Building a l2tp/IPsec VPN based around a OpenBSD head-end – Part 1

This is the first in a series of posts to cover building a l2tp/IPsec VPN service which remote users (road warriors) connect to.
In this post I will begin with getting OpenBSD setup as the head-end & follow up with subsequent posts to cover configuration of various platforms as clients which compose the road warriors.
Undeadly featured an article on configuring OpenBSD in 2012, things have improved since this article was posted and some of the steps are no longer required, hence I will go over the process again here.

It’s assumed you have an install of OpenBSD running that’s setup as a gateway and communicating on the network, we will continue from there.

The following snippet of config needs to be added to your PF config (/etc/pf.conf by default). It unconditionally permits the IPsec ESP & AH protocols intended for the OpenBSD host, as well as any UDP traffic for ISAKMP and to support NAT traversal.
pass quick proto { esp, ah } from any to self
pass quick proto udp from any to self port {isakmp, ipsec-nat-t} keep state
pass on enc0 from any to self keep state (if-bound)

A minimal PF config which just permits the establishment of a VPN tunnel might look like the following

set skip lo
block return
pass quick proto { esp, ah } from any to self
pass quick proto udp from any to self port {isakmp, ipsec-nat-t} keep state
pass on enc0 from any to self keep state (if-bound)

By only permitting isakmp, it enforces having a working IPsec config before anything else happens whereas permitting UDP port 1701 would permit the establishment of a l2tp tunnel without IPsec which in this scenario would likely be undesired.

A basic IPsec config to use a pre-shared key.The default ciphers used for main & quick mode are documented in ipsec.conf(5). The IP address 1.2.3.4 is configured on the OpenBSD host which connections will be accepted on.

ike passive esp transport proto udp from 1.2.3.4 to any port 1701 psk "password"

Note, the OpenBSD defaults are too high for establishing a connection using the networking preferences on Apple devices and so would need to be restricted down to auth "hmac-sha1" enc "3des" group modp1024 which is not recommended, configuring Apple systems will be covered as a separate article.

The default npppd config (/etc/npppd/nppd.conf) works as-is, without any further changes required. That is unless you prefer to use RADIUS for accounting, instead of local user accounts.

myuser:\
    :password=mypass:\
    :framed-ip-address=10.0.0.111:

npppd is set to use pppx(4) interfaces for established sessions, in order for these interfaces to work correctly, pipex(4) needs to be enabled.

sysctl net.pipex.enable=1

and adding net.pipex.enable=1 to /etc/sysctl.conf so it’s set on boot.

Note, hosts missing this commit (5.8-RELEASE and snapshots from today & prior) will suffer a panic on the OpenBSD host upon establishment of a session by clients, if pipex(4) is not enabled.

Start isakmpd & npppd with

isakmpd -K
npppd

Load your ipsec.conf with
ipsecctl -f /etc/ipsec.conf

Your host should be ready to accept VPN connections, set this services to be started on boot by adding the following to /etc/rc.conf.local
isakmpd_flags="-K"
ipsec=YES
npppd_flags=""

2 thoughts on “Building a l2tp/IPsec VPN based around a OpenBSD head-end – Part 1

  1. Venture37 Post author

    The high level, non technical answer is that you’re more likely to find a modern operating system shipping with IPsec support or if you’re lucky support for l2tp as well.
    Certainly the current releases of commercial operating systems such as Android, iOS, OS X, Windows all ship with a native support for l2tp/IPsec. Meaning there is no requirement to install another piece of software. For open source operating systems, it’s safe to assume IPsec is supported by the kernel (enabled by default or not). You’ll need a l2tp client installed however. If you’re happy with OpenVPN, I’m not trying to convince you to move.

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