For example in ubuntu 20.04 LTS where the /etc/hosts
file is empty:
>>> cat /etc/hosts
127.0.0.1 localhost
ping
still works for any subdomain of localhost:
>>> ping test.localhost
PING test.localhost(ip6-localhost (::1)) 56 data bytes
64 bytes from ip6-localhost (::1): icmp_seq=1 ttl=64 time=0.058 ms
64 bytes from ip6-localhost (::1): icmp_seq=2 ttl=64 time=0.049 ms
^C
--- test.localhost ping statistics ---
2 packets transmitted, 2 received, 0% packet loss, time 1028ms
rtt min/avg/max/mdev = 0.049/0.053/0.058/0.004 ms
or:
>>> ping test2.localhost
PING test2.localhost(ip6-localhost (::1)) 56 data bytes
64 bytes from ip6-localhost (::1): icmp_seq=1 ttl=64 time=0.042 ms
64 bytes from ip6-localhost (::1): icmp_seq=2 ttl=64 time=0.063 ms
^C
--- test2.localhost ping statistics ---
2 packets transmitted, 2 received, 0% packet loss, time 1004ms
rtt min/avg/max/mdev = 0.042/0.052/0.063/0.010 ms
This answer alludes that the system resolver is involved but how does it actually happen?
If localhost
in /etc/hosts
is replaced with another value the subdomains no longer work:
>>> cat /etc/hosts
127.0.0.1 testname
>>> ping testname
PING testname (127.0.0.1) 56(84) bytes of data.
64 bytes from testname (127.0.0.1): icmp_seq=1 ttl=64 time=0.071 ms
64 bytes from testname (127.0.0.1): icmp_seq=2 ttl=64 time=0.080 ms
^C
--- testname ping statistics ---
2 packets transmitted, 2 received, 0% packet loss, time 1011ms
rtt min/avg/max/mdev = 0.071/0.075/0.080/0.004 ms
>>> ping new.testname
ping: new.testname: Name or service not known
Why does it work for localhost and no other hostname and how is it achieved?