Add support for IPv6 networks (on Linux clients) (#1459)

* Feat add basic support for IPv6 networks

Newly generated networks automatically generate an IPv6 prefix of size
64 within the ULA address range, devices obtain a randomly generated
address within this prefix.

Currently, this is Linux only and does not yet support all features
(routes currently cause an error).

* Fix firewall configuration for IPv6 networks

* Fix routing configuration for IPv6 networks

* Feat provide info on IPv6 support for specific client to mgmt server

* Feat allow configuration of IPv6 support through API, improve stability

* Feat add IPv6 support to new firewall implementation

* Fix peer list item response not containing IPv6 address

* Fix nftables breaking on IPv6 address change

* Fix build issues for non-linux systems

* Fix intermittent disconnections when IPv6 is enabled

* Fix test issues and make some minor revisions

* Fix some more testing issues

* Fix more CI issues due to IPv6

* Fix more testing issues

* Add inheritance of IPv6 enablement status from groups

* Fix IPv6 events not having associated messages

* Address first review comments regarding IPv6 support

* Fix IPv6 table being created even when IPv6 is disabled

Also improved stability of IPv6 route and firewall handling on client side

* Fix IPv6 routes not being removed

* Fix DNS IPv6 issues, limit IPv6 nameservers to IPv6 peers

* Improve code for IPv6 DNS server selection, add AAAA custom records

* Ensure IPv6 routes can only exist for IPv6 routing peers

* Fix IPv6 network generation randomness

* Fix a bunch of compilation issues and test failures

* Replace method calls that are unavailable in Go 1.21

* Fix nil dereference in cleanUpDefaultForwardRules6

* Fix nil pointer dereference when persisting IPv6 network in sqlite

* Clean up of client-side code changes for IPv6

* Fix nil dereference in rule mangling and compilation issues

* Add a bunch of client-side test cases for IPv6

* Fix IPv6 tests running on unsupported environments

* Fix import cycle in tests

* Add missing method SupportsIPv6() for windows

* Require IPv6 default route for IPv6 tests

* Fix panics in routemanager tests on non-linux

* Fix some more route manager tests concerning IPv6

* Add some final client-side tests

* Add IPv6 tests for management code, small fixes

* Fix linting issues

* Fix small test suite issues

* Fix linter issues and builds on macOS and Windows again

* fix builds for iOS because of IPv6 breakage

client/internal/routemanager/client.go

@@ -38,6 +38,7 @@ type clientNetwork struct {
 	peerStateUpdate     chan struct{}
 	routePeersNotifiers map[string]chan struct{}
 	chosenRoute         *route.Route
+	chosenIP            *net.IP
 	network             netip.Prefix
 	updateSerial        uint64
 }
@@ -221,6 +222,7 @@ func (c *clientNetwork) removeRouteFromWireguardPeer(peerKey string) error {
 
 func (c *clientNetwork) removeRouteFromPeerAndSystem() error {
 	if c.chosenRoute != nil {
+		// TODO IPv6 (pass wgInterface)
 		if err := removeVPNRoute(c.network, c.getAsInterface()); err != nil {
 			return fmt.Errorf("remove route %s from system, err: %v", c.network, err)
 		}
@@ -261,10 +263,20 @@ func (c *clientNetwork) recalculateRouteAndUpdatePeerAndSystem() error {
 			return fmt.Errorf("remove route from peer: %v", err)
 		}
 	} else {
+		// TODO recheck IPv6
+		gwAddr := c.wgInterface.Address().IP
+		c.chosenIP = &gwAddr
+		if c.network.Addr().Is6() {
+			if c.wgInterface.Address6() == nil {
+				return fmt.Errorf("Could not assign IPv6 route %s for peer %s because no IPv6 address is assigned",
+					c.network.String(), c.wgInterface.Address().IP.String())
+			}
+			c.chosenIP = &c.wgInterface.Address6().IP
+		}
 		// otherwise add the route to the system
 		if err := addVPNRoute(c.network, c.getAsInterface()); err != nil {
 			return fmt.Errorf("route %s couldn't be added for peer %s, err: %v",
-				c.network.String(), c.wgInterface.Address().IP.String(), err)
+				c.network.String(), c.chosenIP.String(), err)
 		}
 	}