Linux oracle ORA-01031: insufficient privileges

[oracle@localhost ~]$ sqlplus /nolog

 

SQL*Plus: Release 10.2.0.4.0 - Production on Thu Aug 30 18:35:30 2012

Copyright (c) 1982, 2007, Oracle.  All Rights Reserved.

SQL> conn / as sysdba
ERROR:
ORA-01031: insufficient privileges

SQL> exit

[root@localhost ~]#vi /etc/group
root:x:0:root
bin:x:1:root,bin,daemon
daemon:x:2:root,bin,daemon
sys:x:3:root,bin,adm
adm:x:4:root,adm,daemon
tty:x:5:
disk:x:6:root
lp:x:7:daemon,lp
mem:x:8:
kmem:x:9:
wheel:x:10:root
mail:x:12:mail
news:x:13:news
uucp:x:14:uucp
man:x:15:
games:x:20:
gopher:x:30:
dip:x:40:
ftp:x:50:
lock:x:54:
nobody:x:99:
users:x:100:
rpm:x:37:
dbus:x:81:
nscd:x:28:
utmp:x:22:
utempter:x:35:
floppy:x:19:
vcsa:x:69:
pcap:x:77:
slocate:x:21:
rpc:x:32:
mailnull:x:47:
smmsp:x:51:
sshd:x:74:
rpcuser:x:29:
nfsnobody:x:4294967294:
haldaemon:x:68:
distcache:x:94:
apache:x:48:
avahi:x:70:
postgres:x:26:
mysql:x:27:
dovecot:x:97:
webalizer:x:67:
squid:x:23:
ntp:x:38:
stapdev:x:101:
stapusr:x:102:
named:x:25:
xfs:x:43:
gdm:x:42:
sabayon:x:86:
screen:x:84:
oinstall:x:500:
dba:x:500:oracle
"/etc/group" 56L, 773C written
[root@localhost ~]#exit
logout

[oracle@localhost ~]$ sqlplus /nolog

SQL*Plus: Release 10.2.0.4.0 - Production on Thu Aug 30 18:41:08 2012

Copyright (c) 1982, 2007, Oracle.  All Rights Reserved.

SQL> conn / as sysdba
Connected to an idle instance.
SQL> exit
Disconnected