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IPSEC L2TP VPN on Ubuntu 14.04 with OpenSwan, xl2tpd and ppp

Published: 01-12-2014 | Author: Remy van Elst | Text only version of this article


❗ This post is over ten years old. It may no longer be up to date. Opinions may have changed.

This is a guide on setting up an IPSEC/L2TP vpn server with Ubuntu 14.04 using Openswan as the IPsec server, xl2tpd as the l2tp provider and ppp or local users / PAM for authentication. It has a detailed explanation with every step. We choose the IPSEC/L2TP protocol stack because of recent vulnerabilities found in pptpd VPNs and because it is supported on all major operating systems by default.

Why a VPN?

More than ever, your freedom and privacy when online is under threat. Governments and ISPs want to control what you can and can't see while keeping a record of everything you do, and even the shady-looking guy lurking around your coffee shop or the airport gate can grab your bank details easier than you may think. A self hosted VPN lets you surf the web the way it was intended: anonymously and without oversight.

A VPN (virtual private network) creates a secure, encrypted tunnel through which all of your online data passes back and forth. Any application that requires an internet connection works with this self hosted VPN, including your web browser, email client, and instant messaging program, keeping everything you do online hidden from prying eyes while masking your physical location and giving you unfettered access to any website or web service no matter where you happen to live or travel to.

This tutorial is available for the following platforms:

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IPSec encrypts your IP packets to provide encryption and authentication, so no one can decrypt or forge data between your clients and your server. L2TP provides a tunnel to send data. It does not provide encryption and authentication though, that is why we need to use it together with IPSec.

To work trough this tutorial you should have:

  • 1 Ubuntu 14.04 server with at least 1 public IP address and root access
  • 1 (or more) clients running an OS that support IPsec/L2tp vpns (Ubuntu, Mac OS, Windows, Android).
  • Ports 1701 TCP, 4500 UDP and 500 UDP opened in the firewall.

I do all the steps as the root user. You should do to, but only via * -i* or * su -*. Do not allow root to login via SSH!

Install ppp openswan and xl2tpd

First we will install the required packages:

apt-get install openswan xl2tpd ppp lsof

The openswan installation will ask some questions, this tutorial works with the default answers (just enter through it).

Firewall and sysctl

We are going to set the firewall and make sure the kernel forwards IP packets:

Execute this command to enable the iptables firewall to allow vpn traffic:

iptables -t nat -A POSTROUTING -j SNAT --to-source %SERVERIP% -o eth+

Replace %SERVERIP% with the external IP of your VPS. If your external interface is not named ethX (+ is a wildcard) then rename appropriately.

Execute the below commands to enable kernel IP packet forwarding and disable ICP redirects.

echo "net.ipv4.ip_forward = 1" |  tee -a /etc/sysctl.conf
echo "net.ipv4.conf.all.accept_redirects = 0" |  tee -a /etc/sysctl.conf
echo "net.ipv4.conf.all.send_redirects = 0" |  tee -a /etc/sysctl.conf
echo "net.ipv4.conf.default.rp_filter = 0" |  tee -a /etc/sysctl.conf
echo "net.ipv4.conf.default.accept_source_route = 0" |  tee -a /etc/sysctl.conf
echo "net.ipv4.conf.default.send_redirects = 0" |  tee -a /etc/sysctl.conf
echo "net.ipv4.icmp_ignore_bogus_error_responses = 1" |  tee -a /etc/sysctl.conf

Set these settings for other network interfaces:

for vpn in /proc/sys/net/ipv4/conf/*; do echo 0 > $vpn/accept_redirects; echo 0 > $vpn/send_redirects; done

Apply them:

sysctl -p
Persistent settings via /etc/rc.local

To make sure this keeps working at boot you might want to add the following to /etc/rc.local:

for vpn in /proc/sys/net/ipv4/conf/*; do echo 0 > $vpn/accept_redirects; echo 0 > $vpn/send_redirects; done
iptables -t nat -A POSTROUTING -j SNAT --to-source %SERVERIP% -o eth+

Add it before the exit 0 line and replace %SERVERIP% with the external IP of your VPS.

Configure Openswan (IPSEC)

Use your favorite editor to edit the following file:

/etc/ipsec.conf

Replace the contents with the following:

(Most lines have a comment below it explaining what it does.)

version 2 # conforms to second version of ipsec.conf specification

config setup
    dumpdir=/var/run/pluto/
    #in what directory should things started by setup (notably the Pluto daemon) be allowed to dump core?

    nat_traversal=yes
    #whether to accept/offer to support NAT (NAPT, also known as "IP Masqurade") workaround for IPsec

    virtual_private=%v4:10.0.0.0/8,%v4:192.168.0.0/16,%v4:172.16.0.0/12,%v6:fd00::/8,%v6:fe80::/10
    #contains the networks that are allowed as subnet= for the remote client. In other words, the address ranges that may live behind a NAT router through which a client connects.

    protostack=netkey
    #decide which protocol stack is going to be used.

    force_keepalive=yes
    keep_alive=60
    # Send a keep-alive packet every 60 seconds.

conn L2TP-PSK-noNAT
    authby=secret
    #shared secret. Use rsasig for certificates.

    pfs=no
    #Disable pfs

    auto=add
    #the ipsec tunnel should be started and routes created when the ipsec daemon itself starts.

    keyingtries=3
    #Only negotiate a conn. 3 times.

    ikelifetime=8h
    keylife=1h

    ike=aes256-sha1,aes128-sha1,3des-sha1
    phase2alg=aes256-sha1,aes128-sha1,3des-sha1
    # https://lists.openswan.org/pipermail/users/2014-April/022947.html
    # specifies the phase 1 encryption scheme, the hashing algorithm, and the diffie-hellman group. The modp1024 is for Diffie-Hellman 2. Why 'modp' instead of dh? DH2 is a 1028 bit encryption algorithm that modulo's a prime number, e.g. modp1028. See RFC 5114 for details or the wiki page on diffie hellmann, if interested.

    type=transport
    #because we use l2tp as tunnel protocol

    left=%SERVERIP%
    #fill in server IP above

    leftprotoport=17/1701
    right=%any
    rightprotoport=17/%any

    dpddelay=10
    # Dead Peer Dectection (RFC 3706) keepalives delay
    dpdtimeout=20
    #  length of time (in seconds) we will idle without hearing either an R_U_THERE poll from our peer, or an R_U_THERE_ACK reply.
    dpdaction=clear
    # When a DPD enabled peer is declared dead, what action should be taken. clear means the eroute and SA with both be cleared.

Replace %SERVERIP% with the external IP of your server. You can find it out by:

curl http://ip.mtak.nl

Do note that the config file has changed with this Ubuntu release. If you have upgraded Ubuntu or followed an earlier tutorial, make sure you change the config for ipsec.

The shared secret

The shared secret is defined in the /etc/ipsec.secrets file. Make sure it is long and random:

%SERVERIP%  %any:   PSK "69EA16F2C529E74A7D1B0FE99E69F6BDCD3E44"

Yet again, replace %SERVERIP% with the IP of your server here. If you want to generate a random key you can use the following openssl command:

openssl rand -hex 30

Example output:

c12cf75b47c210b9d7094ce10e3b3544c6927ff49ca2d949252b5a94ccf5
Verify IPSEC Settings

Now to make sure IPSEC works, execute the following command:

ipsec verify

My output looks like this:

Checking your system to see if IPsec got installed and started correctly:
Version check and ipsec on-path                                 [OK]
Linux Openswan U2.6.38/K3.13.0-24-generic (netkey)
Checking for IPsec support in kernel                            [OK]
 SAref kernel support                                           [N/A]
 NETKEY:  Testing XFRM related proc values                      [OK]
    [OK]
    [OK]
Checking that pluto is running                                  [OK]
 Pluto listening for IKE on udp 500                             [OK]
 Pluto listening for NAT-T on udp 4500                          [OK]
Checking for 'ip' command                                       [OK]
Checking /bin/sh is not /bin/dash                               [WARNING]
Checking for 'iptables' command                                 [OK]
Opportunistic Encryption Support                                [DISABLED]

The /bin/sh and Opportunistic Encryption warnings can be ignored. The first one is a openswan bug and the second one causes xl2tpd to trip.

Configure xl2tpd

Use your favorite editor to edit the following file:

/etc/xl2tpd/xl2tpd.conf

Replace the contents with the following:

[global]
ipsec saref = yes
saref refinfo = 30

;debug avp = yes
;debug network = yes
;debug state = yes
;debug tunnel = yes

[lns default]
ip range = 172.16.1.30-172.16.1.100
local ip = 172.16.1.1
refuse pap = yes
require authentication = yes
;ppp debug = yes
pppoptfile = /etc/ppp/options.xl2tpd
length bit = yes
  • ip range = range of IPs to give to the connecting clients
  • local ip = IP of VPN server
  • refuse pap = refure pap authentication
  • ppp debug = yes when testing, no when in production

Local user (PAM / /etc/passwd) authentication

To use local user accounts via pam (or /etc/passwd), and thus not having plain text user passwords in a text file you have to do a few extra steps.

In your /etc/xl2tpd/xl2tpd.conf add the following line:

unix authentication = yes

and remove the following line:

refuse pap = yes

In the file /etc/ppp/options.xl2tpd make sure you do not add the following line (below it states to add it, but not if you want to use UNIX authentication):

require-mschap-v2

Also in that file (/etc/ppp/options.xl2tpd) add the following extra line:

login

Change /etc/pam.d/ppp to this:

auth    required        pam_nologin.so
auth    required        pam_unix.so
account required        pam_unix.so
session required        pam_unix.so

(As in, remove existing lines and add these)

Add the following to /etc/ppp/pap-secrets:

*       l2tpd           ""              *

(And, skip the chap-secrets file below (adding users).)

Configuring PPP

Use your favorite editor to edit the following file:

/etc/ppp/options.xl2tpd

Replace the contents with the following:

require-mschap-v2
ms-dns 8.8.8.8
ms-dns 8.8.4.4
auth
mtu 1200
mru 1000
crtscts
hide-password
modem
name l2tpd
proxyarp
lcp-echo-interval 30
lcp-echo-failure 4
  • ms-dns = The dns to give to the client. I use googles public DNS.
  • proxyarp = Add an entry to this systems ARP [Address Resolution Protocol] table with the IP address of the peer and the Ethernet address of this system. This will have the effect of making the peer appear to other systems to be on the local ethernet.
  • name l2tpd = is used in the ppp authentication file.

Adding users

Every user should be defined in the /etc/ppp/chap-secrets file. Below is an example file.

# Secrets for authentication using CHAP
# client       server  secret                  IP addresses
alice          l2tpd   0F92E5FC2414101EA            *
bob            l2tpd   DF98F09F74C06A2F             *
  • client = username for the user
  • server = the name we define in the ppp.options file for xl2tpd
  • secret = password for the user
  • IP Addresses = leave to * for any address or define addresses from were a user can login.

Testing it

To make sure everything has the newest config files restart openswan and xl2tpd:

/etc/init.d/ipsec restart 
/etc/init.d/xl2tpd restart

On the client connect to the server IP address (or add a DNS name) with a valid user, password and the shared secret. Test if you have internet access and which IP you have (via for example http://whatsmyip.org. If it is the VPN servers IP then it works.

If you experience problems make sure to check the client log files and the ubuntu /var/log/syslog and /var/log/auth.log files. If you google the error messages you most of the time get a good answer.

Tags: debian , ipsec , l2tp , openswan , openvpn , pptp , tutorials , ubuntu , vpn