feat(netlink): detect ipv6 support level (#2523)

- add option `IPV6_CHECK_ADDRESSESES=[2001:4860:4860::8888]:53,[2606:4700:4700::1111]:53`
- gluetun needs access to the addresses above through the host firewall, to test ipv6 support before setting up the vpn

internal/firewall/interfaces.go

@@ -26,6 +26,8 @@ type firewallImpl interface { //nolint:interfacebloat
 	AcceptInputToPort(ctx context.Context, intf string, port uint16, remove bool) error
 	AcceptInputToSubnet(ctx context.Context, intf string, subnet netip.Prefix) error
 	AcceptIpv6MulticastOutput(ctx context.Context, intf string) error
+	AcceptOutput(ctx context.Context, protocol, intf string,
+		ip netip.Addr, port uint16, remove bool) error
 	AcceptOutputFromIPToSubnet(ctx context.Context, intf string, assignedIP netip.Addr,
 		subnet netip.Prefix, remove bool) error
 	AcceptOutputThroughInterface(ctx context.Context, intf string, remove bool) error

internal/firewall/iptables/iptables.go

@@ -162,6 +162,24 @@ func (c *Config) AcceptOutputTrafficToVPN(ctx context.Context,
 	return c.runIP6tablesInstruction(ctx, instruction)
 }
 
+func (c *Config) AcceptOutput(ctx context.Context,
+	protocol, intf string, ip netip.Addr, port uint16, remove bool,
+) error {
+	interfaceFlag := "-o " + intf
+	if intf == "*" { // all interfaces
+		interfaceFlag = ""
+	}
+
+	instruction := fmt.Sprintf("%s OUTPUT -d %s %s -p %s -m %s --dport %d -j ACCEPT",
+		appendOrDelete(remove), ip, interfaceFlag, protocol, protocol, port)
+	if ip.Is4() {
+		return c.runIptablesInstruction(ctx, instruction)
+	} else if c.ip6Tables == "" {
+		return fmt.Errorf("accept output to VPN server: %w", ErrNeedIP6Tables)
+	}
+	return c.runIP6tablesInstruction(ctx, instruction)
+}
+
 // AcceptOutputFromIPToSubnet accepts outgoing traffic from sourceIP to destinationSubnet
 // on the interface intf. If intf is empty, it is set to "*" which means all interfaces.
 // If remove is true, the rule is removed instead of added.

internal/firewall/wrappers.go

@@ -19,3 +19,9 @@ func (c *Config) TempDropOutputTCPRST(ctx context.Context,
 ) {
 	return c.impl.TempDropOutputTCPRST(ctx, src, dst, excludeMark)
 }
+
+func (c *Config) AcceptOutput(ctx context.Context, protocol, intf string,
+	ip netip.Addr, port uint16, remove bool,
+) error {
+	return c.impl.AcceptOutput(ctx, protocol, intf, ip, port, remove)
+}