Difference between revisions of "OpenVPN"

Revision as of 16:36, 16 February 2012

OpenVPN is a secure tunnel. I use it to connect to my home network.

OpenVPN uses certificates to both authenticate the client with the server, and the server with the client. The only hard part about OpenVPN is setting up the certificate infrastructure. You need a root certificate (the certificate authority, CA), and certificates for each server and each client, signed by this root certificate.

Setting up a public key infrastructure

Main article: Create a OpenVPN Certificate Authority

The use of client certificates enhances the security, but makes it harder to deploy, as you need to distribute client certificates to each host. You can not use an existing public key infrastructure (PKI); you would allow anyone with a certificate of that PKI to connect to your server (the tls-remote and tls-verify options can limits the allowed clients). You either need to to set up your own certificate infrastructure, or limit OpenVPN to use password-based authentication (see the options auth-user-pass-verify, client-cert-not-required and username-as-common-name).

The easiest method it to use easy-rsa, as described in the OpenVPN How-to.

The article Create a OpenVPN Certificate Authority describes the steps in more detail, but basically gives the same result.

Certificates, when deployed correctly, are much more secure than passwords, since the secret (key) does not need to be exchanged or shared between the different hosts. However, doing so requires a security hygiene which may not be required for a small-scale deployment.

Running IPv6 through the Tunnel

OpenVPN can both use a tap or a tun interface. A tap interface tunnels Ethernet traffic, so that includes both IPv4, IPv6 and NetBIOS. A tun interface tunnels IP traffic. Version 2.3 of OpenVPN supports tunnelling bot IPv4 and IPv6 traffic through the tunnel. OpenVPN 2.2 can also tunnel IPv6 traffic, but requires a custom scripts to configure the IPv6 addresses and routing tables. At the time of writing, OpenVPN 2.3 is not yet released, so the following three options are available:

• Use a tap interface to bridge all Ethernet traffic, including IPv4 and IPv6 traffic. The disadvantage is that is less efficient since the tunnel contains broadcast traffic not targeted to the client;
• Wait for OpenVPN 2.3 or compile a development version;
• Use OpenVPN 2.2 with a custom connect- and disconnect script to set the IPv6 address of the tunnel.

I choose to compile a development version.

Install Development Version

On FreeBSD:

cd /usr/ports/security/openvpn-devel
sudo make install

On Mac OS X:

sudo port install lzo2
git clone git://openvpn.git.sourceforge.net/gitroot/openvpn/openvpn.git
cd openvpn
git checkout 32ab329bc69c6292c205d4f33a4b8069341798d3
autoreconf -i -v
./configure --with-lzo-headers=/opt/local/include --with-lzo-lib=/opt/local/lib
make
make check
sudo make install

Configure IPv6 Tunnel

Imagine the following setup:

A sample server and client configuration follow. These examples could probably be reduced in size; some parameters are default or some can be combined (e.g. "server" can replace "mode server", "tls-server" "ifconfig" and "ifconfig-pool").

See the OpenVPN man page and Gert Döring's IPv6 Payload Patch for information on the different configuration options.

OpenVPN server configuration

# The IP to listen on for incoming VPN connections
local 192.0.2.206
proto udp   # or tcp-server is UDP is blocked
dev tun1

# PKI Stuff
ca          ca.crt
cert        server.crt
key         server.key
dh          dh2048.pem
crl-verify  crl.pem
# Option TLS shared secret for HMAC signing for additional security
tls-auth    tls-auth.key

mode server
tls-server

topology net30
ifconfig 10.240.177.1 10.240.177.2
ifconfig-pool 10.240.177.4 10.240.177.251
route 10.240.177.0 255.255.255.0

tun-ipv6
ifconfig-ipv6 2001:db8:f0:b1::1 2001:db8:f0:b1::2
ifconfig-ipv6-pool 2001:db8:f0:b1::4/64
route-ipv6 2001:db8:f0:b1::/64

# the actual routes are 10.240.129.0/24 and 2001:db8:f0:81::/64 but this avoids 
# a bug when connecting from the private network
push route 10.240.128.0 255.255.254.0
push route-ipv6 2001:db8:f0:80::0/63

ifconfig-pool-persist ipp.txt

# server and client ping every 10 seconds, client reconnects after 60 seconds.
keepalive 10 60

user nobody
group nobody
persist-key
persist-tun
ns-cert-type client
remote-cert-tls client
# Allow a client to log in multiple times
duplicate-cn
#opt-verify

# Logging 
#
log         /var/log/openvpn-server.log
log-append  /var/log/openvpn.log
status      /var/log/openvpn-status.log
verb 3

OpenVPN client configuration

# First try UDP
remote 145.99.1.67 1194 udp

# If UDP fails, try TCP
remote 145.99.1.67 1194 tcp-client

dev tun
nobind
tls-client
pull

ca       ca.crt
cert     cert.crt
key      key.key
tls-auth tls-auth.key

tun-ipv6
persist-key
persist-tun
ns-cert-type server

Problems

This section describes some of the problems I encountered and how I solved them.

LZO Dependency

Problem

configure: checking for LZO Library and Header files...
checking lzo/lzo1x.h usability... no
checking lzo/lzo1x.h presence... no
checking for lzo/lzo1x.h... no
checking lzo1x.h usability... no
checking lzo1x.h presence... no
checking for lzo1x.h... no
LZO headers were not found
LZO library available from http://www.oberhumer.com/opensource/lzo/
configure: error: Or try ./configure --disable-lzo OR ./configure --enable-lzo-stub

Solution

If lzo2 library isn't yet install, install it first:

sudo port install lzo2

Check where lzo2 is installed:

% locate lzo1x.h
/opt/local/include/lzo/lzo1x.h

and tell openvpn where to find it:

./configure --with-lzo-headers=/opt/local/include --with-lzo-lib=/opt/local/lib

BF-CFC cipher not found

Problem

% make check
...
Sat Feb 11 22:46:37 2012 OpenVPN 2.x-master x86_64-apple-darwin11.2.0 [SSL (OpenSSL)] [LZO2] [eurephia] [MH] [PF_INET6] [IPv6 payload 20110522-1 (2.2.0)] built on Feb 11 2012
Sat Feb 11 22:46:37 2012 Cipher 'BF-CBC' uses a mode not supported by OpenVPN in your current configuration.  CBC mode is always supported, while CFB and OFB modes are supported only when using SSL/TLS authentication and key exchange mode, and when OpenVPN has been built with ALLOW_NON_CBC_CIPHERS.
Sat Feb 11 22:46:37 2012 Exiting due to fatal error
FAIL: t_lpback.sh
...

The above is strange, as BF-CBC (Blowfish) is the default Cipher

BF-CBC cipher seem normally compiled in:

% /usr/local/sbin/openvpn --show-ciphers
...
BF-CBC 128 bit default key (variable)
...

It is also build in into OpenSSL:

% openssl list-cipher-algorithms
...
BF => BF-CBC
...

This seem to be bug introduced in revision 670f9dd91aed7ac435b79c0e28e49fa7c256642c. Unfortunately, as of this writing it has not yet been fixed.

Solution

Revert to a revision before this bug was introduced:

cd openvpn
make distclean
git checkout 32ab329bc69c6292c205d4f33a4b8069341798d3 

VPN from Private Network

There is no benefit in a connection from the private network to the VPN server. However, such connection may sometimes occur if OpenVPN is set to automatically connect to the VPN server. This leads to a problem when the VPN connection is subsequently terminated.

Problem

After the VPN connection is terminated, while the client is still in the local network, the client loses all its connectivity.

If the VPN tunnel is established in the above example, the routes 10.240.129.0/24 and 2001:db8:f0:81::/64 are pushed to the client. Since the client is connecting from this network, the routing table already contains these networks. The OpenVPN nevertheless tries to add them, which gives the error "File exists":

route: writing to routing socket: File exists
add net 2001:db8:f0:81::: gateway tun0: File exists
route: writing to routing socket: File exists
add net 10.240.129.0 gateway 10.240.129.5: File exists

The connection proceeds as normal. The actual routing table is not changed (traffic to these ranges still takes the direct route, not the tunnel). As soon as the VPN connection is brought down, so are these routes:

delete net 10.240.129.0: gateway 10.240.129.5
delete_route_ipv6(2001:db8:f0:81::/64) 
delete net 2001:db8:f0:81::: gateway tun0
delete_route_ipv6(2001:db8:f0:81::/64)

Despite the mismatch of the gateway, Mac OS X still removes these routes from the routing table, meaning that the OS no longer knows how to make local connections.

Workaround

There are a few possible solutions:

• Simply don't connect to the OpenVPN server when the laptop is the private network. This can perhaps even be automated with a script that checks the local IP address before connecting.
• Create a custom connect and disconnect script that brings the routes up and down.
• Make sure the route that is pushed by the OpenVPN server is different from the local subnet. What I did is push the larger subnets:

# the actual routes are 10.240.129.0/24 and 2001:db8:f0:81::/64 but pushing a larger 
# subnet avoids a bug when connecting from the private network.
push route 10.240.128.0 255.255.254.0
push route-ipv6 2001:db8:f0:80::0/63

If you are using the Viscosity GUI, an alternative solution is to enable the preference "Reset network interfaces on disconnect" in the Advanced preferences tab.

IPv6 Tunnel is not explicitly supported by Mac OS X

Problem

While using the above example scripts, I get the following warnings:

NOTE: explicit support for IPv6 tun devices is not provided for this OS
...
add_route_ipv6(2001:db8:f0:b1::/64 -> 2001:db8:f0:b1::4 metric -1683000573) dev tun0
route: writing to routing socket: File exists
add net 2001:db8:f0:b1::: gateway tun0: File exists

Solution

Despite these warnings, everything works for me.

If it does not for you, I recommend to:

• Update your tun/tap driver. I got mine from Tuntap OS X.
• Ask on the OpenVPN mailing list or OpenVPN Forum.