Interface Binding Support for frpc

README.md

@@ -1207,6 +1207,41 @@ serverPort = 7000
 transport.proxyURL = "http://user:pwd@192.168.1.128:8080"
 ```
 
+### Network Interface Binding
+
+frpc can bind to specific network interfaces when connecting to frps, which is useful for multi-network systems or when you need to route traffic through specific network paths.
+
+```toml
+# frpc.toml
+serverAddr = "x.x.x.x"
+serverPort = 7000
+
+[transport]
+# Bind to a specific network interface
+connectServerInterface = "eth0"
+
+# Or use auto-detection for first available interface
+# connectServerInterface = "auto"
+
+# Or bind to a specific IP address (existing feature)
+# connectServerLocalIP = "192.168.1.100"
+```
+
+You can also specify interface binding via command line:
+
+```bash
+# Bind to specific interface
+./frpc --bind-interface eth0 -c frpc.toml
+
+# Auto-detect interface
+./frpc --bind-interface auto -c frpc.toml
+
+# Bind to specific IP
+./frpc --bind-ip 192.168.1.100 -c frpc.toml
+```
+
+This feature supports all protocols except kcp and works across Linux, macOS, and Windows platforms.
+
 ### Port range mapping
 
 *Added in v0.56.0*

client/connector.go

@@ -196,8 +196,14 @@ func (c *defaultConnectorImpl) realConnect() (net.Conn, error) {
 		dialOptions = append(dialOptions, libnet.WithTLSConfig(tlsConfig))
 	}
 
-	if c.cfg.Transport.ConnectServerLocalIP != "" {
-		dialOptions = append(dialOptions, libnet.WithLocalAddr(c.cfg.Transport.ConnectServerLocalIP))
+	// Resolve binding address from interface name or IP address
+	bindAddr, err := c.resolveBindAddress()
+	if err != nil {
+		xl.Errorf("failed to resolve bind address: %v", err)
+		return nil, err
+	}
+	if bindAddr != "" {
+		dialOptions = append(dialOptions, libnet.WithLocalAddr(bindAddr))
 	}
 	dialOptions = append(dialOptions,
 		libnet.WithProtocol(protocol),
@@ -214,6 +220,20 @@ func (c *defaultConnectorImpl) realConnect() (net.Conn, error) {
 	return conn, err
 }
 
+// resolveBindAddress resolves the binding address from interface name or IP address
+func (c *defaultConnectorImpl) resolveBindAddress() (string, error) {
+	// Priority: Interface name > IP address
+	if c.cfg.Transport.ConnectServerInterface != "" {
+		return netpkg.ResolveBindAddress(c.cfg.Transport.ConnectServerInterface)
+	}
+
+	if c.cfg.Transport.ConnectServerLocalIP != "" {
+		return c.cfg.Transport.ConnectServerLocalIP, nil
+	}
+
+	return "", nil // No binding specified
+}
+
 func (c *defaultConnectorImpl) Close() error {
 	c.closeOnce.Do(func() {
 		if c.quicConn != nil {

conf/frpc_full_example.toml

@@ -88,9 +88,16 @@ transport.poolCount = 5
 transport.protocol = "tcp"
 
 # set client binding ip when connect server, default is empty.
-# only when protocol = tcp or websocket, the value will be used.
+# works with tcp, websocket, http, https, and quic protocols. Not supported with kcp protocol.
 transport.connectServerLocalIP = "0.0.0.0"
 
+# set client binding interface when connect server, default is empty.
+# valid values are interface names (e.g., "eth0", "wlan0") or "auto" for auto-detection.
+# works with tcp, websocket, http, https, and quic protocols. Not supported with kcp protocol.
+# priority: if both connectServerLocalIP and connectServerInterface are set, connectServerInterface takes precedence.
+# transport.connectServerInterface = "eth0"
+# transport.connectServerInterface = "auto"
+
 # if you want to connect frps by http proxy or socks5 proxy or ntlm proxy, you can set proxyURL here or in global environment variables
 # it only works when protocol is tcp
 # transport.proxyURL = "http://user:passwd@192.168.1.128:8080"

conf/legacy/frpc_legacy_full.ini

@@ -99,9 +99,16 @@ login_fail_exit = true
 protocol = tcp
 
 # set client binding ip when connect server, default is empty.
-# only when protocol = tcp or websocket, the value will be used.
+# works with tcp, websocket, http, https, and quic protocols. Not supported with kcp protocol.
 connect_server_local_ip = 0.0.0.0
 
+# set client binding interface when connect server, default is empty.
+# valid values are interface names (e.g., "eth0", "wlan0") or "auto" for auto-detection.
+# works with tcp, websocket, http, https, and quic protocols. Not supported with kcp protocol.
+# priority: if both connect_server_local_ip and connect_server_interface are set, connect_server_interface takes precedence.
+# connect_server_interface = eth0
+# connect_server_interface = auto
+
 # quic protocol options
 # quic_keepalive_period = 10
 # quic_max_idle_timeout = 30

pkg/config/flags.go

@@ -164,6 +164,8 @@ func RegisterClientCommonConfigFlags(cmd *cobra.Command, c *v1.ClientCommonConfi
 		cmd.PersistentFlags().BoolVarP(&c.Log.DisablePrintColor, "disable_log_color", "", false, "disable log color in console")
 		cmd.PersistentFlags().StringVarP(&c.Transport.TLS.ServerName, "tls_server_name", "", "", "specify the custom server name of tls certificate")
 		cmd.PersistentFlags().StringVarP(&c.DNSServer, "dns_server", "", "", "specify dns server instead of using system default one")
+		cmd.PersistentFlags().StringVarP(&c.Transport.ConnectServerInterface, "bind-interface", "", "", "network interface to bind when connecting to server (e.g., eth0, wlan0, auto)")
+		cmd.PersistentFlags().StringVarP(&c.Transport.ConnectServerLocalIP, "bind-ip", "", "", "IP address to bind when connecting to server")
 		c.Transport.TLS.Enable = cmd.PersistentFlags().BoolP("tls_enable", "", true, "enable frpc tls")
 	}
 	cmd.PersistentFlags().StringVarP(&c.User, "user", "u", "", "user")

pkg/config/legacy/client.go

@@ -49,8 +49,14 @@ type ClientCommonConf struct {
 	DialServerKeepAlive int64 `ini:"dial_server_keepalive" json:"dial_server_keepalive"`
 	// ConnectServerLocalIP specifies the address of the client bind when it connect to server.
 	// By default, this value is empty.
-	// this value only use in TCP/Websocket protocol. Not support in KCP protocol.
+	// this value works with tcp, websocket, http, https, and quic protocols. Not supported with kcp protocol.
 	ConnectServerLocalIP string `ini:"connect_server_local_ip" json:"connect_server_local_ip"`
+	// ConnectServerInterface specifies the network interface name to bind when connecting to server.
+	// Valid values are interface names (e.g., "eth0", "wlan0") or "auto" for auto-detection.
+	// By default, this value is empty.
+	// this value works with tcp, websocket, http, https, and quic protocols. Not supported with kcp protocol.
+	// Priority: If both connect_server_local_ip and connect_server_interface are set, connect_server_interface takes precedence.
+	ConnectServerInterface string `ini:"connect_server_interface" json:"connect_server_interface"`
 	// HTTPProxy specifies a proxy address to connect to the server through. If
 	// this value is "", the server will be connected to directly. By default,
 	// this value is read from the "http_proxy" environment variable.

pkg/config/legacy/conversion.go

@@ -47,6 +47,7 @@ func Convert_ClientCommonConf_To_v1(conf *ClientCommonConf) *v1.ClientCommonConf
 	out.Transport.DialServerTimeout = conf.DialServerTimeout
 	out.Transport.DialServerKeepAlive = conf.DialServerKeepAlive
 	out.Transport.ConnectServerLocalIP = conf.ConnectServerLocalIP
+	out.Transport.ConnectServerInterface = conf.ConnectServerInterface
 	out.Transport.ProxyURL = conf.HTTPProxy
 	out.Transport.PoolCount = conf.PoolCount
 	out.Transport.TCPMux = lo.ToPtr(conf.TCPMux)

pkg/config/v1/client.go

@@ -107,8 +107,13 @@ type ClientTransportConfig struct {
 	// If negative, keep-alive probes are disabled.
 	DialServerKeepAlive int64 `json:"dialServerKeepalive,omitempty"`
 	// ConnectServerLocalIP specifies the address of the client bind when it connect to server.
-	// Note: This value only use in TCP/Websocket protocol. Not support in KCP protocol.
+	// Note: This value works with tcp, websocket, http, https, and quic protocols. Not supported with kcp protocol.
 	ConnectServerLocalIP string `json:"connectServerLocalIP,omitempty"`
+	// ConnectServerInterface specifies the network interface name to bind when connecting to server.
+	// Valid values are interface names (e.g., "eth0", "wlan0") or "auto" for auto-detection.
+	// Note: This value works with tcp, websocket, http, https, and quic protocols. Not supported with kcp protocol.
+	// Priority: If both ConnectServerLocalIP and ConnectServerInterface are set, ConnectServerInterface takes precedence.
+	ConnectServerInterface string `json:"connectServerInterface,omitempty"`
 	// ProxyURL specifies a proxy address to connect to the server through. If
 	// this value is "", the server will be connected to directly. By default,
 	// this value is read from the "http_proxy" environment variable.

pkg/config/v1/validation/client.go

@@ -24,6 +24,7 @@ import (
 
 	v1 "github.com/fatedier/frp/pkg/config/v1"
 	"github.com/fatedier/frp/pkg/featuregate"
+	netpkg "github.com/fatedier/frp/pkg/util/net"
 )
 
 func ValidateClientCommonConfig(c *v1.ClientCommonConfig) (Warning, error) {
@@ -88,6 +89,10 @@ func ValidateClientCommonConfig(c *v1.ClientCommonConfig) (Warning, error) {
 		errs = AppendError(errs, fmt.Errorf("invalid transport.protocol, optional values are %v", SupportedTransportProtocols))
 	}
 
+	if err := validateInterfaceBinding(&c.Transport); err != nil {
+		errs = AppendError(errs, err)
+	}
+
 	for _, f := range c.IncludeConfigFiles {
 		absDir, err := filepath.Abs(filepath.Dir(f))
 		if err != nil {
@@ -124,3 +129,26 @@ func ValidateAllClientConfig(c *v1.ClientCommonConfig, proxyCfgs []v1.ProxyConfi
 	}
 	return warnings, nil
 }
+
+func validateInterfaceBinding(c *v1.ClientTransportConfig) error {
+	// Check if both interface and IP are specified (they are mutually exclusive)
+	if c.ConnectServerInterface != "" && c.ConnectServerLocalIP != "" {
+		return fmt.Errorf("cannot specify both transport.connectServerInterface and transport.connectServerLocalIP")
+	}
+
+	// Validate interface name if specified
+	if c.ConnectServerInterface != "" {
+		if err := netpkg.ValidateInterfaceOrIP(c.ConnectServerInterface); err != nil {
+			return fmt.Errorf("invalid transport.connectServerInterface: %v", err)
+		}
+	}
+
+	// Validate IP address if specified
+	if c.ConnectServerLocalIP != "" {
+		if err := netpkg.ValidateInterfaceOrIP(c.ConnectServerLocalIP); err != nil {
+			return fmt.Errorf("invalid transport.connectServerLocalIP: %v", err)
+		}
+	}
+
+	return nil
+}

pkg/util/net/interface.go

@@ -0,0 +1,206 @@
+// Copyright 2025 Satyajeet Singh, jeet.0733@gmail.com
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package net
+
+import (
+	"fmt"
+	"net"
+	"strings"
+)
+
+// InterfaceInfo represents information about a network interface
+type InterfaceInfo struct {
+	Name string
+	IP   string
+	Addr net.Addr
+}
+
+// GetInterfaceIP resolves an interface name to its primary IPv4 address
+func GetInterfaceIP(interfaceName string) (string, error) {
+	if interfaceName == "" {
+		return "", fmt.Errorf("interface name cannot be empty")
+	}
+
+	iface, err := net.InterfaceByName(interfaceName)
+	if err != nil {
+		return "", fmt.Errorf("interface '%s' not found: %v", interfaceName, err)
+	}
+
+	addrs, err := iface.Addrs()
+	if err != nil {
+		return "", fmt.Errorf("failed to get addresses for interface '%s': %v", interfaceName, err)
+	}
+
+	// Look for IPv4 address first
+	for _, addr := range addrs {
+		if ipnet, ok := addr.(*net.IPNet); ok {
+			if ip := ipnet.IP.To4(); ip != nil && !ip.IsLoopback() {
+				return ip.String(), nil
+			}
+		}
+	}
+
+	// If no IPv4, look for IPv6
+	for _, addr := range addrs {
+		if ipnet, ok := addr.(*net.IPNet); ok {
+			if ip := ipnet.IP.To16(); ip != nil && !ip.IsLoopback() {
+				return ip.String(), nil
+			}
+		}
+	}
+
+	return "", fmt.Errorf("interface '%s' has no valid IP address assigned", interfaceName)
+}
+
+// ListNetworkInterfaces returns all available network interfaces with their IP addresses
+func ListNetworkInterfaces() ([]InterfaceInfo, error) {
+	interfaces, err := net.Interfaces()
+	if err != nil {
+		return nil, fmt.Errorf("failed to enumerate network interfaces: %v", err)
+	}
+
+	var result []InterfaceInfo
+	for _, iface := range interfaces {
+		// Skip down interfaces
+		if iface.Flags&net.FlagUp == 0 {
+			continue
+		}
+
+		addrs, err := iface.Addrs()
+		if err != nil {
+			continue // Skip interfaces with address errors
+		}
+
+		for _, addr := range addrs {
+			if ipnet, ok := addr.(*net.IPNet); ok {
+				// Skip loopback addresses
+				if ipnet.IP.IsLoopback() {
+					continue
+				}
+
+				result = append(result, InterfaceInfo{
+					Name: iface.Name,
+					IP:   ipnet.IP.String(),
+					Addr: addr,
+				})
+			}
+		}
+	}
+
+	return result, nil
+}
+
+// GetFirstNonLoopbackIP returns the first available non-loopback IPv4 address
+func GetFirstNonLoopbackIP() (string, error) {
+	interfaces, err := net.Interfaces()
+	if err != nil {
+		return "", fmt.Errorf("failed to enumerate network interfaces: %v", err)
+	}
+
+	for _, iface := range interfaces {
+		// Skip down interfaces
+		if iface.Flags&net.FlagUp == 0 {
+			continue
+		}
+
+		addrs, err := iface.Addrs()
+		if err != nil {
+			continue
+		}
+
+		for _, addr := range addrs {
+			if ipnet, ok := addr.(*net.IPNet); ok {
+				if ip := ipnet.IP.To4(); ip != nil && !ip.IsLoopback() {
+					return ip.String(), nil
+				}
+			}
+		}
+	}
+
+	return "", fmt.Errorf("no suitable network interface found")
+}
+
+// ValidateInterfaceOrIP validates if the given value is a valid interface name or IP address
+func ValidateInterfaceOrIP(value string) error {
+	if value == "" {
+		return fmt.Errorf("value cannot be empty")
+	}
+
+	// Check if it's a valid IP address
+	if ip := net.ParseIP(value); ip != nil {
+		if ip.IsLoopback() {
+			return fmt.Errorf("loopback IP address '%s' is not allowed", value)
+		}
+		return nil
+	}
+
+	// Check if it's "auto" (special keyword)
+	if value == "auto" {
+		return nil
+	}
+
+	// Check if it's a valid interface name
+	if _, err := net.InterfaceByName(value); err != nil {
+		return fmt.Errorf("'%s' is not a valid interface name or IP address", value)
+	}
+
+	return nil
+}
+
+// ResolveBindAddress resolves the binding address from interface name or IP address
+func ResolveBindAddress(value string) (string, error) {
+	if value == "" {
+		return "", nil // No binding specified
+	}
+
+	if value == "auto" {
+		return GetFirstNonLoopbackIP()
+	}
+
+	// Check if it's already an IP address
+	if ip := net.ParseIP(value); ip != nil {
+		if ip.IsLoopback() {
+			return "", fmt.Errorf("loopback IP address '%s' is not allowed", value)
+		}
+		return value, nil
+	}
+
+	// Treat as interface name
+	return GetInterfaceIP(value)
+}
+
+// GetAvailableInterfaces returns a formatted string of available interfaces for error messages
+func GetAvailableInterfaces() string {
+	interfaces, err := ListNetworkInterfaces()
+	if err != nil {
+		return "failed to enumerate interfaces"
+	}
+
+	if len(interfaces) == 0 {
+		return "no interfaces available"
+	}
+
+	var names []string
+	seen := make(map[string]bool)
+	for _, iface := range interfaces {
+		if !seen[iface.Name] {
+			names = append(names, iface.Name)
+			seen[iface.Name] = true
+		}
+	}
+
+	return strings.Join(names, ", ")
+}
+

pkg/util/net/interface_test.go

@@ -0,0 +1,111 @@
+// Copyright 2025 Satyajeet Singh, jeet.0733@gmail.com
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package net
+
+import (
+	"testing"
+)
+
+func TestValidateInterfaceOrIP(t *testing.T) {
+	tests := []struct {
+		name    string
+		value   string
+		wantErr bool
+	}{
+		{"empty", "", true},
+		{"valid ip", "192.168.1.1", false},
+		{"valid ipv6", "2001:db8::1", false},
+		{"loopback ip", "127.0.0.1", true},
+		{"auto keyword", "auto", false},
+		{"invalid interface", "nonexistent", true},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			err := ValidateInterfaceOrIP(tt.value)
+			if (err != nil) != tt.wantErr {
+				t.Errorf("ValidateInterfaceOrIP() error = %v, wantErr %v", err, tt.wantErr)
+			}
+		})
+	}
+}
+
+func TestResolveBindAddress(t *testing.T) {
+	tests := []struct {
+		name    string
+		value   string
+		want    string
+		wantErr bool
+	}{
+		{"empty", "", "", false},
+		{"auto", "auto", "", false}, // Will return IP if interfaces available, empty if not
+		{"valid ip", "192.168.1.1", "192.168.1.1", false},
+		{"loopback ip", "127.0.0.1", "", true},
+		{"invalid interface", "nonexistent", "", true},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got, err := ResolveBindAddress(tt.value)
+			if (err != nil) != tt.wantErr {
+				t.Errorf("ResolveBindAddress() error = %v, wantErr %v", err, tt.wantErr)
+				return
+			}
+			// For "auto" case, we can't predict the exact IP, so just check it's not empty if no error
+			if tt.name == "auto" && err == nil {
+				if got == "" {
+					t.Errorf("ResolveBindAddress() returned empty for auto, expected an IP address")
+				}
+			} else if got != tt.want {
+				t.Errorf("ResolveBindAddress() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}
+
+func TestListNetworkInterfaces(t *testing.T) {
+	interfaces, err := ListNetworkInterfaces()
+	if err != nil {
+		t.Fatalf("ListNetworkInterfaces() error = %v", err)
+	}
+
+	// Should have at least one interface
+	if len(interfaces) == 0 {
+		t.Log("No network interfaces found (this might be normal in some environments)")
+	}
+
+	for _, iface := range interfaces {
+		if iface.Name == "" {
+			t.Errorf("Interface name should not be empty")
+		}
+		if iface.IP == "" {
+			t.Errorf("Interface IP should not be empty for interface %s", iface.Name)
+		}
+		if iface.Addr == nil {
+			t.Errorf("Interface address should not be nil for interface %s", iface.Name)
+		}
+	}
+}
+
+func TestGetAvailableInterfaces(t *testing.T) {
+	result := GetAvailableInterfaces()
+	if result == "" {
+		t.Log("No available interfaces found (this might be normal in some environments)")
+	}
+	// Should not return an error message
+	if result == "failed to enumerate interfaces" || result == "no interfaces available" {
+		t.Logf("Interface enumeration result: %s", result)
+	}
+}