Openswan L2TP/IPsec VPN client setup

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This article describes how to configure and use a L2TP/IPsec Virtual Private Network client on Arch Linux. It covers the installation and setup of several needed software packages. L2TP refers to the w:Layer 2 Tunneling Protocol and for w:IPsec, the Openswan implementation is employed.

This guide is primarily targeted for clients connecting to a Windows Server machine, as it uses some settings that are specific to the Microsoft implementation of L2TP/IPsec. However, it is adaptable with any other common L2TP/IPsec setup. The Openswan wiki features instructions to set up a corresponding L2TP/IPSec Linux server.

Installation

Install the xl2tpd and openswanAUR packages.

Now you can start openswan.service. If it's not running you may get an error message about a missing pluto_ctl connect(pluto_ctl) failed: No such file or directory.

Run ipsec verify to check your configuration and resolve possible issues before continuing.

Configuration

OpenSwan

Edit /etc/ipsec.conf to contain the following lines:

config setup
     virtual_private=%v4:10.0.0.0/8,%v4:192.168.0.0/16,%v4:172.16.0.0/12
     nat_traversal=yes
     protostack=netkey            # default is auto, which will try netkey first
     plutoopts="--interface=eth0" # Replace eth0 with your network interface or use %defaultroute to use default route

conn L2TP-PSK
     authby=secret
     pfs=no
     auto=add
     keyingtries=3
     dpddelay=30
     dpdtimeout=120
     dpdaction=clear
     rekey=yes
     ikelifetime=8h
     keylife=1h
     type=transport
     left=192.168.0.123           # Replace with your local IP address (private, behind NAT IP is okay as well)
     leftprotoport=17/1701
     right=68.68.32.79            # Replace with your VPN server's IP
     rightprotoport=17/1701

This file contains the basic information to establish a secure IPsec tunnel to the VPN server. It enables NAT Traversal for if your machine is behind a NAT'ing router (most people are), and various other options that are necessary to connect correctly to the remote IPsec server. The next file contains your pre-shared key (PSK) for the server.

Create the file /etc/ipsec.secrets: It should contain the following line:

192.168.0.123 68.68.32.79 : PSK "your_pre_shared_key"

Remember to replace the local (192.168.0.123) and remote (68.68.32.79) IP addresses with the correct numbers for your location. The pre-shared key will be supplied by the VPN provider and will need to be placed in this file in cleartext form. You may find this file already exists and already have some data, try to back it up and create a new file only with your PSK if you'll see "Can't authenticate: no preshared key found for ..." when enabling connection in next section.

Add the connection, so it's available to use:

# ipsec auto --add L2TP-PSK

At this point the IPsec configuration is complete and we can move on to the L2TP configuration.

xl2tpd

Edit /etc/xl2tpd/xl2tpd.conf so it has the following contents:

[lac vpn-connection]
lns = 68.68.32.79
ppp debug = yes
pppoptfile = /etc/ppp/options.l2tpd.client
length bit = yes

This file configures xl2tpd with the connection name, server IP address (which again, please remember to change to your servers address) and various options that will be passed to pppd once the tunnel is set up.

Now create /etc/ppp/options.l2tpd.client with the following contents:

ipcp-accept-local
ipcp-accept-remote
refuse-eap
require-mschap-v2
noccp
noauth
idle 1800
mtu 1410
mru 1410
defaultroute
usepeerdns
debug
connect-delay 5000
name your_vpn_username
password your_password

Place your assigned username and password for the VPN server in this file. A lot of these options are for interoperability with Windows Server L2TP servers. If your VPN server uses PAP authentication, replace require-mschap-v2 with require-pap.

This concludes the configuration of the applicable software suites to connect to a L2TP/IPsec server. To start the connection do the following:

# systemctl start openswan
# systemctl start xl2tpd
# ipsec auto --up L2TP-PSK
# echo "c vpn-connection" > /var/run/xl2tpd/l2tp-control

At this point the tunnel is up and you should be able to see the interface for it if you type:

$ ip link

You should see a pppX device that represents the tunnel. Right now, nothing is going to get routed through it. You need to add some routing rules to make it work right:

Routing

Routing traffic to a single IP address or Subnet through the tunnel

This is as easy as adding a routing rule to your kernel table:

# ip route add xxx.xxx.xxx.xxx via yyy.yyy.yyy.yyy dev pppX

Note xxx.xxx.xxx.xxx is the specific ip address (e.g. 192.168.3.10) or subnet (e.g. 192.168.3.0/24) that you wish to communicate with through the tunnel device (e.g. ppp0).

Note yyy.yyy.yyy.yyy is "peer ip" of your pppX device used to route traffic to tunnel destination xxx.xxx.xxx.xxx.


See example below for command to identify tunnel device name and peer ip and then add route. :

$ ip address
4: ppp0: <POINTOPOINT,MULTICAST,NOARP,UP,LOWER_UP> mtu 1400 qdisc fq_codel state UNKNOWN group default qlen 3
    link/ppp 
    inet 10.192.168.40 peer 192.0.2.1/32 scope global ppp0
       valid_lft forever preferred_lft forever
# ip route add 192.168.3.0/24 via 192.0.2.1 dev ppp0

Routing all traffic through the tunnel

This is a lot more complex, but all your traffic will travel through the tunnel. Start by adding a special route for the actual VPN server through your current gateway:

# ip route add 68.68.32.79 via 192.168.1.1 dev eth0

This will ensure that once the default gateway is changed to the ppp interface that your network stack can still find the VPN server by routing around the tunnel. If you miss this step you will lose connectivity to the Internet and the tunnel will collapse. Now add a default route that routes to the PPP remote end:

# ip route add default via yyy.yyy.yyy.yyy dev eth0

The remote PPP end can be discovered by following the step in the previous section. Now to ensure that ALL traffic is routing through the tunnel, delete the original default route:

# ip route delete default via 192.168.1.1 dev eth0

To restore your system to the previous state, you can reboot or reverse all of the above steps.

The route creation can also be automated by placing a script in /etc/ppp/ip-up.d.

Troubleshooting

Note: The first step may be to use the ipsec verify command to check the configuration of the installed IPSEC.

Issue: I get a message from pppd saying "Failed to authenticate ourselves to peer" and I've verified my password is correct. What could be wrong?

Solution: If you see the following in your /var/log/daemon.log:

Dec 20 15:14:03 myhost pppd[26529]: rcvd [CHAP Challenge id=0x1 <some_or_another_hash>, name = "SonicWALL"]
Dec 20 15:14:03 myhost pppd[26529]: sent [CHAP Response id=0x1 <some_or_another_hash>, name = "your_vpn_username"]
Dec 20 15:14:03 myhost pppd[26529]: rcvd [LCP EchoRep id=0x0 magic=0x45c269c6]
Dec 20 15:14:03 myhost pppd[26529]: rcvd [CHAP Failure id=0x1 ""]
Dec 20 15:14:03 myhost pppd[26529]: CHAP authentication failed
Dec 20 15:14:03 myhost pppd[26529]: CHAP authentication failed
Dec 20 15:14:03 myhost pppd[26529]: sent [LCP TermReq id=0x3 "Failed to authenticate ourselves to peer"]
Dec 20 15:14:03 myhost pppd[26529]: rcvd [LCP TermReq id=0x2]
Dec 20 15:14:03 myhost pppd[26529]: sent [LCP TermAck id=0x2]
Dec 20 15:14:03 myhost pppd[26529]: rcvd [LCP TermAck id=0x3]

then you are authenticating against a SonicWALL LNS that does not know how to handle CHAP-style authentication correctly.

The solution to this is to add the following to your options.l2tp.client file:

   refuse-chap

This will cause the SonicWALL to default to the next authentication mechanism, namely MSCHAP-v2. This should authenticate successfully, and from this point xl2tpd should successfully construct a tunnel between you and the remote L2TP server.

Tips and Tricks

Script start up and shut down

You can create some scripts either in your home directory or elsewhere(remember where you put them) to bring up the tunnel then shut it back down.

First, a utility script to automatically discover PPP distant ends:

getip.sh
#!/bin/bash

ifconfig $1 | grep "P-t-P" | gawk -F: '{print $2}' | gawk '{print $1}'

Next, the script to bring the tunnel up. This will replace the default route, so all traffic will pass via the tunnel:

startvpn.sh
#!/bin/bash

systemctl start openswan
sleep 2                                                   #delay to ensure that IPsec is started before overlaying L2TP
systemctl start xl2tpd
ipsec auto --up L2TP-PSK                        
echo "c vpn-connection" > /var/run/xl2tpd/l2tp-control     
sleep 2                                                   #delay again to make that the PPP connection is up.
PPP_GW_ADD=`./getip.sh ppp0`

ip route add 68.68.32.79 via 192.168.1.1 dev eth0
ip route add default via $PPP_GW_ADD
ip route del default via 192.168.1.1

Finally, the shutdown script, it simply reverses the process:

stopvpn.sh
#!/bin/bash

ipsec auto --down L2TP-PSK
echo "d vpn-connection" > /var/run/xl2tpd/l2tp-control
systemctl stop xl2tpd
systemctl stop openswan

ip route del 68.68.32.79 via 192.168.1.1 dev eth0
ip route add default via 192.168.1.1

A further script

Above script really help me work. And notice the script use fixed ip, and someone like me may change net vpn addr, i'd like to put my further script below(not sure how to add attachment, so just raw ):

#!/bin/bash
if [ $# != 1 ] ; then
	echo "Usage: (sudo) sh $0 {init|start|stop}" 
	exit 1;
fi

VPN_ADDR=XXX
IFACE=wlan0

function getIP(){
	ip addr show $1 | grep "inet " | awk '{print $2}' | sed 's:/.*::'       
}

function getGateWay(){
	route -n | grep -m 1 "^0\.0\.0\.0" | awk '{print $2}'
}
function getVPNGateWay(){
	route -n | grep -m 1 "$VPN_ADDR" | awk '{print $2}'
}

GW_ADDR=$(getGateWay)  

function init(){
	cp ./options.l2tpd.client /etc/ppp/
	cp ./ipsec.conf /etc/
	cp ./ipsec.secrets /etc/
	cp ./xl2tpd.conf /etc/xl2tpd/
}

function start(){
	sed -i "s/^lns =.*/lns = $VPN_ADDR/g" /etc/xl2tpd/xl2tpd.conf
	sed -i "s/plutoopts=.*/plutoopts=\"--interface=$IFACE\"/g" /etc/ipsec.conf
	sed -i "s/left=.*$/left=$(getIP $IFACE)/g" /etc/ipsec.conf
	sed -i "s/right=.*$/right=$VPN_ADDR/g" /etc/ipsec.conf
	sed -i "s/^.*: PSK/$(getIP $IFACE) $VPN_ADDR : PSK/g" /etc/ipsec.secrets
	systemctl start openswan
	sleep 2    #delay to ensure that IPsec is started before overlaying L2TP

	systemctl start xl2tpd
	ipsec auto --up L2TP-PSK                        
	echo "c vpn-connection" > /var/run/xl2tpd/l2tp-control     
	sleep 2    #delay again to make that the PPP connection is up.

	route add $VPN_ADDR gw $GW_ADDR $IFACE
	route add default gw $(getIP ppp0)
	route delete default gw $GW_ADDR
}

function stop(){
	ipsec auto --down L2TP-PSK
	echo "d vpn-connection" > /var/run/xl2tpd/l2tp-control
	systemctl stop xl2tpd
	systemctl stop openswan
	
	VPN_GW=$(getVPNGateWay)
	route delete $VPN_ADDR gw $VPN_GW $IFACE
	route add default gw $VPN_GW
}

$1
exit 0

Script to resolve dns names and connect

Tango-edit-clear.pngThis article or section needs language, wiki syntax or style improvements. See Help:Style for reference.Tango-edit-clear.png

Reason: The wiki should not host scripts. (Discuss in Talk:Openswan L2TP/IPsec VPN client setup#)

Very useful if you have dynamic IP for the server.

#!/bin/python

from os import system
from socket import gethostbyname
from netifaces import ifaddresses, AF_INET
from time import sleep

# netifaces is a library installed with pip, not part of default insatllation of python
# The script is useful if you have dynamic IP, or need to use a domain for the vpn server
# gist: https://gist.github.com/physicalit/bf9e27c7ecbc12843cd68e442358616c
# The template files are identical to the examples from the link above, except they use the sign `<` as placeholder for the server ip
# can be added to cron, don't forghet to modify your domain and the ip/subnet from the `ip add route ...`

ip = gethostbyname('your.domain.tld')

file_list = ['/etc/xl2tpd/xl2tpd.conf_tmp', '/etc/ipsec.secrets_tmp', '/etc/ipsec.conf_tmp']

def read_file(file):
    with open(file, 'r') as f:
        result = f.readlines()
        #result = [l.rstrip('\n') for l in result]  # l.split('_')[0] 
        return result

def write_ip(ip):
    for l in file_list:
        result = [ip.join(e.split('<')) if "<" in e else e for e in read_file(l)]
        with open(l.split('_')[0], 'w') as f:
            for e in result:
                f.write(e)

if __name__ == "__main__":
    write_ip(ip)
    [ system('systemctl restart {}'.format(l)) for l in ['openswan', 'xl2tpd']]
    vpn = system('ipsec auto --up L2TP-PSK')
    system('echo "c vpn-connection" > /var/run/xl2tpd/l2tp-control')
    sleep(2) # very important or is not going to see ppp0 interface
    if not vpn:
        peer = ifaddresses('ppp0')[AF_INET][0]['peer']
        route = system('ip route add 192.168.88.0/24 via {0} dev ppp0'.format(peer))
        if not route:
            print("VPN sucesfully connected. Route created.")
        else:
            print("VPN KO")

See also