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

management/server/policy.go

@@ -195,6 +195,8 @@ type FirewallRule struct {
 	// PeerIP of the peer
 	PeerIP string
 
+	PeerIP6 string
+
 	// Direction of the traffic
 	Direction int
 
@@ -278,8 +280,14 @@ func (a *Account) connResourcesGenerator() (func(*PolicyRule, []*nbpeer.Peer, in
 					peersExists[peer.ID] = struct{}{}
 				}
 
+				ip6 := ""
+				if peer.IP6 != nil {
+					ip6 = peer.IP6.String()
+				}
+
 				fr := FirewallRule{
 					PeerIP:    peer.IP.String(),
+					PeerIP6:   ip6,
 					Direction: direction,
 					Action:    string(rule.Action),
 					Protocol:  string(rule.Protocol),
@@ -287,6 +295,7 @@ func (a *Account) connResourcesGenerator() (func(*PolicyRule, []*nbpeer.Peer, in
 
 				if isAll {
 					fr.PeerIP = "0.0.0.0"
+					fr.PeerIP6 = "::"
 				}
 
 				ruleID := (rule.ID + fr.PeerIP + strconv.Itoa(direction) +
@@ -474,6 +483,7 @@ func toProtocolFirewallRules(update []*FirewallRule) []*proto.FirewallRule {
 
 		result[i] = &proto.FirewallRule{
 			PeerIP:    update[i].PeerIP,
+			PeerIP6:   update[i].PeerIP6,
 			Direction: direction,
 			Action:    action,
 			Protocol:  protocol,