Add public-ipv4/public-ipv6 config options for manual IP override

On some servers ifconfig.co is unreachable (e.g. Hetzner, AdGuard DNS
blocklists), causing 'mtg doctor' SNI-DNS check and 'mtg access' link
generation to fail. New config options allow specifying public IPs
manually, with automatic detection as fallback.

Fixes #405

internal/config/config.go

@@ -35,6 +35,8 @@ type Config struct {
 	DomainFrontingProxyProtocol TypeBool        `json:"domainFrontingProxyProtocol"`
 	TolerateTimeSkewness        TypeDuration    `json:"tolerateTimeSkewness"`
 	Concurrency                 TypeConcurrency `json:"concurrency"`
+	PublicIPv4                  TypeIP          `json:"publicIpv4"`
+	PublicIPv6                  TypeIP          `json:"publicIpv6"`
 	DomainFronting              struct {
 		IP            TypeIP   `json:"ip"`
 		Port          TypePort `json:"port"`

internal/config/config_test.go

@@ -42,6 +42,32 @@ func (suite *ConfigTestSuite) TestParseMinimalConfig() {
 	suite.Equal("0.0.0.0:3128", conf.BindTo.String())
 }
 
+func (suite *ConfigTestSuite) TestParsePublicIP() {
+	conf, err := config.Parse(suite.ReadConfig("public_ip.toml"))
+	suite.NoError(err)
+	suite.Equal("203.0.113.1", conf.PublicIPv4.Get(nil).String())
+	suite.Equal("2001:db8::1", conf.PublicIPv6.Get(nil).String())
+}
+
+func (suite *ConfigTestSuite) TestParsePublicIPv4Only() {
+	conf, err := config.Parse(suite.ReadConfig("public_ip_v4_only.toml"))
+	suite.NoError(err)
+	suite.Equal("203.0.113.1", conf.PublicIPv4.Get(nil).String())
+	suite.Nil(conf.PublicIPv6.Get(nil))
+}
+
+func (suite *ConfigTestSuite) TestParsePublicIPInvalid() {
+	_, err := config.Parse(suite.ReadConfig("public_ip_invalid.toml"))
+	suite.Error(err)
+}
+
+func (suite *ConfigTestSuite) TestParsePublicIPNotSet() {
+	conf, err := config.Parse(suite.ReadConfig("minimal.toml"))
+	suite.NoError(err)
+	suite.Nil(conf.PublicIPv4.Get(nil))
+	suite.Nil(conf.PublicIPv6.Get(nil))
+}
+
 func (suite *ConfigTestSuite) TestString() {
 	conf, err := config.Parse(suite.ReadConfig("minimal.toml"))
 	suite.NoError(err)

internal/config/parse.go

@@ -21,6 +21,8 @@ type tomlConfig struct {
 	DomainFrontingProxyProtocol bool   `toml:"domain-fronting-proxy-protocol" json:"domainFrontingProxyProtocol,omitempty"`
 	TolerateTimeSkewness        string `toml:"tolerate-time-skewness" json:"tolerateTimeSkewness,omitempty"`
 	Concurrency                 uint   `toml:"concurrency" json:"concurrency,omitempty"`
+	PublicIPv4                  string `toml:"public-ipv4" json:"publicIpv4,omitempty"`
+	PublicIPv6                  string `toml:"public-ipv6" json:"publicIpv6,omitempty"`
 	DomainFronting              struct {
 		IP            string `toml:"ip" json:"ip,omitempty"`
 		Port          uint   `toml:"port" json:"port,omitempty"`

internal/config/testdata/public_ip.toml

@@ -0,0 +1,4 @@
+secret = "7oe1GqLy6TBc38CV3jx7q09nb29nbGUuY29t"
+bind-to = "0.0.0.0:3128"
+public-ipv4 = "203.0.113.1"
+public-ipv6 = "2001:db8::1"

internal/config/testdata/public_ip_invalid.toml

@@ -0,0 +1,3 @@
+secret = "7oe1GqLy6TBc38CV3jx7q09nb29nbGUuY29t"
+bind-to = "0.0.0.0:3128"
+public-ipv4 = "not-an-ip"

internal/config/testdata/public_ip_v4_only.toml

@@ -0,0 +1,3 @@
+secret = "7oe1GqLy6TBc38CV3jx7q09nb29nbGUuY29t"
+bind-to = "0.0.0.0:3128"
+public-ipv4 = "203.0.113.1"