Reuse Cisco ASA’s LAN IP Addressing for AnyConnect Clients

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Why use a Remote Access VPN?

Using a VPN (virtual private network) is by far the best way to provide your employees with secure remote access to corporate resources.  VPNs like Cisco AnyConnect are generally both simple to set up and extremely secure.  I have seen companies open holes in their firewall to allow people to access servers from home but that is a HUGE security risk.  Instead, companies should use VPNs for mobile or remote access.  Believe it or not, most every business-grade firewall today offers remote access VPN capabilities, including the cheap ones you’d find at Office Depot or Staples.  You just have to turn it on.

Remote Access VPN with Dedicated IP Addresses

Remote Access VPN Improves Network Security and Cybersecurity!When you first set up the the remote access VPN on a firewall, it’s usually easiest to create a new dedicated IP address pool for your VPN users.  That way when you create access control lists or traffic filtering policies for those users you can easily specify all VPN clients by referring to that new dedicated IP block.

Cisco AnyConnect VPN with Shared IP Addresses

Sometimes it makes sense to give VPN users an IP address from an existing range though.  I ran into this last week when a manufacturer needed to add Cisco AnyConnect (Cisco’s remote access VPN client) functionality to a Cisco ASA.  This particular manufacturer did not manage their corporate WAN so they really didn’t want to have to request a new subnet be added to the enterprise routing table.  In this case I agreed to allow AnyConnect clients to use addresses from the inside interface but only because the following conditions were true:

  • This was a small sales office with a /24 subnet but only a couple dozen users (so plenty of IP addresses left in the range).
  • Every client had static IPs so there was no chance of the AnyConnect client IP address pool overlapping with a DHCP lease as long as everyone is aware of how the address pool is chopped up.
  • There were expected to be only a few clients using the VPN at any given point in time.
  • Involving the (overseas) WAN team would have greatly extended the project timeline.
  • This was a pretty easy request to accommodate.

After gathering a list of all existing clients at the sales office it turned out this facility was assigning IP addresses from the bottom of the range to the top and hadn’t gotten past yet.  I chose to configure the AnyConnect address pool for to

Remote Access Reach

In addition to providing branch office employees with access to resources in their own office, I also had to make sure they could reach the rest of the enterprise.  The branch office already has access to these locations as shown below, so I just had to make sure remote users inherited the same capabilities.


Remote Office Cisco VPN Design

Rough sketch of the client’s network setup. The headquarters and primary production facility has an ASA5525-X terminating an IPSec VPN tunnel to the ASA5505 at the branch office (orange). Remote users attach to the ASA5505 (blue) but also need access to resources at the corporate HQ (yellow) and other international sites across the MPLS WAN (also yello).


As I mentioned before, it’s not often that I’m asked to have AnyConnect client addresses overlap with an existing ASA interface, but here’s the configuration I used to do it.  Hopefully this will help another engineer looking to do the same thing.  Please feel free to leave questions or comments below.  I’ve left out parts of the config that aren’t relevant to this topic (site-to-site VPN, etc) to keep the template concise.

ip local pool AnyConnect mask
interface Ethernet0/0
switchport access vlan 2
interface Ethernet0/1
interface Ethernet0/2
interface Ethernet0/3
interface Ethernet0/4
interface Ethernet0/5
interface Ethernet0/6
interface Ethernet0/7
interface Vlan1
nameif inside
security-level 100
 ip address
interface Vlan2
nameif outside
security-level 0
ip address x.x.x.x y.y.y.y
dns server-group DefaultDNS
domain-name company.local
same-security-traffic permit inter-interface
!This is required for "hairpin" or "U-Turn" of AnyConnect traffic to go back out the outside interface on a VPN tunnel towards corporate
same-security-traffic permit intra-interface
!Removed all object-groups to hide address details
!Split Tunnel VPN ACL
access-list split-acl extended permit ip object WAN-hosts object AnyConnect
access-list split-acl extended permit ip object corporate-hosts object AnyConnect
access-list split-acl extended permit ip object-group Branch-LAN-Hosts object AnyConnect
!Allow LAN to reach WAN and Corporate hosts
nat (inside,any) source static Branch-LAN-Hosts Branch-LAN-Hosts destination static WAN-hosts WAN-hosts no-proxy-arp route-lookup
nat (inside,any) source static Branch-LAN-Hosts Branch-LAN-Hosts destination static corporate-hosts corporate-hosts no-proxy-arp route-lookup
!Without this NAT exemption, traffic flowing from inside to outside (even stateful replies to VPN clients) will follow the interface NAT overload statement a few lines below

nat (inside,outside) source static Branch-LAN-Hosts Branch-LAN-Hosts destination static AnyConnect AnyConnect description NAT exemption for VPN clients

!All outbound traffic gets NATted to outside interface address unless a NAT exemption exists above
object network obj_any
nat (inside,outside) dynamic interface
route outside z.z.z.z 1
!Cut all IKE details
ssl trust-point ASDM_AnyConnect outside
enable outside
anyconnect image disk0:/anyconnect-win-4.1.00028-k9.pkg 1
anyconnect enable
tunnel-group-list enable
group-policy DfltGrpPolicy attributes
vpn-tunnel-protocol ikev1 l2tp-ipsec ssl-clientless
group-policy GroupPolicy1 internal
group-policy GroupPolicy1 attributes
dns-server value
vpn-tunnel-protocol ikev1 ikev2 ssl-client ssl-clientless
split-tunnel-policy tunnelspecified
split-tunnel-network-list value split-acl
default-domain value company.local
tunnel-group DefaultL2LGroup ipsec-attributes
isakmp keepalive threshold 15 retry 2
tunnel-group DefaultRAGroup ipsec-attributes
isakmp keepalive threshold 15 retry 2
tunnel-group DefaultWEBVPNGroup ipsec-attributes
isakmp keepalive threshold 15 retry 2
tunnel-group AnyConnect type remote-access
tunnel-group AnyConnect general-attributes
address-pool AnyConnect
default-group-policy GroupPolicy1
tunnel-group AnyConnect webvpn-attributes
group-alias AnyConnect enable

About the Author:

Phil is the President and founder of Denton Business Solutions. He's had a fascination with technology for as long as he can remember. Phil uses that fascination to guide manufacturers to technology solutions for enhancing their plant safety, cybersecurity, and productivity. Outside the office, Phil enjoys cooking, spending time with his wife and son, and reading up on the latest manufacturing news. You can connect with Phil on Facebook, Twitter, and LinkedIn!

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