At this time, I need a working VPN for my iPhone and other clients. I used to have PPTP VPN, because is’s easy to configure. However it suggest that PPTP maybe not available on certain mobile networks. But L2TP/IPSec can be used, and its security of IPSec is also nice.
We need several components in order to run L2TP/IPSec:
Internet Protocol Security (IPsec) is a technology protocol suite for securing Internet Protocol (IP) communications by authenticating and/or encrypting each IP packet of a communication session. IPSec encrypts your IP packets to provide encryption and authentication, so no one can decrypt or forge data between your server and client.
Openswan is the preferred daemon to run IPSec, install it on your Debian server:
# apt-get install openswan
There are several ways to handle encryption for IPSec. We use Pre-Shared Key since it is easy to tweak. Edit
/etc/ipsec.conf like this:
version 2.0 config setup nat_traversal=yes virtual_private=%v4:10.0.0.0/8,%v4:192.168.0.0/16,%v4:172.16.0.0/12 oe=off protostack=netkey conn L2TP-PSK-NAT rightsubnet=vhost:%priv also=L2TP-PSK-noNAT conn L2TP-PSK-noNAT authby=secret pfs=no auto=add keyingtries=3 rekey=no ikelifetime=8h keylife=1h type=transport left=YOUR.SERVER.IP.ADDRESS leftprotoport=17/1701 right=%any rightprotoport=17/%any
And then Edit
YOUR.SERVER.IP.ADDRESS %any: PSK "YourSharedSecret"
Note: Remember to change YOUR.SERVER.IP.ADDRESS and YourSharedSecret accordingly.
Run the following commands for openswan to stop complaining
for each in /proc/sys/net/ipv4/conf/* do echo 0 > $each/accept_redirects echo 0 > $each/send_redirects done
Check if IPSec is correctly setup
# ipsec verify
Don’t worry about the disabled Opportunistic Encryption Support. Just make sure other checks are passed OK. Then restart openswan by running
# /etc/init.d/ipsec restart
Now you can add a L2TP/IPSec connection on your client and check IPSec is working. Use whatever account and password. We are not there yet. The only thing you need to make sure is that you connect to the right server with the right shared secret as specified in
/etc/ipsec.secrets on your server.
/var/log/auth.log on your server by running:
while OS X is trying to connect to your server via L2TP/IPSec. It will fail eventually because we haven’t configured L2TP yet, but if you see a line in the system log saying something like “IPSec connection established”.
OK, now IPSec is configure done.
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. Interestingly, both Apple and Microsoft tend to refer L2TP as the secure VPN technology but totally ignore the fact that security is provided by IPSec.
The commonly used L2TP daemon is xl2tpd from the same buys behind openswan. Install it by running:
# apt-get install xl2tpd
[global] ipsec saref = yes [lns default] ip range = 10.1.2.2-10.1.2.255 local ip = 10.1.2.1 refuse chap = yes refuse pap = yes require authentication = yes ppp debug = yes pppoptfile = /etc/ppp/options.xl2tpd length bit = yes
ip range is the set of internal IP addresses that will allocate to clients connected. Make sure it does not overlap with your exisiting IP addresses being used, and not in conflict with the ones on the client’s network. Since most home routers use 172.16.X.X and 192.168.X.X range, you might want to avoid that. local ip is the internal IP for the L2TP server. Make sure it is NOT in the ip range allocated to clients.
I also run PPTP service using PPP, so I would like to use the same daemon to handle user managenet. Install ppp by running
# apt-get install ppp
if you do not have it. Create this file
/etc/ppp/options.xl2tpd with the following content
require-mschap-v2 ms-dns 126.96.36.199 ms-dns 188.8.131.52 asyncmap 0 auth crtscts lock hide-password modem debug name l2tpd proxyarp lcp-echo-interval 30 lcp-echo-failure 4
Note I am using Google Public DNS in the ms-dns field. If you want to use other DNS servers, change the IP addresses accordingly.
Add a test user in
/etc/ppp/chap-secrets to try out if L2TP works.
# user server password ip test l2tpd testpassword *
Now restart xl2tpd by running
sudo /etc/init.d/xl2tpd restart
In addition, if you use iptables for firewalling, make sure it forwards packets so you can browse the Interent after connecting to VPN. Run the following command
iptables --table nat --append POSTROUTING --jump MASQUERADE
/etc/sysctl.conf and uncomment following line:
or just type:
echo 1 > /proc/sys/net/ipv4/ip_forward
Update the L2TP/IPSec VPN connection on your OS X with the test user account and try connect. If it can connect and authenticate successfully, congrats! You are done. Now go enjoy the better security.