客户端连接不上，服务端提示
VERIFY ERROR: depth=0, error=CRL has expired: CN=vpn-client-01, serial=xx
OpenSSL: error:14089086:SSL routines:ssl3_get_client_certificate:certificate verify failed
重新生成证书，或者修改server.conf注释crl-verify crl.pem重启
参考自https://www.likecs.com/show-690776.html#sc=1883