Oracle數據庫服務器DG從庫重啟后，無法完成數據同步，具體報錯信息如下：
一、報錯信息
alter log報錯
*********************************************************************** Fatal NI connect error 12547, connecting to: VERSION INFORMATION: TNS for Linux: Version 11.2.0.3.0 - Production TCP/IP NT Protocol Adapter for Linux: Version 11.2.0.3.0 - Production Time: 23-APR-2018 10:30:45 Tracing not turned on. Tns error struct: ns main err code: 12547 TNS-12547: TNS:lost contact ns secondary err code: 12560 nt main err code: 517 TNS-00517: Lost contact nt secondary err code: 104 nt OS err code: 0 Error 12547 received logging on to the standby FAL[client, USER]: Error 12547 connecting to LOGDB for fetching gap sequence
二、錯誤原因分析
根據報錯信息可以看到通過tns連接logdb服務名存在問題，可通過排查監聽是否啟動，是否有防火牆限制等。
三、處理步驟
通過排查監聽和防火牆限制，發現沒有問題，主庫通過tns服務名連接正常，從庫通過tnsping報錯
[oracle@dg ~]$ tnsping logdb TNS Ping Utility for Linux: Version 11.2.0.3.0 - Production on 23-APR-2018 11:26:28 Copyright (c) 1997, 2011, Oracle. All rights reserved. Used parameter files: Used TNSNAMES adapter to resolve the alias Attempting to contact (DESCRIPTION = (ADDRESS_LIST = (ADDRESS = (PROTOCOL = TCP)(HOST = 1xx.xx.xx.xx)(PORT = 1521))) (CONNECT_DATA = (SERVER = DEDICATED) (SERVICE_NAME = logdb))) TNS-12547: TNS:lost contact
從庫通過telnet 主庫IPaddr 1521端口檢查，發現連接上后直接被關閉
# telnet 1xx.xx.xx.xx 1521 Trying 1xx.xx.xx.xx... Connected to 1xx.xx.xx.xx. Escape character is '^]'. Connection closed by foreign host.
這里基本可以斷定是監聽問題，具體檢查后發現只sqlnet.ora做了監聽限制
tcp.validnode_checking=yes tcp.invited_nodes=(ip1,ip2,ip3,ip4,ip5,ip6)
將備庫IP添加進允許訪問列表即可，然后通過tnsping檢查連接正常，然后通過手動恢復主備同步。
（DG主備同步恢復過程略）