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Wireless LAN Configuration Guide, Cisco IOS Release 12.4T
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Securing a Wireless LAN
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Contents
Securing a Wireless LANLast Updated: October 13, 2011
This module describes how to apply strong wireless security mechanisms on a Cisco 800, 1800, 2800, or 3800 series integrated services router, hereafter referred to as an access point (AP), to ensure that a wireless LAN is protected against unauthorized access and eavesdropping. Finding Feature Information in This ModuleYour Cisco IOS software release may not support all of the features documented in this module. To reach links to specific feature documentation in this module and to see a list of the releases in which each feature is supported, use the Feature Information for Securing a Wireless LAN. Finding Support Information for Platforms and Cisco IOS Software ImagesUse Cisco Feature Navigator to find information about platform support and Cisco IOS software image support. Access Cisco Feature Navigator at http://www.cisco.com/go/fn . You must have an account on Cisco.com. If you do not have an account or have forgotten your username or password, click Cancel at the login dialog box and follow the instructions that appear. Finding Feature InformationYour 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 Securing a Wireless LANThe following prerequisites apply to securing a wireless LAN:
Information About Securing a Wireless LAN
Wired Equivalent Privacy in a Wireless LANThe first, most basic level of a secure wireless LAN is the presence of a Wired Equivalent Privacy (WEP) key. The WEP key is unique to the client and provides the client with the appropriate level of network access. WEP keys encrypt both unicast and multicast messages. Because WEP is the first line of defense against intruders, we recommend that you use full encryption on your wireless network. WEP WeaknessesWEP is vulnerable to attack for several reasons:
Wi-Fi Protected Access in a Wireless LANWi-Fi Protected Access (WPA) was designed as a more secure replacement for WEP. The Temporal Key Integrity Protocol (TKIP), also known as WEP key hashing , is an improvement over WEP. It causes keys to automatically change, and when used in conjunction with a larger initialization vector (IV), it makes discovering keys highly unlikely. In addition to improving authentication and encryption, WPA secures the payload better than in WEP. With WEP, cyclic redundancy checks (CRC) are used to ensure packet integrity. However, it is possible to alter the payload and update the message CRC without knowing the WEP key because the CRC is not encrypted. WPA uses Message Integrity Check (MIC) to ensure packet integrity. The MICs also employ a frame counter, which prevents replay attacks. Breaking into a WLAN that uses WPA is more difficult than breaking into one that uses WEP because the IVs are larger, there are more keys in use, and there is a sturdier message verification system. Broadcast Key Rotation in a Wireless LANExtensible Authentication Protocol (EAP) authentication provides dynamic unicast WEP keys for client devices but uses static broadcast keys. When you enable broadcast key rotation, the access point provides a dynamic broadcast WEP key and changes it at the interval you select. Because broadcast key rotation is used to protect multicast traffic and TKIP is used to protect unicast traffic, they can be enabled at the same time on a wireless LAN. You should enable broadcast key rotation if you are running multicast applications on your wireless LAN. Client devices using static WEP cannot use the access point when you enable broadcast key rotation. Only wireless client devices using 802.1x authentication, such as Lightweight Extensible Authentication Protocol (LEAP), EAP with Transport Layer Security (EAP-TLS), or Protected Extensible Authentication Protocol (PEAP), can use the access point when you enable broadcast key rotation. Types of Access Point AuthenticationThis section describes the authentication types that you can configure to the access point. The authentication types correspond to the SSIDs that you configure for the access point. If you want to serve different types of client devices with the same access point, you can configure multiple SSIDs. See the Separating a Wireless Network by Configuring Multiple SSIDs section for instructions on how to configure multiple SSIDs. Before a wireless client device can communicate on your network through the access point, it must authenticate to the access point using open or shared-key authentication. For maximum security, client devices should also authenticate to your network using MAC-address or EAP authentication, authentication types that rely on an authentication server on your network. The access point uses four authentication mechanisms or types and can use more than one at the same time. The following sections explain each authentication type:
Open Authentication to the Access PointOpen authentication allows any device to authenticate and then attempt to communicate with the access point. If encryption is enabled, any wireless device using open authentication can authenticate to the access point, but the device can communicate only if its WEP keys match the access point's. Open authentication with no encryption is normally used for guest access. Any wireless client can communicate with the AP if open authentication and no encryption are configured. Devices not using WEP do not attempt to authenticate with an access point that is using WEP. Open authentication does not rely on a RADIUS server on your network. The figure below shows the authentication sequence between a device trying to authenticate and an access point using open authentication. In this example, the device's WEP key does not match the access point's key, so it can authenticate but not pass data. EAP Authentication to the Access PointEAP provides the highest level of security for a wireless network. By using EAP to interact with an EAP-compatible RADIUS server, the access point helps a wireless client device and the RADIUS server to perform mutual authentication and derive a dynamic unicast WEP key. The RADIUS server sends the WEP key to the access point, which uses it for all unicast data signals that it sends to or receives from the client. The access point also encrypts its broadcast WEP key (entered in the access point's WEP key slot 1) with the client's unicast key and sends it to the client. EAP authentication provides dynamic WEP keys to wireless users. Dynamic WEP keys are more secure than static, or unchanging, WEP keys. If an intruder passively receives enough packets encrypted by the same WEP key, the intruder can perform a calculation to learn the key and use it to join a network. Because they change frequently, dynamic WEP keys prevent intruders from performing the calculation and learning the key. When you enable EAP on your access points and client devices, authentication to the network occurs in the sequence shown in the figure below. In Steps 1 through 9 in EAP Authentication to the Access Point, a wireless client device and a RADIUS server on the wired LAN use 802.1x and EAP to perform a mutual authentication through the access point. The RADIUS server sends an authentication challenge to the client. The client uses a one-way encryption of the user-supplied password to generate a response to the challenge and sends that response to the RADIUS server. Using information from its user database, the RADIUS server creates its own response and compares that to the response from the client. When the RADIUS server authenticates the client, the process repeats in reverse, and the client authenticates the RADIUS server. When mutual authentication is complete, the RADIUS server and the client determine a WEP key that is unique to the client and provides the client with the appropriate level of network access, thereby approximating the level of security in a wired switched segment to an individual desktop. The client loads this key and prepares to use it for the login session. During the login session, the RADIUS server encrypts and sends the WEP key, called a session key, over the wired LAN to the access point. The access point encrypts its broadcast key with the session key and sends the encrypted broadcast key to the client, which uses the session key to decrypt it. The client and access point activate WEP and use the session and broadcast WEP keys for all communications during the remainder of the session. There is more than one type of EAP authentication, but the access point behaves the same way for each type: It relays authentication messages from the wireless client device to the RADIUS server and from the RADIUS server to the wireless client device. To set up EAP authentication on the access point, see the Separating a Wireless Network by Configuring Multiple SSIDs task.
MAC Address Authentication to the Access PointThe access point relays the wireless client device's MAC address to a RADIUS server on your network, and the server compares the address to a list of allowed MAC addresses. Intruders can create counterfeit MAC addresses, so MAC-based authentication is less secure than EAP authentication. However, MAC-based authentication provides an alternate authentication method for client devices that do not have EAP capability. See the Separating a Wireless Network by Configuring Multiple SSIDs section for instructions on enabling MAC-based authentication. If you do not have a RADIUS server on your network, you can create a list of allowed MAC addresses on the access point. Devices with MAC addresses not on the list are not allowed to authenticate. When you create the list of allowed MAC addresses, use lowercase for all letters in the addresses that you enter. The figure below shows the authentication sequence for MAC-based authentication. MAC-Based EAP and Open AuthenticationYou can set up the access point to authenticate client devices using a combination of MAC-based and EAP authentication. When you enable this feature, client devices that associate to the access point using 802.11 open authentication first attempt MAC authentication; if MAC authentication succeeds, the client device joins the network. If MAC authentication fails, the access point waits for the client device to attempt EAP authentication. See the "Assigning Authentication Types to SSIDs" section for instructions on setting up this combination of authentications. Shared Key Authentication to the Access Point
During shared key authentication, the access point sends an unencrypted challenge text string to any device attempting to communicate with the access point. The device requesting authentication encrypts the challenge text and sends it back to the access point. If the challenge text is encrypted correctly, the access point allows the requesting device to authenticate. Both the unencrypted challenge and the encrypted challenge can be monitored, however, which leaves the access point open to attack from an intruder that calculates the WEP key by comparing the unencrypted and encrypted text strings. Because of this weakness, shared key authentication can be less secure than open authentication. Like open authentication, shared key authentication does not rely on a RADIUS server on your network. The figure below shows the authentication sequence between a device trying to authenticate and an access point using shared key authentication. In this example the device's WEP key matches the access point's key, so it can authenticate and communicate. Correspondence Between Access Point and Client Authentication TypesThe authentication settings on the access point must match the authentication settings on the clients that associate to the access point. Refer to the installation guide for your wireless LAN client adapter for instructions on setting authentication types. The table below lists the client and access point settings required for each authentication type.
1 Some non-Cisco Aironet client adapters do not perform 802.1x authentication to the access point unless you configure open authentication with EAP. To allow both Cisco Aironet clients using LEAP and non-Cisco Aironet clients using LEAP to associate using the same SSID, you might need to configure the SSID for both network EAP authentication and open authentication with EAP.
MAC Address and IP Filters on Access Point InterfacesIn addition to managing access to a WLAN through WEP keys or authentication, you can configure access to be restricted according to device; to do this, you use the MAC address or IP address. For example, you can employ filtering on your APs to keep out clients that do not have an authorized client adapter. Without an explicitly approved MAC address on the network adapter, it does not matter if the correct username and password are presented because the AP does not allow access. Simply put, filtering checks a wireless client's MAC or IP address against a list of authorized MAC or IP addresses maintained on the access point. When a client tries to connect to the access point, it must be on the list. If it is not, the client cannot connect. Filtering should not be the only security measure, however. Both MAC and IP addresses can be spoofed, thus circumventing this layer of security. To configure filters, you use access control lists (ACLs) and bridge groups. MAC Address FiltersMAC address filters allow or disallow the forwarding of unicast and multicast packets either sent from or addressed to specific MAC addresses. You can create a filter that passes or blocks traffic to all MAC addresses except those you specify. You can apply the filters you create to either or both the Ethernet and radio ports and to either or both incoming and outgoing packets. MAC address filters are powerful, and you can lock yourself out of the access point if you make a mistake setting up the filters. If you accidentally lock yourself out of your access point, you may need to attach to the AP using a console, disable the filters, then correct each filter accordingly. IP FiltersYou can limit access to your AP with IP filters. IP filters can be applied based on IP address, IP protocol, and IP port. IP filters prevent or allow the use of specific protocols through the access point's Ethernet and radio ports, and IP address filters allow or prevent the forwarding of unicast and multicast packets either sent from or addressed to specific IP addresses. You can create a filter that passes traffic to all addresses except those you specify, or you can create a filter that blocks traffic to all addresses except those you specify. You can create filters that contain elements of one, two, or all three IP filtering methods. You can apply the filters you create to either or both the Ethernet and radio ports and to either or both incoming and outgoing packets. How to Secure a Wireless LAN
Configuring WEP Encryption and Key Management FeaturesPerform this task to configure WEP encryption and additional key management features, such as MIC, TKIP, and broadcast key rotation. Configure static WEP keys only if the access point must support client devices that use static WEP. If all the client devices that associate to the access point use key management (WPA or 802.1x authentication) you do not need to configure static WEP keys. WEP, TKIP, MIC, and broadcast key rotation are disabled by default. Before You Begin
SUMMARY STEPS
Determine if all the clients that will associate to the access point are capable of key management. If they are, use the encryption mode ciphers command rather than the encryption mode wep command to configure WEP. See the relevant command pages in the Cisco IOS Wireless LAN Command Reference for more details.
DETAILED STEPS
What to Do NextAfter you have configured encryption, configure authentication mechanisms as shown in the Controlling Access to a Wireless Network by Using Authentication Mechanisms section. Controlling Access to a Wireless Network by Using Authentication MechanismsIn a wireless network, you need to ascertain the identity of the users and devices using authentication mechanisms. This is important because access control is established depending on the user's identity. Perform this task to configure authentication mechanisms. Before You Begin
SUMMARY STEPS
The following prerequisites apply to using authentication mechanisms:
DETAILED STEPS
What to Do NextAfter you configure authentication mechanisms, you can configure authentication timeouts and reauthentication periods on the access point by completing the optional task in the Configuring Authentication Timeouts and Reauthentication Periods section. Separating a Wireless Network by Configuring Multiple SSIDsThe SSID is a unique identifier that wireless networking devices use to establish and maintain wireless connectivity. You can configure up to 10 SSIDs on the Cisco 800 and 1800 series fixed-configuration routers and up to 16 SSIDs on the Cisco 1800 modular, 2800, and 3800 series routers and assign different configuration settings to each SSID. All the SSIDs are active at the same time; that is, client devices can associate to the access point using any of the SSIDs. These are the settings you can assign to each SSID:
DETAILED STEPS
What to Do NextAfter you have configured the SSIDs, configure authentication mechanisms by completing the task in the Controlling Access to a Wireless Network by Using Authentication Mechanisms section. Configuring Authentication Timeouts and Reauthentication PeriodsPerform this task to configure authentication timeouts and reauthentication periods for client devices authenticating through your access point. This task is optional and can be used only if 802.1x authentication is configured. DETAILED STEPS
Configuration Examples for Securing a Wireless LAN
Configuring an Access Point in Bridging Mode with Open Authentication and Static WEP Encryption ExampleThe following configuration example shows how to:
configure terminal bridge irb bridge 1 route ip dot11 ssid ssid1 authentication open exit interface dot11Radio 0/0/0 encryption mode wep mandatory encryption key 1 size 40bit 11aa33bb55 ssid ssid1 exit bridge-group 1 bridge-group 1 subscriber-loop-control bridge-group 1 spanning-disabled bridge-group 1 block-unknown-source no bridge-group 1 source-learning no bridge-group 1 unicast-flooding no shutdown exit interface vlan 1 bridge-group 1 bridge-group 1 spanning-disabled exit interface bvi 1 ip address 10.0.1.2 255.255.255.0 end copy running-config startup-config Configuring an Access Point in Bridging Mode with WPA-PSK ExampleThe following example shows how to:
configure terminal bridge irb bridge 1 route ip dot11 ssid ssid1 authentication open authentication key-management wpa wpa-psk ascii shared-key-name exit interface dot11Radio 0/3/0 encryption mode ciphers tkip ssid ssid1 exit bridge-group 1 bridge-group 1 subscriber-loop-control bridge-group 1 spanning-disabled bridge-group 1 block-unknown-source no bridge-group 1 source-learning no bridge-group 1 unicast-flooding no shutdown exit interface vlan 1 bridge-group 1 bridge-group 1 spanning-disabled exit interface bvi 1 ip address 10.0.1.2 255.255.255.0 end copy running-config startup-config Configuring an Access Point in Bridging Mode with MAC Authentication ExampleThe following example shows how to:
configure terminal bridge irb bridge 1 route ip dot11 ssid ssid1 authentication open mac-address maclist1 exit interface dot11Radio 0/3/0 ssid ssid1 exit bridge-group 1 bridge-group 1 subscriber-loop-control bridge-group 1 spanning-disabled bridge-group 1 block-unknown-source no bridge-group 1 source-learning no bridge-group 1 unicast-flooding no shutdown exit interface vlan 1 bridge-group 1 bridge-group 1 spanning-disabled exit interface bvi 1 ip address 10.0.1.2 255.255.255.0 end configure terminal username 000011111111 password 000011111111 aaa new-model aaa authentication login maclist1 local end copy running-config startup-config This example shows how to:
configure terminal bridge irb bridge 1 route ip dot11 ssid ssid1 authentication open mac-address maclist1 exit interface dot11Radio 0/3/0 ssid ssid1 exit bridge-group 1 bridge-group 1 subscriber-loop-control bridge-group 1 spanning-disabled bridge-group 1 block-unknown-source no bridge-group 1 source-learning no bridge-group 1 unicast-flooding no shutdown exit interface vlan 1 bridge-group 1 bridge-group 1 spanning-disabled exit interface bvi 1 ip address 10.0.1.2 255.255.255.0 end configure terminal aaa new-model ip radius source-interface bvi 1 radius-server host 11.2.0.1 auth-port 1812 acct-port 1813 key sharedsecret aaa group server radius rad_mac server 11.2.0.1 auth-port 1812 acct-port 1813 exit aaa authentication login maclist1 group rad_mac end copy running-config startup-config Configuring an Access Point in Bridging Mode with 802.1x Authentication ExampleThe following example shows how to:
configure terminal bridge irb bridge 1 route ip dot11 ssid ssid1 authentication network-eap eaplist1 authentication open eap eaplist1 exit interface dot11Radio 0/3/0 ssid ssid1 exit bridge-group 1 bridge-group 1 subscriber-loop-control bridge-group 1 spanning-disabled bridge-group 1 block-unknown-source no bridge-group 1 source-learning no bridge-group 1 unicast-flooding no shutdown exit interface vlan 1 bridge-group 1 bridge-group 1 spanning-disabled exit interface bvi 1 ip address 10.0.1.2 255.255.255.0 end copy running-config startup-config Configuring an Access Point in Routing Mode with Open Authentication and Static WEP Encryption ExampleThe following example shows how to:
configure terminal dot11 ssid ssid2 authentication open exit interface dot11Radio 0/3/0 ip address 10.0.1.1 255.255.255.0 encryption mode wep mandatory encryption key 1 size 40bit 11aa33bb55 ssid ssid2 no shutdown end copy running-config startup-config Configuring an Access Point in Routing Mode with WPA-PSK ExampleThe following example shows how to:
configure terminal dot11 ssid ssid2 authentication key-management wpa wpa-psk ascii shared-key-name authentication open exit interface dot11Radio 0/3/0 ip address 10.0.1.1 255.255.255.0 encryption mode ciphers tkip ssid ssid2 no shutdown end copy running-config startup-config Configuring an Access Point in Routing Mode with MAC Authentication ExampleThe following example shows how to:
configure terminal dot11 ssid ssid2 authentication open mac-address maclist1 exit interface dot11Radio 0/3/0 ip address 10.0.1.1 255.255.255.0 ssid ssid2 no shutdown end configure terminal username 000011111111 password 000011111111 aaa new-model aaa authentication login maclist1 local end copy running-config startup-config This example shows how to:
configure terminal dot11 ssid2 authentication open mac-address maclist1 exit interface dot11Radio 0/3/0 ip address 10.0.1.1 255.255.255.0 ssid ssid2 no shutdown end configure terminal aaa new-model ip radius source-interface bvi 1 radius-server host 11.2.0.1 auth-port 1812 acct-port 1813 key sharedsecret aaa group server radius rad_mac server 11.2.0.1 auth-port 1812 acct-port 1813 exit aaa authentication login maclist1 group rad_mac end copy running-config startup-config Configuring an Access Point in Routing Mode with 802.1x Authentication ExampleThe following example shows how to:
configure terminal dot11 ssid ssid2 authentication open eap eaplist1 authentication network-eap eaplist1 exit interface dot11Radio 0/3/0 ip address 10.0.1.1 255.255.255.0 ssid ssid2 no shutdown end copy running-config startup-config Where to Go Next
Additional ReferencesRelated Documents
MIBsFeature Information for Securing a Wireless LANThe 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.
Cisco and the Cisco Logo are trademarks of Cisco Systems, Inc. and/or its affiliates in the U.S. and other countries. A listing of Cisco's trademarks can be found at 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. (1005R) 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. 1 Some non-Cisco Aironet client adapters do not perform 802.1x authentication to the access point unless you configure open authentication with EAP. To allow both Cisco Aironet clients using LEAP and non-Cisco Aironet clients using LEAP to associate using the same SSID, you might need to configure the SSID for both network EAP authentication and open authentication with EAP. © 2011 Cisco Systems, Inc. All rights reserved.
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