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IPSec provides security for transmission of sensitive information over unprotected networks such as the Internet. IPSec acts at the network layer, protecting and authenticating IP packets between participating IPSec devices (peers), such as PIX Firewall units.
With IPSec, data can be transmitted across a public network without fear of observation, modification, or spoofing. This enables applications such as VPNs, which are categorized by intranets, extranets, and remote dial access. Each VPN type has different security service needs.With a VPN, customers, business partners, and remote users, such as telecommuters, can access enterprise computing resources securely. VPNs essentially extend a network's capability by accommodating the demands of a networked economy for diverse secured connectivity.
IPSec provides the following network security services. These services are optional. In general, a local security policy will dictate the use of one or more of these services:
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Note The term data authentication is generally used to mean data integrity and data origin authentication. Within this chapter, it also includes anti-replay services, unless otherwise specified. |
In simple terms, IPSec provides secure tunnels between two peers, such as two PIX Firewall units. You define which packets are considered sensitive and should be sent through these secure tunnels, and you define the parameters that should be used to protect these sensitive packets, by specifying the characteristics of these tunnels. Then, when the IPSec peer sees such a sensitive packet, it sets up the appropriate secure tunnel and sends the packet through the tunnel to the remote peer.
More accurately, these tunnels are sets of security associations that are established between two remote IPSec peers. The security associations define which protocols and algorithms should be applied to sensitive packets, and also specify the keying material to be used by the two peers. Security associations are uni-directional and are established per security protocol (AH or ESP).
With IPSec, you define what traffic should be protected between two remote IPSec peers by configuring access lists and applying these access lists to interfaces by way of crypto map sets. Therefore, traffic may be selected on the basis of source and destination address. (Access lists used for IPSec are used only to determine which traffic should be protected by IPSec, not which traffic should be blocked or permitted through the interface. Separate access lists define blocking and permitting at the interface or inbound and outbound from the PIX Firewall.)
A crypto map set can contain multiple entries, each with a different access list. The crypto map entries are searched in order—the PIX Firewall attempts to match the packet to the access list specified in that entry.
When a packet matches a permit entry in a particular access list, and the corresponding crypto map entry is tagged as ipsec-isakmp, IPSec is triggered.
If no security association exists that IPSec can use to protect this traffic to the peer, IPSec uses IKE to negotiate with the peer to set up the necessary IPSec security associations on behalf of the data flow. The negotiation uses information specified in the crypto map entry as well as the data flow information from the specific access list entry. (The behavior is different for dynamic crypto map entries. Refer to "Dynamic Crypto Maps.")
If the crypto map entry is tagged as ipsec-manual, IPSec is triggered. If no security association exists that IPSec can use to protect this traffic to the peer, the traffic is dropped. In this case, the security associations are installed via the configuration, without the intervention of IKE. If the security associations did not exist, IPSec did not have all the necessary pieces configured.
Once established, the set of security associations (outbound, to the remote peer) is then applied to the triggering packet as well as to subsequent applicable packets as those packets exit the PIX Firewall. "Applicable" packets are packets that match the same access list criteria that the original packet matched. For example, all applicable packets could be encrypted before being forwarded to the remote peer. The corresponding inbound security associations are used when processing the incoming traffic from that peer.
If IKE is used to establish the security associations, the security associations will have lifetimes so that they will periodically expire and require renegotiation. (This provides an additional level of security.)
Multiple IPSec tunnels can exist between two peers to secure different data streams, with each tunnel using a separate set of security associations. For example, some data streams might be just authenticated while other data streams must be both encrypted and authenticated.
Access lists associated with IPSec crypto map entries also represent which traffic the PIX Firewall requires to be protected by IPSec. Inbound traffic is processed against the crypto map entries—if an unprotected packet matches a permit entry in a particular access list associated with an IPSec crypto map entry, that packet is dropped because it was not sent as an IPSec-protected packet.
Crypto map entries also include transform sets. A transform set is an acceptable combination of security protocols, algorithms, and other settings to apply to IPSec-protected traffic. During the IPSec security association negotiation, the peers agree to use a particular transform set when protecting a particular data flow.
This chapter includes the following sections, which provide more IPSec background information you will need to know prior to configuring IPSec on a PIX Firewall The steps for configuring IPSec are covered in "Configuring IPSec."
By default, IPSec and all packets that traverse the PIX Firewall are subjected to blocking as specified by inbound conduit, outbound list, or interface access-list. To enable IPSec packets to traverse the PIX Firewall, ensure that you have statements in conduits, outbound lists, or interface access-lists that permit the packets. Optionally, the sysopt connection permit-ipsec command can be configured to enable IPSec packets to bypass the conduit, outbound list, and interface access-list blocking.
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Note The sysopt connection permit-ipsec command enables packets that have been processed by IPSec to bypass the conduit, outbound list, and interface access-list checks. |
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Note IPSec packets that are destined to an IPSec tunnel are selected by the crypto map access-list bound to the outgoing interface. IPSec packets that arrive from an IPSec tunnel are authenticated/deciphered by IPSec and subjected to the proxy identity match of the tunnel. |
You can change the global lifetime values that are used when negotiating new IPSec security associations. (These global lifetime values can be overridden for a particular crypto map entry.)
These lifetimes only apply to security associations established via IKE. Manually established security associations do not expire.
There are two lifetimes: a "timed" lifetime and a "traffic-volume" lifetime. A security association expires after the respective lifetime is reached and negotiations will be initiated 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 new lifetime value will not be applied to currently existing security associations, but will be used in the negotiation of subsequently established security associations. If you wish to use the new values immediately, you can clear all or part of the security association database. See the clear [crypto] ipsec sa command for more information within the crypto ipsec command page of "Command Reference."
IPSec security associations use one or more shared secret keys. These keys and their security associations time out together.
Assuming that the particular crypto map entry does not have lifetime values configured, when the PIX Firewall requests new security associations it will specify its global lifetime values in the request to the peer; it will use this value as the lifetime of the new security associations. When the PIX Firewall receives a negotiation request from the peer, it will use the smaller of either the lifetime value proposed by the peer or the locally configured lifetime value as the lifetime of the new security associations.
The security association (and corresponding keys) will expire according to whichever occurs sooner, either after the seconds time out or after the kilobytes amount of traffic is passed.
A new security association is negotiated before the lifetime threshold of the existing security association is reached to ensure that a new security association is ready for use when the old one expires. The new security association is negotiated either 30 seconds before the seconds lifetime expires or when the volume of traffic through the tunnel reaches 256 kilobytes less than the kilobytes lifetime (whichever occurs first).
If no traffic has passed through the tunnel during the entire life of the security association, a new security association is not negotiated when the lifetime expires. Instead, a new security association will be negotiated only when IPSec sees another packet that should be protected.
Crypto access lists are used to define which IP traffic will be protected by crypto and which traffic will not be protected by crypto. For example, access lists can be created to protect all IP traffic between Subnet A and Subnet Y or between Host A and Host B. (These access lists are similar to access lists used with the access-group command. With the access-group command, the access-list determines which traffic to forward or block at an interface.)
The access lists themselves are not specific to IPSec. It is the crypto map entry referencing the specific access list that defines whether IPSec processing is applied to the traffic matching a permit in the access list.
Crypto access lists associated with IPSec crypto map entries have four primary functions:
If you want certain traffic to receive one combination of IPSec protection (for example, authentication only) and other traffic to receive a different combination of IPSec protection (for example, both authentication and encryption), you need to create two different crypto access lists to define the two different types of traffic. These different access lists are then used in different crypto map entries which specify different IPSec policies.
This section includes the following topics:
Using the permit keyword causes all IP traffic that matches the specified conditions to be protected by crypto, using the policy described by the corresponding crypto map entry. Using the deny keyword prevents traffic from being protected by crypto IPSec in the context of that particular crypto map entry. (In other words, it does not allow the policy as specified in this crypto map entry to be applied to this traffic.) If this traffic is denied in all the crypto map entries for that interface, the traffic is not protected by crypto IPSec.
The crypto access list you define will be applied to an interface after you define the corresponding crypto map entry and apply the crypto map set to the interface. Different access lists must be used in different entries of the same crypto map set. However, both inbound and outbound traffic will be evaluated against the same "outbound" IPSec access list.
Therefore, the access list's criteria are applied in the forward direction to traffic exiting your PIX Firewall, and the reverse direction to traffic entering your PIX Firewall. In Figure 2-1, IPSec protection is applied to traffic between Host 10.0.0.1 and Host 10.2.2.2 as the data exits PIX Firewall A's outside interface en route to Host 10.2.2.2. For traffic from Host 10.0.0.1 to Host 10.2.2.2, the access list entry on PIX Firewall A is evaluated as follows:
source = host 10.0.0.1
dest = host 10.2.2.2
For traffic from Host 10.2.2.2 to Host 10.0.0.1, that same access list entry on PIX Firewall A is evaluated as follows:
source = host 10.2.2.2
dest = host 10.0.0.1
If you configure multiple statements for a given crypto access list that is used for IPSec, in general the first permit statement that is matched will be the statement used to determine the scope of the IPSec security association. That is, the IPSec security association will be set up to protect traffic that meets the criteria of the matched statement only. Later, if traffic matches a different permit statement of the crypto access list, a new, separate IPSec security association will be negotiated to protect traffic matching the newly matched access list statement.
Any unprotected inbound traffic that matches a permit entry in the crypto access list for a crypto map entry flagged as IPSec will be dropped because this traffic was expected to be protected by IPSec.
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Note If you clear or delete the last element from an access list, the crypto map references to the destroyed access list are also removed. |
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Note If you modify an access list that is currently referenced by one or more crypto map entries, the run-time security association database will need to be re initialized using the crypto map interface command. See the crypto map command page for information on the crypto map interface command. |
Cisco recommends that for every crypto access list specified for a static crypto map entry that you define at the local peer, you define a "mirror image" crypto access list at the remote peer. This ensures that traffic that has IPSec protection applied locally can be processed correctly at the remote peer. (The crypto map entries themselves must also support common transforms and must refer to the other system as a peer.)
When you create crypto access lists, using the any keyword could cause problems. Cisco discourages the use of the any keyword to specify source or destination addresses.
The permit any any statement is strongly discouraged, as this will cause all outbound traffic to be protected (and all protected traffic sent to the peer specified in the corresponding crypto map entry) and will require protection for all inbound traffic. Then, all inbound packets that lack IPSec protection will be silently dropped.
You need to be sure you define which packets to protect. If you must use the any keyword in a permit statement, you must preface that statement with a series of deny statements to filter out any traffic (that would otherwise fall within that permit statement) that you do not want to be protected.
A transform set represents a certain combination of security protocols and algorithms. During the IPSec security association negotiation, the peers agree to use a particular transform set for protecting a particular data flow.
You can specify multiple transform sets, and then specify one or more of these transform sets in a crypto map entry. The transform set defined in the crypto map entry will be used in the IPSec security association negotiation to protect the data flows specified by that crypto map entry's access list.
During IPSec security association negotiations with IKE, the peers search for a transform set that is the same at both peers. When such a transform set is found, it is selected and will be applied to the protected traffic as part of both peers' IPSec security associations.
With manually established security associations, there is no negotiation with the peer, so both sides must specify the same transform set.
If you change a transform set definition, the change is only applied to crypto map entries that reference the transform set. The change will not be applied to existing security associations, but will be used in subsequent negotiations to establish new security associations.
If you want the new settings to take effect sooner, you can clear all or part of the security association database by using the clear [crypto] ipsec sa command. See the clear [crypto] ipsec sa command for more information within the crypto ipsec command page of "Command Reference."
Choosing IPSec transforms combination can be complex. The following tips may help you select transforms that are appropriate for your situation:
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Note Some transforms may not be supported by the peer. |
Suggested transform combinations:
To create crypto map entries, follow the guidelines provided in the following sections:
Crypto maps specify IPSec policy. Crypto map entries created for IPSec pull together the various parts used to set up IPSec security associations, including the following:
Crypto map entries with the same crypto map name (but different map sequence numbers) are grouped into a crypto map set. Later, you will apply these crypto map sets to interfaces; then, all IP traffic passing through the interface is evaluated against the applied crypto map set. If a crypto map entry sees outbound IP traffic that should be protected and the crypto map specifies the use of IKE, a security association is negotiated with the peer according to the parameters included in the crypto map entry; otherwise, if the crypto map entry specifies the use of manual security associations, a security association should have already been established via configuration. (If a dynamic crypto map entry sees outbound traffic that should be protected and no security association exists, the packet is dropped.)
The policy described in the crypto map entries is used during the negotiation of security associations. If the local PIX Firewall initiates the negotiation, it will use the policy specified in the static crypto map entries to create the offer to be sent to the specified peer. If the peer initiates the negotiation, the PIX Firewall will check the policy from the static crypto map entries, as well as any referenced dynamic crypto map entries to decide whether to accept or reject the peer's request (offer).
For IPSec to succeed between two peers, both peers' crypto map entries must contain compatible configuration statements.
When two peers try to establish a security association, they must each have at least one crypto map entry that is compatible with one of the other peer's crypto map entries. For two crypto map entries to be compatible, they must at a minimum meet the following criteria:
You can define multiple peers by using crypto maps to allow for redundancy. If one peer fails, there will still be a protected path. The peer that packets are actually sent to is determined by the last peer that the PIX Firewall heard from (received either traffic or a negotiation request from) for a given data flow. If the attempt fails with the first peer, IKE tries the next peer on the crypto map list.
If you are not sure how to configure each crypto map parameter to guarantee compatibility with other peers, you might consider configuring dynamic crypto maps as described in the section "Dynamic Crypto Maps." Dynamic crypto maps are useful when the establishment of the IPSec tunnels is initiated by the peer. They are not useful if the establishment of the IPSec tunnels is locally initiated, because the dynamic crypto maps are policy templates, not complete statements of policy. (Although the access lists in any referenced dynamic crypto map entry are used for crypto packet filtering.)
You can apply only one crypto map set to a single interface. The crypto map set can include a combination of IPSec/IKE and IPSec/manual entries.
If you create more than one crypto map entry for a given interface, use the seq-num of each map entry to rank the map entries: the lower the seq-num, the higher the priority. At the interface that has the crypto map set, traffic is evaluated against higher priority map entries first.
You must create multiple crypto map entries for a given PIX Firewall interface, if any of the following conditions exist:
The use of manual IPSec security associations is a result of a prior arrangement between the users of the PIX Firewall and its peer. There is no negotiation of security associations, so the configuration information in both systems must be the same for traffic to be processed successfully by IPSec.
The PIX Firewall can simultaneously support manual and IKE-established security associations, even within a single crypto map set.
When IKE is used to establish IPSec security associations, the peers can negotiate the settings they will use for the new security associations. This means that you can specify lists (such as lists of acceptable transforms) within the crypto map entry.
Dynamic crypto maps (this requires IKE) can ease IPSec configuration and are recommended for use with networks where the peers are not always predetermined. An example of this is mobile users (VPN clients), who obtain dynamically assigned IP addresses. First, the mobile clients need to authenticate themselves to the local PIX Firewall IKE by something other than an IP address, such as a fully qualified domain name. Once authenticated, the security association request can be processed against a dynamic crypto map that is set up to accept requests (matching the specified local policy) from previously unknown peers.
Dynamic crypto maps are only available for use by IKE.
A dynamic crypto map entry is essentially a crypto map entry without all the parameters configured. It acts as a policy template where the missing parameters are later dynamically configured (as the result of an IPSec negotiation) to match a peer's requirements. This allows peers to exchange IPSec traffic with the PIX Firewall even if the PIX Firewall does not have a crypto map entry specifically configured to meet all the peer's requirements.
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Note Only the transform-set field is required to be configured within each dynamic crypto map entry. |
Dynamic crypto maps are not used by the PIX Firewall to initiate new IPSec security associations with peers. Dynamic crypto maps are used when a peer tries to initiate an IPSec security association with the PIX Firewall. Dynamic crypto maps are also used in evaluating traffic.
A dynamic crypto map set is included by reference as part of a crypto map set. Any crypto map entries that reference dynamic crypto map sets should be the lowest priority crypto map entries in the crypto map set (that is, have the highest sequence numbers) so that the other crypto map entries are evaluated first; that way, the dynamic crypto map set is examined only when the other (static) map entries are not successfully matched.
If the PIX Firewall accepts the peer's request at the point that it installs the new IPSec security associations, it also installs a temporary crypto map entry. This entry is filled in with the results of the negotiation. At this point, the PIX Firewall performs normal processing, using this temporary crypto map entry as a normal entry, even requesting new security associations if the current ones are expiring (based upon the policy specified in the temporary crypto map entry). Once the flow expires (that is, all the corresponding security associations expire), the temporary crypto map entry is then removed.
For both static and dynamic crypto maps, if unprotected inbound traffic matches a permit statement in an access list, and the corresponding crypto map entry is tagged as "IPSec," the traffic is dropped because it is not IPSec protected. (This is because the security policy as specified by the crypto map entry states that this traffic must be IPSec protected.)
For static crypto map entries, if outbound traffic matches a permit statement in an access list and the corresponding security association is not yet established, the PIX Firewall will initiate new security associations with the peer. In the case of dynamic crypto map entries, if no security association existed, the traffic would simply be dropped (because dynamic crypto maps are not used for initiating new security associations).
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Note Use care when using the any keyword in permit entries in dynamic crypto maps. If it is possible for the traffic covered by such a permit entry to include multicast or broadcast traffic, the access list should include deny entries for the appropriate address range. Access lists should also include deny entries for network and subnet broadcast traffic, and for any other traffic that should not be IPSec protected. |
Dynamic Crypto Map Set
If this is configured, the data flow identity proposed by the IPSec peer must fall within a permit statement for this crypto access list.
If this is not configured, the PIX Firewall will accept any data flow identity proposed by the peer.
Care must be taken if the any keyword is used in the access list, because the access list is used for packet filtering, as well as for negotiation.
Dynamic crypto map entries specify crypto access lists that limit traffic for which IPSec security associations can be established. A dynamic crypto map entry that does not specify an access list will be ignored during traffic filtering. If there is only one dynamic crypto map entry in the crypto map set, it must specify acceptable transform sets.
Add the Dynamic Crypto Map Set into a Regular (Static) Crypto Map Set
You can add one or more dynamic crypto map sets into a crypto map set via crypto map entries that reference the dynamic crypto map sets. You should set the crypto map entries referencing dynamic maps to be the lowest priority entries in a crypto map set (that is, use the highest sequence numbers).
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Note Binding a crypto map to an interface will also initialize the run-time data structures, such as the security association database and the security policy database. If the crypto map is modified in any significant manner, reapplying the crypto map to the interface will resynchronize the various run-time data structures with the crypto map configuration. |
Certain configuration changes will only take effect when negotiating subsequent security associations. If you want the new settings to take immediate effect, you must clear the existing security associations so that they will be re-established with the changed configuration. For manually established security associations, you must clear and reinitialize the security associations or the changes will never take effect. If the PIX Firewall is actively processing IPSec traffic, it is desirable to clear only the portion of the security association database that would be affected by the configuration changes (that is, clear only the security associations established by a given crypto map set). Clearing the full security association database should be reserved for large-scale changes, or when the PIX Firewall is processing a small number of other IPSec traffic.
Table 2-1 lists commands you can use to clear and reinitialize IPSec security associations.
| Command | Purpose | ||
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crypto map map-name interface interface-name | Reinitialize the IPSec run-time security association database and security policy database. | ||
clear [crypto] ipsec sa or clear [crypto] ipsec sa peer ip-address | peer-name or clear [crypto] ipsec sa map map-name or clear [crypto] ipsec sa entry destination-address protocol spi | Clear IPSec security associations.
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Table 2-2 lists commands you can use to view information about your IPSec configuration.
| Command | Purpose |
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show crypto ipsec transform-set | View your transform set configuration. |
show crypto map [interface interface-name | tag map-name] | View your crypto map configuration. |
show crypto ipsec sa [map map-name | address | identity] [detail] | View information about IPSec security associations. |
show crypto dynamic-map [tag map-name] | View information about dynamic crypto maps. |
show crypto ipsec security-association lifetime | View global security association lifetime values. |
Posted: Thu Aug 31 19:50:44 PDT 2000
Copyright 1989-2000©Cisco Systems Inc.