Introduction

Advanced Configuration

Including configuration files in config files

One of the lesser-known possibilities when using configuration files is the ability to include other configuration files. This can be especially handy when setting up a complex OpenVPN server, where multiple OpenVPN instances are offered simultaneously. The common configuration directives can be stored in a single file, whereas the connection-specific parts can be stored in a file for each instance. In this recipe, we will set up two OpenVPN instances, one using UDP and the other using TCP as the transport protocol.

Note that this option does not allow for the sharing of VPN IP address ranges between instances.

Getting ready

Set up the client and server certificates using the first recipe from Chapter 2, Client-server IP-only Networks. For this recipe, the server computer was running CentOS 5 Linux and OpenVPN 2.1.3.

How to do it...

1.First, create the common configuration file: dev tun

tls-auth /etc/openvpn/cookbook/ta.key 0

persist-key persist-tun keepalive 10 60

push "route 10.198.0.0 255.255.0.0" topology subnet

user nobody group nobody

daemon

Save it as example11-1-common.conf. Note that this configuration file does not include a protocol specification or server line. Also, note that we will be using the same server certificate for both OpenVPN instances.

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2.Next, create two server configuration files, one for UDP-based connections: config example11-1-common.conf

proto udp port 1194

server 192.168.100.0 255.255.255.0

log-append /var/log/openvpn-udp.log

Save it as example11-1-server1.conf.

3.And one for TCP-based connections: config example11-1-common.conf

proto tcp port 443

server 192.168.200.0 255.255.255.0

log-append /var/log/openvpn-tcp.log

Save it as example11-1-server2.conf. The second instance is listening on the

HTTPS port 443, which is an often-used trick to circumvent very strict firewalls, or to work around a badly configured firewall.

4.Start both servers:

[root@server]# openvpn --config example11-1-server1.conf [root@server]# openvpn --config example11-1-server2.conf

Check the log files to see if both the servers have successfully started.

How it works...

OpenVPN configuration files are treated very similarly to command-line options. As the

--config command-line option is used almost always, it is also possible to use it inside a configuration file again. This allows for a split in the configuration options, where

directives that are common to all OpenVPN instances can be stored in a single file for easy maintenance. The instance-specific directives (such as the server directive) can then be stored in much smaller configuration files, which are also less likely to change over time.

This again eases maintenance of a large-scale OpenVPN server setup.

OpenVPN has a built-in protection mechanism to avoid including the same configuration file recursively.

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Advanced Configuration

Multiple remotes and remote-random

OpenVPN has built-in support for automatic failover and load-balancing: if the connection to one OpenVPN server cannot be established, then the next configured server is chosen.

The remote-random directive can be used to load-balance many OpenVPN clients across multiple OpenVPN servers. In this recipe, we will set up two OpenVPN servers and then use the remote-random directive to have a client choose either one of the two servers.

Note that OpenVPN does not offer transparent failover, in which case the existing connections are transparently migrated to another server. Transparent failover is much harder to achieve with a VPN setup (not just OpenVPN), as the secure session keys need to be migrated from one server to the other as well. This is currently not possible with OpenVPN.

Getting ready

We use the following network layout:

Set up the client and server certificates using the first recipe from Chapter 2, Client-server IP-only Networks. For this recipe, the server computers were running CentOS 5 Linux and OpenVPN 2.1.3. The client was running Fedora 13 Linux and OpenVPN 2.1.1. Keep the configuration file basic-udp-server.conf from the Chapter 2 recipe Server-side routing at hand.

How to do it...

1.Start both servers:

[root@server1]# openvpn --config basic-udp-server.conf

[root@server2]# openvpn --config basic-udp-server.conf

Check the log files to see that both the servers have successfully started.

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Note that we can use the exact same configuration file on both servers. By using masquerading, the VPN clients will appear to come from either server1

or server2.

2.Set up masquerading on both servers:

[root@server1]# iptables -t nat -I POSTROUTING -o eth0 \ -j MASQUERADE

[root@server2]# iptables -t nat -I POSTROUTING -o eth0 \ -j MASQUERADE

3.Create the client configuration file:

client proto udp

remote openvpnserver1.example.com 1194 remote openvpnserver2.example.com 1194 remote-random

dev tun nobind

tls-auth /etc/openvpn/cookbook/ta.key 1

ns-cert-type server

Save it as example11-2-client.conf.

4.Start the client:

[root@client]# openvpn --config example11-2-client.conf

The OpenVPN client will randomly choose which server to connect to.

After the connection has been established, stop the first OpenVPN process on the server that the client connected to:

[root@server1]# killall openvpn

And wait for the client to reconnect. After the default timeout period, the client will reconnect to an alternate server.

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Advanced Configuration

How it works...

When the OpenVPN client starts up and remote-random is specified, it randomly picks a server from the list of available remote servers. If the VPN connection to this server cannot be established, it will pick the next server from the list, and so on. When the VPN connection is dropped, for example, due to a failing server, the OpenVPN client will try to reconnect after a default timeout period. In the server configuration file used in the Chapter 2 recipe Server-side routing, the timeout period is configured using the keepalive option.

There's more...

When setting up a failover OpenVPN solution there are many things to consider, some of which are outlined here.

Mixing TCP and UDP-based setups

It is also possible to mix TCP and UDP-based setups by specifying the protocol type with the remote directive:

remote openvpnserver1.example.com 1194 udp remote openvpnserver2.example.com 1194 tcp

An OpenVPN 2.1-specific feature known as connection blocks is much handier to use in this case. The use of connection blocks is explained in the next chapter.

Advantage of using TCP-based connections

There is one major advantage when using a TCP-based setup in combination with a failover solution. If the OpenVPN server to which a client is connected is unavailable, the TCP connection will fail almost immediately. This leads to a very short timeout period after which the OpenVPN client will try to reconnect. With a UDP-based setup, the client cannot so easily detect whether the server is unavailable and must first wait for the keepalive timeout to pass.

Automatically reverting to the first OpenVPN server

A question that is asked from time to time is whether it is possible to configure OpenVPN to also support automatic revert: a second OpenVPN instance is set up to provide a failover solution. When the main OpenVPN server is unavailable, the backup instance takes over. However, when the main OpenVPN server comes back online, the clients are not automatically reconnected to the main server. For this, a client reset (or server reset of the second OpenVPN instance) is required. It is possible to achieve this using scripting but it depends largely on what type of connectivity is considered acceptable: it takes some time for an OpenVPN client to detect when the remote server is not responding and to reconnect. The VPN connectivity will be intermittent in such a setup. Especially when the network connection to the main OpenVPN server is not stable, this can lead to very low availability.

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