Real-Time Resolution for IPsec Tunnel Peer

First Published: November 3, 2003
Last Updated: March 28, 2011

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.

Finding Feature Information

Your 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 for Real-Time Resolution for IPsec Tunnel Peer" section.

Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.

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

•Feature Information for Real-Time Resolution for IPsec Tunnel Peer

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 works only if they are used as an initiator. The first packet that is to be encrypted triggers a DNS lookup; after the DNS lookup is complete, subsequent packets triggers 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:

•Real-Time Resolution Through Secure DNS

Real-Time Resolution Through Secure DNS

When specifying the host name of a remote IPsec peer through 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 task:

•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 through 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] [default] | ip-address [default] }

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.

•The host-name argument specifies the IPsec peer by its hostname. This is the peer's hostname concatenated with its domain name (for example, myhost.example.com).

•The optional dynamic keyword allows the hostname of the IPsec peer to be resolved through a domain name server (DNS) lookup immediately before the router establishes the IPsec tunnel.

•The optional default keyword designates that the first peer is the default peer if there are multiple IPsec peers.

•The ip-address argument specifies the IPsec peer by its IP address.

Repeat this step if there are 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 flows. 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 through 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

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


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


Technical Assistance

Description
Link

The Cisco Support and Documentation website provides online resources to download documentation, software, and tools. Use these resources to install and configure the software and to troubleshoot and resolve technical issues with Cisco products and technologies. Access to most tools on the Cisco Support and Documentation website requires a Cisco.com user ID and password.

http://www.cisco.com/cisco/web/support/index.html


Feature Information for Real-Time Resolution for IPsec Tunnel Peer

Table 1 lists the release history for this feature.

Use Cisco Feature Navigator to find information about platform support and software image support. Cisco Feature Navigator enables you to determine which software images support a specific software release, feature set, or platform. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.

Note Table 1 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.

Table 1 Feature Information for Real-Time Resolution for IPsec Tunnel Peer

Feature Name
Releases
Feature Information

Real-Time Resolution for IPsec Tunnel Peer

11.2
12.3(4)T
12.2(18)SXD
12.3(14)T
12.2(33)SRA

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.

This feature was introduced in Cisco IOS Release 11.2.

In Cisco IOS Release 12.3(4)T, the dynamic keyword was added to the set peer (IPsec) command.

In Cisco IOS Release 12.3(14)T, the dynamic keyword was added to the set peer (IPsec) command.

The following command was introduced or modified: set peer (IPsec).


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