Add support for kTLS

Reference: https://gitlab.com/go-extension/tls

common/ktls/ktls_alert.go

@@ -0,0 +1,80 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build linux && go1.25 && !without_badtls
+
+package ktls
+
+import (
+	"crypto/tls"
+	"net"
+)
+
+const (
+	// alert level
+	alertLevelWarning = 1
+	alertLevelError   = 2
+)
+
+const (
+	alertCloseNotify                  = 0
+	alertUnexpectedMessage            = 10
+	alertBadRecordMAC                 = 20
+	alertDecryptionFailed             = 21
+	alertRecordOverflow               = 22
+	alertDecompressionFailure         = 30
+	alertHandshakeFailure             = 40
+	alertBadCertificate               = 42
+	alertUnsupportedCertificate       = 43
+	alertCertificateRevoked           = 44
+	alertCertificateExpired           = 45
+	alertCertificateUnknown           = 46
+	alertIllegalParameter             = 47
+	alertUnknownCA                    = 48
+	alertAccessDenied                 = 49
+	alertDecodeError                  = 50
+	alertDecryptError                 = 51
+	alertExportRestriction            = 60
+	alertProtocolVersion              = 70
+	alertInsufficientSecurity         = 71
+	alertInternalError                = 80
+	alertInappropriateFallback        = 86
+	alertUserCanceled                 = 90
+	alertNoRenegotiation              = 100
+	alertMissingExtension             = 109
+	alertUnsupportedExtension         = 110
+	alertCertificateUnobtainable      = 111
+	alertUnrecognizedName             = 112
+	alertBadCertificateStatusResponse = 113
+	alertBadCertificateHashValue      = 114
+	alertUnknownPSKIdentity           = 115
+	alertCertificateRequired          = 116
+	alertNoApplicationProtocol        = 120
+	alertECHRequired                  = 121
+)
+
+func (c *Conn) sendAlertLocked(err uint8) error {
+	switch err {
+	case alertNoRenegotiation, alertCloseNotify:
+		c.rawConn.Tmp[0] = alertLevelWarning
+	default:
+		c.rawConn.Tmp[0] = alertLevelError
+	}
+	c.rawConn.Tmp[1] = byte(err)
+
+	_, writeErr := c.writeRecordLocked(recordTypeAlert, c.rawConn.Tmp[0:2])
+	if err == alertCloseNotify {
+		// closeNotify is a special case in that it isn't an error.
+		return writeErr
+	}
+
+	return c.rawConn.Out.SetErrorLocked(&net.OpError{Op: "local error", Err: tls.AlertError(err)})
+}
+
+// sendAlert sends a TLS alert message.
+func (c *Conn) sendAlert(err uint8) error {
+	c.rawConn.Out.Lock()
+	defer c.rawConn.Out.Unlock()
+	return c.sendAlertLocked(err)
+}