Linux L2tp client setup for Mac OS X vpn server

Instructions below are based on the work at https://github.com/hwdsl2/setup-ipsec-vpn/blob/master/docs/clients.md#linux
Commands must be run as root on your VPN client.
To set up the VPN client, first install the following packages:

# Ubuntu & Debian
apt-get update
apt-get -y install strongswan xl2tpd

# CentOS & RHEL
yum -y install epel-release
yum -y install strongswan xl2tpd

# Fedora
yum -y install strongswan xl2tpd

Create VPN variables (replace with actual values):

VPN_SERVER_IP='your_vpn_server_ip'
VPN_IPSEC_PSK='your_ipsec_pre_shared_key'
VPN_USER='your_vpn_username'
VPN_PASSWORD='your_vpn_password'

Configure strongSwan:

cat > /etc/ipsec.conf <<EOF
# ipsec.conf - strongSwan IPsec configuration file

# basic configuration

config setup
  # strictcrlpolicy=yes
  # uniqueids = no

# Add connections here.

# Sample VPN connections

conn %default
  ikelifetime=60m
  keylife=20m
  rekeymargin=3m
  keyingtries=1
  keyexchange=ikev1
  authby=secret
  ike=aes128-sha1-modp1024,3des-sha1-modp1024!
  esp=aes128-sha1-modp1024,3des-sha1-modp1024!

conn myvpn
  keyexchange=ikev1
  left=%defaultroute
  auto=add
  authby=secret
  type=transport
  leftprotoport=17/1701
  rightprotoport=17/1701
  right=$VPN_SERVER_IP

  rightid=%any

EOF

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

chmod 600 /etc/ipsec.secrets

# For CentOS/RHEL & Fedora ONLY
mv /etc/strongswan/ipsec.conf /etc/strongswan/ipsec.conf.old 2>/dev/null
mv /etc/strongswan/ipsec.secrets /etc/strongswan/ipsec.secrets.old 2>/dev/null
ln -s /etc/ipsec.conf /etc/strongswan/ipsec.conf
ln -s /etc/ipsec.secrets /etc/strongswan/ipsec.secrets

Configure xl2tpd:

cat > /etc/xl2tpd/xl2tpd.conf <<EOF
[lac myvpn]
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

The VPN client setup is now complete. Follow the steps below to connect.

Note: You must repeat all steps below every time you try to connect to the VPN.
Create xl2tpd control file:

mkdir -p /var/run/xl2tpd
touch /var/run/xl2tpd/l2tp-control

Restart services:

service strongswan restart
service xl2tpd restart

Start the IPsec connection:

# Ubuntu & Debian
ipsec up myvpn

# CentOS/RHEL & Fedora
strongswan up myvpn

Start the L2TP connection:

echo "c myvpn" > /var/run/xl2tpd/l2tp-control

Run ifconfig and check the output. You should now see a new interface ppp0.
Check your existing default route:

ip route