$ ping localhost ping: unknown host localhost
If you ever find yourself in a situation where you cannot ping localhost, I’ll show you a few things to check!
Verify that the /etc/hosts file has entires for localhost which looks like the following:
127.0.0.1 localhost ::1 localhost ip6-localhost ip6-loopback
The file /etc/nsswitch.conf requires specific permissions. Verify that it is set to 644.
$ sudo chmod 644 /etc/nsswitch.conf
Also verify that /etc/nsswitch.conf has an entry like the following:
hosts: files dns
Still no luck?
– Temporarily disable iptables, ufw, or any other firewall that may be in place
– Verify you have proper DNS in /etc/resolv.conf
– Restart all network services
– Verify that the output of sudo ifconfig provides something like this:
lo Link encap:Local Loopback inet addr:127.0.0.1 Mask:255.0.0.0 inet6 addr: ::1/128 Scope:Host UP LOOPBACK RUNNING MTU:16436 Metric:1 RX packets:295 errors:0 dropped:0 overruns:0 frame:0 TX packets:295 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:0 RX bytes:43479 (42.4 KiB) TX bytes:43479 (42.4 KiB)
If no local loopback is provided with the above command, inspect the network interface configuration in /etc/network/interfaces (debian/ubuntu) or /etc/sysconfig/network-scripts/ifcfg-lo (red hat/centos).
– Another way to verify the loopback interface is up is with the following command:
$ ip link show lo 1: lo:
mtu 16436 qdisc noqueue state UNKNOWN link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00