ASA/PIX 7.2: Block Certain Websites (URLs) Using Regular Expressions with MPF Configuration Examples

Introduction

This document describes how to configure the Cisco Security Appliances ASA/PIX 7.2 with Regular Expressions with Modular Policy Framework (MPF) in order to block certain websites (URLs).

Note: This configuration does not block all application downloads. For reliable file blocks, a dedicated appliance, such as Websense, etc., or module, such as the CSC module for the ASA, must be used.

HTTPS filtering is not supported on ASA. ASA cannot do deep packet inspection or inspection based on regular expression for HTTPS traffic because, in HTTPS, the content of packet is encrypted (ssl).

Prerequisites

Requirements

This document assumes that Cisco Security Appliance is configured and works properly.

Components Used

• Cisco 5500 Series Adaptive Security Appliance (ASA) that runs Software Version 7.2(2)

• Cisco Adaptive Security Device Manager (ASDM) Version 5.2(2) for ASA 7.2(2)

The information in this document was created from the devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If your network is live, make sure that you understand the potential impact of any command.

Related Products

This configuration can also be used with the Cisco 500 Series PIX that runs Software Version 7.2(2).

Conventions

Refer to the Cisco Technical Tips Conventions for more information on document conventions.

Background Information

Modular Policy Framework Overview

MPF provides a consistent and flexible way to configure security appliance features. For example, you can use MPF to create a timeout configuration that is specific to a particular TCP application, as opposed to one that applies to all TCP applications.

MPF supports these features:

• TCP normalization, TCP and UDP connection limits and timeouts, and TCP sequence number randomization

• CSC

• Application inspection

• IPS

• QoS input policing

• QoS output policing

• QoS priority queue

The configuration of the MPF consists of four tasks:

1. Identify the Layer 3 and 4 traffic to which you want to apply actions. Refer to Identifying Traffic Using a Layer 3/4 Class Map for more information.

2. (Application inspection only) Define special actions for application inspection traffic. Refer to Configuring Special Actions for Application Inspections for more information.

3. Apply actions to the Layer 3 and 4 traffic. Refer to Defining Actions Using a Layer 3/4 Policy Map for more information.

4. Activate the actions on an interface. Refer to Applying a Layer 3/4 Policy to an Interface Using a Service Policy for more information.

Regular Expression

A regular expression matches text strings either literally as an exact string, or with metacharacters, so you can match multiple variants of a text string. You can use a regular expression to match the content of certain application traffic; for example, you can match a URL string inside an HTTP packet.

Note: Use Ctrl+V to escape all the special characters in the CLI, such as a question mark (?) or tab. For example, type d[Ctrl+V]g to enter d?g in the configuration.

In order to create a regular expression, use the regex command, which can be used for various features that require text matching. For example, you can configure special actions for application inspection with Modular Policy Framework with an inspection policy map (see the policy map type inspect command). In the inspection policy map, you can identify the traffic you want to act upon if you create an inspection class map that contains one or more match commands, or you can use match commands directly in the inspection policy map. Some match commands let you identify text in a packet with a regular expression; for example, you can match URL strings inside HTTP packets. You can group regular expressions in a regular expression class map (see the class-map type regex command).

Table 1 lists the metacharacters that have special meanings.

Configure

In this section, you are presented with the information to configure the features described in this document.

Note: Use the Command Lookup Tool (registered customers only) to obtain more information on the commands used in this section.

Network Diagram

This document uses this network setup:

Configurations

This document uses these configurations:

• ASA CLI Configuration

• ASA Configuration 7.2(x) with ASDM 5.2

ASA CLI Configuration

ciscoasa#show running-config
: Saved
:
ASA Version 7.2(2)
!
hostname ciscoasa
domain-name default.domain.invalid
enable password 8Ry2YjIyt7RRXU24 encrypted
names
!
interface Ethernet0/0
 nameif inside
 security-level 100
 ip address 10.1.1.1 255.255.255.0
!
interface Ethernet0/1
 nameif outside
 security-level 0
 ip address 192.168.1.5 255.255.255.0
!
interface Ethernet0/2
 nameif DMZ
 security-level 90
 ip address 10.77.241.142 255.255.255.192
!
interface Ethernet0/3
 shutdown
 no nameif
 no security-level
 no ip address
!
interface Management0/0
 shutdown
 no nameif
 no security-level
 no ip address
!
passwd 2KFQnbNIdI.2KYOU encrypted
regex urllist1 ".*\.([Ee][Xx][Ee]|[Cc][Oo][Mm]|[Bb][Aa][Tt]) HTTP/1.[01]"


!--- Extensions such as .exe, .com, .bat to be captured and !--- provided the http version being used by web browser must be either 1.0 or 1.1


regex urllist2 ".*\.([Pp][Ii][Ff]|[Vv][Bb][Ss]|[Ww][Ss][Hh]) HTTP/1.[01]"

!--- Extensions such as .pif, .vbs, .wsh to be captured !--- and provided the http version being used by web browser must be either !--- 1.0 or 1.1


regex urllist3 ".*\.([Dd][Oo][Cc]|[Xx][Ll][Ss]|[Pp][Pp][Tt]) HTTP/1.[01]"


!--- Extensions such as .doc(word), .xls(ms-excel), .ppt to be captured and provided !--- the http version being used by web browser must be either 1.0 or 1.1


regex urllist4 ".*\.([Zz][Ii][Pp]|[Tt][Aa][Rr]|[Tt][Gg][Zz]) HTTP/1.[01]"


!--- Extensions such as .zip, .tar, .tgz to be captured and provided !--- the http version being used by web browser must be either 1.0 or 1.1


regex domainlist1 "\.yahoo\.com"
regex domainlist2 "\.myspace\.com"
regex domainlist3 "\.youtube\.com"


!--- Captures the URLs with domain name like yahoo.com, !--- youtube.com and myspace.com


regex contenttype "Content-Type"
regex applicationheader "application/.*"


!--- Captures the application header and type of !--- content in order for analysis


boot system disk0:/asa802-k8.bin
ftp mode passive
dns server-group DefaultDNS
 domain-name default.domain.invalid

access-list inside_mpc extended permit tcp any any eq www

access-list inside_mpc extended permit tcp any any eq 8080


!--- Filters the http and port 8080 !--- traffic in order to block the specific traffic with regular !--- expressions


pager lines 24
mtu inside 1500
mtu outside 1500
mtu DMZ 1500
no failover
icmp unreachable rate-limit 1 burst-size 1
asdm image disk0:/asdm-602.bin
no asdm history enable
arp timeout 14400
route DMZ 0.0.0.0 0.0.0.0 10.77.241.129 1
timeout xlate 3:00:00
timeout conn 1:00:00 half-closed 0:10:00 udp 0:02:00 icmp 0:00:02
timeout sunrpc 0:10:00 h323 0:05:00 h225 1:00:00 mgcp 0:05:00 mgcp-pat 0:05:00
timeout sip 0:30:00 sip_media 0:02:00 sip-invite 0:03:00 sip-disconnect 0:02:00
timeout uauth 0:05:00 absolute
dynamic-access-policy-record DfltAccessPolicy
http server enable
http 0.0.0.0 0.0.0.0 DMZ
no snmp-server location
no snmp-server contact
snmp-server enable traps snmp authentication linkup linkdown coldstart
no crypto isakmp nat-traversal
telnet timeout 5
ssh timeout 5
console timeout 0
threat-detection basic-threat
threat-detection statistics access-list
!
class-map type regex match-any DomainBlockList
 match regex domainlist1
 match regex domainlist2
 match regex domainlist3


!--- Class map created in order to match the domain names !--- to be blocked


class-map type inspect http match-all BlockDomainsClass
 match request header host regex class DomainBlockList


!--- Inspect the identified traffic by class !--- "DomainBlockList"


class-map type regex match-any URLBlockList
 match regex urllist1
 match regex urllist2
 match regex urllist3
 match regex urllist4


!--- Class map created in order to match the URLs !--- to be blocked


class-map inspection_default
 match default-inspection-traffic

class-map type inspect http match-all AppHeaderClass
 match response header regex contenttype regex applicationheader


!--- Inspect the captured traffic by regular !--- expressions "content-type" and "applicationheader"


class-map httptraffic
 match access-list inside_mpc


!--- Class map created in order to match the !--- filtered traffic by ACL


class-map type inspect http match-all BlockURLsClass
 match request uri regex class URLBlockList
!

!--- Inspect the identified traffic by class !--- "URLBlockList"


!
policy-map type inspect dns preset_dns_map
 parameters
  message-length maximum 512
policy-map type inspect http http_inspection_policy
 parameters
  protocol-violation action drop-connection
 class AppHeaderClass
  drop-connection log
 match request method connect
  drop-connection log
 class BlockDomainsClass
  reset log
 class BlockURLsClass
  reset log


!--- Define the actions such as drop, reset or log !--- in the inspection policy map


policy-map global_policy
 class inspection_default
  inspect dns preset_dns_map
  inspect ftp
  inspect h323 h225
  inspect h323 ras
  inspect netbios
  inspect rsh
  inspect rtsp
  inspect skinny
  inspect esmtp
  inspect sqlnet
  inspect sunrpc
  inspect tftp
  inspect sip
  inspect xdmcp

policy-map inside-policy
 class httptraffic
  inspect http http_inspection_policy


!--- Map the inspection policy map to the class !--- "httptraffic" under the policy map created for the !--- inside network traffic


!
service-policy global_policy global
service-policy inside-policy interface inside


!--- Apply the policy to the interface inside where the websites will be blocked


prompt hostname context
Cryptochecksum:e629251a7c37af205c289cf78629fc11
: end
ciscoasa#

ASA Configuration 7.2(x) with ASDM 5.2

Complete these steps in order to configure the regular expressions and apply them to MPF to block the specific websites:

1. Create Regular Expressions

   Choose Configuration > Global Objects > Regular Expressions and click Add under the Regular Expression tab in order to create regular expressions.

   1. Create a regular expression domainlist1 in order to capture the domain name yahoo.com. Click OK.

      

   2. Create a regular expression domainlist2 in order to capture the domain name myspace.com. Click OK.

      

   3. Create a regular expression domainlist3 in order to capture the domain name youtube.com. Click OK.

      

   4. Create a regular expression urllist1 in order to capture the file extensions such as exe, com, and bat provided that the http version used by the web browser must be either 1.0 or 1.1. Click OK.

      

   5. Create a regular expression urllist2 in order to capture the file extensions, such as pif, vbs, and wsh provided that the HTTP version that is used by the web browser is either 1.0 or 1.1. Click OK.

      

   6. Create a regular expression urllist3 in order to capture the file extensions, such as doc, xls, and ppt provided that the HTTP version that is used by the web browser is either 1.0 or 1.1. Click OK.

      

   7. Create a regular expression urllist4 in order to capture the file extensions, such as zip, tar, and tgz provided that the HTTP version that is used by the web browser is either 1.0 or 1.1. Click OK.

      

   8. Create a regular expression contenttype in order to capture the content type. Click OK.

      

   9. Create a regular expression applicationheader in order to capture the various application header. Click OK.

      

   Equivalent CLI Configuration

   ASA CLI Configuration

   ciscoasa#configure terminal ciscoasa(config)#regex urllist1 ".*\.([Ee][Xx][Ee]|[Cc][Oo][Mm]|[Bb][Aa][Tt])$ ciscoasa(config)#regex urllist2 ".*\.([Pp][Ii][Ff]|[Vv][Bb][Ss]|[Ww][Ss][Hh])$ ciscoasa(config)#regex urllist3 ".*\.([Dd][Oo][Cc]|[Xx][Ll][Ss]|[Pp][Pp][Tt])$ ciscoasa(config)#regex urllist4 ".*\.([Zz][Ii][Pp]|[Tt][Aa][Rr]|[Tt][Gg][Zz])$ ciscoasa(config)#regex domainlist1 "\.yahoo\.com" ciscoasa(config)#regex domainlist2 "\.myspace\.com" ciscoasa(config)#regex domainlist3 "\.youtube\.com" ciscoasa(config)#regex contenttype "Content-Type" ciscoasa(config)#regex applicationheader "application/.*"

2. Create Regular Expression Classes

   Choose Configuration > Global Objects > Regular Expressions, and click Add under the Regular Expression Classes tab in order to create the various classes.

   1. Create a regular expression class DomainBlockList in order to match any of the regular expressions: domainlist1, domainlist2, and domainlist3. Click OK.

      

   2. Create a regular expression class URLBlockList in order to match any of the regular expressions: urllist1, urllist2, urllist3, and urllist4. Click OK.

      

      Equivalent CLI Configuration

      ASA CLI Configuration

      ciscoasa#configure terminal ciscoasa(config)#class-map type inspect http match-all BlockDomainsClass ciscoasa(config-cmap)#match request header host regex class DomainBlockList ciscoasa(config-cmap)#exit ciscoasa(config)#class-map type regex match-any URLBlockList ciscoasa(config-cmap)#match regex urllist1 ciscoasa(config-cmap)#match regex urllist2 ciscoasa(config-cmap)#match regex urllist3 ciscoasa(config-cmap)#match regex urllist4 ciscoasa(config-cmap)#exit

3. Inspect the identified traffic with Class maps

   Choose Configuration > Global Objects > Class Maps > HTTP > Add in order to create a class map to inspect the HTTP traffic identified by various regular expressions.

   1. Create a class map AppHeaderClass in order to match the response header with regular expression captures.

      

      Click OK.

   2. Create a class map BlockDomainsClass in order to match the request header with regular expression captures.

      

      Click OK.

   3. Create a class map BlockURLsClass in order to match the request URI with regular expression captures.

      

      Click OK.

      Equivalent CLI Configuration

      ASA CLI Configuration

      ciscoasa#configure terminal ciscoasa(config)#class-map type inspect http match-all AppHeaderClass ciscoasa(config-cmap)#match response header regex contenttype regex applicationheader ciscoasa(config-cmap)#exit ciscoasa(config)#class-map type inspect http match-all BlockDomainsClass ciscoasa(config-cmap)#match request header host regex class DomainBlockList ciscoasa(config-cmap)#exit ciscoasa(config)#class-map type inspect http match-all BlockURLsClass ciscoasa(config-cmap)#match request uri regex class URLBlockList ciscoasa(config-cmap)#exit

4. Set the actions for the matched traffic in the inspection policy

   Choose Configuration > Global Objects > Inspect Maps > HTTP in order to create a http_inspection_policy to set the action for the matched traffic. Click Add and Apply.

   

   1. Choose Configuration > Global Objects > Inspect Maps > HTTP > http_inspection_policy and click Advanced View > Inspections > Add in order to set the actions for the various Classes created so far.

      

      Click OK.

   2. Set the action as Drop Connection; Enable the logging for the Criterion as Request Method and Value as connect.

      

      Click OK.

   3. Set the action as Drop Connection, and Enable the logging for the class AppHeaderClass.

      

      Click OK.

   4. Set the action as Reset, and Enable the logging for the class BlockDomainsClass.

      

      Click OK.

   5. Set the action as Reset, and Enable the logging for the class BlockURLsClass.

      

      Click OK.

      Click Apply.

      Equivalent CLI Configuration

      ASA CLI Configuration

      ciscoasa#configure terminal ciscoasa(config)#policy-map type inspect http http_inspection_policy ciscoasa(config-pmap)#parameters ciscoasa(config-pmap-p)#match request method connect ciscoasa(config-pmap-c)#drop-connection log ciscoasa(config-pmap-c)#class AppHeaderClass ciscoasa(config-pmap-c)#drop-connection log ciscoasa(config-pmap-c)#class BlockDomainsClass ciscoasa(config-pmap-c)#reset log ciscoasa(config-pmap-c)#class BlockURLsClass ciscoasa(config-pmap-c)#reset log ciscoasa(config-pmap-c)#exit ciscoasa(config-pmap)#exit

5. Apply the inspection http policy to the interface

   Choose Configuration > Security Policy > Service Policy Rules > Add > Add Service Policy Rule under the Service Policy Rules tab.

   

   1. HTTP Traffic

      1. Choose the Interface radio button with the inside interface from the drop-down menu and the Policy Name as inside-policy. Click Next.

         

      2. Create a class map httptraffic, and check the Source and Destination IP Address (uses ACL). Click Next.

         

      3. Choose the Source and Destination as any with the TCP port as HTTP. Click Next.

         

      4. Check the HTTP radio button, and click Configure.

         

      5. Check the radio button Select a HTTP inspect map for the control over inspection. Click OK.

         

      6. Click Finish.

         

   2. Port 8080 Traffic

      1. Again, click Add > Add Service Policy Rule.

         

      2. Click Next.

         

      3. Choose the Add rule to existing traffic class radio button, and choose httptraffic from the drop-down menu. Click Next.

         

      4. Choose the Source and Destination as any with the TCP port as 8080. Click Next.

         

      5. Click Finish.

         

         

      Click Apply.

      Equivalent CLI Configuration

      ASA CLI Configuration

      ciscoasa#configure terminal ciscoasa(config)#access-list inside_mpc extended permit tcp any any eq www ciscoasa(config)#access-list inside_mpc extended permit tcp any any eq 8080 ciscoasa(config)#class-map httptraffic ciscoasa(config-cmap)#match access-list inside_mpc ciscoasa(config-cmap)#exit ciscoasa(config)#policy-map inside-policy ciscoasa(config-pmap)#class httptraffic ciscoasa(config-pmap-c)#inspect http http_inspection_policy ciscoasa(config-pmap-c)#exit ciscoasa(config-pmap)#exit ciscoasa(config)#service-policy inside-policy interface inside

Verify

Use this section to confirm that your configuration works properly.

The Output Interpreter Tool (registered customers only) (OIT) supports certain show commands. Use the OIT to view an analysis of show command output.

• show running-config regex—Shows the regular expressions that have been configured

  ciscoasa#show running-config regex
  regex urllist1 ".*\.([Ee][Xx][Ee]|[Cc][Oo][Mm]|[Bb][Aa][Tt]) HTTP/1.[01]"
  regex urllist2 ".*\.([Pp][Ii][Ff]|[Vv][Bb][Ss]|[Ww][Ss][Hh]) HTTP/1.[01]"
  regex urllist3 ".*\.([Dd][Oo][Cc]|[Xx][Ll][Ss]|[Pp][Pp][Tt]) HTTP/1.[01]"
  regex urllist4 ".*\.([Zz][Ii][Pp]|[Tt][Aa][Rr]|[Tt][Gg][Zz]) HTTP/1.[01]"
  regex domainlist1 "\.yahoo\.com"
  regex domainlist2 "\.myspace\.com"
  regex domainlist3 "\.youtube\.com"
  regex contenttype "Content-Type"
  regex applicationheader "application/.*"
  ciscoasa#

• show running-config class-map—Shows the class maps that have been configured

  ciscoasa#show running-config class-map
  !
  class-map type regex match-any DomainBlockList
   match regex domainlist1
   match regex domainlist2
   match regex domainlist3
  class-map type inspect http match-all BlockDomainsClass
   match request header host regex class DomainBlockList
  class-map type regex match-any URLBlockList
   match regex urllist1
   match regex urllist2
   match regex urllist3
   match regex urllist4
  class-map inspection_default
   match default-inspection-traffic
  class-map type inspect http match-all AppHeaderClass
   match response header regex contenttype regex applicationheader
  class-map httptraffic
   match access-list inside_mpc
  class-map type inspect http match-all BlockURLsClass
   match request uri regex class URLBlockList
  !
  ciscoasa#

• show running-config policy-map type inspect http—Shows the policy maps that inspects the http traffic that have been configured

  ciscoasa#show running-config policy-map type inspect http
  !
  policy-map type inspect http http_inspection_policy
   parameters
    protocol-violation action drop-connection
   class AppHeaderClass
    drop-connection log
   match request method connect
    drop-connection log
   class BlockDomainsClass
    reset log
   class BlockURLsClass
    reset log
  !
  ciscoasa#

• show running-config policy-map—Displays all the policy-map configurations as well as the default policy-map configuration

  ciscoasa#show running-config policy-map
  !
  policy-map type inspect dns preset_dns_map
   parameters
    message-length maximum 512
  policy-map type inspect http http_inspection_policy
   parameters
    protocol-violation action drop-connection
   class AppHeaderClass
    drop-connection log
   match request method connect
    drop-connection log
   class BlockDomainsClass
    reset log
   class BlockURLsClass
    reset log
  policy-map global_policy
   class inspection_default
    inspect dns preset_dns_map
    inspect ftp
    inspect h323 h225
    inspect h323 ras
    inspect netbios
    inspect rsh
    inspect rtsp
    inspect skinny
    inspect esmtp
    inspect sqlnet
    inspect sunrpc
    inspect tftp
    inspect sip
    inspect xdmcp
  policy-map inside-policy
   class httptraffic
    inspect http http_inspection_policy
  !
  ciscoasa#

• show running-config service-policy—Displays all currently running service policy configurations

  ciscoasa#show running-config service-policy
  service-policy global_policy global
  service-policy inside-policy interface inside

• show running-config access-list—Displays the access-list configuration that runs on the security appliance

  ciscoasa#show running-config access-list
  access-list inside_mpc extended permit tcp any any eq www
  
  access-list inside_mpc extended permit tcp any any eq 8080
  ciscoasa#

Troubleshoot

This section provides information you can use to troubleshoot your configuration.

Note: Refer to Important Information on Debug Commands before you use debug commands.

• debug http—Shows the debug messages for HTTP traffic.

Related Information

• Cisco Adaptive Security Appliance Support Page
• Cisco Adaptive Security Device Manager (ASDM) Support Page
• Cisco 500 Series PIX Support Page
• Technical Support & Documentation - Cisco Systems