[Feature]: Add UDP SO_REUSEADDR Support to make mosdns compatible with UDP TPROXY

coremain/config.go

@@ -77,6 +77,15 @@ type ServerListenerConfig struct {
 	ProxyProtocol       bool   `yaml:"proxy_protocol"`          // accepting the PROXYProtocol
 
 	IdleTimeout uint `yaml:"idle_timeout"` // (sec) used by tcp, dot, doh as connection idle timeout.
+
+	// Issue: https://github.com/IrineSistiana/mosdns/pull/657
+	// When a TPROXY is processing a UDP connection,
+	// for example: if the Dest Addr is 8.8.8.8:53,
+	// the TPROXY program must bind to 8.8.8.8:53 on TPROXY host os
+	// and send the UDP packet back to the Client to make sure the Src Addr and Port is 8.8.8.8:53 ,
+	// IF, when mosdns has bind :53 on this host os without SO_REUSEADDR,
+	// the behavior of the TPROXY program will fail with EADDRINUSE (Address already in use)
+	ReuseAddr bool `yaml:"reuse_addr"` // used by udp on unix alike systems.
 }
 
 type APIConfig struct {

coremain/server.go

@@ -20,6 +20,7 @@
 package coremain
 
 import (
+	"context"
 	"errors"
 	"fmt"
 	"github.com/IrineSistiana/mosdns/v4/pkg/server"
@@ -28,6 +29,7 @@ import (
 	"github.com/pires/go-proxyproto"
 	"go.uber.org/zap"
 	"net"
+	"syscall"
 	"time"
 )
 
@@ -115,7 +117,13 @@ func (m *Mosdns) startServerListener(cfg *ServerListenerConfig, dnsHandler dns_h
 	var run func() error
 	switch cfg.Protocol {
 	case "", "udp":
-		conn, err := net.ListenPacket("udp", cfg.Addr)
+		// see also: https://github.com/IrineSistiana/mosdns/pull/657
+		var lc net.ListenConfig
+		lc.Control = func(network, address string, c syscall.RawConn) error {
+			return serverSocketOption(c, cfg, network, address)
+		}
+
+		conn, err := lc.ListenPacket(context.Background(), "udp", cfg.Addr)
 		if err != nil {
 			return err
 		}

coremain/utilities_other.go

@@ -0,0 +1,15 @@
+//go:build !(linux || darwin || freebsd || netbsd || openbsd)
+
+package coremain
+
+import (
+	"errors"
+	"syscall"
+)
+
+func serverSocketOption(c syscall.RawConn, cfg *ServerListenerConfig, network, address string) error {
+	if cfg.ReuseAddr {
+		return errors.New("reuseaddr not supported on this platform")
+	}
+	return nil
+}

coremain/utilities_unix.go

@@ -0,0 +1,20 @@
+//go:build linux || darwin || freebsd || netbsd || openbsd
+
+package coremain
+
+import (
+	"syscall"
+)
+
+func serverSocketOption(c syscall.RawConn, cfg *ServerListenerConfig, network, address string) error {
+	var errSysCall error
+	errControl := c.Control(func(fd uintptr) {
+		if cfg.ReuseAddr {
+			errSysCall = syscall.SetsockoptInt(int(fd), syscall.SOL_SOCKET, syscall.SO_REUSEADDR, 1)
+		}
+	})
+	if errSysCall != nil {
+		return errSysCall
+	}
+	return errControl
+}