Refining an IP Access List
There are several ways to refine an access list while or after you create it. You can change the order of the entries in an access list or add entries to an access list. You can restrict access list entries to a certain time of day or week, or achieve finer granularity when filtering packets by filtering noninitial fragments of packets.
Finding Feature Information
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.
Information About Refining an IP Access List
- Access List Sequence Numbers
- Benefits of Access List Sequence Numbers
- Sequence Numbering Behavior
- Benefits of Time Ranges
- Benefits Filtering Noninitial Fragments of Packets
- Access List Processing of Fragments
Access List Sequence Numbers
The ability to apply sequence numbers to IP access list entries simplifies access list changes. Prior to the IP Access List Entry Sequence Numbering feature, there was no way to specify the position of an entry within an access list. If you wanted to insert an entry in the middle of an existing list, all of the entries after the desired position had to be removed, then the new entry was added, and then all the removed entries had to be reentered. This method was cumbersome and error prone.
Sequence numbers allow users to add access list entries and resequence them. When you add a new entry, you specify the sequence number so that it is in a desired position in the access list. If necessary, entries currently in the access list can be resequenced to create room to insert the new entry.
Benefits of Access List Sequence Numbers
An access list sequence number is a number at the beginning of a permit or deny command in an access list. The sequence number determines the order that the entry appears in the access list. The ability to apply sequence numbers to IP access list entries simplifies access list changes.
Prior to having sequence numbers, users could only add access list entries to the end of an access list; therefore, needing to add statements anywhere except the end of the list required reconfiguring the entire access list. There was no way to specify the position of an entry within an access list. If a user wanted to insert an entry (statement) in the middle of an existing list, all of the entries after the desired position had to be removed, then the new entry was added, and then all the removed entries had to be reentered. This method was cumbersome and error prone.
This feature allows users to add sequence numbers to access list entries and resequence them. When a user adds a new entry, the user chooses the sequence number so that it is in a desired position in the access list. If necessary, entries currently in the access list can be resequenced to create room to insert the new entry. Sequence numbers make revising an access list much easier.
Sequence Numbering Behavior
- For backward compatibility with previous releases, if entries with no sequence numbers are applied, the first entry is assigned a sequence number of 10, and successive entries are incremented by 10. The maximum sequence number is 2147483647. If the generated sequence number exceeds this maximum number, the following message is displayed:
Exceeded maximum sequence number.
- If the user enters an entry without a sequence number, it is assigned a sequence number that is 10 greater than the last sequence number in that access list and is placed at the end of the list.
- If the user enters an entry that matches an already existing entry (except for the sequence number), then no changes are made.
- If the user enters a sequence number that is already present, the following error message is generated:
Duplicate sequence number.
- If a new access list is entered from global configuration mode, then sequence numbers for that access list are generated automatically.
- Sequence numbers are not nvgened. That is, the sequence numbers themselves are not saved. In the event that the system is reloaded, the configured sequence numbers revert to the default sequence starting number and increment. The function is provided for backward compatibility with software releases that do not support sequence numbering.
- This feature works with named and numbered, standard and extended IP access lists.
Benefits of Time Ranges
Benefits and possible uses of time ranges include the following:
- The network administrator has more control over permitting or denying a user access to resources. These resources could be an application (identified by an IP address/mask pair and a port number), policy routing, or an on-demand link (identified as interesting traffic to the dialer).
- Network administrators can set time-based security policy, including the following:
- Perimeter security using access lists
- Data confidentiality with IP Security Protocol (IPsec)
- When provider access rates vary by time of day, it is possible to automatically reroute traffic cost effectively.
- Network administrators can control logging messages. Access list entries can log traffic at certain times of the day, but not constantly. Therefore, administrators can simply deny access without needing to analyze many logs generated during peak hours.
Benefits Filtering Noninitial Fragments of Packets
Filter noninitial fragments of packets with an extended access list if you want to block more of the traffic you intended to block, not just the initial fragment of such packets. You should first understand the following concepts.
If the fragmentskeyword is used in additional IP access list entries that deny fragments, the fragment control feature provides the following benefits:
Additional Security
You are able to block more of the traffic you intended to block, not just the initial fragment of such packets. The unwanted fragments no longer linger at the receiver until the reassembly timeout is reached because they are blocked before being sent to the receiver. Blocking a greater portion of unwanted traffic improves security and reduces the risk from potential hackers.
Reduced Cost
By blocking unwanted noninitial fragments of packets, you are not paying for traffic you intended to block.
Reduced Storage
By blocking unwanted noninitial fragments of packets from ever reaching the receiver, that destination does not have to store the fragments until the reassembly timeout period is reached.
Expected Behavior Is Achieved
The noninitial fragments will be handled in the same way as the initial fragment, which is what you would expect. There are fewer unexpected policy routing results and fewer fragments of packets being routed when they should not be.
Access List Processing of Fragments
The behavior of access list entries regarding the use or lack of use of the fragments keyword can be summarized as follows:
Be aware that you should not add the fragments keyword to every access list entry because the first fragment of the IP packet is considered a nonfragment and is treated independently of the subsequent fragments. An initial fragment will not match an access list permit or deny entry that contains the fragments keyword. The packet is compared to the next access list entry, and so on, until it is either permitted or denied by an access list entry that does not contain the fragments keyword. Therefore, you may need two access list entries for every deny entry. The first deny entry of the pair will not include the fragments keyword and applies to the initial fragment. The second deny entry of the pair will include the fragments keyword and applies to the subsequent fragments. In the cases in which there are multiple deny entries for the same host but with different Layer 4 ports, a single deny access list entry with the fragments keyword for that host is all that needs to be added. Thus all the fragments of a packet are handled in the same manner by the access list.
Packet fragments of IP datagrams are considered individual packets, and each counts individually as a packet in access list accounting and access list violation counts.
How to Refine an IP Access List
The tasks in this module provide you with various ways to refine an access list if you did not already do so while you were creating it. You can change the order of the entries in an access list, add entries to an access list, restrict access list entries to a certain time of day or week, or achieve finer granularity when filtering packets by filtering on noninitial fragments of packets.
- Revising an Access List Using Sequence Numbers
- Restricting an Access List Entry to a Time of Day or Week
Revising an Access List Using Sequence Numbers
Perform this task if you want to add entries to an existing access list, change the order of entries, or simply number the entries in an access list to accommodate future changes.
 Note |
Remember that if you want to delete an entry from an access list, you can simply use the no deny or no permit form of the command, or the no sequence-number command if the statement already has a sequence number. |
Note |
DETAILED STEPS
| Command or Action | Purpose | |
|---|---|---|
|
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Example: Router> enable |
Enables privileged EXEC mode. |
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Example: Router# configure terminal |
Enters global configuration mode. |
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Example: Router(config)# ip access-list resequence kmd1 100 15 |
Resequences the specified IP access list using the starting sequence number and the increment of sequence numbers. |
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Example: Router(config)# ip access-list standard xyz123 |
Specifies the IP access list by name and enters named access list configuration mode. |
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Example: Router(config-std-nacl)# 105 permit 10.5.5.5 0.0.0.255 |
Specifies a permit statement in named IP access list mode.
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Example: Router(config-std-nacl)# 110 deny 10.6.6.7 0.0.0.255 |
(Optional) Specifies a deny statement in named IP access list mode.
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|
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Allows you to revise the access list. |
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Example: Router(config-std-nacl)# end |
(Optional) Exits the configuration mode and returns to privileged EXEC mode. |
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Example: Router# show ip access-lists xyz123 |
(Optional) Displays the contents of the IP access list. |
Examples
The following is sample output from the show ip access-lists command when the xyz123 access list is specified.
Router# show ip access-lists xyz123
Standard IP access list xyz123
100 permit 10.4.4.0, wildcard bits 0.0.0.255
105 permit 10.5.5.5, wildcard bits 0.0.0.255
115 permit 10.0.0.0, wildcard bits 0.0.0.255
130 permit 10.5.5.0, wildcard bits 0.0.0.255
145 permit 10.0.0.0, wildcard bits 0.0.0.255
Restricting an Access List Entry to a Time of Day or Week
By default, access list statements are always in effect once they are applied. However, you can define the times of the day or week that permit or deny statements are in effect by defining a time range, and then referencing the time range by name in an individual access list statement. IP and Internetwork Packet Exchange (IPX) named or numbered extended access lists can use time ranges.
DETAILED STEPS
| Command or Action | Purpose | |
|---|---|---|
|
|
Example: Router> enable |
Enables privileged EXEC mode.
|
|
|
Example: Router# configure terminal |
Enters global configuration mode. |
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Example: Router(config)# ip access-list extended rstrct4 |
Defines an extended IP access list using a name and enters extended named access list configuration mode. |
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Example: Router(config-ext-nacl)# deny ip any 172.20.1.1 |
(Optional) Denies any packet that matches all of the conditions specified in the statement.
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Example: Router(config-ext-nacl)# deny ip any 172.20.1.1 fragments |
(Optional) Denies any packet that matches all of the conditions specified in the statement
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Example: Router(config-ext-nacl)# permit tcp any any |
Permits any packet that matches all of the conditions specified in the statement.
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|
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Remember that all sources not specifically permitted are denied by an implicit deny statement at the end of the access list. |
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Example: Router(config-ext-nacl)# end |
Ends configuration mode and returns the system to privileged EXEC mode. |
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Example: Router# show ip access-list |
(Optional) Displays the contents of all current IP access lists. |
What to Do Next
Apply the access list to an interface or reference it from a command that accepts an access list.
Configuration Examples for Refining an IP Access List
- Example Resequencing Entries in an Access List
- Example Adding an Entry with a Sequence Number
- Example Adding an Entry with No Sequence Number
- Example Time Ranges Applied to IP Access List Entries
- Example Filtering IP Packet Fragments
Example Resequencing Entries in an Access List
The following example shows an access list before and after resequencing. The starting value is 1, and increment value is 2. The subsequent entries are ordered based on the increment values that users provide, and the range is from 1 to 2147483647.
When an entry with no sequence number is entered, by default it has a sequence number of 10 more than the last entry in the access list.
Router# show access-list carls Extended IP access list carls 10 permit ip host 10.3.3.3 host 172.16.5.34 20 permit icmp any any 30 permit tcp any host 10.3.3.3 40 permit ip host 10.4.4.4 any 50 Dynamic test permit ip any any 60 permit ip host 172.16.2.2 host 10.3.3.12 70 permit ip host 10.3.3.3 any log 80 permit tcp host 10.3.3.3 host 10.1.2.2 90 permit ip host 10.3.3.3 any 100 permit ip any any Router(config)# ip access-list extended carls Router(config)# ip access-list resequence carls 1 2 Router(config)# end Router# show access-list carls Extended IP access list carls 1 permit ip host 10.3.3.3 host 172.16.5.34 3 permit icmp any any 5 permit tcp any host 10.3.3.3 7 permit ip host 10.4.4.4 any 9 Dynamic test permit ip any any 11 permit ip host 172.16.2.2 host 10.3.3.12 13 permit ip host 10.3.3.3 any log 15 permit tcp host 10.3.3.3 host 10.1.2.2 17 permit ip host 10.3.3.3 any 19 permit ip any any
Example Adding an Entry with a Sequence Number
In the following example, an new entry (sequence number 15) is added to an access list:
Router# show ip access-list Standard IP access list tryon 2 permit 10.4.4.2, wildcard bits 0.0.255.255 5 permit 10.0.0.44, wildcard bits 0.0.0.255 10 permit 10.0.0.1, wildcard bits 0.0.0.255 20 permit 10.0.0.2, wildcard bits 0.0.0.255 Router(config)# ip access-list standard tryon Router(config-std-nacl)# 15 permit 10.5.5.5 0.0.0.255 Router# show ip access-list Standard IP access list tryon 2 permit 10.4.0.0, wildcard bits 0.0.255.255 5 permit 10.0.0.0, wildcard bits 0.0.0.255 10 permit 10.0.0.0, wildcard bits 0.0.0.255 15 permit 10.5.5.0, wildcard bits 0.0.0.255 20 permit 10.0.0.0, wildcard bits 0.0.0.255
Example Adding an Entry with No Sequence Number
The following example shows how an entry with no specified sequence number is added to the end of an access list. When an entry is added without a sequence number, it is automatically given a sequence number that puts it at the end of the access list. Because the default increment is 10, the entry will have a sequence number 10 higher than the last entry in the existing access list.
Router(config)# ip access-list standard resources Router(config-std-nacl)# permit 10.1.1.1 0.0.0.255 Router(config-std-nacl)# permit 10.2.2.2 0.0.0.255 Router(config-std-nacl)# permit 10.3.3.3 0.0.0.255 Router# show access-list Standard IP access list resources 10 permit 10.1.1.1, wildcard bits 0.0.0.255 20 permit 10.2.2.2, wildcard bits 0.0.0.255 30 permit 10.3.3.3, wildcard bits 0.0.0.255 Router(config)# ip access-list standard resources Router(config-std-nacl)# permit 10.4.4.4 0.0.0.255 Router(config-std-nacl)# end Router# show access-list Standard IP access list resources 10 permit 10.1.1.1, wildcard bits 0.0.0.255 20 permit 10.2.2.2, wildcard bits 0.0.0.255 30 permit 10.3.3.3, wildcard bits 0.0.0.255 40 permit 10.4.4.4, wildcard bits 0.0.0.255
Example Time Ranges Applied to IP Access List Entries
The following example creates a time range called no-http, which extends from Monday to Friday from 8:00 a.m. to 6:00 p.m. That time range is applied to the deny statement, thereby denying HTTP traffic on Monday through Friday from 8:00 a.m. to 6:00 p.m.
The time range called udp-yes defines weekends from noon to 8:00 p.m. That time range is applied to the permit statement, thereby allowing UDP traffic on Saturday and Sunday from noon to 8:00 p.m. only. The access list containing both statements is applied to inbound packets on Fast Ethernet interface 0/0/0.
time-range no-http periodic weekdays 8:00 to 18:00 ! time-range udp-yes periodic weekend 12:00 to 20:00 ! ip access-list extended strict deny tcp any any eq http time-range no-http permit udp any any time-range udp-yes ! interface fastethernet 0/0/0 ip access-group strict in
Example Filtering IP Packet Fragments
In the following access list, the first statement will deny only noninitial fragments destined for host 172.16.1.1. The second statement will permit only the remaining nonfragmented and initial fragments that are destined for host 172.16.1.1 TCP port 80. The third statement will deny all other traffic. In order to block noninitial fragments for any TCP port, we must block noninitial fragments for all TCP ports, including port 80 for host 172.16.1.1. That is, non-initial fragments will not contain Layer 4 port information, so, in order to block such traffic for a given port, we have to block fragments for all ports.
access-list 101 deny ip any host 172.16.1.1 fragments access-list 101 permit tcp any host 172.16.1.1 eq 80 access-list 101 deny ip any any
Additional References
Related Documents
| Related Topic |
Document Title |
|---|---|
| Cisco IOS commands |
|
| Using the time-range command to establish time ranges |
The chapter "Performing Basic System Management" in the Cisco IOS XE Network Management Configuration Guide |
| Network management command descriptions |
Cisco IOS Network Management Command Reference |
Standards
| Standard |
Title |
|---|---|
| No new or modified standards are supported by this feature, and support for existing standards has not been modified by this feature. |
-- |
MIBs
| MIB |
MIBs Link |
|---|---|
| No new or modified MIBs are supported by this feature, and support for existing MIBs has not been modified by this feature. |
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: |
RFCs
| RFC |
Title |
|---|---|
| No new or modified RFCs are supported by this feature, and support for existing RFCs has not been modified by this feature. |
-- |
Technical Assistance
| Description |
Link |
|---|---|
| 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. |
Feature Information for Refining an IP Access List
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 1 | Feature Information for Refining an IP Access List |
| Feature Name |
Releases |
Feature Configuration Information |
|---|---|---|
| Time-Based Access Lists |
Cisco IOS XE Release 2.1 |
This feature was introduced on Cisco ASR 1000 Series Aggregation Services Routers. No commands were introduced or modified for this feature. |
Cisco and the Cisco logo are trademarks or registered trademarks of Cisco and/or its affiliates in the U.S. and other countries. To view a list of Cisco trademarks, go to this URL: www.cisco.com/go/trademarks. Third-party trademarks mentioned are the property of their respective owners. The use of the word partner does not imply a partnership relationship between Cisco and any other company. (1110R)
Any Internet Protocol (IP) addresses and phone numbers used in this document are not intended to be actual addresses and phone numbers. Any examples, command display output, network topology diagrams, and other figures included in the document are shown for illustrative purposes only. Any use of actual IP addresses or phone numbers in illustrative content is unintentional and coincidental.
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