Implement handling of overlapping IPv6 routes with IPv6 remote VPN server address

Implement handling of overlapping IPv6 routes with IPv6 remote VPN server address

- socket.[ch]: add link_socket_current_remote_ipv6() helper to extract
   current address of remote VPN server (if IPv6, NULL otherwise), IPv6
   equivalent to link_socket_current_remote()

- init.c: pass remote VPN server address to init_route_ipv6_list()
   (link_socket_current_remote_ipv6())

- route.h: add route_ipv6_gateway_info to route_ipv6_list, and reorder
   structures so that this actually compiles.  Add iface/adapter_index
   to struct route_ipv6 (for non-tun/tap routes).

- route.[ch]: add "const" to *dest argument to get_default_gateway_ipv6()

- route.c: add route_ipv6_match_host() helper to check whether an IPv6
   address is matched by a given "route_ipv6" IPv6 route)

- route.c: init_route_ipv6_list()
   - call get_default_gateway_ipv6()
   - check to-be-installed IPv6 routes against VPN server address (if IPv6)
   - if an overlap is seen, add a host route for the VPN server address
     via the just-discovered gateway to the list of IPv6 routes to be
     installed (rl6->routes_ipv6)
   - warn if overlap is detected but platform code has not been able to
     discover IPv6 default gateway

- route.c: add_route_ipv6() / delete_route_ipv6(): set "device" to
   "external default gateway interface" (r6->iface) instead of TUN/TAP
   device (if set), which nicely enables arbitrary gateway/interface
   combinations for Linux

- ssl.c: add "IV_RGI6=1" to push-peer-info data to let server know we can
  handle pushed IPv6 routes that overlap with server IPv6 address

- tun.c: when adding/removing on-link routes, CLEAR(r6) first to
  ensure new struct route_ipv6 members are cleared

Tested on Linux with iproute2 and /bin/route, on eth and tun routes.

Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <1441985627-14822-7-git-send-email-gert@greenie.muc.de>
URL: http://article.gmane.org/gmane.network.openvpn.devel/10089