Feat: OPENVPN_INTERFACE defaulting to tun0

- Fix: custom config with custom network interface name for firewall
- Keep VPN tunnel interface in firewall state
- Vul fix: only allow traffic through vpn interface when needed
- Adapt code to adapt to network interface name
- Remove outdated TUN and TAP constants

internal/openvpn/custom/modify.go

@@ -11,7 +11,7 @@ import (
 )
 
 func modifyCustomConfig(lines []string, settings configuration.OpenVPN,
-	connection models.Connection) (modified []string) {
+	connection models.Connection, intf string) (modified []string) {
 	// Remove some lines
 	for _, line := range lines {
 		switch {
@@ -22,6 +22,7 @@ func modifyCustomConfig(lines []string, settings configuration.OpenVPN,
 			strings.HasPrefix(line, "user "),
 			strings.HasPrefix(line, "proto "),
 			strings.HasPrefix(line, "remote "),
+			strings.HasPrefix(line, "dev "),
 			settings.Cipher != "" && strings.HasPrefix(line, "cipher "),
 			settings.Cipher != "" && strings.HasPrefix(line, "data-ciphers "),
 			settings.Auth != "" && strings.HasPrefix(line, "auth "),
@@ -35,6 +36,7 @@ func modifyCustomConfig(lines []string, settings configuration.OpenVPN,
 	// Add values
 	modified = append(modified, connection.OpenVPNProtoLine())
 	modified = append(modified, connection.OpenVPNRemoteLine())
+	modified = append(modified, "dev "+intf)
 	modified = append(modified, "mute-replay-warnings")
 	modified = append(modified, "auth-nocache")
 	modified = append(modified, "pull-filter ignore \"auth-token\"") // prevent auth failed loop