vpn-slice

Table of Contents

• Introduction
  • Who this is for
  • Requirements
  • First steps
  • Usage
  • Diagnostics
• Inspiration and credits
• License
  • TODO

Introduction

This is a replacement for the vpnc-script used by OpenConnect or VPNC.

Instead of trying to copy the behavior of standard corporate VPN clients, which normally reroute all your network traffic through the VPN, this one tries to minimize your contact with an intrusive VPN. This is also known as a split-tunnel VPN, since it splits your traffic between the VPN tunnel and your normal network interfaces.

vpn-slice makes it easy to set up a split-tunnel VPN:

• By default, it only routes traffic for specific hosts or subnets through the VPN.
• It automatically looks up named hosts, using the VPN's DNS servers, and adds entries for them to your /etc/hosts (which it cleans up after VPN disconnection), however it does not otherwise alter your /etc/resolv.conf at all.
• It has many additional options to customize routing and lookup (for example, --route-splits to additionally route traffic for specific subnets requested by the server). Run vpn-slice --help to see them all.

Who this is for

If you are using a VPN to route all your traffic for privacy reasons (or to avoid censorship in repressive countries), then you do not want to use this.

The purpose of this tool is almost the opposite; it makes it easy to connect to a VPN while minimizing the traffic that passes over the VPN.

This is for people who have to connect to the high-security VPNs of corporations or other bureaucracies (which monitor and filter and otherwise impede network traffic), and thus wish to route as little traffic as possible through those VPNs.

Requirements

• Python 3.3+
• Either of the following:
  • dnspython module (preferred, tested with v1.16.0)
  • dig command-line DNS lookup tool (tested with v9.9.5 and v9.10.3)
• Supported OSes:
  • Linux kernel 3.x+ with iproute2 and iptables utilities (used for all routing setup)
  • macOS 10.x with BSD route
  • FreeBSD with BSD route if procfs is mounted

You can install the latest build from PyPI with pip (make sure you are using the Python 3.x version, usually invoked with pip3).

You should install as root (e.g. using sudo), because openconnect or vpnc will need to be able to invoke vpn-slice while running as root:

# latest release from PyPI
$ sudo pip3 install vpn-slice

# latest development version
$ sudo pip3 install https://github.com/dlenski/vpn-slice/archive/master.zip

On macOS, you can also install from the Homebrew repository:

$ brew install vpn-slice

First steps

Before trying to use vpn-slice with openconnect or vpnc, check that it works properly on your platform, and can verify that it has all of the access and dependencies that it needs (to modify /etc/hosts, alter routing table, etc.):

$ sudo vpn-slice --self-test
***************************************************************************
*** Self-test passed. Try using vpn-slice with openconnect or vpnc now. ***
***************************************************************************

If you run the self-test as a non-root user, it will tell you what required access it is unable to obtain:

$ vpn-slice --self-test
WARNING: Couldn't configure hosts provider: Cannot read/write /etc/hosts
******************************************************************************************
*** Self-test did not pass. Double-check that you are running as root (e.g. with sudo) ***
******************************************************************************************
Aborting because providers for hosts are required; use --help for more information

When you start trying to use vpn-slice for real, you should use the diagnostic options (e.g openconnect -s 'vpn-slice --verbose --dump') to troubleshoot and understand its behavior.

Usage

You should specify vpn-slice as your connection script with openconnect or vpnc. It has been tested with vpnc v0.5.3, OpenConnect v7.06+ (Cisco AnyConnect and Juniper protocols) and v8.0+ (PAN GlobalProtect protocol).

For example:

$ sudo openconnect gateway.bigcorp.com -u user1234 \
    -s 'vpn-slice 192.168.1.0/24 hostname1 alias2=alias2.bigcorp.com=192.168.1.43'
$ cat /etc/hosts
...
192.168.1.1 dns0.tun0					# vpn-slice-tun0 AUTOCREATED
192.168.1.2 dns1.tun0					# vpn-slice-tun0 AUTOCREATED
192.168.1.57 hostname1 hostname1.bigcorp.com		# vpn-slice-tun0 AUTOCREATED
192.168.1.43 alias2 alias2.bigcorp.com		# vpn-slice-tun0 AUTOCREATED

or

# With most versions of vpnc, you *must* specify an absolute path
# for the disconnect hook to work correctly, due to a bug.
#
# I reported this bug, but the original maintainers no longer maintain vpnc.
#   https://lists.unix-ag.uni-kl.de/pipermail/vpnc-devel/2016-August/004199.html
#
# However, some Linux distro packagers have picked up my patch in recent
# releases, e.g. Ubuntu 17.04:
#   https://changelogs.ubuntu.com/changelogs/pool/universe/v/vpnc/vpnc_0.5.3r550-3/changelog
#
$ sudo vpnc config_file \
       --script '/path/to/vpn-slice 192.168.1.0/24 hostname1 alias2=alias2.bigcorp.com=192.168.1.43'

Notice that vpn-slice accepts several different kinds of routes and hostnames on the command line:

• Hostnames alone (hostname1) as well as host-to-IP aliases (alias2=alias2.bigcorp.com=192.168.1.43). The former are first looked up using the VPN's DNS servers. Both are also added to the routing table, as well as to /etc/hosts (unless --no-host-names is specified). As in this example, multiple aliases can be specified for a single IP address.
• Subnets to include (10.0.0.0/8) in the VPN routes as well as subnets to explicitly exclude (%10.123.0.0/24).

There are many command-line options to alter the behavior of vpn-slice; try vpn-slice --help to show them all.

Diagnostics

Running with --verbose makes it explain what it is doing, while running with --dump shows the environment variables passed in by the caller.

Inspiration and credits

• @jagtesh's split-tunnelling tutorial gist taught me the basics of how to set up a split-tunnel VPN by wrapping the standard vpnc-script.
• @apenwarr's sshuttle has the excellent --auto-hosts and --seed-hosts options. These inspired the automatic host lookup feature.
• @gmacon's PR #11 substantially refactored the code to separate the OS-dependent parts more cleanly, and added macOS support.
• @joelbu's PR #30 added support for IPv6 DNS lookups using dig.

License

GPLv3 or later.

TODO / Help Wanted

• Better error-explaining
• Fix timing issues
• Improve IPv6 support
• Support OSes other than Linux and macOS
  • Other Unix-like operating systems should be pretty easy