chore(settings): refactor settings processing (#756)

- Better settings tree structure logged using `qdm12/gotree`
- Read settings from environment variables, then files, then secret files
- Settings methods to default them, merge them and override them
- `DNS_PLAINTEXT_ADDRESS` default changed to `127.0.0.1` to use DoT. Warning added if set to something else.
- `HTTPPROXY_LISTENING_ADDRESS` instead of `HTTPPROXY_PORT` (with retro-compatibility)

internal/provider/ivpn/openvpnconf.go

@@ -4,14 +4,14 @@ import (
 	"strconv"
 	"strings"
 
-	"github.com/qdm12/gluetun/internal/configuration"
+	"github.com/qdm12/gluetun/internal/configuration/settings"
 	"github.com/qdm12/gluetun/internal/constants"
 	"github.com/qdm12/gluetun/internal/models"
 	"github.com/qdm12/gluetun/internal/provider/utils"
 )
 
 func (i *Ivpn) BuildConf(connection models.Connection,
-	settings configuration.OpenVPN) (lines []string, err error) {
+	settings settings.OpenVPN) (lines []string, err error) {
 	if len(settings.Ciphers) == 0 {
 		settings.Ciphers = []string{constants.AES256cbc}
 	}
@@ -23,7 +23,7 @@ func (i *Ivpn) BuildConf(connection models.Connection,
 		"nobind",
 		"tls-exit",
 		"dev " + settings.Interface,
-		"verb " + strconv.Itoa(settings.Verbosity),
+		"verb " + strconv.Itoa(*settings.Verbosity),
 
 		// IVPN specific
 		"ping 5",
@@ -47,25 +47,25 @@ func (i *Ivpn) BuildConf(connection models.Connection,
 
 	lines = append(lines, utils.CipherLines(settings.Ciphers, settings.Version)...)
 
-	if settings.Auth != "" {
-		lines = append(lines, "auth "+settings.Auth)
+	if *settings.Auth != "" {
+		lines = append(lines, "auth "+*settings.Auth)
 	}
 
-	if settings.MSSFix > 0 {
-		lines = append(lines, "mssfix "+strconv.Itoa(int(settings.MSSFix)))
+	if *settings.MSSFix > 0 {
+		lines = append(lines, "mssfix "+strconv.Itoa(int(*settings.MSSFix)))
 	}
 
 	if connection.Protocol == constants.UDP {
 		lines = append(lines, "explicit-exit-notify")
 	}
 
-	if !settings.Root {
+	if !*settings.Root {
 		lines = append(lines, "user "+settings.ProcUser)
 		lines = append(lines, "persist-tun")
 		lines = append(lines, "persist-key")
 	}
 
-	if !settings.IPv6 {
+	if !*settings.IPv6 {
 		lines = append(lines, `pull-filter ignore "route-ipv6"`)
 		lines = append(lines, `pull-filter ignore "ifconfig-ipv6"`)
 	}