ORA-12170: TNS:Connect timeout occurred When Exec Rman Duplicate

Symptom:

    The Error details are blow:
We run below on target DB hosts:

RMAN>  connect target sys/****@testdb
connected to target database: testdb (DBID=1516071651)
RMAN>  connect auxiliary sys/*****@testdbaux
connected to auxiliary database: testdbaux(not mounted)
RMAN> DUPLICATE TARGET DATABASE  TO 'testdbaux' FROM ACTIVE DATABASE ;

RMAN-00571: ===========================================================
RMAN-00569: =============== ERROR MESSAGE STACK FOLLOWS ===============
RMAN-00571: ===========================================================
RMAN-03002: failure of Duplicate Db command at 07/30/2018 01:06:30
RMAN-05501: aborting duplication of target database
RMAN-03015: error occurred in stored script Memory Script
RMAN-03009: failure of backup command on ORA_DISK_1 channel at 07/30/2018 01:06:30
ORA-17629: Cannot connect to the remote database server
ORA-17627: ORA-12170: TNS:Connect timeout occurred
ORA-17629: Cannot connect to the remote database server

Diagnosis:

  On Target testdb hosts,   tnsping testdb and testdbaux are both fine.
  It turns out rman duplicate will run sessions from testdbaux hosts to connect to testdb target hosts
  We open fireports from target hosts to aux hosts only.  But rman duplicate need ports open from aux hosts to targets as well.

Solution:

   Open firewall bi-direction firewall ports between target and aux hosts