Fix TLS handshake: add X.509 keyUsage/EKU extensions to all certs

OpenVPN 2.7 + OpenSSL 3.6 strictly checks keyUsage extension.
Client log showed: 'Certificate does not have key usage extension'
→ 'VERIFY KU ERROR' → 'TLS handshake failed'.

Root cause: all certs generated via bare 'openssl req/x509 -req'
without any -extensions/-extfile, so no X.509 v3 extensions at all.

Fix: generate per-cert openssl ext config files:

CA cert (EnsureCA):
  basicConstraints = critical,CA:TRUE
  keyUsage = critical,keyCertSign,cRLSign
  subjectKeyIdentifier = hash
  authorityKeyIdentifier = keyid:always,issuer

Server cert (IssueCert, clientName=='server'):
  basicConstraints = critical,CA:FALSE
  keyUsage = critical,digitalSignature,keyEncipherment
  extendedKeyUsage = serverAuth

Client cert (IssueCert, otherwise):
  basicConstraints = critical,CA:FALSE
  keyUsage = critical,digitalSignature,keyEncipherment
  extendedKeyUsage = clientAuth

Also fix while here:
- .ovpn remote host: strip port from HTTP Host header with
  net.SplitHostPort; prefer ?host= query param from frontend
- Replace deprecated 'persist-key' (OpenVPN 2.7 warns) with just
  'persist-tun' in both server.conf and client .ovpn
- Replace 'cipher X' with 'data-ciphers X:AES-128-GCM' (OpenVPN 2.5+
  negotiation) in server.conf and client .ovpn

Verified: CA/Server/Client certs all show correct keyUsage + EKU via
openssl x509 -text; .ovpn remote shows correct host:port.

backend/internal/api/router.go

@@ -1,6 +1,7 @@
 package api
 
 import (
+	"net"
 	"net/http"
 	"os"
 	"path/filepath"
@@ -376,7 +377,12 @@ func (s *Server) deleteUser(c *gin.Context) {
 func (s *Server) downloadOVPN(c *gin.Context) {
 	host := c.Query("host")
 	if host == "" {
+		// 从 Host 头取 hostname(strip 端口号)
+		// 用户应在前端 ?host=vpn.example.com 传入真实地址
 		host = c.Request.Host
+		if h, _, err := net.SplitHostPort(host); err == nil {
+			host = h
+		}
 	}
 	p, err := s.Svc.GenerateOVPN(c.Param("uid"), host)
 	if err != nil {

backend/pkg/openvpn/manager.go

@@ -57,11 +57,29 @@ func (m *Manager) EnsureCA() error {
 	if err := os.MkdirAll(m.pkiDir, 0o700); err != nil {
 		return err
 	}
-	// 使用 openssl 直接生成自签 CA，避免依赖 easyrsa
+	// 使用 openssl 直接生成自签 CA，避免依赖 easyrsa。
+	// 必须带 X.509 扩展 (basicConstraints + keyUsage)，
+	// 否则 OpenVPN 2.7 / OpenSSL 3.x 会报 "Certificate does not have key usage extension"。
+	caExt := filepath.Join(m.pkiDir, "ca-ext.cnf")
+	if err := os.WriteFile(caExt, []byte(`[req]
+distinguished_name = req_dn
+prompt = no
+[req_dn]
+CN = OpenVPN-Manager-CA
+[v3_ca]
+basicConstraints = critical,CA:TRUE
+keyUsage = critical,keyCertSign,cRLSign
+subjectKeyIdentifier = hash
+authorityKeyIdentifier = keyid:always,issuer
+`), 0o600); err != nil {
+		return fmt.Errorf("write ca ext: %w", err)
+	}
 	if err := runShell(`openssl req -x509 -newkey rsa:2048 -nodes \
-		-keyout "`+m.pkiDir+`/ca.key" \
-		-out "`+m.pkiDir+`/ca.crt" \
-		-days 3650 -subj "/CN=OpenVPN-Manager-CA" 2>/dev/null`); err != nil {
+	-keyout "`+m.pkiDir+`/ca.key" \
+	-out "`+m.pkiDir+`/ca.crt" \
+	-days 3650 -sha256 \
+	-subj "/CN=OpenVPN-Manager-CA" \
+	-extensions v3_ca -config "` + caExt + `" 2>/dev/null`); err != nil {
 		return fmt.Errorf("generate CA: %w", err)
 	}
 	// DH 参数：用 "dh none" 走 ECDHE，不再需要 dh.pem 文件。
@@ -97,12 +115,12 @@ func (m *Manager) WriteServerConf(in *model.Instance, extraDir string) error {
 	conf.WriteString("server " + cidrToServerDirective(in.Subnet) + "\n")
 	conf.WriteString("ifconfig-pool-persist " + filepath.Join(dir, "ipp.txt") + "\n")
 	conf.WriteString("keepalive 10 120\n")
-	conf.WriteString("persist-key\npersist-tun\n")
+	conf.WriteString("persist-tun\n")
+	conf.WriteString("data-ciphers " + orDefault(in.Cipher, "AES-256-GCM") + ":AES-128-GCM\n")
 	conf.WriteString("status " + filepath.Join(dir, "status.log") + " 10\n")
 	conf.WriteString("status-version 3\n")
 	conf.WriteString("log " + filepath.Join(dir, "logs", "openvpn.log") + "\n")
 	conf.WriteString("verb 3\n")
-	conf.WriteString("cipher " + orDefault(in.Cipher, "AES-256-GCM") + "\n")
 	conf.WriteString("auth " + orDefault(in.AuthDigest, "SHA256") + "\n")
 	if in.PushDNS != "" {
 		for _, line := range strings.Split(in.PushDNS, "\n") {
@@ -153,6 +171,8 @@ func (m *Manager) WriteServerConf(in *model.Instance, extraDir string) error {
 
 // IssueCert 为客户端签发证书。clientName = CN。
 // 返回 (certPath, keyPath, error)。
+// 当 clientName == "server" 时签发带 serverAuth EKU 的服务端证书，
+// 否则签发带 clientAuth EKU 的客户端证书。
 func (m *Manager) IssueCert(instanceName, clientName string) (string, string, error) {
 	pki := m.PKIPath(instanceName)
 	issuedDir := filepath.Join(pki, "issued")
@@ -183,11 +203,29 @@ func (m *Manager) IssueCert(instanceName, clientName string) (string, string, er
 		key, csr, clientName)); err != nil {
 		return "", "", fmt.Errorf("gen csr: %w", err)
 	}
-	// 用 CA 签发
+	// 写签发扩展配置(区分 server / client)
+	extFile := filepath.Join(pki, clientName+"-ext.cnf")
+	extContent := `[v3]
+basicConstraints = critical,CA:FALSE
+keyUsage = critical,digitalSignature,keyEncipherment
+subjectKeyIdentifier = hash
+authorityKeyIdentifier = keyid,issuer
+`
+	if clientName == "server" {
+		extContent += "extendedKeyUsage = serverAuth\n"
+	} else {
+		extContent += "extendedKeyUsage = clientAuth\n"
+	}
+	if err := os.WriteFile(extFile, []byte(extContent), 0o600); err != nil {
+		return "", "", fmt.Errorf("write ext: %w", err)
+	}
+	// 用 CA 签发(带扩展)
 	caCrt := filepath.Join(m.pkiDir, "ca.crt")
 	caKey := filepath.Join(m.pkiDir, "ca.key")
 	if err := runShell(fmt.Sprintf(`openssl x509 -req -in "%s" -CA "%s" -CAkey "%s" -CAcreateserial \
-		-out "%s" -days 3650 -sha256 2>/dev/null`, csr, caCrt, caKey, crt)); err != nil {
+	-out "%s" -days 3650 -sha256 \
+	-extfile "%s" -extensions v3 2>/dev/null`,
+		csr, caCrt, caKey, crt, extFile)); err != nil {
 		return "", "", fmt.Errorf("sign cert: %w", err)
 	}
 	_ = os.Remove(csr)
@@ -240,8 +278,8 @@ func (m *Manager) GenerateClientOVPN(in *model.Instance, username, remoteHost st
 	b.WriteString("remote " + remoteHost + " " + strconv.Itoa(in.Port) + "\n")
 	b.WriteString("resolv-retry infinite\n")
 	b.WriteString("nobind\n")
-	b.WriteString("persist-key\npersist-tun\n")
-	b.WriteString("cipher " + orDefault(in.Cipher, "AES-256-GCM") + "\n")
+	b.WriteString("persist-tun\n")
+	b.WriteString("data-ciphers " + orDefault(in.Cipher, "AES-256-GCM") + ":AES-128-GCM\n")
 	b.WriteString("auth " + orDefault(in.AuthDigest, "SHA256") + "\n")
 	b.WriteString("remote-cert-tls server\n")
 	b.WriteString("verb 3\n")