Configuring Lock-and-Key Security (Dynamic Access Lists)

Lock-and-key is a traffic filtering security feature that dynamically filters IP protocol traffic. Lock-and-key is configured using IP dynamic extended access lists. Lock-and-key can be used in conjunction with other standard access lists and static extended access lists.

When lock-and-key is configured, designated users whose IP traffic is normally blocked at a router can gain temporary access through the router. When triggered, lock-and-key reconfigures the interface’s existing IP access list to permit designated users to reach their designated host(s). Afterwards, lock-and-key reconfigures the interface back to its original state.

For a user to gain access to a host through a router with lock-and-key configured, the user must first open a Telnet session to the router. When a user initiates a standard Telnet session to the router, lock-and-key automatically attempts to authenticate the user. If the user is authenticated, they will then gain temporary access through the router and be able to reach their destination host.

Lock-and-key provides the same benefits as standard and static extended access lists (these benefits are discussed in the chapter “Access Control Lists: Overview and Guidelines”). However, lock-and-key also has the following security benefits over standard and static extended access lists:

• Lock-and-key uses a challenge mechanism to authenticate individual users.
• Lock-and-key provides simpler management in large internetworks.
• In many cases, lock-and-key reduces the amount of router processing required for access lists.
• Lock-and-key reduces the opportunity for network break-ins by network hackers.

With lock-and-key, you can specify which users are permitted access to which source and destination hosts. These users must pass a user authentication process before they are permitted access to their designated hosts. Lock-and-key creates dynamic user access through a firewall, without compromising other configured security restrictions.

Two examples of when you might use lock-and-key follow:

• When you want a specific remote user (or group of remote users) to be able to access a host within your network, connecting from their remote hosts via the Internet. Lock-and-key authenticates the user, then permits limited access through your firewall router for the individual’s host or subnet, for a finite period of time.
• When you want a subset of hosts on a local network to access a host on a remote network protected by a firewall. With lock-and-key, you can enable access to the remote host only for the desired set of local user’s hosts. Lock-and-key require the users to authenticate through a TACACS+ server, or other security server, before allowing their hosts to access the remote hosts.

The following process describes the lock-and-key access operation:

1. A user opens a Telnet session to a border (firewall) router configured for lock-and-key. The user connects via the virtual terminal port on the router.
2. The Cisco IOS software receives the Telnet packet, opens a Telnet session, prompts for a password, and performs a user authentication process. The user must pass authentication before access through the router is allowed. The authentication process can be done by the router or by a central access security server such as a TACACS+ or RADIUS server.
3. When the user passes authentication, they are logged out of the Telnet session, and the software creates a temporary entry in the dynamic access list. (Per your configuration, this temporary entry can limit the range of networks to which the user is given temporary access.)
4. The user exchanges data through the firewall.
5. The software deletes the temporary access list entry when a configured timeout is reached, or when the system administrator manually clears it. The configured timeout can either be an idle timeout or an absolute timeout.

Note	The temporary access list entry is not automatically deleted when the user terminates a session. The temporary access list entry remains until a configured timeout is reached or until it is cleared by the system administrator.

Enhancements to the access-list command are used for lock-and-key. These enhancements are backward compatible--if you migrate from a release before Cisco IOS Release 11.1 to a newer release, your access lists will be automatically converted to reflect the enhancements. However, if you try to use lock-and-key with a release before Cisco IOS Release 11.1, you might encounter problems as described in the following caution paragraph:

Caution

Cisco IOS releases before Release 11.1 are not upwardly compatible with the lock-and-key access list enhancements. Therefore, if you save an access list with software older than Release 11.1, and then use this software, the resulting access list will not be interpreted correctly. This could cause you severe security problems. You must save your old configuration files with Cisco IOS Release 11.1 or later software before booting an image with these files.

Caution

Lock-and-key access allows an external event (a Telnet session) to place an opening in the firewall. While this opening exists, the router is susceptible to source address spoofing.

When lock-and-key is triggered, it creates a dynamic opening in the firewall by temporarily reconfiguring an interface to allow user access. While this opening exists, another host might spoof the authenticated user’s address to gain access behind the firewall. Lock-and-key does not cause the address spoofing problem; the problem is only identified here as a concern to the user. Spoofing is a problem inherent to all access lists, and lock-and-key does not specifically address this problem.

To prevent spoofing, configure encryption so that traffic from the remote host is encrypted at a secured remote router, and decrypted locally at the router interface providing lock-and-key. You want to ensure that all traffic using lock-and-key will be encrypted when entering the router; this way no hackers can spoof the source address, because they will be unable to duplicate the encryption or to be authenticated as is a required part of the encryption setup process.

When lock-and-key is configured, router performance can be affected in the following ways:

• When lock-and-key is triggered, the dynamic access list forces an access list rebuild on the silicon switching engine (SSE). This causes the SSE switching path to slow down momentarily.
• Dynamic access lists require the idle timeout facility (even if the timeout is left to default) and therefore cannot be SSE switched. These entries must be handled in the protocol fast-switching path.
• When remote users trigger lock-and-key at a border router, additional access list entries are created on the border router interface. The interface’s access list will grow and shrink dynamically. Entries are dynamically removed from the list after either the idle-timeout or max-timeout period expires. Large access lists can degrade packet switching performance, so if you notice performance problems, you should look at the border router configuration to see if you should remove temporary access list entries generated by lock-and-key.

When lock-and-key is in use, dynamic access lists will dynamically grow and shrink as entries are added and deleted. You need to make sure that entries are being deleted in a timely way, because while entries exist, the risk of a spoofing attack is present. Also, the more entries there are, the bigger the router performance impact will be.

If you do not have an idle or absolute timeout configured, entries will remain in the dynamic access list until you manually remove them. If this is the case, make sure that you are extremely vigilant about removing entries.

Use the following guidelines for configuring dynamic access lists:

• Do not create more than one dynamic access list for any one access list. The software only refers to the first dynamic access list defined.
• Do not assign the same dynamic-name to another access list. Doing so instructs the software to reuse the existing list. All named entries must be globally unique within the configuration.
• Assign attributes to the dynamic access list in the same way you assign attributes for a static access list. The temporary access list entries inherit the attributes assigned to this list.
• Configure Telnet as the protocol so that users must open a Telnet session into the router to be authenticated before they can gain access through the router.
• Either define an idle timeout now with the timeout keyword in the access-enable command in the autocommand command, or define an absolute timeout value later with the access-list command. You must define either an idle timeout or an absolute timeout--otherwise, the temporary access list entry will remain configured indefinitely on the interface (even after the user has terminated their session) until the entry is removed manually by an administrator. (You could configure both idle and absolute timeouts if you wish.)
• If you configure an idle timeout, the idle timeout value should be equal to the WAN idle timeout value.
• If you configure both idle and absolute timeouts, the idle timeout value must be less than the absolute timeout value.
• If you realize that a job will run past the ACL’s absolute timer, use the access-list dynamic-extend command to extend the absolute timer of the dynamic ACL by six minutes. This command allows you to open a new Telnet session into the router to re-authentication yourself using lock-and-key.
• The only values replaced in the temporary entry are the source or destination address, depending whether the access list was in the input access list or output access list. All other attributes, such as port, are inherited from the main dynamic access list.
• Each addition to the dynamic list is always put at the beginning of the dynamic list. You cannot specify the order of temporary access list entries.
• Temporary access list entries are never written to NVRAM.
• To manually clear or to display dynamic access lists, refer to the section “Maintaining Lock-and-Key” later in this chapter.

There are three possible methods to configure an authentication query process. These three methods are described in this section.

Note

Cisco recommends that you use the TACACS+ server for your authentication query process. TACACS+ provides authentication, authorization, and accounting services. It also provides protocol support, protocol specification, and a centralized security database. Using a TACACS+ server is described in the next section, “Method 1--Configuring a Security Server.”

Use a network access security server such as TACACS+ server. This method requires additional configuration steps on the TACACS+ server but allows for stricter authentication queries and more sophisticated tracking capabilities.

Router(config-line)# login tacacs

Use the username command. This method is more effective because authentication is determined on a user basis.

Router(config)# username
 
name
 {nopassword
 | 
password
 {
mutual-password
 | 
encryption-type
 
encryption-password
}}

Use the password and login commands. This method is less effective because the password is configured for the port, not for the user. Therefore, any user who knows the password can authenticate successfully.

R
outer(config-line)# password
 
password
Router(config-line)# login local

The autocommand command configures the system to automatically execute a specified privileged EXEC command when a user connects to a particular line. Use the following guidelines for configuring the autocommand command:

• If you use a TACACS+ server to authenticate the user, you should configure the autocommand command on the TACACS+ server as a per-user autocommand. If you use local authentication, use the autocommand command on the line.
• Configure all virtual terminal (VTY) ports with the same autocommand command. Omitting an autocommand command on a VTY port allows a random host to gain privileged EXEC mode access to the router and does not create a temporary access list entry in the dynamic access list.
• If you do not define an idle timeout with the autocommand access-enable command, you must define an absolute timeout with the access-list command. You must define either an idle timeout or an absolute timeout--otherwise, the temporary access list entry will remain configured indefinitely on the interface (even after the user has terminated the session) until the entry is removed manually by an administrator. (You could configure both idle and absolute timeouts if you wish.)
• If you configure both idle and absolute timeouts, the absolute timeout value must be greater than the idle timeout value.

1.    Router(config)# access-list access-list-number [dynamic dynamic-name [timeout minutes]] {deny | permit} telnet source source-wildcard destination destination-wildcard[precedence precedence] [tos tos] [established] [log]

2.    Router(config)# access-list dynamic-extend

3.    Router(config)# interface type number

4.    Router(config-if)# ip access-group access-list-number

5.    Router(config-if)# exit

6.    Router(config)# line vty line-number [ending-line-number]

7.   Do one of the following:

8.   Do one of the following:

You can verify that lock-and-key is successfully configured on the router by asking a user to test the connection. The user should be at a host that is permitted in the dynamic access list, and the user should have AAA authentication and authorization configured.

To test the connection, the user should Telnet to the router, allow the Telnet session to close, and then attempt to access a host on the other side of the router. This host must be one that is permitted by the dynamic access list. The user should access the host with an application that uses the IP protocol.

The following sample display illustrates what end-users might see if they are successfully authenticated. Notice that the Telnet connection is closed immediately after the password is entered and authenticated. The temporary access list entry is then created, and the host that initiated the Telnet session now has access inside the firewall.

Router% telnet corporate
Trying 172.21.52.1 ...
Connected to corporate.example.com.
Escape character is ‘^]’.
User Access Verification
Password:Connection closed by foreign host.

You can then use the show access-lists command at the router to view the dynamic access lists, which should include an additional entry permitting the user access through the router.s

You can display temporary access list entries when they are in use. After a temporary access list entry is cleared by you or by the absolute or idle timeout parameter, it can no longer be displayed. The number of matches displayed indicates the number of times the access list entry was hit.

To view dynamic access lists and any temporary access list entries that are currently established, use the following command in privileged EXEC mode:

Router# show access-lists [access-list-number] 

To manually delete a temporary access list entry, use the following command in privileged EXEC mode:

Router# clear access-template [access-list-number | name] [dynamic-name] [source] [destination]