Implement the customized dialer

internal/dialer/dialer.go

@@ -0,0 +1,52 @@
+//go:build linux
+
+// Package dialer provides a customized dialer for aproxy.
+package dialer
+
+import (
+	"fmt"
+	"net"
+	"syscall"
+)
+
+// Options customizes the dialer returned by New.
+// The zero value is the default dialer setting.
+type Options struct {
+	// Fwmark sets the fwmark on all outgoing packets dialed by the dialer, so
+	// they can be matched by netfilter rules.
+	// This includes the DNS resolution connections during the dialing process.
+	Fwmark uint32
+}
+
+// New creates a new customized network dialer based on the option.
+// The option is optional, at most one should be given and the rest are ignored.
+func New(option ...Options) *net.Dialer {
+	if len(option) == 0 {
+		option = append(option, Options{})
+	}
+	o := option[0]
+	d := &net.Dialer{}
+	if o.Fwmark != 0 {
+		mark := o.Fwmark
+		d.Control = func(network, address string, c syscall.RawConn) error {
+			var sockErr error
+			err := c.Control(func(fd uintptr) {
+				sockErr = syscall.SetsockoptInt(
+					int(fd), syscall.SOL_SOCKET, syscall.SO_MARK, int(mark),
+				)
+			})
+			if err != nil {
+				return fmt.Errorf("raw control: %w", err)
+			}
+			if sockErr != nil {
+				return fmt.Errorf("failed to set SO_MARK %d: %w", mark, sockErr)
+			}
+			return nil
+		}
+		d.Resolver = &net.Resolver{
+			PreferGo: true,
+			Dial:     d.DialContext,
+		}
+	}
+	return d
+}