Cisco ASA VPN Static to Dynamic IP (DHCP)

KB ID 0001683

Problem

I had a call with a client last week, they are in one of my employer’s DCs, and their servers are behind a vASA. They had purchased some Meraki MX devices for their IT team who were working remotely (during the Covid-19 lockdown), and were struggling. Normally we would just suggest AnyConnect, but these guys were building new machines for  their clients, and needed access directly to the domain from their home networks to facilitate this. So a Site-to-Site VPN was the ‘best fit’ for them.

But being home internet connections they were on DHCP for their public IP addresses. If we were all Meraki, then Meraki Auto VPN would have them up and running in no time. Conversely if they were all ‘proper’ Cisco we would be OK. In fact if they had static IP’s at home we would be OK.

So how to setup a site-to-site VPN ‘into’ a Cisco ASA with a static IP,  from another firewall with a dynamically assigned (DHCP) public IP?

Site-to-Site-Static-to-DHCP

I tested this firstly using a Cisco ASA at the ‘remote/dynamic’ end, then tested with a Meraki MX Device. But the methodology can be applied to any ISAKMP/IPSEC capable firewall with a dynamically assigned public IP that you want to establish a VPN into an ASA with a static IP address.

Solution

Step 1: Investigate Your Remote Device

Do your homework on the remote device, find out what it supports for VPN connectivity, you will need to answer the following questions;

  • IKE Version: Does it use/support IKEv1 or IKEv2 (or both) Set the ASA config accordingly
  • Phase 1 Encryption: What does it support? DES <- Don’t Use this if you can avoid it! 3DES, AES, AES-128, AES-192, AES-256 etc.
  • Phase 1 Hashing: What does it support? MD5, SHA/SHA1, SHA256 etc.
  • Phase 1 Diffie Hellman Group (for IKEv1): Group 1,2,5,14 etc.
  • Lifetime: {in seconds}
  • Pseudorandom function (for IKEv2): MD5, SHA1 or SHA256
  • Phase 2 (IPSEC) Encryption: 3DES, AES, AES-128, AES-192, AES-256 etc.
  • Phase 2 (IPSEC) Hashing: MD5, SHA1 or SHA254
  • Phase 2 PFS: Off or DH1, DH2, DH5, or DH14 etc.
  • Subnet on the remote site: Make sure it does not overlap with your central site, (or anyone else who will also be ‘dynamically’ connecting*).
  • Pre-Shared-Key: MAKE SURE this is long/complicated and hard to guess. 

*Note This may mean your remote sites may need to change their subnets so they don’t overlap!

Step 1 Configure the ‘Central’ ASA

I’ve covered IKEv1 VPNs and IKEv2 VPNs elsewhere on the site, feel free to go and see what what the following configuration is doing. For my Meraki Tunnel I’m going to use IKEv1, Phase 1 (3DES, SHA, Diffie Hellman Group 2, and a Lifetime of 86400 Seconds,) and Phase 2 (3DES, SHA and no PFS). Note: Some would consider this weak, but it’s the default setting on the remote device so lets keep it simple. In production you can slowly make phase1/2 more secure to your liking once you know it’s working.

Change the values in Red;

WARNING: If you already have a crypto map in use then change the keyword CRYPTO-MAP (below) to match yours. (Or all your VPNs will suddenly stop working!) Issue the command show run crypto map, if you’re unsure.

!
object network OBJ-MainSite
subnet 192.168.100.0 255.255.255.0
object network OBJ-RemoteSite-01
subnet 192.168.1.0 255.255.255.0
!
nat (inside,outside) source static OBJ-MainSite OBJ-MainSite destination static OBJ-RemoteSite-01 OBJ-RemoteSite-01 no-proxy-arp route-lookup
!
crypto ikev1 policy 65535
 authentication pre-share
 encryption 3des
 hash sha
 group 2
 lifetime 86400
!
crypto ikev1 enable outside
!
crypto ipsec ikev1 transform-set TS-IPSEC-VPN esp-3des esp-sha-hmac
!
crypto dynamic-map DYNAMIC-CRYPTO-MAP 65535 set ikev1 transform-set TS-IPSEC-VPN
crypto dynamic-map DYNAMIC-CRYPTO-MAP 65535 set reverse-route
crypto map CRYPTO-MAP 65535 ipsec-isakmp dynamic DYNAMIC-CRYPTO-MAP
crypto map CRYPTO-MAP interface outside
!
tunnel-group DefaultL2LGroup ipsec-attributes
 ikev1 pre-shared-key V3ryL0ng&H@rd2Gue55
!

Step 3: Configure the Remote (DHCP) Firewall

If the remote end was going to be another Cisco ASA, here’s the config;

object network OBJ-MainSite
 subnet 192.168.1.0 255.255.255.0 
object network OBJ-RemoteSite
 subnet 192.168.100.0 255.255.255.0 
! 
access-list VPN-INTERESTING-TRAFFIC extended permit ip object OBJ-RemoteSite object OBJ-MainSite 
nat (inside,outside) source static OBJ-RemoteSite OBJ-RemoteSite destination static OBJ-MainSite OBJ-MainSite no-proxy-arp route-lookup 
!
crypto ikev1 policy 10
 authentication pre-share
 encryption 3des
 hash sha
 group 2
 lifetime 86400
!
crypto ikev1 enable outside
!
crypto ipsec ikev1 transform-set VPN-TRANSFORM esp-3des esp-sha-hmac
!
crypto map CRYPTO-MAP 1 match address VPN-INTERESTING-TRAFFIC 
crypto map CRYPTO-MAP 1 set peer 203.0.113.1 
crypto map CRYPTO-MAP 1 set ikev1 transform-set VPN-TRANSFORM 
crypto map CRYPTO-MAP interface outside
!
tunnel-group 203.0.113.1 type ipsec-l2l
tunnel-group 203.0.113.1 ipsec-attributes
pre-shared-key V3ryL0ng&H@rd2Gue55
isakmp keepalive threshold 10 retry 2

Note For IKEv2 sample ASA Configs, Click Here

However if the remote end was going to be a Meraki Device.

Log into the Meraki Portal, and select your ‘customer’ (if you have more than one) > Security & SD WAN > Site-to-Site VPN.

Type: Hub > Scroll down.

Non-Meraki VPN Peers > Add a peer.

  • Name: Give it a sensible name.
  • IKE Version: IKEv1 (Note some versions don’t support IKEv2).
  • Public IP :Of the ASA.
  • Private Subnets: The subnet(s) behind the ASA.
  • Availability: ‘All Networks’ Unless you want to filter it.
  • Save.

Note: Should you wish to change any phase1/phase2 settings, click the link that says ‘Default‘ under IPSec policies.

 

Related Articles, References, Credits, or External Links

Special thanks to Paul White for putting me on the right road, and to Andrew Dorrian for taking the time to test the Meraki VPN config for me.

Author: PeteLong

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1 Comment

  1. If the remote end (DHCP) was an ASA use the command

    crypto isakmp identity hostname

    The ASA will send the hostname and domain as identifier and you can create a seperate tunnel-group for every remote office with DHCP and you can have seperate keys.

    In the main ASA.

    tunnel-group firewallx.domain.local ipsec-attributes
    ikev1 pre-shared-key V3ryL0ng&H@rd2Gue5

    Remote ASA

    hostname firewallx
    domain-name domain.local

    crypto isakmp identity hostname

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