Cisco ASA VPN with over overlapping addresses and twice NAT
IP addressing design is a topic that follows every networker from the basic to the architect level of experience.
Usually we just pick a random range from RFC1918 and address all the devices.
But then VPN happens, and with VPN comes the risk of overlapping.
How do we fix overlapping? With NAT of course!
In this post I’ll show how to use twice NAT to allow VPN connections with overlapping addresses.
- to change addressing of any network is not an option
- no NAT on ASA-REMOTE for the VPN
- Cisco ASA 5510 releas 8.4(2)
- twice nat
We can see in the topology both local and remote network use addressing 10.99.0.0/24. The local PC2 must reach remote PC1.
To fix the overlapping we need to NAT on ASA-NAT both networks to two subnets not used on both sides.
Network LOCAL LAN 10.99.0.0/24 will be seen as 10.101.0.0/24 from REMOTE LAN.
Network REMOTE LAN 10.99.0.0/24 will be seen as 10.100.0.0/24 from LOCAL LAN.
A full network NAT will be used so a host 10.99.0.1 on LOCAL LAN will be natted ad 10.101.0.1 to REMOTE LAN. This is a better choice than nat overload when remote services must be reached.
If you’re familiar with VPN configuration on ASA or IOS you know a cryptomap is involved. A cryptomap is an ACL that defines the traffic that will be encrypted and sent to the remote VPN peer.
In out case the cryptomap on device ASA-NAT will be:
access-list outside_cryptomap extended permit ip 10.100.0.0 255.255.255.0 10.99.0.0 255.255.255.0
This could be quite confusing, the destination address is the same subnet of the LOCAL LAN, but we’ll see why it works (hint: order of operation).
VPN configuration on device ASA-NAT:
crypto ipsec ikev2 ipsec-proposal AES256 protocol esp encryption aes-256 protocol esp integrity sha-1 md5 crypto map outside_map 1 match address outside_cryptomap crypto map outside_map 1 set peer 192.168.2.2 crypto map outside_map 1 set ikev2 ipsec-proposal AES256 crypto map outside_map interface outside crypto ikev2 policy 1 encryption aes-256 integrity sha group 5 2 prf sha lifetime seconds 86400 crypto ikev2 enable outside group-policy GroupPolicy_192.168.2.2 internal group-policy GroupPolicy_192.168.2.2 attributes vpn-tunnel-protocol ikev2 tunnel-group 192.168.2.2 type ipsec-l2l tunnel-group 192.168.2.2 general-attributes default-group-policy GroupPolicy_192.168.2.2 tunnel-group 192.168.2.2 ipsec-attributes ikev2 remote-authentication pre-shared-key cisco123 ikev2 local-authentication pre-shared-key cisco123
VPN configuration on device ASA-REMOTE:
crypto ipsec ikev2 ipsec-proposal AES256 protocol esp encryption aes-256 protocol esp integrity sha-1 md5 crypto map outside_map 1 match address outside_cryptomap crypto map outside_map 1 set peer 192.168.1.1 crypto map outside_map 1 set ikev2 ipsec-proposal AES256 crypto map outside_map interface outside crypto ikev2 policy 1 encryption aes-256 integrity sha group 5 2 prf sha lifetime seconds 86400 crypto ikev2 enable outside group-policy GroupPolicy_192.168.1.1 internal group-policy GroupPolicy_192.168.1.1 attributes vpn-tunnel-protocol ikev2 tunnel-group 192.168.1.1 type ipsec-l2l tunnel-group 192.168.1.1 general-attributes default-group-policy GroupPolicy_192.168.1.1 tunnel-group 192.168.1.1 ipsec-attributes ikev2 remote-authentication pre-shared-key cisco123 ikev2 local-authentication pre-shared-key cisco123
All the magic happens on device ASA-NAT.
First step - create the network objects:
ASA-LAN is the local network:
object network ASA-LAN subnet 10.99.0.0 255.255.255.0
ASA-REMOTE-LAN is the remote network:
object network ASA-REMOTE-LAN subnet 10.99.0.0 255.255.255.0
It is the same as AS-LAN but I used different names to make the nat command more clear.
ASA-NATTED-LAN is how the local network is seen from the remote network:
object network ASA-NATTED-LAN subnet 10.100.0.0 255.255.255.0
ASA-NATTED-REMOTE is how the remote network is seen from the local network:
object network ASA-NATTED-REMOTE subnet 10.101.0.0 255.255.255.0
Second step - the NAT command:
nat (inside,outside) source static ASA-LAN ASA-NATTED-LAN destination static ASA-NATTED-REMOTE ASA-REMOTE-LAN net-to-net
The first part of the command source translates the local network ASA-LAN (10.99.0.0/24) as ASA-NATTED-LAN (10.100.0.0/24) when talking to ASA-NATTED-REMOTE (10.101.0.0/24)
The second part of the command destination translates the remote network ASA-NATTED-REMOTE (10.101.0.0/24) as ASA-REMOTE (10.99.0.0/24).
The final result creates a packet with source 10.100.0.0/24 and destination 10.99.0.0/24 that perfectly matches the cryptomap we created so the traffic correctly enters the VPN.
Order of operations
For a better understanding of the whole process we can use the packet tracer and see the steps.
The command is:
ASA-NAT# packet-tracer input inside tcp 10.99.0.1 http 10.101.0.1 http
I removed some lines for a better output.
Phase: 1 Type: ACCESS-LIST Result: ALLOW
In phase 2 the destination IP of the packet is un-natted from 10.101.0.1 to 10.99.0.1.
Notice: there’s no routing involved here, the traffic is diverted to the outside interface by the NAT rule, overriding the routing table.
Phase: 2 Type: UN-NAT Subtype: static Result: ALLOW Config: nat (inside,outside) source static ASA-LAN ASA-NATTED-LAN destination static ASA-NATTED-REMOTE ASA-LAN Additional Information: NAT divert to egress interface outside Untranslate 10.101.0.1/80 to 10.99.0.1/80 Phase: 3 Type: IP-OPTIONS Result: ALLOW
In phase 4 the source of the packet is translated from 10.99.0.1 to 10.100.0.1:
Phase: 4 Type: NAT Subtype: Result: ALLOW Config: nat (inside,outside) source static ASA-LAN ASA-NATTED-LAN destination static ASA-NATTED-REMOTE ASA-LAN Additional Information: Static translate 10.99.0.1/80 to 10.100.0.1/80
The traffic matches the cryptomap:
Phase: 5 Type: VPN Subtype: encrypt Result: ALLOW Config: Additional Information: Phase: 6 Type: NAT Subtype: rpf-check Result: ALLOW Config: nat (inside,outside) source static ASA-LAN ASA-NATTED-LAN destination static ASA-NATTED-REMOTE ASA-LAN Additional Information: Phase: 7 Type: VPN Subtype: ipsec-tunnel-flow Result: ALLOW Config: Additional Information: Phase: 8 Result: ALLOW Phase: 9 Type: FLOW-CREATION Result: ALLOW
The final result:
Result: input-interface: inside input-status: up input-line-status: up output-interface: outside output-status: up output-line-status: up Action: allow
I use to configure NAT on both firewalls for this kind of overlapping but in this case we wanted to have the same results without any NAT on remote firewall.
Comment if you need clarifications or help on this topic.