Peer to Peer IPSEC VPN with StrongSwan

This will walk you through setting up an Ipsec VPN between 2 networks using 2 hosts using strongswan to build the tunnel.

This was my go to solution to connect Amazon AWS VPCs across regions… that is until AWS allowed peering VPCs across regions in December of 2018. So connecting VPCs using peering is preferred over having 2 machines (1 per VPC) running 24×7 to maintain the connections.

The thing to remember about ipsec is that you should not overlap network subnets across networks. It is ideal to have a unique CIDR for each location that will be used in the peer to peer VPN. There are methods to handle overlapping subnets, but that is another post.

In this example, we have 2 networks.
In Virginia Network 1 is 10.32.252.0/24
In Ohio Network 2 is 10.33.252.0/24

We want to bridge the 2 networks together.

We need to create a host in each network. The host must have a private IP and a Public IP. You may use a physical host with 2 interfaces or even a host with a single internal IP that has a one to one NAT to a public IP with all protocols forwarded. Either way is fine for this solution.

On network 1, let us create a host as 10.32.252.150 as private and 1.1.1.1 as our public ip.
On network 2, let us create a host as 10.33.252.150 as private and 2.2.2.2 as our public ip.

On each host, we want to install strongswan

apt-get install strongswan

Edit the config files for each host as follows:

VA Side (private 10.32.252.150, public 1.1.1.1):
####################################
# /etc/ipsec.conf
#
config setup
conn %default
ikelifetime=60m
keylife=20m
rekeymargin=3m
keyingtries=1
authby=secret
keyexchange=ikev2
mobike=no

conn net-net
left=10.32.252.150
leftsubnet=10.32.0.0/16
leftid=@va
leftfirewall=yes
right=2.2.2.2
rightsubnet=10.33.0.0/16
rightid=@oh
auto=route

####################################
# /etc/ipsec.secrets
# This file holds shared secrets or RSA private keys for authentication.

# RSA private key for this host, authenticating it to any other host
# which knows the public part.
@va @oh : PSK "secret"

Then on the other side:

OH Side (private 10.33.252.150, public 2.2.2.2):
#############################################
# /etc/ipsec.conf

config setup

conn %default
ikelifetime=60m
keylife=20m
rekeymargin=3m
keyingtries=1
authby=secret
keyexchange=ikev2
mobike=no

conn net-net
left=10.33.252.150
leftsubnet=10.33.0.0/16
leftid=@oh
leftfirewall=yes
right=1.1.1.1
rightsubnet=10.32.0.0/16
rightid=@va
auto=route

###############################################
# /etc/ipsec.secrets

# This file holds shared secrets or RSA private keys for authentication.

# RSA private key for this host, authenticating it to any other host
# which knows the public part.

@oh @va : PSK "secret"

This is added to both hosts:

#############################################
# both hosts:
#####################################
# /etc/sysctl.conf
# Add the following:

net.ipv4.ip_forward=1
net.ipv4.conf.all.accept_redirects = 0
net.ipv4.conf.all.send_redirects = 0

After we edit sysctl we need to reload for the changes:

######################################
# Reload sysctl
$ sysctl -p

Now that AWS offers cross region VPC peering, the need to dedicated VPNs isn’t as important. But if you need to use this for AWS hosts we need to disable host check on the EC2 instance.

In AWS, the traffic that is destined for the far end of the VPN tunnel must be routed to the IP of the local VPN endpoint. You do this using a route table assigned to the subnet. Once the host check is disabled on the EC2 instance, the IP becomes eligible for a routing target. Create the route entry in the route table assigned to the subnet.

To Test the VPN, use the following commands.

########################
# Testing
ipsec up net-net
ipsec statusall