add netlink support for Linux

Hi all,

finally this is the first "real" patchset that introduces native netlink support for the Linux platform.

- Description:

At the moment openvpn operates on the tun interface and on the routing table by directly invoking the "ip" command (or ifconfig/route if nettools is selected at compile time).

With this patchset, openvpn would not need to fork new processes to run the "ip" binary any longer, but would directly talk to the kernel by means of the netlink interface.
This means simpler/cleaner code and, possibly, faster execution.

Another important advantage of this change is that the openvpn process will be in charge of directly working with the kernel, thus it can be granted special capabilities so that interfaces/routes operations can be carried out even when running as non-root.

Christian Hesse is working on a follow-up patch to properly allow the above.

This patchset also offers a first step towards a refactoring of the tun.c and route.c code.

The idea moving forward is to drop nettools support once this patchset is merged, but to retain support for ip and the --ifconfig/route-noexec options.

The last patch implements a little framework to test sitnl functionalities. Some functions are actually tested in the framework, but more should be added in the future.

- Git:

This code can also be found on GitHub (based on latest master) at:
https://github.com/ordex/openvpn/tree/sitnl

Regards,

Antonio Quartulli (8):
implement platform generic networking API
implement networking API for iproute2
tun.c: use new networking API to handle tun interface on Linux route.c: use new networking API to handle routing table on Linux introduce sitnl: Simplified Interface To NetLink
configure: don't check for route/ifconfig on linux route.c: use sitnl to implement get_default_gateway_ipv6() unit tests: implement test for sitnl

 configure.ac                               |    5 +-
 src/openvpn/Makefile.am                    |    1 +
 src/openvpn/errlevel.h                     |    1 +
 src/openvpn/networking.h                   |  225 ++++
 src/openvpn/networking_ip.c                |  368 ++++++
 src/openvpn/networking_sitnl.c             | 1206 ++++++++++++++++++++
 src/openvpn/route.c                        |  364 ++----
 src/openvpn/sitnl.h                        |  217 ++++
 src/openvpn/tun.c                          |  199 +---
 tests/Makefile.am                          |    3 +-
 tests/t_net.sh                             |  170 +++
 tests/unit_tests/openvpn/Makefile.am       |   23 +-
 tests/unit_tests/openvpn/test_networking.c |  217 ++++
 13 files changed, 2559 insertions(+), 440 deletions(-)
 create mode 100644 src/openvpn/networking.h
 create mode 100644 src/openvpn/networking_ip.c
 create mode 100644 src/openvpn/networking_sitnl.c
 create mode 100644 src/openvpn/sitnl.h
 create mode 100755 tests/t_net.sh
 create mode 100644 tests/unit_tests/openvpn/test_networking.c

2.17.0