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/expressvpn/openvpnconf.go

@@ -3,23 +3,26 @@ package expressvpn
 import (
 	"strconv"
 
-	"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 (p *Provider) 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}
 	}
-	if settings.Auth == "" {
-		settings.Auth = constants.SHA512
+	auth := *settings.Auth
+	if auth == "" {
+		auth = constants.SHA512
 	}
 
-	if settings.MSSFix == 0 {
-		settings.MSSFix = 1200
+	mssFix := *settings.MSSFix
+	if mssFix == 0 {
+		const defaultMSSFix = 1200
+		mssFix = defaultMSSFix
 	}
 
 	lines = []string{
@@ -27,19 +30,19 @@ func (p *Provider) BuildConf(connection models.Connection,
 		"nobind",
 		"tls-exit",
 		"dev " + settings.Interface,
-		"verb " + strconv.Itoa(settings.Verbosity),
+		"verb " + strconv.Itoa(*settings.Verbosity),
 
 		// Expressvpn specific
 		"fast-io",
 		"fragment 1300",
-		"mssfix " + strconv.Itoa(int(settings.MSSFix)),
+		"mssfix " + strconv.Itoa(int(mssFix)),
 		"sndbuf 524288",
 		"rcvbuf 524288",
 		"verify-x509-name Server name-prefix", // security hole I guess?
 		"remote-cert-tls server",              // updated name of ns-cert-type
 		"key-direction 1",
 		"auth-user-pass " + constants.OpenVPNAuthConf,
-		"auth " + settings.Auth,
+		"auth " + auth,
 
 		// Added constant values
 		"mute-replay-warnings",
@@ -59,13 +62,13 @@ func (p *Provider) BuildConf(connection models.Connection,
 		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"`)
 	}