VPNSP value custom for OpenVPN custom config files (#621)

- Retro-compatibility: `OPENVPN_CUSTOM_CONFIG` set implies `VPNSP=custom`
- Change: `up` and `down` options are not filtered out
- Change: `OPENVPN_INTERFACE` overrides the network interface defined in the configuration file
- Change: `PORT` overrides any port found in the configuration file
- Feat: config file is read when building the OpenVPN configuration, so it's effectively reloaded on VPN restarts
- Feat: extract values from custom file at start to log out valid settings
- Maint: `internal/openvpn/extract` package instead of `internal/openvpn/custom` package
- Maint: All providers' `BuildConf` method return an error
- Maint: rename `CustomConfig` to `ConfFile` in Settings structures

internal/provider/custom/openvpnconf.go

@@ -0,0 +1,101 @@
+package custom
+
+import (
+	"errors"
+	"fmt"
+	"strconv"
+	"strings"
+
+	"github.com/qdm12/gluetun/internal/configuration"
+	"github.com/qdm12/gluetun/internal/constants"
+	"github.com/qdm12/gluetun/internal/models"
+	"github.com/qdm12/gluetun/internal/provider/utils"
+)
+
+var ErrExtractData = errors.New("failed extracting information from custom configuration file")
+
+func (p *Provider) BuildConf(connection models.Connection,
+	settings configuration.OpenVPN) (lines []string, err error) {
+	lines, _, err = p.extractor.Data(settings.ConfFile)
+	if err != nil {
+		return nil, fmt.Errorf("%w: %s", ErrExtractData, err)
+	}
+
+	lines = modifyConfig(lines, connection, settings)
+
+	return lines, nil
+}
+
+func modifyConfig(lines []string, connection models.Connection,
+	settings configuration.OpenVPN) (modified []string) {
+	// Remove some lines
+	for _, line := range lines {
+		switch {
+		case
+			line == "",
+			strings.HasPrefix(line, "verb "),
+			strings.HasPrefix(line, "auth-user-pass "),
+			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 "),
+			settings.MSSFix > 0 && strings.HasPrefix(line, "mssfix "),
+			!settings.IPv6 && strings.HasPrefix(line, "tun-ipv6"):
+		default:
+			modified = append(modified, line)
+		}
+	}
+
+	// Add values
+	modified = append(modified, connection.OpenVPNProtoLine())
+	modified = append(modified, connection.OpenVPNRemoteLine())
+	modified = append(modified, "dev "+settings.Interface)
+	modified = append(modified, "mute-replay-warnings")
+	modified = append(modified, "auth-nocache")
+	modified = append(modified, "pull-filter ignore \"auth-token\"") // prevent auth failed loop
+	modified = append(modified, "auth-retry nointeract")
+	modified = append(modified, "suppress-timestamps")
+	if settings.User != "" {
+		modified = append(modified, "auth-user-pass "+constants.OpenVPNAuthConf)
+	}
+	modified = append(modified, "verb "+strconv.Itoa(settings.Verbosity))
+	if settings.Cipher != "" {
+		modified = append(modified, utils.CipherLines(settings.Cipher, settings.Version)...)
+	}
+	if settings.Auth != "" {
+		modified = append(modified, "auth "+settings.Auth)
+	}
+	if settings.MSSFix > 0 {
+		modified = append(modified, "mssfix "+strconv.Itoa(int(settings.MSSFix)))
+	}
+	if !settings.IPv6 {
+		modified = append(modified, `pull-filter ignore "route-ipv6"`)
+		modified = append(modified, `pull-filter ignore "ifconfig-ipv6"`)
+	}
+	if !settings.Root {
+		modified = append(modified, "user "+settings.ProcUser)
+	}
+
+	modified = append(modified, "") // trailing line
+
+	return uniqueLines(modified)
+}
+
+func uniqueLines(lines []string) (unique []string) {
+	seen := make(map[string]struct{}, len(lines))
+	unique = make([]string, 0, len(lines))
+
+	for _, line := range lines {
+		_, ok := seen[line]
+		if ok {
+			continue
+		}
+		seen[line] = struct{}{}
+		unique = append(unique, line)
+	}
+
+	return unique
+}