Scenario

In this recipe two machines will be configured:

• A server which is a firewall and has access to a private network ¹
• A client which initiates the connections to the server and gains direct access to the private network

 --------         /\_/-\/\/-\       -----------------
| Client |~~~~~~~/ Internet /~~~~~~| Server/Firewall |~~~[ private net ]
 --------        \_/-\/\_/\/      / ----------------- \
    ||\                           \          ||\       \
    || {tun0}                      {eth0}    || {tun0}  {eth1}
    ||                                       ||
    \-================= tunnel ==============-/

For this recipe lets number things like this:

• the private net is 10.99.99.0/24
• eth0 on the server has public IP 5.6.7.8
• eth1 on the server has private IP 10.99.99.1
• the VPN network is 10.254.254.0/30
• tun0 on the server has private IP 10.254.254.1
• tun0 on the client has private IP 10.254.254.2

On the Client

If you do not already have them, generate an SSH keypair for root:

$ sudo ssh-keygen -t rsa

/etc/network/interfaces: Add this stanza to the file:

iface tun0 inet static
      pre-up ssh -S /var/run/ssh-myvpn-tunnel-control -M -f -w 0:0 5.6.7.8 true
      pre-up sleep 5
      address 10.254.254.2
      pointopoint 10.254.254.1
      netmask 255.255.255.252
      up route add -net 10.99.99.0 netmask 255.255.255.0 gw 10.254.254.1 tun0
      post-down ssh -S /var/run/ssh-myvpn-tunnel-control -O exit 5.6.7.8

The first time we connect to the server as root we may need to acknowledge saving the servers SSH key fingerprint:

$ sudo ssh 5.6.7.8
The authenticity of host '5.6.7.8 (5.6.7.8)' can't be established.
RSA key fingerprint is aa:fe:a0:38:7d:11:78:60:01:b0:80:78:90:ab:6a:d2.
Are you sure you want to continue connecting (yes/no)? yes
Warning: Permanently added '5.6.7.8' (RSA) to the list of known hosts.

Don’t bother logging in, just hit CTRL-C.

On the server

/etc/ssh/sshd_config: Add/modify the two keywords to have the same values as below.

PermitTunnel point-to-point
PermitRootLogin forced-commands-only

The PermitRootLogin line is changed from the default of no. You do restrict root SSH login, right?

/root/.ssh/authorized_keys: Add the following line.

tunnel="0",command="/sbin/ifdown tun0;/sbin/ifup tun0" ssh-rsa AAAA ..snipped.. == root@server

Replace everything starting with “ssh-rsa” with the contents of root’s public SSH key from the client(/root/.ssh/id_rsa.pub on the client).

/etc/network/interfaces: Add the following stanza.

iface tun0 inet static
      address 10.254.254.1
      netmask 255.255.255.252
      pointopoint 10.254.254.2

/etc/sysctl.conf: Make sure net.ipv4.conf.default.forwarding is set to 1

net.ipv4.conf.default.forwarding=1

This will take effect upon the next reboot so make it active now:

$ sudo sysctl net.ipv4.conf.default.forwarding=1

Using the VPN

user@client:~$ sudo ifup tun0
RTNETLINK answers: File exists
run-parts: /etc/network/if-up.d/avahi-autoipd exited with return code 2

user@client:~$ ping -c 2 10.99.99.1
PING 10.99.99.1 (10.99.99.1) 56(84) bytes of data.
64 bytes from 10.99.99.1 icmp_seq=1 ttl=64 time=96.3 ms
64 bytes from 10.99.99.1 icmp_seq=2 ttl=64 time=94.9 ms

--- 10.99.99.1 ping statistics ---
2 packets transmitted, 2 received, 0% packet loss, time 999ms
rtt min/avg/max/mdev = 94.954/95.670/96.387/0.780 ms
user@client:~$ sudo ifdown tun0
Exit request sent.

You may get the two errors after running ifup. No problem, they are harmless.