I have set the owner of /home/jack/.ssh to the jack and also give it the permissions as follows:
-rw------- 1 jack jack 1679 Oct 10 01:41 id_rsa
-rw------- 1 jack jack 396 Oct 10 01:41 id_rsa.pub
-rwxrwx---+ 1 jack jack 884 Oct 10 01:48 known_hosts
and
drw------- 2 jack jack 4096 Oct 13 18:00 .ssh
but jack can not open /home/jack/known_hosts file. What is wrong with that?