OpenVPN – creating a routed VPN
In this article, I will show you how I created a routed VPN using
In this network, multiple clients can attach to the server, each of which
has access to the network attached to the server. Each client can also
contact any other client, subject to firewall rules.
In my case, I wanted a way for all my servers (on the internet, in data
centers) to contact my CVS repository behind my firewall at home. Given that
home has a dynamic IP address, it complicates matters. A VPN solves this issue
and provides several benefits. I have outlined the problems
in my other diary
and I urge you
to read that before proceeding. It will provide valuable background
as to why I have chosen this particular solution.
Two people have been of great help while I struggled with OpenVPN. ecrist
and krzee have both pointed me to examples and features. Others within
the FreeNode OpenVPN channel also listened to my goals and provided suggestions.
Here are a few good references:
This article relies upon previous articles on this website:
Most of this article will concentrate on the configuration and setup. It will
not cover certificates or installation.
NOTE: since this was originally written, I have changed the configuration. The article
used to show both ifconfig-pool-persist
The latter can be used with ifconfig-push to guarantee static IP addresses. That is
what I need. I have since changed the configuration shown below to reflect what I am
This section shows you the setup of my OpenVPN server. The main configuration
file is /usr/local/etc/openvpn/openvpn.conf. This is mine:
server 10.8.1.0 255.255.255.0
keepalive 10 120
tls-auth /usr/local/etc/openvpn/keys/ta.key 0
push "route 10.55.0.0 255.255.255.0"
Some of the options from above are outlined below. For full details,
please refer to the OpenVPN man page.
, and dh
directives are straight from the basic setup
given in the previous article.
specifies the directory for custom client config files.
We use these files for assigning static IPs, but they have additional uses.
indicates that our VPN server will use the 10.8.1.0/24 subnet.
allows each VPN client to see all other VPN clients.
is a shared secret, same file in each client, that is optional
but allows n additional layer of HMAC authentication on top of the TLS control
channel to protect against DoS attacks.
ensures that each client can access the 10.55.0.0/24 network on
the VPN server. This push adds an entry to the client’s routing table.
The configuration file, /usr/local/etc/openvpn/openvpn.conf, contains this:
remote myserver.example.com 1194
tls-auth /usr/local/etc/openvpn/keys/ta.key 1
As expected, I will outline some of these directives.
designates this as a client configuration.
avoids a man-in-the-middle attack.
is the same shared secret file as mentioned in the server
section. I will show you how to generate it later. Notice that the
direction is 1 on the client and 2 on the server. See the –secret option
on the OpenVPN man page for more information.
, and dh
have been previously
Generating the tls-auth
You can generate a tls-auth file with this command:
openvpn --genkey --secret /usr/local/etc/openvpn/keys/ta.key
Copy the file contents to each client.
Getting it running
Start both the server and the client. You should see this on the server:
tun0: flags=8051<UP,POINTOPOINT,RUNNING,MULTICAST> mtu 1500
inet6 fe80::204:acff:fea3:74af%tun0 prefixlen 64 scopeid 0x9
inet 10.8.1.1 --> 10.8.1.2 netmask 0xffffffff
Opened by PID 59658
Notice that this configuration uses tun0, not tap0.
On the client, you should see something like this:
tun0: flags=8051<UP,POINTOPOINT,RUNNING,MULTICAST> metric 0 mtu 1500
inet 10.8.1.14 --> 10.8.1.13 netmask 0xffffffff
Opened by PID 39769
You will also see that the server is configured for IPv6 but the client
is not (i.e. the inet6 line).
The client should be able to ping the server:
$ ping 10.8.1.1
PING 10.8.1.1 (10.8.1.1): 56 data bytes
64 bytes from 10.8.1.1: icmp_seq=0 ttl=64 time=24.077 ms
64 bytes from 10.8.1.1: icmp_seq=1 ttl=64 time=23.429 ms
64 bytes from 10.8.1.1: icmp_seq=2 ttl=64 time=23.045 ms
64 bytes from 10.8.1.1: icmp_seq=3 ttl=64 time=31.404 ms
--- 10.8.1.1 ping statistics ---
4 packets transmitted, 4 packets received, 0.0% packet loss
round-trip min/avg/max/stddev = 23.045/25.489/31.404/3.435 ms
That’s it. Now, on to static IP addresses
Static IP addresses
I want static IP addresses for my clients. This will make things
easier when it comes to running jobs on them. These hosts already
have public IP addresses and hostnames. I will add private hostnames
for them. For example, if the host can be accessed publicly by
the name nyi.example.com, then I will add a new entry to my private
DNS server (accessible only from my LAN at home) for
nyi-vpn IN A 10.8.1.20
After reloading the name server, I can resolve that hostname:
$ host nyi-vpn
nyi-vpn.example.com has address 10.8.1.20
To assign that IP address to that host, I create a file in the
client-config-dir, namely: /usr/local/etc/openvpn/ccd
That file must have the same name as the client’s X509 common name. In this
The file will contain:
ifconfig-push nyi-vpn.example.com 255.255.255.0
You can provide an IP address instead of a hostname. Restart your client
and you should see something similar to this in /var/log/messages on your
OPTIONS IMPORT: reading client specific options from: /usr/local/etc/openvpn/ccd/nyi.example.com
MULTI: Learn: 10.8.1.20 -> nyi.example.com/172.16.1.23:51376
MULTI: primary virtual IP for nyi.example.com/172.16.1.23:51376: 10.8.1.20
nyi.example.com/172.16.1.23:51376 PUSH: Received control message: 'PUSH_REQUEST'
nyi.example.com/172.16.1.23:51376 SENT CONTROL [nyi.example.com]: 'PUSH_REPLY,route 10.55.0.0
255.255.255.0,route 10.8.1.0 255.255.255.0,ping 10,ping-restart 120,
ifconfig 10.8.1.20 255.255.255.0' (status=1)
And on the client, you’ll see messages similar to this:
TUN/TAP device /dev/tun0 opened
/sbin/ifconfig tun0 10.8.1.40 255.255.255.0 mtu 1500 netmask 255.255.255.255 up
/sbin/route add -net 10.55.0.0 255.255.255.0 255.255.255.0
/sbin/route add -net 10.8.1.0 255.255.255.0 255.255.255.0
There, you should be running now. I think this VPN solution will be ideal to me.
Time will tell. Let’s see what happens when my first IP address change happens.
For now, I’m about to alter my Nagios monitoring to check my remote clients
over the VPN rather than the public networks. This little bit of work now
will save me a great deal of time when my IP address changes.