"localhost" is not known on CentOS

localhost is not known

My linux system could not recognize its own localhost.

Here is several ways to check "localhost" work or not:

1. Enter following command in terminal, and localhost is the system's hostname:
# ping localhost

It will show you something looks like this:

PING localhost (127.0.0.1) 56(84) bytes of data.
64 bytes from localhost (127.0.0.1): icmp_seq=1 ttl=64 time=0.022 ms
64 bytes from localhost (127.0.0.1): icmp_seq=2 ttl=64 time=0.029 ms
64 bytes from localhost (127.0.0.1): icmp_seq=3 ttl=64 time=0.086 ms
64 bytes from localhost (127.0.0.1): icmp_seq=4 ttl=64 time=0.121 ms

if it does not show you the above thing, it does not work, or you can check ping directly with your local ip address:

#ping 127.0.0.1   

2. Check localhost using ssh

#ssh localhost
It will show you something looks like this:

The authenticity of host 'localhost (127.0.0.1)' can't be established.
RSA key fingerprint is a3:0a:4f:d4:f1:73:6f:25:eb:92:70:9d:93:b8:45:4f.
Are you sure you want to continue connecting (yes/no)?

If not so, it means your hostname does not work properly.

HOWTO solve it?

1. Modify /etc/hosts

It looks like the following:

# Do not remove the following line, or various programs
# that require network functionality will fail.
127.0.0.1       localhost.localdomain localhost mycentos

I added localhost and mycentos as my host name.

2.Try to flush DNS cache :

#/etc/init.d/nscd restart

It works for my system.