Install StrongSwan on Ubuntu 24.04

• When installing StrongSwan on a server, the item that takes the most preparation is the setup of the PKI.
• Although it is easy once you figured it out, because StrongSwan has been around for such a long time there are lots of outdated or over complicated documentation out there.
• Even when I asked one of the AI engines for instructions it provided me with old outdated instructions.
• The following instructions should work well on any of the recent versions of StrongSwan

• As stated on the StrongSwan Primer wiki page, StrongSwan has gone through an aggressive redesign and you should take care when installing it not to install the older legacy version.
• The natural behavior would be to install the StrongSwan meta package. This should not be done
• Instead we install charon-systemd and strongswan-swanctl

#Make sure the old StrongSwan versions are not installed or running
sudo systemctl disable strongswan.service
sudo systemctl stop strongswan
sudo apt-get remove strongswan-starter
sudo apt-get remove strongswan-charon
#Install the new style StrongSwan
sudo apt-get install charon-systemd strongswan-swanctl
#Enable its startup
sudo systemctl enable strongswan.service
#This will link the strongswan-swanctl
#Created symlink /etc/systemd/system/strongswan-swanctl.service → /usr/lib/systemd/system/strongswan.service.
#Created symlink /etc/systemd/system/multi-user.target.wants/strongswan.service → /usr/lib/systemd/system/strongswan.service.
 
#Start it up
sudo systemctl start strongswan-swanctl.service
#=== Or for the same result alternatively ===
sudo systemctl start strongswan
sudo service strongswan start
 
#Check Its status
sudo systemctl status strongswan-swanctl.service
#=== Or for the same result alternatively ===
sudo systemctl status strongswan
sudo service strongswan status

• This is the result of the status command on our server:

● strongswan.service - strongSwan IPsec IKEv1/IKEv2 daemon using swanctl
     Loaded: loaded (/usr/lib/systemd/system/strongswan.service; enabled; preset: enabled)
     Active: active (running) since Sun 2026-01-11 10:17:49 UTC; 11min ago
   Main PID: 1777399 (charon-systemd)
     Status: "charon-systemd running, strongSwan 5.9.13, Linux 6.8.0-90-generic, x86_64"
      Tasks: 17 (limit: 1107)
     Memory: 4.6M (peak: 20.0M)
        CPU: 2.445s
     CGroup: /system.slice/strongswan.service
             └─1777399 /usr/sbin/charon-systemd

• As you can see it has the charon-systemd program running.

• Next we will creaate the PKI.
• We need to install a helper package that is part of StrongSwan first.

sudo apt-get install strongswan-pki

• We issue the following commands.

mkdir -p ~/pki/{cacerts,certs,private}
chmod 700 ~/pki
cd ~/pki
 
#Create the CA Certificate
pki --gen --type rsa --size 4096 --outform pem > ca/ca.key
pki --self --ca --lifetime 3650 --in ca/ca.key --type rsa --dn "CN=VPN Root CA" --outform pem > ca/ca.crt
 
#Create the server certificate:
pki --gen --type rsa --size 4096 --outform pem > private/server.key
pki --issue --lifetime 825 --in private/server.key --type rsa --cacert ca/ca.crt --cakey ca/ca.key --dn "CN=cloud.radiusdesk.com" --san cloud.radiusdesk.com --flag serverAuth --flag ikeIntermediate --outform pem > certs/server.crt
 
#Create a client certificate
pki --gen --type rsa --size 4096 --outform pem > private/carol.key
pki --issue --lifetime 825 --in private/carol.key --type rsa --cacert ca/ca.crt --cakey ca/ca.key --dn "CN=Carol" --san carol@strongswan.org --flag clientAuth --outform pem > certs/carolCert.pem
 
#View it
pki --print --in certs/carolCert.pem

• The way we configure StrongSwan is again a bit different compared to the 'traditional' way of configuring services running on a server.
• With StrongSwan we have the /etc/swanctl folder.
• Inside this folder are various sub-folders that are pre-installed.
• This setup relies strong on convention where swanctl expect certain items to be located under certain folders.
• We also have the swanctl.conf config file which we will cover in this section.

• The config file has a JSON like structure.
• Please note that it is not valid JSON but rather a JSON like structure.
• Below is our demo server's config:

connections {
    xfrm-gw {
 
        local_addrs = %any
        remote_addrs = %any
        pools = rw_pool
        # XFRM interface binding - CRITICAL
        if_id_in = 100
        if_id_out = 100
        version = 2
        proposals = aes128-sha1-modp2048
        local {
            auth = pubkey
            certs = server.crt
            id = cloud.radiusdesk.com
         }
        remote {
            auth = pubkey
      }
      children {
        xfrm-gw {
          local_ts  = 0.0.0.0/0
          remote_ts = 0.0.0.0/0
          if_id_in  = 100
          if_id_out = 100
          esp_proposals =  aes128-sha1-modp2048
          start_action = start
          dpd_action = restart
        }
      }
      send_cert = always
      dpd_delay = 30s
      rekey_time = 1h
      #send_certreq = no
    }
}
pools {
    rw_pool {
      addrs = 10.3.1.0/16
    }
}

• As we stated there are some pre-installed sub folders under the /etc/swanctl folder.
• The following should be used to contain the CA, server cert and server key files.
• Copy the ca.crt file to /etc/swanctl/x509ca.
• Copy the server.key file to /etc/swanctl/private.
• Copy the server.crt file to /etc/swanctl/x509.

• When a client connects establishes a connection to the StrongSwan server, it is referred to as a Security Association (SA).
• We can use the swanctl command to see if there are any existing SAs.

sudo swanctl --list-sa
xfrm-gw: #11, ESTABLISHED, IKEv2, 8c4f8f2882625d72_i* f423c783942a5006_r
  local  'cloud.radiusdesk.com' @ 164.160.89.129[4500]
  remote 'carol@strongswan.org' @ 197.64.146.11[4500] [10.3.1.0]
  AES_CBC-128/HMAC_SHA1_96/PRF_HMAC_SHA1/MODP_2048
  established 679s ago, rekeying in 2626s
  xfrm-gw: #7, reqid 1, INSTALLED, TUNNEL-in-UDP, ESP:AES_CBC-128/HMAC_SHA1_96/MODP_2048
    installed 1372s ago, rekeying in 2128s, expires in 2588s
    in  c9f9936f (-|0x00000064),   2645 bytes,    29 packets,   139s ago
    out c33bfaa6 (-|0x00000064),  22938 bytes,    38 packets,   139s ago
    local  0.0.0.0/0
    remote 0.0.0.0/0

• We implement a route based IPsec VPN (The other option is policy based).
• Route based IPsec specify a if_id_in and if_id_out.
• This is used to tag traffic inside a SA.
• With Wireguard, you can have multiple instances running on different ports.
• With StrongSwan there is one instance, but you can have multiple connections defined in the config file each using a unique if_id_in and if_id_out.
• The if_id_in and if_if_out in tern have to terminate into a xfrm interface.
• We will create a startup script that prepare this interface for us BEFORE we start StrongSwan.
• Create the file /usr/local/sbin/xfrm-up.sh with the following contents.
• We assume eth0 is the interface name where the server gets it Internet from. Please adapt if your server is different.

#!/bin/sh
set -e
#
IFACE=xfrm0
IF_ID=100
ADDR=10.3.0.1/32
SUBNET=10.3.0.0/24
 
# Create XFRM interface
ip link show "$IFACE" >/dev/null 2>&1 || \
ip link add "$IFACE" type xfrm if_id "$IF_ID"
 
# Assign IP
ip addr show "$IFACE" | grep -q "$ADDR" || \
ip addr add "$ADDR" dev "$IFACE"
 
# Bring interface up
ip link set "$IFACE" up
 
# Route for remote side
ip route show "$SUBNET" | grep -q "$IFACE" || \
ip route add "$SUBNET" dev "$IFACE"
 
# ---- NAT via nftables ----
 
# Create table if missing
nft list table ip nat >/dev/null 2>&1 || \
nft add table ip nat
 
# Create postrouting chain if missing
nft list chain ip nat postrouting >/dev/null 2>&1 || \
nft add chain ip nat postrouting { type nat hook postrouting priority 100\; }
 
# Add SNAT/MASQUERADE rule (idempotent)
nft list chain ip nat postrouting | grep -q "$IFACE" || \
nft add rule ip nat postrouting oifname "$IFACE" masquerade
nft add rule ip nat postrouting oifname "eth0" masquerade

• Create a startup script called /etc/systemd/system/xfrm0.service which calls the script above.

[Unit]
Description=XFRM Interface xfrm0
Before=strongswan.service
Wants=network-online.target
After=network-online.target
 
[Service]
Type=oneshot
ExecStart=/usr/local/sbin/xfrm-up.sh
RemainAfterExit=yes
 
[Install]
WantedBy=multi-user.target

• Enable the startup script and start it up

systemctl enable xfrm0
systemctl start xfrm0