L2TP VPN client on Linux Debian

l2tpclient.sh

#!/usr/bin/env bash
# Requirements
# debian/ubuntu

apt-get -y update && apt-get -y upgrade
apt-get -y install strongswan xl2tpd libstrongswan-standard-plugins libstrongswan-extra-plugins

VPN_SERVER_IP=''
VPN_IPSEC_PSK=''
VPN_USER=''
VPN_PASSWORD=''
VPN_LOCAL_NETWORK='172.25.1.1/24'
VPN_NAME='VPN1'

cat > /etc/ipsec.conf <<EOF
config setup
conn %default
    ikelifetime=60m
    keylife=20m
    rekeymargin=3m
    keyingtries=1
    keyexchange=ikev1
    authby=secret

conn ${VPN_NAME}
    keyexchange=ikev1
    ike=3des-sha1-modp1024!
    esp=3des-sha1!
    left=%defaultroute
    auto=add
    authby=secret
    type=transport
    leftprotoport=17/1701
    rightprotoport=17/1701
    right=${VPN_SERVER_IP}
EOF

cat > /etc/ipsec.secrets <<EOF
: PSK "${VPN_IPSEC_PSK}"
EOF

chmod 600 /etc/ipsec.secrets

cat > /etc/xl2tpd/xl2tpd.conf <<EOF
[lac ${VPN_NAME}]
lns = ${VPN_SERVER_IP}
ppp debug = yes
pppoptfile = /etc/ppp/options.l2tpd.client
length bit = yes
EOF

cat > /etc/ppp/options.l2tpd.client <<EOF
ipcp-accept-local
ipcp-accept-remote
refuse-eap
require-chap
noccp
noauth
mtu 1280
mru 1280
noipdefault
defaultroute
usepeerdns
connect-delay 5000
name ${VPN_USER}
password ${VPN_PASSWORD}
EOF

chmod 600 /etc/ppp/options.l2tpd.client

service strongswan restart
service xl2tpd restart

cat > /usr/local/bin/start-vpn <<EOF
#!/usr/bin/env bash

VPN_LOCAL_NETWORK="${VPN_LOCAL_NETWORK}"
VPN_NAME="${VPN_NAME}"
(
    service strongswan start
    sleep 2
    service xl2tpd start
) && (
    ipsec up ${VPN_NAME}
    echo "c ${VPN_NAME}" > /var/run/xl2tpd/l2tp-control
    sleep 5
    if (( ${#VPN_LOCAL_NETWORK} != 0 )); then
        ip route add ${VPN_LOCAL_NETWORK} dev ppp0
    fi
    ipsec statusall
    route -n
)
EOF
chmod +x /usr/local/bin/start-vpn

cat > /usr/local/bin/stop-vpn <<EOF
#!/usr/bin/env bash

VPN_LOCAL_NETWORK="${VPN_LOCAL_NETWORK}"
(
    echo "d ${VPN_NAME}" > /var/run/xl2tpd/l2tp-control
    ipsec down ${VPN_NAME}
) && (
    service xl2tpd stop
    service strongswan stop
    if (( ${#VPN_LOCAL_NETWORK} != 0 )); then
        ip route del ${VPN_LOCAL_NETWORK} dev ppp0
    fi
    ipsec statusall
    route -n
)
EOF
chmod +x /usr/local/bin/stop-vpn

echo "To start VPN type: start-vpn"
echo "To stop VPN type: stop-vpn"

add

ike=3des-sha1-modp1024!
esp=3des-sha1!

https://unix.stackexchange.com/a/381479