Cisco ASA 5500 Series Configuration Guide using the CLI, 8.4 and 8.6

IPsec Overview

The ASA uses IPsec for LAN-to-LAN VPN connections and provides the option of using IPsec for client-to-LAN VPN connections. In IPsec terminology, a peer is a remote-access client or another secure gateway. For both connection types, the ASA supports only Cisco peers. Because we adhere to VPN industry standards, ASAs can work with other vendors' peers; however, we do not support them.

During tunnel establishment, the two peers negotiate security associations that govern authentication, encryption, encapsulation, and key management. These negotiations involve two phases: first, to establish the tunnel (the IKE SA) and second, to govern traffic within the tunnel (the IPsec SA).

A LAN-to-LAN VPN connects networks in different geographic locations. In IPsec LAN-to-LAN connections, the ASA can function as initiator or responder. In IPsec client-to-LAN connections, the ASA functions only as responder. Initiators propose SAs; responders accept, reject, or make counter-proposals—all in accordance with configured SA parameters. To establish a connection, both entities must agree on the SAs.

Note When the ASA is configured for IPsec VPN, you cannot enable security contexts (also called firewall multimode) or Active/Active stateful failover. Therefore, these features are unavailable.

ISAKMP and IKE Overview

ISAKMP is the negotiation protocol that lets two hosts agree on how to build an IPsec security association (SA). It provides a common framework for agreeing on the format of SA attributes. This security association includes negotiating with the peer about the SA and modifying or deleting the SA. ISAKMP separates negotiation into two phases: Phase 1 and Phase 2. Phase 1 creates the first tunnel, which protects later ISAKMP negotiation messages. Phase 2 creates the tunnel that protects data.

IKE uses ISAKMP to set up the SA for IPsec to use. IKE creates the cryptographic keys used to authenticate peers.

The ASA supports IKEv1 for connections from the legacy Cisco VPN client, and IKEv2 for the AnyConnect VPN client.

To set the terms of the ISAKMP negotiations, you create an IKE policy, which includes the following:

• The authentication type required of the IKEv1 peer, either RSA signature using certificates or preshared key (PSK).
• An encryption method to protect the data and ensure privacy.
• A Hashed Message Authentication Codes (HMAC) method to ensure the identity of the sender, and to ensure that the message has not been modified in transit.
• A Diffie-Hellman group to determine the strength of the encryption-key-determination algorithm. The ASA uses this algorithm to derive the encryption and hash keys.
• For IKEv2, a separate pseudo-random function (PRF) used as the algorithm to derive keying material and hashing operations required for the IKEv2 tunnel encryption and so on.
• A limit to the time the ASA uses an encryption key before replacing it.

With IKEv1 policies, you set one value for each parameter. For IKEv2, you can configure multiple encryption and authentication types, and multiple integrity algorithms for a single policy. The ASA orders the settings from the most secure to the least secure and negotiates with the peer using that order. This ordering allows you to potentially send a single proposal to convey all the allowed transforms instead of sending each allowed combination as with IKEv1.

Configuring IKEv1 and IKEv2 Policies

To create an IKE policy, enter the crypto ikev1 | ikev2 policy command from global configuration mode. The prompt displays IKE policy configuration mode. For example:

After creating the policy, you can specify the settings for the policy.

Table 64-1 and Table 64-2 provide information about the IKEv1 and IKEv2 policy keywords and their values.

IKEv1 and IKEv2 each support a maximum of 20 IKE policies, each with a different set of values. Assign a unique priority to each policy that you create. The lower the priority number, the higher the priority.

When IKE negotiations begin, the peer that initiates the negotiation sends all of its policies to the remote peer, and the remote peer tries to find a match. The remote peer checks all of the peer's policies against each of its configured policies in priority order (highest priority first) until it discovers a match.

A match exists when both policies from the two peers contain the same encryption, hash, authentication, and Diffie-Hellman parameter values. For IKEv1, the remote peer policy must also specify a lifetime less than or equal to the lifetime in the policy the initiator sent. If the lifetimes are not identical, the ASA uses the shorter lifetime. For IKEv2 the lifetime is not negotiated but managed locally between each peer, making it possible to configure lifetime independently on each peer. If no acceptable match exists, IKE refuses negotiation and the SA is not established.

There is an implicit trade-off between security and performance when you choose a specific value for each parameter. The level of security the default values provide is adequate for the security requirements of most organizations. If you are interoperating with a peer that supports only one of the values for a parameter, your choice is limited to that value.

Note New ASA configurations do not have a default IKEv1 or IKEv2 policy.

To configure IKE policies, in global configuration mode, use the crypto ikev1 | ikev2 policy command to enter IKE policy configuration mode:

crypto ikev1 | ikev2 policy priority

You must include the priority in each of the ISAKMP commands. The priority number uniquely identifies the policy and determines the priority of the policy in IKE negotiations.

To enable and configure IKE, complete the following steps, using the IKEv1 examples as a guide:

Note If you do not specify a value for a given policy parameter, the default value applies.

Step 1 Enter IKEv1 policy configuration mode:

Step 2 Specify the encryption algorithm. The default is Triple DES. This example sets encryption to DES.

For example:

Step 3 Specify the hash algorithm. The default is SHA-1. This example configures MD5.

For example:

Step 4 Specify the authentication method. The default is preshared keys. This example configures RSA signatures.

For example:

Step 5 Specify the Diffie-Hellman group identifier. The default is Group 2. This example configures Group 5.

For example:

Step 6 Specify the SA lifetime. This examples sets a lifetime of 4 hours (14400 seconds). The default is 86400 seconds (24 hours).

For example:

Enabling IKE on the Outside Interface

You must enable IKE on the interface that terminates the VPN tunnel. Typically this is the outside, or public interface. To enable IKEv1 or IKEv2, use the crypto ikev1 | ikev2 enable command from global configuration mode:

crypto ikev1 | ikev2 enable interface-name

For example:

Disabling IKEv1 Aggressive Mode

Phase 1 IKEv1 negotiations can use either main mode or aggressive mode. Both provide the same services, but aggressive mode requires only two exchanges between the peers totaling three messages, rather than three exchanges totaling six messages. Aggressive mode is faster, but does not provide identity protection for the communicating parties. Therefore, the peers must exchange identification information before establishing a secure SA. Aggressive mode is enabled by default.

• Main mode is slower, using more exchanges, but it protects the identities of the communicating peers.
• Aggressive mode is faster, but does not protect the identities of the peers.

To disable aggressive mode, enter the following command:

For example:

If you have disabled aggressive mode, and want to revert to back to it, use the no form of the command. For example:

Note Disabling aggressive mode prevents Cisco VPN clients from using preshared key authentication to establish tunnels to the ASA. However, they may use certificate-based authentication (that is, ASA or RSA) to establish tunnels.

Determining an ID Method for IKEv1 and IKEv2 ISAKMP Peers

During ISAKMP Phase I negotiations, either IKEv1 or IKEv2, the peers must identify themselves to each other. You can choose the identification method from the following options:

The ASA uses the Phase I ID to send to the peer. This is true for all VPN scenarios except LAN-to-LAN IKEv1 connections in main mode that authenticate with preshared keys.

The default setting is auto.

To change the peer identification method, enter the following command:

For example, the following command sets the peer identification method to hostname:

Enabling IPsec over NAT-T

NAT-T lets IPsec peers establish a connection through a NAT device. It does this by encapsulating IPsec traffic in UDP datagrams, using port 4500, which provides NAT devices with port information. NAT-T auto-detects any NAT devices and only encapsulates IPsec traffic when necessary. This feature is disabled by default.

Note Due to a limitation of the AnyConnect client, you must enable NAT-T for the AnyConnect client to successfully connect using IKEv2. This requirement applies even if the client is not behind a NAT-T device.

With the exception of the home zone on the Cisco ASA 5505, the ASA can simultaneously support standard IPsec, IPsec over TCP, NAT-T, and IPsec over UDP, depending on the client with which it is exchanging data.

The following breakdown shows the connections with each option enabled:

Note When IPsec over TCP is enabled, it takes precedence over all other connection methods.

When you enable NAT-T, the ASA automatically opens port 4500 on all IPsec-enabled interfaces.

The ASA supports multiple IPsec peers behind a single NAT/PAT device operating in one of the following networks, but not both:

• LAN-to-LAN
• Remote access

In a mixed environment, the remote access tunnels fail the negotiation because all peers appear to be coming from the same public IP address, address of the NAT device. Also, remote access tunnels fail in a mixed environment because they often use the same name as the LAN-to-LAN tunnel group (that is, the IP address of the NAT device). This match can cause negotiation failures among multiple peers in a mixed LAN-to-LAN and remote access network of peers behind the NAT device.

Using NAT-T

To use NAT-T, you must perform the following tasks:

Step 1 Enter the following command to enable IPsec over NAT-T globally on the ASA:

The range for the natkeepalive argument is 10 to 3600 seconds. The default is 20 seconds.

For example, enter the following command to enable NAT-T and set the keepalive value to one hour.

Step 2 Select the before-encryption option for the IPsec fragmentation policy by entering this command:

This option lets traffic travel across NAT devices that do not support IP fragmentation. It does not impede the operation of NAT devices that do support IP fragmentation.

Enabling IPsec with IKEv1 over TCP

IPsec/IKEv1 over TCP enables a Cisco VPN client to operate in an environment in which standard ESP or IKEv1 cannot function or can function only with modification to existing firewall rules. IPsec over TCP encapsulates both the IKEv1 and IPsec protocols within a TCP-like packet and enables secure tunneling through both NAT and PAT devices and firewalls. This feature is disabled by default.

Note This feature does not work with proxy-based firewalls.

IPsec over TCP works with remote access clients. You enable it globally, and it works on all IKEv1-enabled interfaces. It is a client to the ASA feature only. It does not work for LAN-to-LAN connections.

The ASA can simultaneously support standard IPsec, IPsec over TCP, NAT-Traversal, and IPsec over UDP, depending on the client with which it is exchanging data. IPsec over TCP, if enabled, takes precedence over all other connection methods.

The VPN 3002 hardware client, which supports one tunnel at a time, can connect using standard IPsec, IPsec over TCP, NAT-Traversal, or IPsec over UDP.

You enable IPsec over TCP on both the ASA and the client to which it connects.

You can enable IPsec over TCP for up to 10 ports that you specify. If you enter a well-known port, for example port 80 (HTTP) or port 443 (HTTPS), the system displays a warning that the protocol associated with that port no longer works on the public interface. The consequence is that you can no longer use a browser to manage the ASA through the public interface. To solve this problem, reconfigure the HTTP/HTTPS management to different ports.

The default port is 10000.

You must configure TCP port(s) on the client as well as on the ASA. The client configuration must include at least one of the ports you set for the ASA.

To enable IPsec over TCP for IKEv1 globally on the ASA, enter the following command:

This example enables IPsec over TCP on port 45:

Waiting for Active Sessions to Terminate Before Rebooting

You can schedule an ASA reboot to occur only when all active sessions have terminated voluntarily. This feature is disabled by default.

To enable waiting for all active sessions to voluntarily terminate before the ASA reboots, enter the following command:

For example:

Use the reload command to reboot the ASA. If you set the reload-wait command, you can use the reload quick command to override the reload-wait setting. The reload and reload-wait commands are available in privileged EXEC mode; neither includes the isakmp prefix.

Alerting Peers Before Disconnecting

Remote access or LAN-to-LAN sessions can drop for several reasons, such as an ASA shutdown or reboot, session idle timeout, maximum connection time exceeded, or administrator cut-off.

The ASA can notify qualified peers (in LAN-to-LAN configurations), Cisco VPN clients, and VPN 3002 hardware clients of sessions that are about to be disconnected. The peer or client receiving the alert decodes the reason and displays it in the event log or in a pop-up pane. This feature is disabled by default.

Qualified clients and peers include the following:

• Security appliances with Alerts enabled
• Cisco VPN clients running version 4.0 or later software (no configuration required)
• VPN 3002 hardware clients running version 4.0 or later software, with Alerts enabled
• VPN 3000 series concentrators running version 4.0 or later software with Alerts enabled

To enable disconnect notification to IPsec peers, enter the crypto isakmp disconnect-notify command.

For example:

Creating a Certificate Group Matching Rule and Policy

To configure the policy and rules by which certificate-based ISAKMP sessions map to tunnel groups, and to associate the certificate map entries with tunnel groups, enter the tunnel-group-map command in global configuration mode.

The syntax follows:

tunnel-group-map enable { rules | ou | ike-id | peer ip }

tunnel-group-map [ rule-index ] enable policy

Be aware of the following:

• You can invoke this command multiple times as long as each invocation is unique and you do not reference a map index more than once.
• Rules cannot be longer than 255 characters.
• You can assign multiple rules to the same group. To do that, you add the rule priority and group first. Then you define as many criteria statements as you need for each group. When multiple rules are assigned to the same group, a match results for the first rule that tests true.
• By creating a single rule, you can require all criteria to match before assigning a user to a specific tunnel group. Requiring all criteria to match is equivalent to a logical AND operation. Alternatively, create one rule for each criterion if you want to require that only one match before assigning a user to a specific tunnel group. Requiring only one criterion to match is equivalent to a logical OR operation.

The following example enables mapping of certificate-based ISAKMP sessions to a tunnel group based on the content of the phase1 ISAKMP ID:

The following example enables mapping of certificate-based ISAKMP sessions to a tunnel group based on the IP address of the peer:

The following example enables mapping of certificate-based ISAKMP sessions based on the organizational unit (OU) in the subject distinguished name (DN):

The following example enables mapping of certificate-based ISAKMP sessions based on established rules:

Using the Tunnel-group-map default-group Command

This command specifies a default tunnel group to use when the configuration does not specify a tunnel group.

The syntax is tunnel-group-map [ rule-index ] default-group tunnel-group-name where rule-index is the priority for the rule, and tunnel-group name must be for a tunnel group that already exists.

Understanding IPsec Tunnels

IPsec tunnels are sets of SAs that the ASA establishes between peers. The SAs specify the protocols and algorithms to apply to sensitive data and also specify the keying material that the peers use. IPsec SAs control the actual transmission of user traffic. SAs are unidirectional, but are generally established in pairs (inbound and outbound).

The peers negotiate the settings to use for each SA. Each SA consists of the following:

• IKEv1 transform sets or IKEv2 proposals
• Crypto maps
• Access lists
• Tunnel groups
• Prefragmentation policies

Understanding IKEv1 Transform Sets and IKEv2 Proposals

An IKEv1 transform set or an IKEv2 proposal is a combination of security protocols and algorithms that define how the ASA protects data. During IPsec SA negotiations, the peers must identify a transform set or proposal that is the same at both peers. The ASA then applies the matching transform set or proposal to create an SA that protects data flows in the access list for that crypto map.

With IKEv1 transform sets, you set one value for each parameter. For IKEv2 proposals, you can configure multiple encryption and authentication types and multiple integrity algorithms for a single proposal. The ASA orders the settings from the most secure to the least secure and negotiates with the peer using that order. This allows you to potentially send a single proposal to convey all the allowed combinations instead of the need to send each allowed combination individually as with IKEv1.

The ASA tears down the tunnel if you change the definition of the transform set or proposal used to create its SA. See “Clearing Security Associations” for further information.

Note If you clear or delete the only element in a transform set or proposal, the ASA automatically removes the crypto map references to it.

Defining Crypto Maps

Crypto maps define the IPsec policy to be negotiated in the IPsec SA. They include the following:

• Access list to identify the packets that the IPsec connection permits and protects.
• Peer identification.
• Local address for the IPsec traffic. (See “Applying Crypto Maps to Interfaces” for more details.)
• Up to 11 IKEv1 transform sets or IKEv2 proposals, with which to attempt to match the peer security settings.

A crypto map set consists of one or more crypto maps that have the same map name. You create a crypto map set when you create its first crypto map. The following command syntax creates or adds to a crypto map:

You can continue to enter this command to add crypto maps to the crypto map set. In the following example, mymap is the name of the crypto map set to which you might want to add crypto maps:

The sequence number ( seq-num ) shown in the syntax above distinguishes one crypto map from another one with the same name. The sequence num ber assigned to a crypto map also determines its priority among the other crypto maps within a crypto map set. The lower the sequence number, the higher the priority. After you assign a crypto map set to an interface, the ASA evaluates all IP traffic passing through the interface against the crypto maps in the set, beginning with the crypto map with the lowest sequence number.

The ACL assigned to a crypto map consists of all of the ACEs that have the same access list name, as shown in the following command syntax:

Each ACL consists of one or more ACEs that have the same access list name. You create an ACL when you create its first ACE. The following command syntax creates or adds to an ACL:

In the following example, the ASA applies the IPsec protections assigned to the crypto map to all traffic flowing from the 10.0.0.0 subnet to the 10.1.1.0 subnet:

The crypto map that matches the packet determines the security settings used in the SA negotiations. If the local ASA initiates the negotiation, it uses the policy specified in the static crypto map to create the offer to send to the specified peer. If the peer initiates the negotiation, the ASA attempts to match the policy to a static crypto map, and if that fails, then it attempts to match any dynamic crypto maps in the crypto map set, to decide whether to accept or reject the peer offer.

For two peers to succeed in establishing an SA, they must have at least one compatible crypto map. To be compatible, a crypto map must meet the following criteria:

• The crypto map must contain compatible crypto ACLs (for example, mirror image ACLs). If the responding peer uses dynamic crypto maps, so the ASA also must contain compatible crypto ACLs as a requirement to apply IPsec.
• Each crypto map identifies the other peer (unless the responding peer uses dynamic crypto maps).
• The crypto maps have at least one transform set or proposal in common.

You can apply only one crypto map set to a single interface. Create more than one crypto map for a particular interface on the ASA if any of the following conditions exist:

• You want specific peers to handle different data flows.
• You want different IPsec security to apply to different types of traffic.

For example, create a crypto map and assign an ACL to identify traffic between two subnets and assign one IKEv1 transform set or IKEv2 proposal. Create another crypto map with a different ACL to identify traffic between another two subnets and apply a transform set or proposal with different VPN parameters.

If you create more than one crypto map for an interface, specify a sequence number (seq-num) for each map entry to determine its priority within the crypto map set.

Each ACE contains a permit or deny statement. Table 64-3 explains the special meanings of permit and deny ACEs in ACLs applied to crypto maps.

ACEs containing deny statements filter out outbound traffic that does not require IPsec protection (for example, routing protocol traffic). Therefore, insert initial deny statements to filter outbound traffic that should not be evaluated against permit statements in a crypto access list.

For an inbound, encrypted packet, the security appliance uses the source address and ESP SPI to determine the decryption parameters. After the security appliance decrypts the packet, it compares the inner header of the decrypted packet to the permit ACEs in the ACL associated with the packet SA. If the inner header fails to match the proxy, the security appliance drops the packet. It the inner header matches the proxy, the security appliance routes the packet.

When comparing the inner header of an inbound packet that was not encrypted, the security appliance ignores all deny rules because they would prevent the establishment of a Phase 2 SA.

Note To route inbound, unencrypted traffic as clear text, insert deny ACEs before permit ACEs.

Figure 64-1 shows an example LAN-to-LAN network of ASAs.

Figure 64-1 Effect of Permit and Deny ACEs on Traffic (Conceptual Addresses)

The simple address notation shown in this figure and used in the following explanation is an abstraction. An example with real IP addresses follows the explanation.

The objective in configuring Security Appliances A, B, and C in this example LAN-to-LAN network is to permit tunneling of all traffic originating from one of the hosts shown in Figure 64-1 and destined for one of the other hosts. However, because traffic from Host A.3 contains sensitive data from the Human Resources department, it requires strong encryption and more frequent rekeying than the other traffic. So you will want to assign a special transform set for traffic from Host A.3.

To configure Security Appliance A for outbound traffic, you create two crypto maps, one for traffic from Host A.3 and the other for traffic from the other hosts in Network A, as shown in the following example:

After creating the ACLs, you assign a transform set to each crypto map to apply the required IPsec to each matching packet.

Cascading ACLs involves the insertion of deny ACEs to bypass evaluation against an ACL and resume evaluation against a subsequent ACL in the crypto map set. Because you can associate each crypto map with different IPsec settings, you can use deny ACEs to exclude special traffic from further evaluation in the corresponding crypto map, and match the special traffic to permit statements in another crypto map to provide or require different security. The sequence number assigned to the crypto ACL determines its position in the evaluation sequence within the crypto map set.

Figure 64-2 shows the cascading ACLs created from the conceptual ACEs above. The meaning of each symbol in the figure follows.

	Crypto map within a crypto map set.
	(Gap in a straight line) Exit from a crypto map when a packet matches an ACE.
	Packet that fits the description of one ACE. Each size ball represents a different packet matching the respective ACE in the figure. The differences in size merely represent differences in the source and destination of each packet.
	Redirection to the next crypto map in the crypto map set.
	Response when a packet either matches an ACE or fails to match all of the permit ACEs in a crypto map set.

Figure 64-2 Cascading ACLs in a Crypto Map Set

Security Appliance A evaluates a packet originating from Host A.3 until it matches a permit ACE and attempts to assign the IPsec security associated with the crypto map. Whenever the packet matches a deny ACE, the ASA ignores the remaining ACEs in the crypto map and resumes evaluation against the next crypto map, as determined by the sequence number assigned to it. So in the example, if Security Appliance A receives a packet from Host A.3, it matches the packet to a deny ACE in the first crypto map and resumes evaluation of the packet against the next crypto map. When it matches the packet to the permit ACE in that crypto map, it applies the associated IPsec security (strong encryption and frequent rekeying).

To complete the security appliance configuration in the example network, we assign mirror crypto maps to Security Appliances B and C. However, because security appliances ignore deny ACEs when evaluating inbound, encrypted traffic, we can omit the mirror equivalents of the deny A.3 B and deny A.3 C ACEs, and therefore omit the mirror equivalents of Crypto Map 2. So the configuration of cascading ACLs in Security Appliances B and C is unnecessary.

Table 64-4 shows the ACLs assigned to the crypto maps configured for all three ASAs in Figure 64-1.

Figure 64-3 maps the conceptual addresses shown in Figure 64-1 to real IP addresses.

Figure 64-3 Effect of Permit and Deny ACEs on Traffic (Real Addresses)

The tables that follow combine the IP addresses shown in Figure 64-3 to the concepts shown in Table 64-4 . The real ACEs shown in these tables ensure that all IPsec packets under evaluation within this network receive the proper IPsec settings.

You can apply the same reasoning shown in the example network to use cascading ACLs to assign different security settings to different hosts or subnets protected by a Cisco ASA.

Note By default, the ASA does not support IPsec traffic destined for the same interface from which it enters. Names for this type of traffic include U-turn, hub-and-spoke, and hairpinning. However, you can configure IPsec to support U-turn traffic by inserting an ACE to permit traffic to and from the network. For example, to support U-turn traffic on Security Appliance B, add a conceptual “permit B B” ACE to ACL1. The actual ACE would be as follows:
permit 192.168.12.0 255.255.255.248 192.168.12.0 255.255.255.248

Applying Crypto Maps to Interfaces

You must assign a crypto map set to each interface through which IPsec traffic flows. The ASA supports IPsec on all interfaces. Assigning the crypto map set to an interface instructs the ASA to evaluate all the traffic against the crypto map set and to use the specified policy during connection or SA negotiation.

Assigning a crypto map to an interface also initializes run-time data structures, such as the SA database and the security policy database. Reassigning a modified crypto map to the interface resynchronizes the run-time data structures with the crypto map configuration. Also, adding new peers through the use of new sequence numbers and reassigning the crypto map does not tear down existing connections.

Using Interface Access Lists

By default, the ASA lets IPsec packets bypass interface ACLs. If you want to apply interface access lists to IPsec traffic, use the no form of the sysopt connection permit-vpn command.

The crypto map access list bound to the outgoing interface either permits or denies IPsec packets through the VPN tunnel. IPsec authenticates and deciphers packets that arrive from an IPsec tunnel, and subjects them to evaluation against the ACL associated with the tunnel.

Access lists define which IP traffic to protect. For example, you can create access lists to protect all IP traffic between two subnets or two hosts. (These access lists are similar to access lists used with the access-group command. However, with the access-group command, the access list determines which traffic to forward or block at an interface.)

Before the assignment to crypto maps, the access lists are not specific to IPsec. Each crypto map references the access lists and determines the IPsec properties to apply to a packet if it matches a permit in one of the access lists.

Access lists assigned to IPsec crypto maps have four primary functions:

• Select outbound traffic to be protected by IPsec (permit = protect).
• Trigger an ISAKMP negotiation for data travelling without an established SA.
• Process inbound traffic to filter out and discard traffic that should have been protected by IPsec.
• Determine whether to accept requests for IPsec SAs when processing IKE negotiation from the peer. (Negotiation applies only to ipsec-isakmp crypto map entries.) The peer must permit a data flow associated with an ipsec-isakmp crypto map command entry to ensure acceptance during negotiation.

Regardless of whether the traffic is inbound or outbound, the ASA evaluates traffic against the access lists assigned to an interface. You assign IPsec to an interface as follows:

Step 1 Create the access lists to be used for IPsec.

Step 2 Map the lists to one or more crypto maps, using the same crypto map name.

Step 3 Map the IKEv1 transform sets or IKEv2 proposals to the crypto maps to apply IPsec to the data flows.

Step 4 Apply the crypto maps collectively as a crypto map set by assigning the crypto map name they share to the interface.

In Figure 64-4, IPsec protection applies to traffic between Host 10.0.0.1 and Host 10.2.2.2 as the data exits the outside interface on Security Appliance A toward Host 10.2.2.2.

Figure 64-4 How Crypto Access Lists Apply to IPsec

Security Appliance A evaluates traffic from Host 10.0.0.1 to Host 10.2.2.2, as follows:

• source = host 10.0.0.1
• dest = host 10.2.2.2

Security Appliance A also evaluates traffic from Host 10.2.2.2 to Host 10.0.0.1, as follows:

• source = host 10.2.2.2
• dest = host 10.0.0.1

The first permit statement that matches the packet under evaluation determines the scope of the IPsec SA.

Note If you delete the only element in an access list, the ASA also removes the associated crypto map.

If you modify an access list currently referenced by one or more crypto maps, use the crypto map interface command to reinitialize the run-time SA database. See the crypto map command for more information.

We recommend that for every crypto access list specified for a static crypto map that you define at the local peer, you define a “mirror image” crypto access list at the remote peer. The crypto maps should also support common transforms and refer to the other system as a peer. This ensures correct processing of IPsec by both peers.

Note Every static crypto map must define an access list and an IPsec peer. If either is missing, the crypto map is incomplete and the ASA drops any traffic that it has not already matched to an earlier, complete crypto map. Use the show conf command to ensure that every crypto map is complete. To fix an incomplete crypto map, remove the crypto map, add the missing entries, and reapply it.

We discourage the use of the any keyword to specify source or destination addresses in crypto access lists because they cause problems. We strongly discourage the permit any any command statement because it does the following:

• Protects all outbound traffic, including all protected traffic sent to the peer specified in the corresponding crypto map.
• Requires protection for all inbound traffic.

In this scenario, the ASA silently drops all inbound packets that lack IPsec protection.

Be sure that you define which packets to protect. If you use the any keyword in a permit statement, preface it with a series of deny statements to filter out traffic that would otherwise fall within that permit statement that you do not want to protect.

Note Decrypted through traffic is permitted from the client despite having an access group on the outside interface, which calls a deny ip any any access-list, while no sysopt connection permit-vpn is configured.

Users who want to control access to the protected network via site-to-site or remote access VPN using the no sysopt permit command in conjunction with an access control list (ACL) on the outside interface are not successful.

In this situation, when management-access inside is enabled, the ACL is not applied, and users can still connect using SSH to the security appliance. Traffic to hosts on the inside network are blocked correctly by the ACL, but cannot block decrypted through traffic to the inside interface.

The ssh and http commands are of a higher priority than the ACLs. In other words, to deny SSH, Telnet, or ICMP traffic to the device from the VPN session, use ssh, telnet and icmp commands, which deny the IP local pool should be added.

Changing IPsec SA Lifetimes

You can change the global lifetime values that the ASA uses when negotiating new IPsec SAs. You can override these global lifetime values for a particular crypto map.

IPsec SAs use a derived, shared, secret key. The key is an integral part of the SA; the keys time out together to require the key to refresh. Each SA has two lifetimes: timed and traffic-volume. An SA expires after the respective lifetime and negotiations begin for a new one. The default lifetimes are 28,800 seconds (eight hours) and 4,608,000 kilobytes (10 megabytes per second for one hour).

If you change a global lifetime, the ASA drops the tunnel. It uses the new value in the negotiation of subsequently established SAs.

When a crypto map does not have configured lifetime values and the ASA requests a new SA, it inserts the global lifetime values used in the existing SA into the request sent to the peer. When a peer receives a negotiation request, it uses the smaller of either the lifetime value the peer proposes or the locally configured lifetime value as the lifetime of the new SA.

The peers negotiate a new SA before crossing the lifetime threshold of the existing SA to ensure that a new SA is ready when the existing one expires. The peers negotiate a new SA when about 5 to 15 percent of the lifetime of the existing SA remains.

Creating a Basic IPsec Configuration

You can create basic IPsec configurations with static or dynamic crypto maps.

To create a basic IPsec configuration using a static crypto map, perform the following steps:

Step 1 To create an access list to define the traffic to protect, enter the following command:

For example:

In this example, the permit keyword causes all traffic that matches the specified conditions to be protected by crypto.

Step 2 To configure an IKEv1 transform set that defines how to protect the traffic, enter the following command:

For example:

In this example, myset1 and myset2 and aes_set are the names of the transform sets.

To configure an IKEv2 proposal that also defines how to protect the traffic, enter the crypto ipsec ikev2 ipsec-proposal command to create the proposal and enter the ipsec proposal configuration mode where you can specify multiple encryption and integrity types for the proposal:

For example:

In this example, secure is the name of the proposal. Enter a protocol and encryption types:

Step 3 To create a crypto map, perform the following steps:

a. Assign an access list to a crypto map:

In the following example, mymap is the name of the crypto map set. The map set sequence number 10, which is used to rank multiple entries within one crypto map set. The lower the sequence number, the higher the priority.

In this example, the access list named 101 is assigned to crypto map mymap.

b. Specify the peer to which the IPsec-protected traffic can be forwarded:

For example:

The ASA sets up an SA with the peer assigned the IP address 192.168.1.100. Specify multiple peers by repeating this command.

c. Specify which IKEv1 transform sets or IKEv2 proposals are allowed for this crypto map. List multiple transform sets or proposals in order of priority (highest priority first). You can specify up to 11 transform sets or proposals in a crypto map using either of these two commands:

For example (for IKEv1):

In this example, when traffic matches access list 101, the SA can use either myset1 (first priority) or myset2 (second priority) depending on which transform set matches the transform set of the peer.

d. (Optional) Specify an SA lifetime for the crypto map if you want to override the global lifetime.

For example:

This example shortens the timed lifetime for the crypto map mymap 10 to 2700 seconds
(45 minutes). The traffic volume lifetime is not changed.

e. (Optional) Specify that IPsec require perfect forward secrecy when requesting new SA for this crypto map, or require PFS in requests received from the peer:

For example:

This example requires PFS when negotiating a new SA for the crypto map mymap 10. The ASA uses the 1024-bit Diffie-Hellman prime modulus group in the new SA.

Step 4 Apply a crypto map set to an interface for evaluating IPsec traffic:

For example:

In this example, the ASA evaluates the traffic going through the outside interface against the crypto map mymap to determine whether it needs to be protected.

Using Dynamic Crypto Maps

A dynamic crypto map is a crypto map without all of the parameters configured. It acts as a policy template where the missing parameters are later dynamically learned, as the result of an IPsec negotiation, to match the peer requirements. The ASA applies a dynamic crypto map to let a peer negotiate a tunnel if its IP address is not already identified in a static crypto map. This occurs with the following types of peers:

• Peers with dynamically assigned public IP addresses.

Both LAN-to-LAN and remote access peers can use DHCP to obtain a public IP address. The ASA uses this address only to initiate the tunnel.

• Peers with dynamically assigned private IP addresses.

Peers requesting remote access tunnels typically have private IP addresses assigned by the headend. Generally, LAN-to-LAN tunnels have a predetermined set of private networks that are used to configure static maps and therefore used to establish IPsec SAs.

As an administrator configuring static crypto maps, you might not know the IP addresses that are dynamically assigned (via DHCP or some other method), and you might not know the private IP addresses of other clients, regardless of how they were assigned. VPN clients typically do not have static IP addresses; they require a dynamic crypto map to allow IPsec negotiation to occur. For example, the headend assigns the IP address to a Cisco VPN client during IKE negotiation, which the client then uses to negotiate IPsec SAs.

Note A dynamic crypto map requires only the transform-set parameter.

Dynamic crypto maps can ease IPsec configuration, and we recommend them for use in networks where the peers are not always predetermined. Use dynamic crypto maps for Cisco VPN clients (such as mobile users) and routers that obtain dynamically assigned IP addresses.

Tip Use care when using the any keyword in permit entries in dynamic crypto maps. If the traffic covered by such a permit entry could include multicast or broadcast traffic, insert deny entries for the appropriate address range into the access list. Remember to insert deny entries for network and subnet broadcast traffic, and for any other traffic that IPsec should not protect.

Dynamic crypto maps work only to negotiate SAs with remote peers that initiate the connection. The ASA cannot use dynamic crypto maps to initiate connections to a remote peer. With a dynamic crypto map, if outbound traffic matches a permit entry in an access list and the corresponding SA does not yet exist, the ASA drops the traffic.

A crypto map set may include a dynamic crypto map. Dynamic crypto map sets should be the lowest priority crypto maps in the crypto map set (that is, they should have the highest sequence numbers) so that the ASA evaluates other crypto maps first. It examines the dynamic crypto map set only when the other (static) map entries do not match.

Similar to static crypto map sets, a dynamic crypto map set consists of all of the dynamic crypto maps with the same dynamic-map-name. The dynamic-seq-num differentiates the dynamic crypto maps in a set. If you configure a dynamic crypto map, insert a permit ACL to identify the data flow of the IPsec peer for the crypto access list. Otherwise the ASA accepts any data flow identity the peer proposes.

Caution Do not assign module default routes for traffic to be tunneled to a ASA interface configured with a dynamic crypto map set. To identify the traffic that should be tunneled, add the ACLs to the dynamic crypto map. Use care to identify the proper address pools when configuring the ACLs associated with remote access tunnels. Use Reverse Route Injection to install routes only after the tunnel is up.

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The procedure for using a dynamic crypto map entry is the same as the basic configuration described in “Creating a Basic IPsec Configuration,” except that instead of creating a static crypto map, you create a dynamic crypto map entry. You can also combine static and dynamic map entries within a single crypto map set.

Create a crypto dynamic map entry as follows:

Step 1 (Optional) Assign an access list to a dynamic crypto map:

This determines which traffic should be protected and not protected.

For example:

In this example, access list 101 is assigned to dynamic crypto map dyn1. The map sequence number is 10.

Step 2 Specify which IKEv1 transform sets or IKEv2 proposals are allowed for this dynamic crypto map. List multiple transform sets or proposals in order of priority (highest priority first) using the command for IKEv1 transform sets or IKEv2 proposals:

For example (for IKEv1):

In this example, when traffic matches access list 101, the SA can use either myset1 (first priority) or myset2 (second priority), depending on which transform set matches the transform sets of the peer.

Step 3 (Optional) Specify the SA lifetime for the crypto dynamic map entry if you want to override the global lifetime value:

For example:

This example shortens the timed lifetime for dynamic crypto map dyn1 10 to 2700 seconds
(45 minutes). The time volume lifetime is not changed.

Step 4 (Optional) Specify that IPsec ask for PFS when requesting new SAs for this dynamic crypto map, or should demand PFS in requests received from the peer:

For example:

Step 5 Add the dynamic crypto map set into a static crypto map set.

Be sure to set the crypto maps referencing dynamic maps to be the lowest priority entries (highest sequence numbers) in a crypto map set.

For example:

Providing Site-to-Site Redundancy

You can define multiple IKEv1 peers by using crypto maps to provide redundancy. This configuration is useful for site-to-site VPNs. This feature is not supported with IKEv2.

If one peer fails, the ASA establishes a tunnel to the next peer associated with the crypto map. It sends data to the peer that it has successfully negotiated with, and that peer becomes the active peer. The active peer is the peer that the ASA keeps trying first for follow-on negotiations until a negotiation fails. At that point the ASA goes on to the next peer. The ASA cycles back to the first peer when all peers associated with the crypto map have failed.

Viewing an IPsec Configuration

Table 64-6 lists commands that you can enter to view information about your IPsec configuration.