strongSwan : Configure Server2025/03/24

Install strongSwan that is the IPsec-based VPN solution.

This example is based on the environment like follows.

First, it needs to configure IP masquerade setting on your router that UDP packets to global IP address of strongSwan server from strongSwan client via internet are forwared to local IP address of strongSwan server.

  +------------------------+
  | [  strongSwan Server ] |
  |      dlp.srv.world     +--------+
  |                        |        |
  +-----------+------------+        |
        enp1s0|10.0.0.30/24         |
              |                     |
              |       Local Network |
       +------+-----+               |
-------|  Router#1  |---------------|-----
       +------+-----+               |
              |                     |
    Internet  |  Internet           |
              |                     |
       +------+-----+               |
-------|  Router#2  |---------------|-----
       +------+-----+               |
              |       Local Network |
              |                     |
        enp1s0|192.168.10.30/24     |
  +-----------+------------+        |
  |  [ strongSwan Client]  |        |
  |                        +--------+
  |                        |172.16.100.x (VPN IP)
  +------------------------+

[1]	Get SSL certificate, refer to here.
[2]	Install strongSwan.

# install from EPEL

[root@dlp ~]#

dnf --enablerepo=epel -y install strongswan

[3]	Configure strongSwan. It needs Firewalld is running for the example of settings below.

# create symbolic links for certificate

[root@dlp ~]#

ln -s /etc/letsencrypt/live/dlp.srv.world/fullchain.pem \
/etc/strongswan/swanctl/x509/fullchain.pem

[root@dlp ~]#

ln -s /etc/letsencrypt/live/dlp.srv.world/privkey.pem \
/etc/strongswan/swanctl/private/privkey.pem

[root@dlp ~]#

ln -s /etc/letsencrypt/live/dlp.srv.world/chain.pem \
/etc/strongswan/swanctl/x509ca/chain.pem

[root@dlp ~]#

vi /etc/strongswan/swanctl/conf.d/ikev2.conf

# create new

connections {
    ikev2 {
        version = 2
        proposals = default,aes256-sha256-modp4096,aes256-sha256-modp2048,aes256gcm16-sha256-modp1024
        unique = never
        send_cert = always
        pools = ipv4
        fragmentation = yes

        local {
            # set hostname of strongSwan
            # match the hostname registered in the certificate
            id = dlp.srv.world
            certs = fullchain.pem
        }
        remote {
            auth = eap-mschapv2
            eap_id = %any
        }
        children {
            child-ikev2 {
                # set your local network
                local_ts = 10.0.0.0/24
            }
        }
    }
}

pools {
    ipv4 {
        # network that is used for VPN clients
        addrs = 172.16.100.0/24
        # name servers that is used for VPN clients
        dns = 8.8.8.8
    }
}

secrets {
    # set VPN users
    eap-1 {
        id = vpnuser01
        secret = "userPassword01"
    }
    eap-2 {
        id = vpnuser02
        secret = "userPassword02"
    }
}

[root@dlp ~]#

chmod 600 /etc/strongswan/swanctl/conf.d/ikev2.conf

[root@dlp ~]#

vi /etc/strongswan/masquerade.sh

# create new

#!/bin/bash

# network interface that can connect to a local network
IF=enp1s0

case "$1" in
    "start")
        firewall-cmd --zone=public --add-masquerade
        firewall-cmd --direct --add-rule ipv4 filter FORWARD 0 -o ${IF} -j ACCEPT
        firewall-cmd --direct --add-rule ipv4 nat POSTROUTING 0 -o ${IF} -j MASQUERADE
        firewall-cmd --add-service=ipsec
        ;;
    "stop")
        firewall-cmd --zone=public --remove-masquerade
        firewall-cmd --direct --remove-rule ipv4 filter FORWARD 0 -o ${IF} -j ACCEPT
        firewall-cmd --direct --remove-rule ipv4 nat POSTROUTING 0 -o ${IF} -j MASQUERADE
        firewall-cmd --remove-service=ipsec
        ;;
    *)
        echo "Unknown option is specified"
        ;;
esac

[root@dlp ~]#

chmod 700 /etc/strongswan/masquerade.sh

[root@dlp ~]#

systemctl edit strongswan

# create new

[Service]
ExecStartPost=/etc/strongswan/masquerade.sh start
ExecStopPost=/etc/strongswan/masquerade.sh stop

[root@dlp ~]#

systemctl enable --now strongswan

[4]	If SELinux is enabled, change policy.

[root@dlp ~]#

vi swan-ipsec.te

# create new

module swan-ipsec 1.0;

require {
        type ipsec_conf_file_t;
        type init_t;
        type lsmd_t;
        type passt_repair_exec_t;
        class file { getattr execute execute_no_trans };
}

#============= init_t ==============
allow init_t ipsec_conf_file_t:file { execute execute_no_trans };;
allow lsmd_t passt_repair_exec_t:file getattr;

[root@dlp ~]#

checkmodule -m -M -o swan-ipsec.mod swan-ipsec.te

[root@dlp ~]#

semodule_package --outfile swan-ipsec.pp --module swan-ipsec.mod

[root@dlp ~]#

semodule -i swan-ipsec.pp

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