that the connection was denied. For example, the ASA changes code 250 to “550 Requested file is prohibited by URL filtering policy.”

To enable FTP filtering, enter the following command:

 

Command
Purpose
filter ftp port[-port] localIP local_mask foreign_IP foreign_mask [allow] [ interact-block ]
 

ciscoasa# filter ftp 21 0 0 0 0 0 0 0 0 allow

Enables FTP filtering.

Replaces port[-port] with a range of port numbers if a different port than the default port for FTP (21) is used.

Replaces local_ip and local_mask with the IP address and subnet mask of a user or subnetwork making requests.

Replaces foreign_ip and foreign_mask with the IP address and subnet mask of a server or subnetwork responding to requests.

The allow option causes the ASA to forward HTTPS traffic without filtering when the primary filtering server is unavailable.

Use the interact-block option to prevent interactive FTP sessions that do not provide the entire directory path. An interactive FTP client allows you to change directories without typing the entire path. For example, you might enter cd./files instead of cd /public/files.

Monitoring Filtering Statistics

To monitor filtering statistics, enter one of the following commands:

 

Command
Purpose

show url-server

Shows information about the URL filtering server.

show url-server statistics

Shows URL filtering statistics.

show url-block

Shows the number of packets held in the url-block buffer and the number (if any) dropped because of exceeding the buffer limit or retransmission.

show url-block block statistics

Shows the URL block statistics.

show url-cache stats

Shows the URL cache statistics.

show perfmon

Shows URL filtering performance statistics, along with other performance statistics.

show filter

Shows the filtering configuration.

Examples

The following is sample output from the show url-server command:

ciscoasa# show url-server
url-server (outside) vendor n2h2 host 128.107.254.202 port 4005 timeout 5 protocol TCP
 

The following is sample output from the show url-server statistics command:

ciscoasa# show url-server statistics
 
Global Statistics:
--------------------
URLs total/allowed/denied 13/3/10
URLs allowed by cache/server 0/3
URLs denied by cache/server 0/10
HTTPSs total/allowed/denied 138/137/1
HTTPSs allowed by cache/server 0/137
HTTPSs denied by cache/server 0/1
FTPs total/allowed/denied 0/0/0
FTPs allowed by cache/server 0/0
FTPs denied by cache/server 0/0
Requests dropped 0
Server timeouts/retries 0/0
Processed rate average 60s/300s 0/0 requests/second
Denied rate average 60s/300s 0/0 requests/second
Dropped rate average 60s/300s 0/0 requests/second
 
Server Statistics:
--------------------
10.125.76.20 UP
Vendor websense
Port 15868
Requests total/allowed/denied 151/140/11
Server timeouts/retries 0/0
Responses received 151
Response time average 60s/300s 0/0
 
URL Packets Sent and Received Stats:
------------------------------------
Message Sent Received
STATUS_REQUEST 1609 1601
LOOKUP_REQUEST 1526 1526
LOG_REQUEST 0 NA
 
Errors:
-------
RFC noncompliant GET method 0
URL buffer update failure 0
 

The following is sample output from the show url-block command:

ciscoasa# show url-block
url-block url-mempool 128
url-block url-size 4
url-block block 128
 

The following is sample output from the show url-block block statistics command:

ciscoasa# show url-block block statistics
 
URL Pending Packet Buffer Stats with max block 128
-----------------------------------------------------
Cumulative number of packets held: 896
Maximum number of packets held (per URL): 3
Current number of packets held (global): 38
Packets dropped due to
exceeding url-block buffer limit: 7546
HTTP server retransmission: 10
Number of packets released back to client: 0
 

The following is sample output from the show url-cache stats command:

ciscoasa# show url-cache stats
URL Filter Cache Stats
----------------------
Size : 128KB
Entries : 1724
In Use : 456
Lookups : 45
Hits : 8
 
This shows how the cache is used.
 

The following is sample output from the show perfmon command:

ciscoasa# show perfmon
PERFMON STATS: Current Average
Xlates 0/s 0/s
Connections 0/s 2/s
TCP Conns 0/s 2/s
UDP Conns 0/s 0/s
URL Access 0/s 2/s
URL Server Req 0/s 3/s
TCP Fixup 0/s 0/s
TCPIntercept 0/s 0/s
HTTP Fixup 0/s 3/s
FTP Fixup 0/s 0/s
AAA Authen 0/s 0/s
AAA Author 0/s 0/s
AAA Account 0/s 0/s
 

The following is sample output from the show filter command:

ciscoasa# show filter
filter url http 0.0.0.0 0.0.0.0 0.0.0.0 0.0.0.0
 

Feature History for URL Filtering

Table 29-5 lists the release history for URL filtering. ASDM is backwards-compatible with multiple platform releases, so the specific ASDM release in which support was added is not listed.

 

Table 29-5 Feature History for URL Filtering
Feature Name
Platform Releases
Feature Information

URL filtering

7.0(1)

Filters URLs based on an established set of filtering criteria.

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