Table Of Contents

Real-Time Resolution for IPsec Tunnel Peer

Contents

Restrictions for Real-Time Resolution for IPsec Tunnel Peer

Information About Real-Time Resolution for IPsec Tunnel Peer

Benefits of Real-Time Resolution Via Secure DNS

How to Configure Real-Time Resolution

Configuring Real-Time Resolution for IPsec Peers

Prerequisites

Troubleshooting Tips

What to Do Next

Configuration Examples for Real-Time Resolution

Configuring Real-Time Resolution for an IPsec Peer: Example

Additional References

Related Documents

Standards

MIBs

RFCs

Technical Assistance

Command Reference

Real-Time Resolution for IPsec Tunnel Peer

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After a user specifies a host name (instead of an IP address) for remote IP Security (IPsec) peer, the Real-Time Resolution for IPsec Tunnel Peer feature allows the host name to be domain name server (DNS) resolved before the router establishes the IPsec tunnel. Thus, the router can immediately discover whether the IP address of the peer has changed.

Release	Modification
12.3(4)T	This feature was introduced.
Cisco IOS XE Release 2.1	This feature was introduced on Cisco ASR 1000 Series Routers.

Feature History for Real-Time Resolution for IPsec Tunnel Peer

Finding Support Information for Platforms and Cisco IOS Software Images

Use Cisco Feature Navigator to find information about platform support and Cisco IOS software image support. Access Cisco Feature Navigator at http://tools.cisco.com/ITDIT/CFN/jsp/index.jsp. 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.

Contents

•Restrictions for Real-Time Resolution for IPsec Tunnel Peer

•Information About Real-Time Resolution for IPsec Tunnel Peer

•How to Configure Real-Time Resolution

•Configuration Examples for Real-Time Resolution

•Additional References

•Command Reference

Restrictions for Real-Time Resolution for IPsec Tunnel Peer

Secure DNS Requirement

It is recommended that you use this feature only with secure DNS and when the DNS responses can be authenticated. Otherwise, an attacker can spoof or forge DNS responses and have access to Internet Key Exchange (IKE) authentication data, such as a certificate. If an attacker has a certificate that is trusted by the initiating host, the attacker can successfully establish Phase 1 IKE security association (SA), or the attacker can try to guess the preshared key that is shared between the initiator and the actual responder.

DNS Initiator

DNS names resolution for remote IPsec peers will work only if they are used as an initiator. The first packet that is to be encrypted will trigger a DNS lookup; after the DNS lookup is complete, subsequent packets will trigger IKE.

Information About Real-Time Resolution for IPsec Tunnel Peer

To configure real-time resolution for your IPsec peer, you should understand the following concept:

•Benefits of Real-Time Resolution Via Secure DNS

Benefits of Real-Time Resolution Via Secure DNS

When specifying the host name of a remote IPsec peer via the set peer command, you can also issue the dynamic keyword, which defers DNS resolution of the host name until right before the IPsec tunnel has been established. Deferring resolution enables the Cisco IOS software to detect whether the IP address of the remote IPsec peer has changed. Thus, the software can contact the peer at the new IP address.

If the dynamic keyword is not issued, the host name is resolved immediately after it is specified. So, the Cisco IOS software cannot detect an IP address change and, therefore, attempts to connect to the IP address that it previously resolved.

DNS resolution assures users that their established IPsec tunnel is secure and authenticated.

How to Configure Real-Time Resolution

This section contains the following procedure:

•Configuring Real-Time Resolution for IPsec Peers

Configuring Real-Time Resolution for IPsec Peers

Use this task to configure a router to perform real-time DNS resolution with a remote IPsec peer; that is, the host name of peer is resolved via a DNS lookup right before the router establishes a connection (an IPsec tunnel) with the peer.

Prerequisites

Before creating a crypto map, you should perform the following tasks:

•Define Internet Security Association Key Management Protocol (ISAKMP) policies.

•Define IPsec transform sets.

SUMMARY STEPS

1. enable

2. configure terminal

3. crypto map map-name seq-num ipsec-isakmp

4. match address access-list-id

5. set peer {host-name [dynamic] | ip-address}

6. set transform-set transform-set-name1 [transform-set-name2...transform-set-name6]

DETAILED STEPS

	Command or Action	Purpose
Step 1	enable Example: Router> enable	Enables privileged EXEC mode. •Enter your password if prompted.
Step 2	configure terminal Example: Router# configure terminal	Enters global configuration mode.
Step 3	crypto map map-name seq-num ipsec-isakmp Example: Router(config)# crypto map secure_b 10 ipsec-isakmp	Specifies the crypto map entry to create (or modify) and enters crypto map configuration mode.
Step 4	match address access-list-id Example: Router(config-crypto-m)# match address 140	Names an extended access list. This access list determines which traffic should be protected by IPsec and which traffic should not be protected by IPsec in the context of this crypto map entry.
Step 5	set peer {host-name [dynamic] | ip-address} Example: Router(config-crypto-m)# set peer b.cisco.com dynamic	Specifies a remote IPsec peer. This is the peer to which IPsec-protected traffic can be forwarded. •dynamic—Allows the host name to be resolved via a DNS lookup just before the router establishes the IPsec tunnel with the remote peer. If this keyword is not specified, the host name will be resolved immediately after the host name is specified. Repeat for multiple remote peers.
Step 6	set transform-set transform-set-name1 [transform-set-name2...transform-set-name6] Example: Router(config-crypto-m)# set transform-set myset	Specifies which transform sets are allowed for this crypto map entry. List multiple transform sets in order of priority (highest priority first).

Troubleshooting Tips

To display crypto map configuration information, use the show crypto map command.

What to Do Next

You need to apply a crypto map set to each interface through which IPsec traffic will flow. Applying the crypto map set to an interface instructs the router to evaluate all the interface's traffic against the crypto map set and to use the specified policy during connection or security association (SA) negotiation on behalf of traffic to be protected by crypto.

Configuration Examples for Real-Time Resolution

This section provides the following configuration example:

•Configuring Real-Time Resolution for an IPsec Peer: Example

Configuring Real-Time Resolution for an IPsec Peer: Example

Figure 1 and the following example illustrate how to create a crypto map that configures the host name of a remote IPsec peer to DNS resolved via a DNS lookup right before the Cisco IOS software attempts to establish a connection with that peer.

Figure 1 Real-Time Resolution Sample Topology

! Configure the initiating router.

hostname a.cisco.com

ip domain name cisco.com

ip name server 10.0.0.1 

! 

crypto map secure_b 10 ipsec-isakmp

  match address 140

  set peer b.cisco.com dynamic 

  set transform-set xset

interface serial1

  ip address 30.0.0.1

  crypto map secure_b

access-list 140 permit ...

!

! Configure the responding router (the remote IPSec peer).

hostname b.cisco.com

!

crypto map secure_a 10 ipsec-isakmp

  match address 150

  set peer 30.0.0.1

  set transform-set

interface serial0/1

  ip address 40.0.0.1

  crypto map secure_a

access-list 150 ...

! DNS server configuration

b.cisco.com    40.0.0.1       # the address of serial0/1 of b.cisco.com

Additional References

The following sections provide references related to Real-Time Resolution for IPsec Tunnel Peer.

Related Documents

Related Topic	Document Title
Crypto maps	Security for VPNs with IPsec
ISAKMP policies	Configuring Internet Key Exchange for IPsec VPNs
IPsec and IKE configuration commands	Cisco IOS Security Command Reference

Standards

Standards	Title
None	—

MIBs

MIBs	MIBs Link
None	To locate and download MIBs for selected platforms, Cisco IOS software releases, and feature sets, use Cisco MIB Locator found at the following URL: http://www.cisco.com/go/mibs

RFCs

RFCs	Title
None	—

Technical Assistance

Description

Link

The Cisco Support website provides extensive online resources, including documentation and tools for troubleshooting and resolving technical issues with Cisco products and technologies. To receive security and technical information about your products, you can subscribe to various services, such as the Product Alert Tool (accessed from Field Notices), the Cisco Technical Services Newsletter, and Really Simple Syndication (RSS) Feeds. Access to most tools on the Cisco Support website requires a Cisco.com user ID and password.

http://www.cisco.com/techsupport

Command Reference

The following commands are introduced or modified in the feature or features

•set peer (IPsec)

For information about these commands, see the Cisco IOS Security Command Reference at

http://www.cisco.com/en/US/docs/ios/security/command/reference/sec_book.html.

For information about all Cisco IOS commands, see the Command Lookup Tool at

http://tools.cisco.com/Support/CLILookup or the Master Command List.

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