ASA 8.3 and Later: NTP with and without an IPsec Tunnel Configuration Example

Introduction

This document provides a sample configuration for synchronizing the Adaptive Security Appliance (ASA) clock with a network time server using Network Time Protocol (NTP). ASA1 communicates directly with the network time server. ASA2 passes NTP traffic through an IPsec tunnel to ASA1, which in turn forwards the packets to the network time server.

Refer to ASA/PIX: NTP with and without an IPsec Tunnel Configuration Example for an identical configuration on Cisco ASA with versions 8.2 and earlier.

Note: A router can also be used as an NTP server for synchronizing the ASA Security Appliance clock.

Prerequisites

Requirements

There are no specific requirements for this document.

Components Used

The information in this document is based on these software and hardware versions:

• Cisco ASA with version 8.3 and later

• Cisco Adaptive Security Device Manager (ASDM) version 6.x and later

Note: Refer to Allowing HTTPS Access for ASDM in order to allow the ASA to be configured by the ASDM.

The information in this document was created from the devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If your network is live, make sure that you understand the potential impact of any command.

Conventions

Refer to the Cisco Technical Tips Conventions for more information on document conventions.

Configuration

Network Diagram

This document uses this network setup:

Note: The IP addressing schemes used in this configuration are not legally routable on the Internet. They are RFC 1918 addresses, which have been used in a lab environment.

• VPN Tunnel ASDM Configuration

• NTP ASDM Configuration

• ASA1 CLI Configuration

• ASA2 CLI Configuration

VPN Tunnel ASDM Configuration

Complete these steps in order to create the VPN tunnel:

1. Open your browser and type https://<Inside_IP_Address_of_ASA> in order to access the ASDM on the ASA.

   Be sure to authorize any warnings your browser gives you related to SSL certificate authenticity. The default username and password are both blank.

   The ASA presents this window in order to allow the download of the ASDM application.

   

   This example loads the application onto the local computer and does not run in a Java applet.

2. Click Download ASDM Launcher and Start ASDM in order to download the installer for the ASDM application.

3. Once the ASDM Launcher downloads, complete the steps directed by the prompts in order to install the software and run the Cisco ASDM Launcher.

4. Enter the IP address for the interface you configured with the http - command, and a username and password if you specified one.

   This example uses the default blank username and password:

   

5. Run the VPN Wizard once the ASDM application connects to the ASA.

   

6. Choose Site-to-Site for the IPsec VPN Tunnel Type, and click Next.

   

7. Specify the outside IP address of the remote peer. Enter the authentication information to use, which is the pre-shared key in this example:

   

8. Specify the attributes to use for IKE, also known as Phase 1. These attributes must be the same on both sides of the tunnel.

   

9. Specify the attributes to use for IPsec, also known as Phase 2. These attributes must match on both sides.

   

10. Specify the hosts whose traffic should be allowed to pass through the VPN tunnel. In this step, you have to provide the Local Networks and Remote Networks for the VPN Tunnel. Click the button next to Local Networks (as shown here) in order to choose the local network address from the drop-down menu:

    

11. Choose the Local Network address, and click OK.

    

12. Click the button next to Remote Networks in order to choose the remote network address from the drop-down menu.

    

13. Choose the Remote Network address, and click OK.

    Note: If you do not have the Remote Network in the list, then the network has to be added to the list. Click Add in order to do so.

    

14. Check the Exempt ASA side host/network from address translation checkbox in order to prevent the tunnel traffic from undergoing Network Address Translation. Click Next.

    

15. The attributes defined by the VPN Wizard are displayed in this summary. Double-check the configuration and click Finish when you are satisfied that the settings are correct.

    

NTP ASDM Configuration

Complete these steps in order to configure NTP on the Cisco Security Appliance:

1. Choose Configuration in the ASDM home page.

   

2. Choose Device Setup > System Time > NTP in order to open the NTP configuration page of ASDM.

   

3. Click Add in order to add a NTP server and provide the required attributes such as IP address, Interface name (Inside or Outside), key number, and key value for Authentication in the new window that comes up. Click OK.

   

   Note: The interface name should be chosen as inside for ASA1 and outside for ASA2.

   Note: The ntp authentication key should be the same in ASA and the NTP server.

   The Authentication attribute configuration in the CLI for ASA1 and ASA2 are shown here:

   ASA1#ntp authentication-key 1 md5 cisco
   ntp trusted-key 1
   ntp server 172.22.1.161 key 1 source inside

   ASA2#ntp authentication-key 1 md5 cisco
   ntp trusted-key 1
   ntp server 172.22.1.161 key 1 source outside

4. Click the checkbox Enable NTP Authentication and click Apply, which completes the NTP configuration task.

   

ASA1 CLI Configuration

ASA#show run
: Saved
ASA Version 8.3(1)
!
hostname ASA1
domain-name default.domain.invalid
enable password 8Ry2YjIyt7RRXU24 encrypted
names

!
interface Ethernet0
 nameif outside
 security-level 0
 ip address 10.10.10.1 255.255.255.0

!--- Configure the outside interface. !

interface Ethernet1
 nameif inside
 security-level 100
 ip address 172.22.1.163 255.255.255.0

!--- Configure the inside interface. !



 !-- Output suppressed !

passwd 2KFQnbNIdI.2KYOU encrypted
ftp mode passive
dns server-group DefaultDNS
 domain-name default.domain.invalid



access-list inside_nat0_outbound extended permit ip 172.22.1.0 255.255.255.0 172
.16.1.0 255.255.255.0

 !--- This access list (inside_nat0_outbound) is used !--- with the nat zero command. This prevents traffic which !--- matches the access list from undergoing network address translation (NAT). !--- The traffic specified by this ACL is traffic that is to be encrypted and !--- sent across the VPN tunnel. This ACL is intentionally !--- the same as (outside_cryptomap_20). !--- Two separate access lists should always be used in this configuration.

access-list outside_cryptomap_20 extended permit ip 172.22.1.0 255.255.255.0 172
.16.1.0 255.255.255.0

!--- This access list (outside_cryptomap_20) is used !--- with the crypto map outside_map !--- to determine which traffic should be encrypted and sent !--- across the tunnel. !--- This ACL is intentionally the same as (inside_nat0_outbound). !--- Two separate access lists should always be used in this configuration.

pager lines 24
mtu inside 1500
mtu outside 1500
no failover

asdm image flash:/asdm-631.bin

!--- Enter this command to specify the location of the ASDM image.

asdm history enable
arp timeout 14400


object network obj-local
subnet 172.22.1.0 255.255.255.0

object network obj-remote
subnet 172.16.1.0 255.255.255.0


nat (inside,outside) 1 source static obj-local obj-local destination static 
obj-remote obj-remote

!--- NAT 0 prevents NAT for networks specified in !--- the ACL inside_nat0_outbound. 

route outside 0.0.0.0 0.0.0.0 10.10.10.2 1

timeout xlate 3:00:00
timeout conn 1:00:00 half-closed 0:10:00 udp 0:02:00 icmp 0:00:02
timeout sunrpc 0:10:00 h323 0:05:00 h225 1:00:00 mgcp 0:05:00
timeout mgcp-pat 0:05:00 sip 0:30:00 sip_media 0:02:00
timeout uauth 0:05:00 absolute


http server enable

!--- Enter this command in order to enable the HTTPS server !--- for ASDM.


http 172.22.1.1 255.255.255.255 inside

!--- Identify the IP addresses from which the security appliance !--- accepts HTTPS connections.


no snmp-server location
no snmp-server contact


!--- PHASE 2 CONFIGURATION ---! !--- The encryption types for Phase 2 are defined here. 

crypto ipsec transform-set ESP-AES-256-SHA esp-aes-256 esp-sha-hmac

!--- Define the transform set for Phase 2.



crypto map outside_map 20 match address outside_cryptomap_20

!--- Define which traffic should be sent to the IPsec peer.


crypto map outside_map 20 set peer 10.20.20.1

!--- Sets the IPsec peer


crypto map outside_map 20 set transform-set ESP-AES-256-SHA

!--- Sets the IPsec transform set "ESP-AES-256-SHA" !--- to be used with the crypto map entry "outside_map".


crypto map outside_map interface outside

!--- Specifies the interface to be used with !--- the settings defined in this configuration.


!--- PHASE 1 CONFIGURATION ---! !--- This configuration uses isakmp policy 10. !--- Policy 65535 is included in the config by default. !--- The configuration commands here define the Phase !--- 1 policy parameters that are used.


isakmp enable outside
isakmp policy 10 authentication pre-share
isakmp policy 10 encryption aes-256
isakmp policy 10 hash sha
isakmp policy 10 group 5
isakmp policy 10 lifetime 86400

isakmp policy 65535 authentication pre-share
isakmp policy 65535 encryption 3des
isakmp policy 65535 hash sha
isakmp policy 65535 group 2
isakmp policy 65535 lifetime 86400

 

tunnel-group 10.20.20.1 type ipsec-l2l

!--- In order to create and manage the database of connection-specific !--- records for ipsec-l2l—IPsec (LAN-to-LAN) tunnels, use the command !--- tunnel-group in global configuration mode. !--- For L2L connections, the name of the tunnel group MUST be the IP !--- address of the IPsec peer.



tunnel-group 10.20.20.1 ipsec-attributes
 pre-shared-key *

!--- Enter the pre-shared-key in order to configure the !--- authentication method.


telnet timeout 5
ssh timeout 5
console timeout 0
!
class-map inspection_default
 match default-inspection-traffic
!
!
policy-map global_policy
 class inspection_default
  inspect dns maximum-length 512
  inspect ftp
  inspect h323 h225
  inspect h323 ras
  inspect netbios
  inspect rsh
  inspect rtsp
  inspect skinny
  inspect esmtp
  inspect sqlnet
  inspect sunrpc
  inspect tftp
  inspect sip
  inspect xdmcp
!
service-policy global_policy global


!--- Define the NTP server authentication-key,Trusted-key !--- and the NTP server address for configuring NTP.

ntp authentication-key 1 md5 *
ntp trusted-key 1


!--- The NTP server source is to be mentioned as inside for ASA1

ntp server 172.22.1.161 key 1 source inside
Cryptochecksum:ce7210254f4a0bd263a9072a4ccb7cf7
: end

This video posted to the Cisco Support Community explains with a demo, the procedure to configure ASA as NTP client:

How to configure a Cisco Adaptive Security Appliance (ASA) to synchronize its clock with a Network Time Protocol (NTP) Server.

ASA2 CLI Configuration

ASA Version 8.3(1)
!
hostname ASA2
domain-name default.domain.invalid
enable password 8Ry2YjIyt7RRXU24 encrypted
names
!
interface Ethernet0
 nameif outside
 security-level 0
 ip address 10.20.20.1 255.255.255.0
!
interface Ethernet1
 nameif inside
 security-level 100
 ip address 172.16.1.1 255.255.255.0
!
passwd 2KFQnbNIdI.2KYOU encrypted
ftp mode passive
dns server-group DefaultDNS
 domain-name default.domain.invalid

access-list inside_nat0_outbound extended permit ip 172.16.1.0 255.255.255.0 172
.22.1.0 255.255.255.0

!--- Note that this ACL is a mirror of the inside_nat0_outbound !--- ACL on ASA1.

access-list outside_cryptomap_20 extended permit ip 172.16.1.0 255.255.255.0 172
.22.1.0 255.255.255.0

!--- Note that this ACL is a mirror of the outside_cryptomap_20 !--- ACL on ASA1.

pager lines 24
mtu inside 1500
mtu outside 1500
no failover
asdm image flash:/asdm-631.bin
no asdm history enable
arp timeout 14400
object network obj-local
subnet 172.22.1.0 255.255.255.0

object network obj-remote
subnet 172.16.1.0 255.255.255.0


nat (inside,outside) 1 source static obj-local obj-local destination static 
obj-remote obj-remote
timeout xlate 3:00:00
timeout conn 1:00:00 half-closed 0:10:00 udp 0:02:00 icmp 0:00:02
timeout sunrpc 0:10:00 h323 0:05:00 h225 1:00:00 mgcp 0:05:00
timeout mgcp-pat 0:05:00 sip 0:30:00 sip_media 0:02:00
timeout uauth 0:05:00 absolute
http server enable
http 0.0.0.0 0.0.0.0 inside
no snmp-server location
no snmp-server contact
crypto ipsec transform-set ESP-AES-256-SHA esp-aes-256 esp-sha-hmac
crypto map outside_map 20 match address outside_cryptomap_20
crypto map outside_map 20 set peer 10.10.10.1
crypto map outside_map 20 set transform-set ESP-AES-256-SHA
crypto map outside_map interface outside
isakmp enable outside
isakmp policy 10 authentication pre-share
isakmp policy 10 encryption aes-256
isakmp policy 10 hash sha
isakmp policy 10 group 5
isakmp policy 10 lifetime 86400
tunnel-group 10.10.10.1 type ipsec-l2l
tunnel-group 10.10.10.1 ipsec-attributes
 pre-shared-key *
telnet timeout 5
ssh timeout 5
console timeout 0
!
class-map inspection_default
 match default-inspection-traffic
!
!
policy-map global_policy
 class inspection_default
  inspect dns maximum-length 512
  inspect ftp
  inspect h323 h225
  inspect h323 ras
  inspect netbios
  inspect rsh
  inspect rtsp
  inspect skinny
  inspect esmtp
  inspect sqlnet
  inspect sunrpc
  inspect tftp
  inspect sip
  inspect xdmcp
!
service-policy global_policy global


!--- Define the NTP server authentication-key,Trusted-key !--- and the NTP server address for configuring NTP.

ntp authentication-key 1 md5 *
ntp trusted-key 1


!--- The NTP server source is to be mentioned as outside for ASA2.

ntp server 172.22.1.161 key 1 source outside
Cryptochecksum:d5e2ee898f5e8bd28e6f027aeed7f41b
: end
ASA#

Verify

This section provides information you can use in order to confirm that your configuration works properly.

Certain show commands are supported by the Output Interpreter Tool (registered customers only) , which allows you to view an analysis of show command output.

• show ntp status - Displays the NTP clock information.

  ASA1#show ntp status
  
  Clock is synchronized, stratum 2, reference is 172.22.1.161
  nominal freq is 99.9984 Hz, actual freq is 99.9983 Hz, precision is 2**6
  reference time is ccf22b77.f7a6e7b6 (13:28:23.967 UTC Tue Dec 16 2008)
  clock offset is 34.8049 msec, root delay is 4.78 msec
  root dispersion is 60.23 msec, peer dispersion is 25.41 msec

• show ntp associations [detail] - Displays the configured network time server associations.

  ASA1#show ntp associations detail
  172.22.1.161 configured, authenticated, our_master, sane, valid, stratum 1
  ref ID .LOCL., time ccf2287d.3668b946 (13:15:41.212 UTC Tue Dec 16 2008)
  our mode client, peer mode server, our poll intvl 64, peer poll intvl 64
  root delay 0.00 msec, root disp 0.03, reach 7, sync dist 23.087
  delay 4.52 msec, offset 9.7649 msec, dispersion 20.80
  precision 2**19, version 3
  org time ccf22896.f1a4fca3 (13:16:06.943 UTC Tue Dec 16 2008)
  rcv time ccf22896.efb94b28 (13:16:06.936 UTC Tue Dec 16 2008)
  xmt time ccf22896.ee5691dc (13:16:06.931 UTC Tue Dec 16 2008)
  filtdelay =     4.52    4.68    4.61    0.00    0.00    0.00    0.00    0.00
  filtoffset =    9.76    7.09    3.85    0.00    0.00    0.00    0.00    0.00
  filterror =    15.63   16.60   17.58 14904.3 14904.3 14904.3 14904.3 14904.3

Troubleshoot

This section provides information you can use to troubleshoot your configuration.

Troubleshooting Commands

Certain show commands are supported by the Output Interpreter Tool (registered customers only) , which allows you to view an analysis of show command output.

Note: Before issuing debug commands, refer to Important Information on Debug Commands.

• debug ntp validity - Displays NTP peer clock validity.

  This is debug output from the key mismatch:

  NTP: packet from 172.22.1.161 failed validity tests 10
   	  Authentication failed
  

• debug ntp packet - Displays NTP packet information.

  When there is no response from the server, only the NTP xmit packet is seen on the ASA with no NTP rcv packet.

  ASA1# NTP: xmit packet to 172.22.1.161:
   leap 0, mode 3, version 3, stratum 2, ppoll 64
   rtdel 012b (4.562), rtdsp 0cb6 (49.652), refid ac1601a1 (172.22.1.161)
   ref ccf22916.f1211384 (13:18:14.941 UTC Tue Dec 16 2008)
   org ccf22916.f426232d (13:18:14.953 UTC Tue Dec 16 2008)
   rec ccf22916.f1211384 (13:18:14.941 UTC Tue Dec 16 2008)
   xmt ccf22956.f08ee8b4 (13:19:18.939 UTC Tue Dec 16 2008)
  NTP: rcv packet from 172.22.1.161 to 172.22.1.163 on inside:
   leap 0, mode 4, version 3, stratum 1, ppoll 64
   rtdel 0000 (0.000), rtdsp 0002 (0.031), refid 4c4f434c (76.79.67.76)
   ref ccf2293d.366a4808 (13:18:53.212 UTC Tue Dec 16 2008)
   org ccf22956.f08ee8b4 (13:19:18.939 UTC Tue Dec 16 2008)
   rec ccf22956.f52e480e (13:19:18.957 UTC Tue Dec 16 2008)
   xmt ccf22956.f5688c29 (13:19:18.958 UTC Tue Dec 16 2008)
   inp ccf22956.f982bcd9 (13:19:18.974 UTC Tue Dec 16 2008)

Related Information

• Cisco Adaptive Security Device Manager
• Cisco ASA 5500 Series Adaptive Security Appliances
• Requests for Comments (RFCs)
• Technical Support & Documentation - Cisco Systems