Creating an IP Access List to Filter IP Options TCP Flags Noncontiguous Ports or TTL Values
Last Updated: May 14, 2011
This module describes how to use an IP access list to filter IP packets that contain certain IP Options, TCP flags, noncontiguous ports, or time-to-live (TTL) values.
Your software release may not support all the features documented in this module. For the latest feature information and caveats, see the release notes for your platform and software release. To find information about the features documented in this module, and to see a list of the releases in which each feature is supported, see the Feature Information Table at the end of this document.
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.
Prerequisites for Creating an IP Access List to Filter IP Options TCP Flags Noncontiguous Ports or TTL Values
Before you perform any of the tasks in this module, you should be familiar with the information in the following modules:
âIP Access List Overviewâ
âCreating an IP Access List and Applying It to an Interfaceâ
Information About Creating an IP Access List to Filter IP Options TCP Flags Noncontiguous Ports or TTL Values
IP uses four key mechanisms in providing its service: Type of Service, Time to Live, Options, and Header Checksum.
The Options, commonly referred to as IP Options, provide for control functions that are required in some situations but unnecessary for the most common communications. IP Options include provisions for time stamps, security, and special routing.
IP Options may or may not appear in datagrams. They must be implemented by all IP modules (host and gateways). What is optional is their transmission in any particular datagram, not their implementation. In some environments the security option may be required in all datagrams.
The option field is variable in length. There may be zero or more options. IP Options can have one of two formats:
Format 1: A single octet of option-type.
Format 2: An option-type octet, an option-length octet, and the actual option-data octets.
The option-length octet counts the option-type octet, the option-length octet, and the option-data octets.
The option-type octet is viewed as having three fields: a 1-bit copied flag, a 2-bit option class, and a 5-bit option number. These fields form an 8-bit value for the option type field. IP Options are commonly referred to by their 8-bit value.
For a complete list and description of IP Options, refer to RFC 791,
Internet Protocol at the following URL: http://www.faqs.org/rfcs/rfc791.html
Benefits of Filtering IP Options
Filtering of packets that contain IP Options from the network relieves downstream routers and hosts of the load from options packets.
This feature also minimizes load to the Route Processor (RP) for packets with IP Options that require RP processing on distributed systems. Previously, the packets were always routed to or processed by the RP CPU. Filtering the packets prevents them from impacting the RP.
Benefits of Filtering on TCP Flags
The ACL TCP Flags Filtering feature provides a flexible mechanism for filtering on TCP flags. Before Cisco IOS Release 12.3(4)T, an incoming packet was matched as long as any TCP flag in the packet matched a flag specified in the access control entry (ACE). This behavior allows for a security loophole, because packets with all flags set could get past the access control list (ACL). The ACL TCP Flags Filtering feature allows you to select any combination of flags on which to filter. The ability to match on a flag set and on a flag not set gives you a greater degree of control for filtering on TCP flags, thus enhancing security.
Because TCP packets can be sent as false synchronization packets that can be accepted by a listening port, it is recommended that administrators of firewall devices set up some filtering rules to drop false TCP packets.
The ACEs that make up an access list can be configured to detect and drop unauthorized TCP packets by allowing only the packets that have a very specific group of TCP flags set or not set. The ACL TCP Flags Filtering feature gives users a greater degree of packet-filtering control in the following ways:
Users can select any desired combination of TCP flags on which to filter TCP packets.
Users can configure ACEs in order to allow matching on a flag that is set, as well as on a flag that is not set.
TCP Flags
The table below lists the TCP flags, which are further described in RFC 793,
Transmission Control Protocol .
Table 1 TCP Flags
TCP Flag
Purpose
ACK
Acknowledge flag--Indicates that the acknowledgment field of a segment specifies the next sequence number the sender of this segment is expecting to receive.
FIN
Finish flag--Used to clear connections.
PSH
Push flag-- Indicates the data in the call should be immediately pushed through to the receiving user.
RST
Reset flag--Indicates that the receiver should delete the connection without further interaction.
SYN
Synchronize flag--Used to establish connections.
URG
Urgent flag--Indicates that the urgent field is meaningful and must be added to the segment sequence number.
Benefits of Using the ACL--Named ACL Support for Noncontiguous Ports on an Access Control Entry Feature
This feature greatly reduces the number of ACEs required in an access control list to handle multiple entries for the same source address, destination address, and protocol. If you maintain large numbers of ACEs, we recommend that you use this feature to consolidate existing groups of access list entries wherever it is possible and also when you create new access list entries. When you configure access list entries with noncontiguous ports, you will have fewer access list entries to maintain.
How Filtering on TTL Works
IP extended named and numbered access lists may filter on the TTL value of packets arriving at or leaving an interface. Packets with any possible TTL values 0 through 255 may be permitted or denied (filtered). Like filtering on other fields, such as source or destination address, the
ipaccess-group command specifies
in or
out, which makes the access list ingress or egress and applies it to incoming or outgoing packets, respectively. The TTL value is checked in conjunction with the specified protocol, application, and any other settings in the access list entry, and all conditions must be met.
Special Handling for Packets with TTL or 0 or 1 Arriving on Ingress Interface
The software switching paths--distributed Cisco Express Forwarding (dCEF), CEF, fast switching, and process switching--will usually permit or discard the packets based on the access list statements. However, when the TTL value of packets arriving on an ingress interface have a TTL of 0 or 1, special handling is required. The packets with a TTL of 0 or 1 get sent to the process level before the ingress access list is checked in CEF, dCEF, or fast switching paths. The ingress access list is applied to packets with TTL values 2 through 255 and a permit or deny decision is made.
Packets with a TTL value of 0 or 1 are sent to the process level because they will never be forwarded out of the device; the process level must check whether each packet is destined for the router or not and whether an Internet Control Message Protocol (ICMP) TTL Expire message needs to be sent back or not. This means that even if an ACL with TTL value 0 or 1 filtering is configured on the ingress interface with the intention to drop packets with a TTL of 0 or 1, the dropping of the packets will not happen in the faster paths. It will instead happen in the process level when the process applies the ACL. This is also true for hardware switching platforms. Packets with TTL 0 or 1 are sent to the process level of the route processor (RP) or Multilayer Switch Feature Card (MSFC).
On egress interfaces, access list filtering on TTL work just like other access list features. The check will happen in the fastest switching path enabled in the device. This is because the faster switching paths handle all the TTL values (0-255) equally on the egress interface.
Control Plane Policing for Filtering TTL Values 0 and 1
The special behavior for packets with a TTL of 0 or 1 results in higher CPU usage for the device. If you are filtering on TTL value 0 or 1, you should use control plane policing (CPP) to protect the CPU from being overwhelmed. In order to leverage CPP, you must configure an access list especially for filtering TTL values 0 and 1 and apply the access list through CPP. This access list will be a separate access list from any interface access lists. Because CPP works for the entire system, not just on individual interfaces, you would need to configure only one such special access list for the entire device. This task is described in the section "Enabling Control Plane Policing to Filter on TTL Values 0 and 1".
Benefits of Filtering on TTL
Filtering on TTL provides a way to control which packets are allowed to reach the router or prevented from reaching the router. By looking at your network layout, you can choose whether to accept or deny packets from a certain router based on how many hops away it is. For example, in a small network, you can deny packets from a location more than three hops away. Filtering on TTL allows you to validate if the traffic originated from a neighboring device, as follows. You can accept only packets that reach you in one hop, for example, by accepting only packets with a TTL of one less than the initial TTL value of a particular protocol.
Many control plane protocols communicate only with their neighbors, but receive packets from everyone. By applying to receiving routers an access list that filters on TTL, you can block unwanted packets.
The Cisco IOS software sends all packets with a TTL of 0 or 1 to the process level to be processed. The device must then send an ICMP TTL expire message to the source. By filtering packets that have a TTL of 0 through 2, you can reduce the load on the process level.
How to Create an IP Access List to Filter IP Options TCP Flags Noncontiguous Ports or TTL Values
The task in this section configures an access list to filter packets that contain IP Options and verifies that the access list has been configured correctly.
Note
The ACL Support for Filtering IP Options feature can be used only with named, extended ACLs.
Resource Reservation Protocol (RSVP) Multiprotocol Label Switching Traffic Engineering (MPLS TE), Internet Group Management Protocol Version 2 (IGMPV2), and other protocols that use IP Options packets may not function in drop or ignore mode if this feature is configured.
On most Cisco routers, a packet with IP Options is not switched in hardware, but requires control plane software processing (primarily because there is a need to process the options and rewrite the IP header), so all IP packets with IP Options will be filtered and switched in software.
Apply the access list to an interface or reference it from a command that accepts an access list.
Note
To effectively eliminate all packets that contain IP Options, we recommend that you configure the global ipoptionsdrop command.
Filtering Packets That Contain TCP Flags
The task in this section configures an access list to filter packets that contain TCP flags and verifies that the access list has been configured correctly.
Caution
If a router having ACEs with the new syntax format is reloaded with an older version of Cisco IOS software that does not support the ACL TCP Flags Filtering feature, the ACEs will not be applied, leading to possible security loopholes.
Note
TCP flag filtering can be used only with named, extended ACLs.
The ACL TCP Flags Filtering feature is supported only for Cisco IOS ACLs.
Before Cisco IOS Release 12.3(4)T, the following command-line interface (CLI) format could be used to configure a TCP flag-checking mechanism:
permittcpanyanyrst
The following format that represents the same ACE can be used with Cisco IOS Release 12.3(4)T and later releases:
permittcpanyanymatch-any+rst
Both the CLI formats are accepted; however, if the new keywords match-all or match-any are chosen, they must be followed by the new flags that are prefixed with â+â or â-â. It is advisable to use only the old format or the new format in a single ACL. You cannot mix and match the old and new CLI formats.
Router(config-ext-nacl)# deny tcp any any match-all -ack -fin
(Optional) Specifies a deny statement in named IP access list mode.
This access list happens to use a permitstatement first, but a deny statement could appear first, depending on the order of statements you need.
Use the TCP command syntax of the denycommand.
Any packet that does not have the ACK flag set, and also does not have the FIN flag set, will not be allowed to pass the named access list kmd1 in Step 3.
See the deny(IP) command for additional command syntax to permit upper-layer protocols (ICMP, IGMP, TCP, and UDP).
Step 6
Repeat Step 4 or Step 5 as necessary, adding statements by sequence number where you planned. Use the nosequence-numbercommand to delete an entry.
Allows you to revise the access list.
Step 7
end
Example:
Router(config-ext-nacl)# end
(Optional) Exits the configuration mode and returns to privileged EXEC mode.
Step 8
showipaccess-listsaccess-list-name
Example:
Router# show ip access-lists kmd1
(Optional) Displays the contents of the IP access list.
Review the output to confirm that the access list includes the new entry.
Apply the access list to an interface or reference it from a command that accepts an access list.
Configuring an Access Control Entry with Noncontiguous Ports
Perform this task to create access list entries that use noncontiguous TCP or UDP port numbers. Although this task uses TCP ports, you could use the UDP syntax of the permit and deny commands to filter noncontiguous UDP ports.
Although this task uses a permit command first, use the permit and deny commands in the order that achieves your filtering goals.
Note
The ACL--Named ACL Support for Noncontiguous Ports on an Access Control Entry feature can be used only with named, extended ACLs.
Router(config-ext-nacl)# permit tcp any eq telnet ftp any eq 450 679
Specifies a permit statement in named IP access list configuration mode.
Operators include lt (less than), gt (greater than), eq (equal), neq (not equal), and range (inclusive range).
If the operator is positioned after the source and source-wildcard arguments, it must match the source port. If the operator is positioned after the destination and destination-wildcard arguments, it must match the destination port.
The range operator requires two port numbers. You can configure up to 10 ports after the eq and neqoperators. All other operators require one port number.
To filter UDP ports, use the UDP syntax of this command.
Router(config-ext-nacl)# deny tcp any neq 45 565 632
(Optional) Specifies a deny statement in named access list configuration mode.
Operators include lt (less than), gt (greater than), eq (equal), neq (not equal), and range (inclusive range).
If the operator is positioned after the source and source-wildcard arguments, it must match the source port. If the operator is positioned after the destination anddestination-wildcard arguments, it must match the destination port.
The range operator requires two port numbers. You can configure up to 10 ports after the eq and neqoperators. All other operators require one port number.
To filter UDP ports, use the UDP syntax of this command.
Step 6
Repeat Step 4 or Step 5 as necessary, adding statements by sequence number where you planned. Use the nosequence-numbercommand to delete an entry.
Allows you to revise the access list.
Step 7
end
Example:
Router(config-ext-nacl)# end
(Optional) Exits named access list configuration mode and returns to privileged EXEC mode.
Step 8
showipaccess-listsaccess-list-name
Example:
Router# show ip access-lists kmd1
(Optional) Displays the contents of the access list.
Review the output to verify that the access list displays the new entries that you created.
Consolidating Access List Entries with Noncontiguous Ports into One Access List Entry
Perform this task to consolidate a group of access list entries with noncontiguous ports into one access list entry.
Although this task uses TCP ports, you could use the UDP syntax of the permit and deny commands to filter noncontiguous UDP ports.
Although this task uses a permit command first, use the permit and deny commands in the order that achieves your filtering goals.
7.
Repeat Steps 5 and 6 as necessary, adding permit or deny statements to consolidate access list entries where possible. Use the nosequence-number command to delete an entry.
8.end
9.showipaccess-listsaccess-list-name
DETAILED STEPS
Command or Action
Purpose
Step 1
enable
Example:
Router> enable
Enables privileged EXEC mode.
Enter your password if prompted.
Step 2
showipaccess-listsaccess-list-name
Example:
Router# show ip access-lists mylist1
(Optional) Displays the contents of the IP access list.
Review the output to see if you can consolidate any access list entries.
Step 3
configureterminal
Example:
Router# configure terminal
Enters global configuration mode.
Step 4
ipaccess-listextendedaccess-list-name
Example:
Router(config)# ip access-list extended mylist1
Specifies the IP access list by name and enters named access list configuration mode.
Step 5
no [sequence-number]permitprotocolsourcesource-wildcarddestinationdestination-wildcard[optionoption-name] [precedenceprecedence][tostos] [log] [time-rangetime-range-name] [fragments]
Example:
Router(config-ext-nacl)# no 10
Removes the redundant access list entry that can be consolidated.
Repeat this step to remove entries to be consolidated because only the port numbers differ.
After this step is repeated to remove the access list entries 20, 30, and 40, for example, those entries are removed because they will be consolidated into one permit statement.
If a sequence-number is specified, the rest of the command syntax is optional.
Router(config-ext-nacl)# permit tcp any neq 45 565 632 any eq 23 45 34 43
Specifies a permit statement in named access list configuration mode.
In this instance, a group of access list entries with noncontiguous ports was consolidated into one permit statement.
You can configure up to 10 ports after the eq and neq operators.
Step 7
Repeat Steps 5 and 6 as necessary, adding permit or deny statements to consolidate access list entries where possible. Use the nosequence-number command to delete an entry.
Allows you to revise the access list.
Step 8
end
Example:
Router(config-std-nacl)# end
(Optional) Exits named access list configuration mode and returns to privileged EXEC mode.
Step 9
showipaccess-listsaccess-list-name
Example:
Router# show ip access-lists mylist1
(Optional) Displays the contents of the access list.
Review the output to verify that the redundant access list entries have been replaced with your new consolidated entries.
Apply the access list to an interface or reference it from a command that accepts an access list.
Filtering Packets Based on TTL Value
Because access lists are very flexible, it is not possible to define only one combination of permit and deny commands to filter packets based on the TTL value. This task illustrates just one example that achieves TTL filtering. Configure the appropriate permit and deny statements that will accomplish your filtering plan.
Note
When the access list specifies the operation EQ or NEQ, routers running Cisco IOS Release 12.2S can have that access list specify up to ten TTL values. However, for Release 12.0S, only one TTL value can be specified.
Router(config-ext-nacl)# permit ip host 172.16.1.1 any ttl lt 2
Sets conditions to allow a packet to pass a named IP access list.
Every access list must have at least one permit statement.
This example permits packets from source 172.16.1.1 to any destination with a TTL value less than 2.
Step 5
Continue to add permit or deny statements to achieve the filtering you want.
--
Step 6
exit
Example:
Router(config-ext-nacl)# exit
Exits any configuration mode to the next highest mode in the CLI mode hierarchy.
Step 7
interfacetypenumber
Example:
Router(config)# interface ethernet 0
Configures an interface type and enters interface configuration mode.
Step 8
ipaccess-groupaccess-list-name {in | out}
Example:
Router(config-if)# ip access-group ttlfilter in
Applies the access list to an interface.
Enabling Control Plane Policing to Filter on TTL Values 0 and 1
Perform this task if you want to filter IP packets based on a TTL value of 0 or 1 and you want to protect the CPU from being overwhelmed. This task configures an access list for classification on TTL 0 and 1, configures Modular QoS CLI (MQC), and applies the policy map to the control plane. Any packets that pass the access list are dropped. This special access list is separate from any interface access lists.
Because access lists are very flexible, it is not possible to define only one combination of permit and deny commands to filter packets based on the TTL value. This task illustrates just one example that achieves TTL filtering. Configure the appropriate permit and deny statements that will accomplish your filtering plan.
SUMMARY STEPS
1.enable
2.configureterminal
3.ipaccess-listextendedaccess-list-name
4.
[sequence-number] permitprotocolsourcesource-wildcarddestinationdestination-wildcardttloperator value
5.
Continue to add permit or deny statements to achieve the filtering you want.
Router(config-cmap)# match access-group name ttlfilter
Configures the match criteria for a class map on the basis of the specified access control list.
Step 9
exit
Example:
Router(config-cmap)# exit
Exits any configuration mode to the next highest mode in the CLI mode hierarchy.
Step 10
policy-mappolicy-map-name
Example:
Router(config)# policy-map acl-filter
Creates or modifies a policy map that can be attached to one or more interfaces to specify a service policy.
Step 11
class{class-name | class-default}
Example:
Router(config-pmap)# class acl-filter-class
Specifies the name of the class whose policy you want to create or change or to specify the default class (commonly known as the class-default class) before you configure its policy.
Step 12
drop
Example:
Router(config-pmap-c)# drop
Configures a traffic class to discard packets belonging to a specific class.
Step 13
exit
Example:
Router(config-pmap-c)# exit
Exits any configuration mode to the next highest mode in the CLI mode hierarchy.
Step 14
exit
Example:
Router(config-pmap)# exit
Exits any configuration mode to the next highest mode in the CLI mode hierarchy.
Step 15
control-plane
Example:
Router(config)# control-plane
Associates or modifies attributes or parameters that are associated with the control plane of the device.
The following example shows an extended access list named mylist2 that contains access list entries (ACEs) that are configured to permit TCP packets only if they contain the IP Options that are specified in the ACEs:
ip access-list extended mylist2
10 permit ip any any option eool
20 permit ip any any option record-route
30 permit ip any any option zsu
40 permit ip any any option mtup
The showaccess-list command has been entered to show how many packets were matched and therefore permitted:
Router# show ip access-list mylist2
Extended IP access list test
10 permit ip any any option eool (1 match)
20 permit ip any any option record-route (1 match)
30 permit ip any any option zsu (1 match)
40 permit ip any any option mtup (1 match)
Example Filtering Packets That Contain TCP Flags
The following access list allows TCP packets only if the TCP flags ACK and SYN are set and the FIN flag is not set:
ip access-list extended aaa
permit tcp any any match-all +ack +syn -fin
end
The showaccess-list command has been entered to display the ACL:
Router# show access-list aaa
Extended IP access list aaa
10 permit tcp any any match-all +ack +syn -fin
Example Creating an Access List Entry with Noncontiguous Ports
The following access list entry can be created because up to ten ports can be entered after the eq and neq operators:
ip access-list extended aaa
permit tcp any eq telnet ftp any eq 23 45 34
end
Enter the showaccess-lists command to display the newly created access list entry.
Router# show access-lists aaa
Extended IP access list aaa
10 permit tcp any eq telnet ftp any eq 23 45 34
Example Consolidating Some Existing Access List Entries into One Access List Entry with Noncontiguous Ports
The showaccess-lists command is used to display a group of access list entries for the access list named abc:
Router# show access-lists abc
Extended IP access list abc
10 permit tcp any eq telnet any eq 450
20 permit tcp any eq telnet any eq 679
30 permit tcp any eq ftp any eq 450
40 permit tcp any eq ftp any eq 679
Because the entries are all for the same permit statement and simply show different ports, they can be consolidated into one new access list entry. The following example shows the removal of the redundant access list entries and the creation of a new access list entry that consolidates the previously displayed group of access list entries:
ip access-list extended abc
no 10
no 20
no 30
no 40
permit tcp any eq telnet ftp any eq 450 679
end
When the showaccess-lists command is reentered, the consolidated access list entry is displayed:
Router# show access-lists abc
Extended IP access list abc
10 permit tcp any eq telnet ftp any eq 450 679
Example Filtering on TTL Value
The following access list filters IP packets containing type of service (ToS) level 3 with TTL values 10 and 20. It also filters IP packets with a TTL greater than 154 and applies that rule to noninitial fragments. It permits IP packets with a precedence level of flash and a TTL not equal to 1, and it sends log messages about such packets to the console. All other packets are denied.
ip access-list extended incomingfilter
deny ip any any tos 3 ttl eq 10 20
deny ip any any ttl gt 154 fragments
permit ip any any precedence flash ttl neq 1 log
!
interface ethernet 0
ip access-group incomingfilter in
Example Control Plane Policing to Filter on TTL Values 0 and 1
The following example configures a traffic class called acl-filter-class for use in a policy map called acl-filter. An access list permits IP packets from any source having a TTL of 0 or 1. Any packets matching the access list are dropped. The policy map is attached to the control plane.
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Feature Information for Creating an IP Access List to Filter
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.
Table 2 Feature Information for Creating an IP Access List to Filter IP Options, TCP Flags, Noncontiguous Ports, or TTL Values
Feature Name
Releases
Feature Configuration Information
ACL Support for Filtering IP Options
12.3(4)T 12.2(25)S
This feature allows you to filter packets having IP Options, in order to prevent routers from becoming saturated with spurious packets.
ACL TCP Flags Filtering
12.3(4)T 12.2(25)S
This feature provides a flexible mechanism for filtering on TCP flags. Before Cisco IOS Release 12.3(4)T, an incoming packet was matched as long as any TCP flag in the packet matched a flag specified in the access control entry (ACE). This behavior allows for a security loophole, because packets with all flags set could get past the access control list (ACL). The ACL TCP Flags Filtering feature allows you to select any combination of flags on which to filter. The ability to match on a flag set and on a flag not set gives you a greater degree of control for filtering on TCP flags, thus enhancing security.
ACL--Named ACL Support for Noncontiguous Ports on an Access Control Entry
12.3(7)T 12.2(25)S
This feature allows you to specify noncontiguous ports in a single access control entry, which greatly reduces the number of entries required in an access control list when several entries have the same source address, destination address, and protocol, but differ only in the ports.
ACL Support for Filtering on TTL Value
12.4(2)T
Customers may use extended IP access lists (named or numbered) to filter packets based on their time-to-live (TTL) value, from 0 to 255. This filtering enhances a customerâs control over which packets reach a router.
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