Fix OpenVPN start: CIDR→netmask, dhcp-option DNS, ccd dir, dh none ECDHE

Four bugs prevented OpenVPN from actually listening on its UDP port:

1. server.conf line 'server 10.8.0.0/24' — OpenVPN 2.5 rejects CIDR.
   Fix: cidrToServerDirective() converts CIDR to 'NETWORK NETMASK'
   (e.g. '10.8.0.0 255.255.255.0') using net.ParseCIDR.

2. push_dns '1.1.1.1 8.8.8.8' was emitted as a single push directive
   instead of multiple 'push dhcp-option DNS x' lines.
   Fix: split space-separated IPs into individual push directives;
   pass through if already 'dhcp-option ...' form.

3. client-config-dir referenced a ccd/ directory that was never created.
   Fix: CreateInstance now MkdirAll(ccd); WriteServerConf also
   defensively MkdirAll(extraDir) before writing the directive.

4. dh.pem generated with 1024-bit DH params → OpenSSL 3.0 refuses
   with 'dh key too small'. Fix: use 'dh none' in server.conf so
   OpenVPN 2.4+ uses ECDHE key exchange — no DH params needed at all.
   Removed the slow openssl dhparam generation from EnsureCA.

Verified: instance starts, OpenVPN parses config successfully,
now fails only at TUNSETIFF (expected: requires root/CAP_NET_ADMIN).

backend/pkg/openvpn/manager.go

@@ -64,12 +64,8 @@ func (m *Manager) EnsureCA() error {
 		-days 3650 -subj "/CN=OpenVPN-Manager-CA" 2>/dev/null`); err != nil {
 		return fmt.Errorf("generate CA: %w", err)
 	}
-	// DH 参数（轻量: 1024，生产可改为 2048/4096）
-	if _, err := os.Stat(filepath.Join(m.pkiDir, "dh.pem")); os.IsNotExist(err) {
-		if err := runShell(`openssl dhparam -out "` + m.pkiDir + `/dh.pem" 1024 2>/dev/null`); err != nil {
-			return fmt.Errorf("generate DH: %w", err)
-		}
-	}
+	// DH 参数：用 "dh none" 走 ECDHE，不再需要 dh.pem 文件。
+	// (保留目录以兼容旧配置)
 	// TLS-Auth key
 	if _, err := os.Stat(filepath.Join(m.pkiDir, "ta.key")); os.IsNotExist(err) {
 		if err := runShell(`openvpn --genkey secret "` + m.pkiDir + `/ta.key"`); err != nil {
@@ -93,10 +89,12 @@ func (m *Manager) WriteServerConf(in *model.Instance, extraDir string) error {
 	conf.WriteString("ca " + filepath.Join(m.pkiDir, "ca.crt") + "\n")
 	conf.WriteString("cert " + filepath.Join(m.PKIPath(in.Name), "issued", "server.crt") + "\n")
 	conf.WriteString("key " + filepath.Join(m.PKIPath(in.Name), "private", "server.key") + "\n")
-	conf.WriteString("dh " + filepath.Join(m.pkiDir, "dh.pem") + "\n")
+	// 用 "dh none" 让 OpenVPN 走 ECDHE 密钥交换(OpenVPN 2.4+),
+	// 不需要 DH 参数文件,既安全又避免 openssl dhparam 2048 的长等待。
+	conf.WriteString("dh none\n")
 	conf.WriteString("tls-auth " + filepath.Join(m.pkiDir, "ta.key") + " 0\n")
 	conf.WriteString("topology subnet\n")
-	conf.WriteString("server " + in.Subnet + "\n")
+	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")
@@ -112,7 +110,14 @@ func (m *Manager) WriteServerConf(in *model.Instance, extraDir string) error {
 			if line == "" {
 				continue
 			}
-			conf.WriteString("push \"" + line + "\"\n")
+			// 支持 "1.1.1.1 8.8.8.8" 或 "dhcp-option DNS 1.1.1.1" 两种写法
+			if strings.HasPrefix(line, "dhcp-option") {
+				conf.WriteString("push \"" + line + "\"\n")
+			} else {
+				for _, ip := range strings.Fields(line) {
+					conf.WriteString("push \"dhcp-option DNS " + ip + "\"\n")
+				}
+			}
 		}
 	}
 	if in.PushRoutes != "" {
@@ -125,6 +130,7 @@ func (m *Manager) WriteServerConf(in *model.Instance, extraDir string) error {
 		}
 	}
 	if extraDir != "" {
+		_ = os.MkdirAll(extraDir, 0o755)
 		conf.WriteString("client-config-dir " + extraDir + "\n")
 	}
 	if in.AccessMode == model.AccessWhitelist {
@@ -562,6 +568,21 @@ func atoi64(s string) int64 {
 	return n
 }
 
+// cidrToServerDirective 把 "10.8.0.0/24" 转成 OpenVPN server 指令需要的
+// "10.8.0.0 255.255.255.0" 格式(网络地址 + 点分掩码)。
+// 如果输入不含 "/"(已经是 "ip mask" 形式),原样返回。
+func cidrToServerDirective(subnet string) string {
+	subnet = strings.TrimSpace(subnet)
+	if !strings.Contains(subnet, "/") {
+		return subnet
+	}
+	_, ipnet, err := net.ParseCIDR(subnet)
+	if err != nil {
+		return subnet // 让 OpenVPN 报错,总比生成错误格式好
+	}
+	return ipnet.IP.String() + " " + net.IP(ipnet.Mask).String()
+}
+
 func pemTrim(b []byte) string {
 	return strings.TrimSpace(string(b))
 }