config: accept hostname for [domain-fronting] target

example.config.toml

@@ -112,11 +112,22 @@ allow-fallback-on-unknown-dc = false
 # required.
 [domain-fronting]
 # By default, mtg resolves the fronting hostname (from the secret) via DNS
-# to establish a TCP connection. If DNS resolution of that hostname is blocked,
-# you can specify an IP address to connect to directly. The hostname is still
-# used for SNI in the TLS handshake.
+# to establish a TCP connection. If that resolution is blocked, or loops
+# back to this server (e.g. mtg sits behind an SNI router whose DNS points
+# at itself), override the destination here.
 #
-# default value is not set (DNS resolution is used).
+# Use `host` — accepts a hostname or a literal IP. Hostnames are resolved
+# at dial time, so a dual-stack DNS record can reach the right backend
+# address family for IPv4 or IPv6 clients.
+#
+# The hostname from the secret is still used for SNI in the TLS handshake.
+#
+# default value is not set (the secret's hostname is used).
+# host = "fronting-backend"
+
+# Deprecated: use `host`. If `ip` is set, mtg logs a warning at startup
+# and ignores the value (domain-fronting falls back to the secret's
+# hostname unless `host` is also set).
 # ip = "10.10.10.11"
 
 # FakeTLS uses domain fronting protection. So it needs to know a port to

internal/cli/doctor.go

@@ -140,7 +140,18 @@ func (d *Doctor) checkDeprecatedConfig() bool {
 			"when":        "2.3.0",
 			"old":         "domain-fronting-ip",
 			"old_section": "",
-			"new":         "ip",
+			"new":         "host",
+			"new_section": "domain-fronting",
+		})
+	}
+
+	if d.conf.DomainFronting.IP.Value != nil {
+		ok = false
+		tplWDeprecatedConfig.Execute(os.Stdout, map[string]string{ //nolint: errcheck
+			"when":        "2.4.0",
+			"old":         "ip",
+			"old_section": "domain-fronting",
+			"new":         "host",
 			"new_section": "domain-fronting",
 		})
 	}
@@ -298,8 +309,8 @@ func (d *Doctor) checkNetworkAddresses(ntw mtglib.Network, addresses []string) e
 
 func (d *Doctor) checkFrontingDomain(ntw mtglib.Network) bool {
 	host := d.conf.Secret.Host
-	if ip := d.conf.GetDomainFrontingIP(nil); ip != "" {
-		host = ip
+	if override := d.conf.GetDomainFrontingHost(); override != "" {
+		host = override
 	}
 
 	port := d.conf.GetDomainFrontingPort(mtglib.DefaultDomainFrontingPort)

internal/cli/run_proxy.go

@@ -287,11 +287,23 @@ func warnSNIMismatch(conf *config.Config, ntw mtglib.Network, log mtglib.Logger)
 		"DPI may detect and block the proxy. See 'mtg doctor' for details")
 }
 
+func warnDeprecatedDomainFronting(conf *config.Config, log mtglib.Logger) {
+	if conf.DomainFrontingIP.Value != nil {
+		log.Warning(`config option "domain-fronting-ip" is deprecated and ignored; use "host" in [domain-fronting] instead`)
+	}
+
+	if conf.DomainFronting.IP.Value != nil {
+		log.Warning(`config option "ip" in [domain-fronting] is deprecated and ignored; use "host" instead`)
+	}
+}
+
 func runProxy(conf *config.Config, version string) error { //nolint: funlen, cyclop
 	logger := makeLogger(conf)
 
 	logger.BindJSON("configuration", conf.String()).Debug("configuration")
 
+	warnDeprecatedDomainFronting(conf, logger)
+
 	eventStream, err := makeEventStream(conf, logger)
 	if err != nil {
 		return fmt.Errorf("cannot build event stream: %w", err)
@@ -343,7 +355,7 @@ func runProxy(conf *config.Config, version string) error { //nolint: funlen, cyc
 		Secret:                      conf.Secret,
 		Concurrency:                 conf.GetConcurrency(mtglib.DefaultConcurrency),
 		DomainFrontingPort:          conf.GetDomainFrontingPort(mtglib.DefaultDomainFrontingPort),
-		DomainFrontingIP:            conf.GetDomainFrontingIP(nil),
+		DomainFrontingHost:          conf.GetDomainFrontingHost(),
 		DomainFrontingProxyProtocol: conf.GetDomainFrontingProxyProtocol(false),
 		PreferIP:                    conf.PreferIP.Get(mtglib.DefaultPreferIP),
 		AutoUpdate:                  conf.AutoUpdate.Get(false),

internal/cli/simple_run.go

@@ -17,8 +17,9 @@ type SimpleRun struct {
 	Concurrency         uint64        `kong:"name='concurrency',short='c',default='8192',help='Max number of concurrent connection to proxy.'"`                        //nolint: lll
 	TCPBuffer           string        `kong:"name='tcp-buffer',short='b',default='4KB',help='Deprecated and ignored'"`                                                 //nolint: lll
 	PreferIP            string        `kong:"name='prefer-ip',short='i',default='prefer-ipv6',help='IP preference. By default we prefer IPv6 with fallback to IPv4.'"` //nolint: lll
-	DomainFrontingPort  uint64        `kong:"name='domain-fronting-port',short='p',default='443',help='A port to access for domain fronting.'"`                        //nolint: lll
-	DomainFrontingIP    string        `kong:"name='domain-fronting-ip',help='An IP address to use for domain fronting instead of resolving the hostname via DNS.'"`    //nolint: lll
+	DomainFrontingPort  uint64        `kong:"name='domain-fronting-port',short='p',default='443',help='A port to access for domain fronting.'"`                                                                  //nolint: lll
+	DomainFrontingHost  string        `kong:"name='domain-fronting-host',help='Hostname or IP to dial for domain fronting instead of resolving the secret hostname.'"`                                           //nolint: lll
+	DomainFrontingIP    string        `kong:"name='domain-fronting-ip',help='Deprecated: use --domain-fronting-host. Setting this flag logs a warning at startup and the value is ignored.'"`                    //nolint: lll
 	DOHIP               net.IP        `kong:"name='doh-ip',short='n',default='1.1.1.1',help='IP address of DNS-over-HTTP to use.'"`                                    //nolint: lll
 	Timeout             time.Duration `kong:"name='timeout',short='t',default='10s',help='Network timeout to use'"`                                                    //nolint: lll
 	Socks5Proxies       []string      `kong:"name='socks5-proxy',short='s',help='Socks5 proxies to use for network access.'"`                                          //nolint: lll
@@ -48,6 +49,15 @@ func (s *SimpleRun) Run(cli *CLI, version string) error { //nolint: cyclop,funle
 		return fmt.Errorf("incorrect domain-fronting-port: %w", err)
 	}
 
+	if s.DomainFrontingHost != "" {
+		if err := conf.DomainFronting.Host.Set(s.DomainFrontingHost); err != nil {
+			return fmt.Errorf("incorrect domain-fronting-host: %w", err)
+		}
+	}
+
+	// --domain-fronting-ip is deprecated; the value is parsed only so the
+	// runtime check in runProxy can detect it and emit the warn-and-ignore
+	// log message. The value never reaches the dial path.
 	if s.DomainFrontingIP != "" {
 		if err := conf.DomainFrontingIP.Set(s.DomainFrontingIP); err != nil {
 			return fmt.Errorf("incorrect domain-fronting-ip: %w", err)

internal/config/config.go

@@ -4,7 +4,6 @@ import (
 	"bytes"
 	"encoding/json"
 	"fmt"
-	"net"
 	"net/url"
 
 	"github.com/9seconds/mtg/v2/mtglib"
@@ -38,6 +37,7 @@ type Config struct {
 	PublicIPv4                  TypeIP          `json:"publicIpv4"`
 	PublicIPv6                  TypeIP          `json:"publicIpv6"`
 	DomainFronting              struct {
+		Host          TypeHost `json:"host"`
 		IP            TypeIP   `json:"ip"`
 		Port          TypePort `json:"port"`
 		ProxyProtocol TypeBool `json:"proxyProtocol"`
@@ -117,14 +117,8 @@ func (c *Config) GetDomainFrontingPort(defaultValue uint) uint {
 	return c.DomainFrontingPort.Get(defaultValue)
 }
 
-func (c *Config) GetDomainFrontingIP(defaultValue net.IP) string {
-	if ip := c.DomainFronting.IP.Get(nil); ip != nil {
-		return ip.String()
-	}
-	if ip := c.DomainFrontingIP.Get(defaultValue); ip != nil {
-		return ip.String()
-	}
-	return ""
+func (c *Config) GetDomainFrontingHost() string {
+	return c.DomainFronting.Host.Get("")
 }
 
 func (c *Config) GetDomainFrontingProxyProtocol(defaultValue bool) bool {

internal/config/config_test.go

@@ -74,6 +74,47 @@ func (suite *ConfigTestSuite) TestString() {
 	suite.NotEmpty(conf.String())
 }
 
+func (suite *ConfigTestSuite) TestDomainFrontingIPIgnoredWhenHostSet() {
+	conf, err := config.Parse(suite.ReadConfig("minimal.toml"))
+	suite.NoError(err)
+
+	suite.NoError(conf.DomainFronting.Host.Set("fronting-backend"))
+	suite.NoError(conf.DomainFronting.IP.Set("10.0.0.10"))
+	suite.NoError(conf.Validate())
+	suite.Equal("fronting-backend", conf.GetDomainFrontingHost())
+}
+
+func (suite *ConfigTestSuite) TestDomainFrontingHostFromTOML() {
+	conf, err := config.Parse(suite.ReadConfig("domain_fronting_host.toml"))
+	suite.NoError(err)
+	suite.NoError(conf.Validate())
+	suite.Equal("fronting-backend", conf.GetDomainFrontingHost())
+}
+
+func (suite *ConfigTestSuite) TestDomainFrontingHostAcceptsLiteralIP() {
+	conf, err := config.Parse(suite.ReadConfig("domain_fronting_host_ip.toml"))
+	suite.NoError(err)
+	suite.NoError(conf.Validate())
+	suite.Equal("10.0.0.1", conf.GetDomainFrontingHost())
+}
+
+func (suite *ConfigTestSuite) TestDomainFrontingIPIgnoredFromTOML() {
+	conf, err := config.Parse(suite.ReadConfig("domain_fronting_ip.toml"))
+	suite.NoError(err)
+	suite.NoError(conf.Validate())
+	// Deprecated [domain-fronting].ip is parsed but never used to derive
+	// the dial target — the user must migrate to [domain-fronting].host.
+	suite.NotNil(conf.DomainFronting.IP.Get(nil))
+	suite.Equal("", conf.GetDomainFrontingHost())
+}
+
+func (suite *ConfigTestSuite) TestDomainFrontingNotSet() {
+	conf, err := config.Parse(suite.ReadConfig("minimal.toml"))
+	suite.NoError(err)
+	suite.NoError(conf.Validate())
+	suite.Equal("", conf.GetDomainFrontingHost())
+}
+
 func TestConfig(t *testing.T) {
 	t.Parallel()
 	suite.Run(t, &ConfigTestSuite{})

internal/config/parse.go

@@ -24,6 +24,7 @@ type tomlConfig struct {
 	PublicIPv4                  string `toml:"public-ipv4" json:"publicIpv4,omitempty"`
 	PublicIPv6                  string `toml:"public-ipv6" json:"publicIpv6,omitempty"`
 	DomainFronting              struct {
+		Host          string `toml:"host" json:"host,omitempty"`
 		IP            string `toml:"ip" json:"ip,omitempty"`
 		Port          uint   `toml:"port" json:"port,omitempty"`
 		ProxyProtocol bool   `toml:"proxy-protocol" json:"proxyProtocol,omitempty"`

internal/config/testdata/domain_fronting_host.toml

@@ -0,0 +1,5 @@
+secret = "7oe1GqLy6TBc38CV3jx7q09nb29nbGUuY29t"
+bind-to = "0.0.0.0:3128"
+
+[domain-fronting]
+host = "fronting-backend"

internal/config/testdata/domain_fronting_host_ip.toml

@@ -0,0 +1,5 @@
+secret = "7oe1GqLy6TBc38CV3jx7q09nb29nbGUuY29t"
+bind-to = "0.0.0.0:3128"
+
+[domain-fronting]
+host = "10.0.0.1"

internal/config/testdata/domain_fronting_ip.toml

@@ -0,0 +1,5 @@
+secret = "7oe1GqLy6TBc38CV3jx7q09nb29nbGUuY29t"
+bind-to = "0.0.0.0:3128"
+
+[domain-fronting]
+ip = "10.0.0.10"

internal/config/type_host.go

@@ -0,0 +1,61 @@
+package config
+
+import (
+	"fmt"
+	"net"
+	"strings"
+)
+
+// TypeHost is a non-empty string that is either a literal IP address
+// (IPv4 or IPv6) or a hostname suitable for DNS resolution. It does not
+// include a port — the port belongs in a separate field.
+type TypeHost struct {
+	Value string
+}
+
+func (t *TypeHost) Set(value string) error {
+	if value == "" {
+		return fmt.Errorf("host cannot be empty")
+	}
+
+	if net.ParseIP(value) != nil {
+		t.Value = value
+
+		return nil
+	}
+
+	if strings.ContainsAny(value, " \t\n/?#") {
+		return fmt.Errorf("incorrect host %q", value)
+	}
+
+	// At this point value is not a parsed IP (IPv6 literals returned
+	// above), so any remaining colon indicates a host:port form, which
+	// belongs in a separate field.
+	if strings.Contains(value, ":") {
+		return fmt.Errorf("host must not contain a port: %q", value)
+	}
+
+	t.Value = value
+
+	return nil
+}
+
+func (t TypeHost) Get(defaultValue string) string {
+	if t.Value == "" {
+		return defaultValue
+	}
+
+	return t.Value
+}
+
+func (t *TypeHost) UnmarshalText(data []byte) error {
+	return t.Set(string(data))
+}
+
+func (t TypeHost) MarshalText() ([]byte, error) {
+	return []byte(t.Value), nil
+}
+
+func (t TypeHost) String() string {
+	return t.Value
+}

internal/config/type_host_test.go

@@ -0,0 +1,77 @@
+package config_test
+
+import (
+	"encoding/json"
+	"testing"
+
+	"github.com/9seconds/mtg/v2/internal/config"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/suite"
+)
+
+type typeHostTestStruct struct {
+	Value config.TypeHost `json:"value"`
+}
+
+type TypeHostTestSuite struct {
+	suite.Suite
+}
+
+func (suite *TypeHostTestSuite) TestUnmarshalFail() {
+	testData := []string{
+		"",
+		"web:8443",
+		"http://example.com",
+		"example.com/path",
+		"two words",
+	}
+
+	for _, v := range testData {
+		data, err := json.Marshal(map[string]string{
+			"value": v,
+		})
+		suite.NoError(err)
+
+		suite.T().Run(v, func(t *testing.T) {
+			assert.Error(t, json.Unmarshal(data, &typeHostTestStruct{}))
+		})
+	}
+}
+
+func (suite *TypeHostTestSuite) TestUnmarshalOk() {
+	testData := []string{
+		"example.com",
+		"web",
+		"sub.example.com",
+		"127.0.0.1",
+		"2001:db8::1",
+	}
+
+	for _, v := range testData {
+		value := v
+
+		data, err := json.Marshal(map[string]string{
+			"value": value,
+		})
+		suite.NoError(err)
+
+		suite.T().Run(value, func(t *testing.T) {
+			testStruct := &typeHostTestStruct{}
+			assert.NoError(t, json.Unmarshal(data, testStruct))
+			assert.Equal(t, value, testStruct.Value.Get(""))
+		})
+	}
+}
+
+func (suite *TypeHostTestSuite) TestGet() {
+	value := config.TypeHost{}
+	suite.Equal("default", value.Get("default"))
+
+	suite.NoError(value.Set("example.com"))
+	suite.Equal("example.com", value.Get("default"))
+}
+
+func TestTypeHost(t *testing.T) {
+	t.Parallel()
+	suite.Run(t, &TypeHostTestSuite{})
+}