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strongSwan

This tutorial explains how to set up strongSwan along with Magic WAN. You will learn how to configure strongSwan, configure an IPsec tunnel and create a Policy Based Routing.

1. Health checks configuration

Start by configuring the bidirectional health checks target for Magic WAN. For this particular tutorial, we are using 172.64.240.252 as the target IP address, and type as the request.

This can be set up with the API. For example:

Terminal window
curl --request PUT \
https://api.cloudflare.com/client/v4/accounts/{account_id}/magic/ipsec_tunnels/{tunnel_id} \
--header "X-Auth-Email: <EMAIL>" \
--header "X-Auth-Key: <API_KEY>" \
--header "Content-Type: application/json" \
--data '{
"health_check": {
"enabled": true,
"target": "172.64.240.252",
"type": "request",
"rate": "mid"
}
}'

2. Configure StrongSwan

  1. Start by installing StrongSwan. For example, open the console and run:
Terminal window
sudo apt-get install strongswan -y
  1. After StrongSwan finishes installing, go to /etc/strongswan.conf to edit the configuration file and add the following settings:
charon {
load_modular = yes
install_routes = no
install_virtual_ip = no
plugins {
include strongswan.d/charon/*.conf
}
}
include strongswan.d/*.conf

3. Configure IPsec file

  1. Go to /etc/ipsec.conf and add the following settings:
# ipsec.conf - strongSwan IPsec configuration file
config setup
charondebug="all"
uniqueids = yes
conn %default
ikelifetime=4h
rekey=yes
reauth=no
keyexchange=ikev2
authby=secret
dpdaction=restart
closeaction=restart
# Sample VPN connections
conn cloudflare-ipsec
auto=start
type=tunnel
fragmentation=no
leftauth=psk
# Private IP of the VM
left=%any
# Tunnel ID from dashboard, in this example FQDN is used
leftid=<YOUR_TUNNEL_ID>.<YOUR_ACCOUNT_ID>.ipsec.cloudflare.com
leftsubnet=0.0.0.0/0
# Cloudflare Anycast IP
right=<YOUR_CLOUDFLARE_ANYCAST_IP>
rightid=<YOUR_CLOUDFLARE_ANYCAST_IP>
rightsubnet=0.0.0.0/0
rightauth=psk
ike=aes256-sha256-modp2048!
esp=aes256-sha256-modp2048!
replay_window=0
mark_in=42
mark_out=42
leftupdown=/etc/strongswan.d/ipsec-vti.sh
  1. Now, you need to create a virtual tunnel interface (VTI) with the IP we configured earlier as the target for Cloudflare’s health checks (172.64.240.252) to route IPsec packets. Go to /etc/strongswan.d/.

  2. Create a script called ipsec-vti.sh and add the following:

#!/bin/bash
set -o nounset
set -o errexit
VTI_IF="vti0"
case "${PLUTO_VERB}" in
up-client)
ip tunnel add "${VTI_IF}" local "${PLUTO_ME}" remote "${PLUTO_PEER}" mode vti \
key "${PLUTO_MARK_OUT%%/*}"
ip link set "${VTI_IF}" up
ip addr add 172.64.240.252/32 dev vti0
sysctl -w "net.ipv4.conf.${VTI_IF}.disable_policy=1"
sysctl -w "net.ipv4.conf.${VTI_IF}.rp_filter=0"
sysctl -w "net.ipv4.conf.all.rp_filter=0"
ip rule add from 172.64.240.252 lookup viatunicmp
ip route add default dev vti0 table viatunicmp
;;
down-client)
ip tunnel del "${VTI_IF}"
ip rule del from 172.64.240.252 lookup viatunicmp
ip route del default dev vti0 table viatunicmp
;;
esac
echo "executed"

4. Add Policy Based Routing (PBR)

Although the IPsec tunnel is working as is, we need to create Policy Based Routing (PBR) to redirect returning traffic via the IPsec tunnel. Without it, the ICMP replies to the health probes sent by Cloudflare will be returned via the Internet, instead of the same IPsec tunnel. This is required to avoid any potential issues.

To accomplish this, the tutorial uses iproute2 to route IP packets from 172.63.240.252 to the tunnel interface.

  1. Go to /etc/iproute2/.

  2. Edit the rt_tables file to add a routing table number and name. In this example, we used viatunicmp as the name and 200 as the number for the routing table.

#
# reserved values
#
255 local
254 main
253 default
0 unspec
200 viatunicmp
#
# local
#
#1 inr.ruhep
  1. Open the console and add a rule to match the routing table just created. This rule instructs the system to use routing table viatunicmp if the packet’s source address is 172.64.240.252:
Terminal window
ip rule add from 172.64.240.252 lookup viatunicmp
  1. Add a route to the newly created routing table viatunicmp. This is the default route via the interface vti0 in the viatunicmp table.
Terminal window
ip route add default dev vti0 table viatunicmp
  1. Now, you can start IPsec. You can also stop, restart and show the status for the IPsec connection:
Terminal window
ipsec start
Security Associations (1 up, 0 connecting):
cloudflare-ipsec[1]: ESTABLISHED 96 minutes ago, <IPSEC_TUNNEL_IDENTIFIER>.ipsec.cloudflare.com]...162.159.67.88[162.159.67.88]
cloudflare-ipsec{4}: INSTALLED, TUNNEL, reqid 1, ESP SPIs: c4e20a95_i c5373d00_o
cloudflare-ipsec{4}: 0.0.0.0/0 === 0.0.0.0/0

5. Check connection status

After you finish configuring StrongSwan with Magic WAN, you can use tcpdump to investigate the status of health checks originated from Cloudflare.

Terminal window
sudo tcpdump -i <OUTGOING_INTERFACE> esp and host <TUNNEL_CLOUDFLARE_ENDPOINT_IP>

In this example, the outgoing Internet interface shows that the IPsec encrypted packets (ESP) from Cloudflare’s health check probes (both the request and response) are going through the IPsec tunnel we configured.

tcpdump shows the IPsec encrypted packets from Cloudflare's health probbes

You can also run tcpdump on vti0 to check the decrypted packets.

Terminal window
sudo tcpdump -i vti0 host 172.64.240.252
If you run tcpdump on vti0 you can check for decrypted packets