Sqlplus / as sysdba Hangs

On test system, when we try "sqlplus / as sysdba" or "rman target /" commands,  it waits 15 seconds then connects, found no error.

We digged the problem, traced sqlplus command and find out some socket issues.

socket(2, 2, 0)                                 = 12
_esendto(12, 0x0FFFFFFFFFFF60A0, 26, 0, 0x09001000A0025530, 16, 0x0000000000000000) = 26
_poll(0x0FFFFFFFFFFF5260, 1, 5000) (sleeping...)
_poll(0x0FFFFFFFFFFF5260, 1, 5000)              = 0
close(12)                  

After talking with network team, they found problem on dns server and
changed dns server address, issue is resolved.

OS: AIX 6.1
DB: 11.2.0.3

You can trace sqlplus with below command;

truss -o truss_sqlplus.out sqlplus / as sysdba