Prerequisites for Flexible Packet Matching
Although access to an XML editor is not required, XML will ease the creation of protocol header description files (PHDFs).
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Flexible Packet Matching (FPM) is an access control list (ACL) pattern matching tool, providing more thorough and customized packet filters. FPM enables users to match on arbitrary bits of a packet at an arbitrary depth in the packet header and payload. FPM removes constraints to specific fields that had limited packet inspection.
FPM enables users to create their own stateless packet classification criteria and to define policies with multiple actions (such as drop, log, or send Internet Control Message Protocol [ICMP] unreachable1) to immediately block new viruses, worms, and attacks.
Although access to an XML editor is not required, XML will ease the creation of protocol header description files (PHDFs).
FPM can search for patterns up to 32 bytes in length within the first 256 bytes of the packet.
A maximum of 32 classes are supported in a policy-map.
For IP option packets, FPM inspects only the fields in the Layer 2 header and the first 20 bytes of the IP header.
For noninitial IP fragments, FPM inspects only the fields in the Layer 2 header and the first 20 bytes of the IP header.
FPM cannot be used to mitigate an attack that requires stateful classification.
Because FPM is stateless, it cannot keep track of port numbers being used by protocols that dynamically negotiate ports. Thus, when using FPM, port numbers must be explicitly specified.
FPM cannot perform IP fragmentation or TCP flow reassembly.
FPM inspects only IPv4 unicast packets.
FPM cannot classify packets with IP options.
FPM does not support multicast packet inspection.
FPM is not supported on tunnel and MPLS interfaces.
Noninitial fragments will not be matched by the FPM engine.
Offset can be only a constant in a match start construct.
FPM cannot match across packets.
Mapping of FPM policies to control-plane is not supported.
FPM allows customers to create their own filtering policies that can immediately detect and block new viruses and attacks.
A filtering policy is defined via the following tasks:
Load a PHDF (for protocol header field matching)
Define a class map and define the protocol stack chain (traffic class)
Define a service policy (traffic policy)
Apply the service policy to an interface
Protocol headers are defined in separate files called PHDFs; the field names that are defined within the PHDFs are used for defining the packet filters. A PHDF is a file that allows the user to leverage the flexibility of XML to describe almost any protocol header. The important components of the PHDF are the version, the XML file schema location, and the protocol field definitions. The protocol field definitions name the appropriate field in the protocol header, allow for a comment describing the field, provide the location of the protocol header field in the header (the offset is relative to the start of the protocol header), and provide the length of the field. Users can choose to specify the measurement in bytes or in bits.
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The total length of the header must be specified at the end of each PHDF. |
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When redundant sup PHDF files are used by FPM policy, the files should also be on standby sup’s corresponding disk. If the files are not available FPM policy will not work after the switch over. |
Users can write their own custom PHDFs via XML for existing or proprietary protocols. However, the following standard PHDFs can also be loaded onto the router via the load protocol command: ip.phdf, ether.phdf, tcp.phdf, and udp.phdf.
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Because PHDFs are defined via XML, they are not shown in a running configuration. However, you can use the show protocol phdf command to verify the loaded PHDF. |
Standard PHDFs are available on Cisco.com at the following URL: http://www.cisco.com/cgi-bin/tablebuild.pl/fpm
A filter description is a definition of a traffic class that can contain the header fields defined in a PHDF (using the match field command). If a PHDF is not loaded, the traffic class can be defined through the datagram header start (Layer 2) or the network header start (Layer 3) (using the match start command). If a PHDF has been loaded onto the router, the class specification begins with a list of the protocol headers in the packet.
A filter definition also includes the policy map; that is, after a class map has been defined, a policy map is needed to bind the match to an action. A policy map is an ordered set of classes and associated actions, such as drop, log, or send ICMP unreachable.
For information on how to configure a class map and a policy map for FPM, see the How to Configure a Flexible Packet Matching Traffic Class and Traffic Policy section.
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If the PHDF protocol fields are referenced in the access-control classmap, the stack classmap is required in order to make FPM work properly |
Command or Action | Purpose | |||||
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Step 1 |
enable Example:
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Enables privileged EXEC mode.
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Step 2 |
configure terminal Example:
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Enters global configuration mode. |
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Step 3 |
load protocol location:filename Example:
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(Optional) Loads a PHDF onto a router.
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Step 4 |
class-map [type {stack | access-control}] class-map-name [match-all | match-any] Example:
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Creates a class map to be used for matching packets to a specified class and enters class-map configuration mode.
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Step 5 |
description character-string Example:
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(Optional) Adds a description to the class map. |
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Step 6 |
match field protocol protocol-field {eq [mask] | neq | [mask] | gt | lt | range range | regex string} value [next next-protocol] Example:
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(Optional) Configures the match criteria for a class map on the basis of the fields defined in the PHDFs.
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Step 7 |
match start {l2-start | l3-start} offset number size number {eq | neq | gt | lt | range range | regex string} {value [value2] | [string]} Example:
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(Optional) Configures the match criteria for a class map on the basis of the datagram header (Layer 2) or the network header (Layer 3). |
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Step 8 |
match class class-name [packet-range low high | byte-range low high] session Example:
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(Optional) Configures match criteria for a class map that identifies a session (flow) containing packets of interest, which is then applied to all packets transmitted during the session. The packet-range and byte-range keywords create a filter mechanism that increases the performance and matching accuracy of regex-based FPM class maps by classifying traffic that resides in the narrow packet number or packet byte ranges of each packet flow. When the session keyword is used with the class-name argument, the classification results are preserved for the subsequent packets of the same packet session. When the session keyword is used with the packet-range or byte-range keywords, the classification results are preserved for the specified packets or bytes of the same packet session. |
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Step 9 |
exit Example:
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Exits class-map configuration mode. |
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Step 10 |
exit Example:
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Exits global configuration mode. |
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Step 11 |
show class-map [type {stack | access-control} | class-map-name] Example:
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(Optional) Displays configured FPM class maps. |
To track all FPM events, issue the debug fpm event command.
The following sample output is from the debug fpm event command:
*Jun 21 09:22:21.607: policy-classification-inline(): matches class: class-default *Jun 21 09:22:21.607: packet-access-control(): policy-map: fpm-policy, dir: input, match. retval: 0x0, ip-flags: 0x80000000
After you have defined at least one class map for your network, you must create a traffic policy and apply that policy to an interface as shown in the following task “Creating a Traffic Policy for Flexible Packet Matching.”
Command or Action | Purpose | |||
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Step 1 |
enable Example:
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Enables privileged EXEC mode.
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Step 2 |
configure terminal Example:
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Enters global configuration mode. |
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Step 3 |
policy-map type access-control policy-map-name Example:
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Creates or modifies a policy map that can be attached to one or more interfaces to specify a service policy, and enters policy-map configuration mode. |
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Step 4 |
description character-string Example:
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(Optional) Adds a description to the policy map. |
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Step 5 |
class class-name insert-before class-name Example:
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Specifies the name of a predefined traffic class, which was configured with the class-map command. The class command also classifies traffic to the traffic policy and enters policy-map class configuration mode.
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Step 6 |
drop [all] Example:
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(Optional) Configures a traffic class to discard packets belonging to a specific class. The all keyword is used to discard the entire stream of packets belonging to the traffic class. If this command is issued, note the following restrictions:
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Step 7 |
log [all] Example:
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(Optional) Generates log messages for the traffic class. The all keyword is used to log the entire stream of discarded packets belonging to the traffic class. This keyword is only available for a class map that is created with the class-map type access-control command. |
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Step 8 |
service-policy policy-map-name Example:
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Creates hierarchical service policies. |
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Step 9 |
exit Example:
Example:
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Exits policy-map class configuration mode and policy-map configuration mode. |
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Step 10 |
interface type number Example:
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Configures an interface type and enters interface configuration mode. |
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Step 11 |
service-policy type access-control {input | output} policy-map-name Example:
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Specifies the type and the name of the traffic policy to be attached to the input or output direction of an interface. |
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Step 12 |
exit Example:
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Exits interface configuration mode. |
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Step 13 |
exit Example:
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Exits global configuration mode. |
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Step 14 |
show policy-map [type access-control | interface type number | input | output] Example:
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(Optional) Verifies the FPM configuration.
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The following example shows how to configure FPM on the ASR platform.
load protocol bootflash:ip.phdf
load protocol bootflash:tcp.phdf
class-map type stack match-all ip_tcp
match field IP protocol eq 6 next TCP
class-map type access-control match-all test_class
match field TCP dest-port gt 10
match start l3-start offset 40 size 32 regex "ABCD"
policy-map type access-control child
class test_class
drop
policy-map type access-control parent
class ip_tcp
service-policy child
interface GigabitEthernet0/3/0
ip address 10.1.1.1 255.0.0.0
service-policy type access-control input parent
In the following sample output, all TCP packets are seen under the class-map “ip_tcp” and all packets matching the specific pattern are seen under the class-map “test_class.” TCP packets without the specific pattern are seen under the child policy “class-default,” while all non-TCP packets are seen under the parent policy “class-default.” (The counter is 0 in this example.)
Router# show policy-map type access-control interface GigabitEthernet0/3/0
GigabitEthernet0/3/0
Service-policy access-control input: parent
Class-map: ip_tcp (match-all)
2024995578 packets, 170099628552 bytes
5 minute offered rate 775915000 bps
Match: field IP version eq 4
Match: field IP ihl eq 5
Match: field IP protocol eq 6 next TCP
Service-policy access-control : child
Class-map: test_class (match-all)
1598134279 packets, 134243279436 bytes
5 minute offered rate 771012000 bps, drop rate 771012000 bps
Match: field TCP dest-port gt 10
Match: start l3-start offset 40 size 32 regex "ABCD"
drop
Class-map: class-default (match-any)
426861294 packets, 35856348696 bytes
5 minute offered rate 4846000 bps, drop rate 0 bps
Match: any
Class-map: class-default (match-any)
0 packets, 0 bytes
5 minute offered rate 0 bps, drop rate 0 bps
Match: any
Router#
Related Topic |
Document Title |
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Cisco IOS commands |
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Security commands |
Cisco IOS Security Command Reference |
Configuring FPM using traffic classification definition files. |
"Flexible Packet Matching XML Configuration" module in the Cisco IOS Security Configuration Guide: Securing the Data Plane |
Complete suite of quality of service (QoS) commands |
Cisco IOS Quality of Service Solutions Command Reference |
MIBs |
MIBs Link |
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None |
To locate and download MIBs for selected platforms, Cisco IOS XE software releases, and feature sets, use Cisco MIB Locator found at the following URL: |
Description |
Link |
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The Cisco Support and Documentation website provides online resources to download documentation, software, and tools. Use these resources to install and configure the software and to troubleshoot and resolve technical issues with Cisco products and technologies. Access to most tools on the Cisco Support and Documentation website requires a Cisco.com user ID and password. |
The following table provides release information about the feature or features described in this module. This table lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature.
Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.
Feature Name |
Releases |
Feature Information |
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Flexible Packet Matching |
Cisco IOS XE Release 2.2 |
FPM is a packet classification feature that allows users to define one or more classes of network traffic by pairing a set of standard matching operators with user-defined protocol header fields. The following commands were introduced or modified: class (policy-map) class-map debug fpm event, description (class-map) load protocol match field match start , policy-map , service-policy , show class-map , show policy-map interface , show protocol phdf . |