How come one can successfully ping 127.0.0.2 on Linux?

On a FreeBSD system with a loopback network interface ...

% ifconfig lo0      
lo0
	link up loopback drv_running running multicast
	nd6 performnud auto_linklocal no_radr
	link rxcsum txcsum hwcsum rxcsum_ipv6 txcsum_ipv6
	link address  metric 0 mtu 16384 
		type 24 linkstate 0 physical 0 baudrate 0 
	inet4 address 127.0.0.1 prefixlen 8 bdaddr 127.0.0.1 
	inet6 address ::1 scope 0 prefixlen 128 bdaddr ::1 scope 0 
	inet6 address fe80::1 scope 3 prefixlen 64 
	inet6 address ::2 scope 0 prefixlen 128 
	inet4 address 127.53.0.1 prefixlen 8 bdaddr 127.53.0.1 
	inet4 address 127.53.1.1 prefixlen 8 bdaddr 127.53.1.1 
%

... pinging the IP address 127.0.0.2, which is not assigned to that (or to any other) network interface results in failure:

% ping -c 1 127.0.0.2  
PING 127.0.0.2 (127.0.0.2): 56 data bytes
ping: sendto: Network is unreachable

--- 127.0.0.2 ping statistics ---
1 packets transmitted, 0 packets received, 100.0% packet loss
% 

The same is true for OpenBSD:

% ping -c 1 127.0.0.2      
PING 127.0.0.2 (127.0.0.2): 56 data bytes
ping: sendto: Network is unreachable
ping: wrote 127.0.0.2 64 chars, ret=-1
--- 127.0.0.2 ping statistics ---
1 packets transmitted, 0 packets received, 100.0% packet loss
% 

But on Linux where there is similarly no 127.0.0.2 configured ...

% ifconfig lo
lo
	link up loopback running
	link address 00:00:00:00:00:00 bdaddr 00:00:00:00:00:00 
	inet4 address 127.0.0.1 prefixlen 8 bdaddr 127.0.0.1 
	inet4 address 127.53.0.1 prefixlen 8 bdaddr 127.255.255.255 
	inet6 address ::2 scope 0 prefixlen 128 
	inet6 address fe80:: scope 1 prefixlen 10 
	inet6 address ::1 scope 0 prefixlen 128 
% 

... the ping is surprisingly successful:

% ping -c 1 127.0.0.2
PING 127.0.0.2 (127.0.0.2) 56(84) bytes of data.
64 bytes from 127.0.0.2: icmp_seq=1 ttl=64 time=0.044 ms

--- 127.0.0.2 ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 0.044/0.044/0.044/0.000 ms
% 

How come? # Related questions * "https://unix.stackexchange.com/q/508007/5132 " — the very roundabout question, asking for an explanation of "virtual interfaces", that this question asks straightforwardly * "https://unix.stackexchange.com/q/169410/5132 " — several questions in one, rather than one question per question, and _not in fact answered_ by the purported duplicate * "https://unix.stackexchange.com/q/347825/5132 " — a question about why on Linux the lo interface seems to _also_ control the pingability of IP addresses that are assigned to _other_ network interfaces (behaviour that is similarly not what one gets on the BSDs) * "https://unix.stackexchange.com/q/140008/5132 " — a question about _non-loopback_ behaviour # Other references * Jonathan de Boyne Pollard (2019). [ifconfig](http://jdebp.eu./Softwares/nosh/guide/commands/ifconfig.xml) . _nosh Guide_. Softwares.