Ttyjfyk

I need one more loopback interface in my OpenBSD 6.1, with the IP address 127.0.0.2.

I can create it by hand with the command:

ifconfig lo1 127.0.0.2

And to have it at boot time, I just inserted that command into /etc/rc.local.

I have researched for a more standard way to do that, was not successful.

Having it in /etc/rc.local also means I only have that interface late in the boot process.

How may I configure it in a cleaner "OpenBSD" way?

As hinted at in lo(4), you may create /etc/hostname.lo1:

inet 127.0.0.2 255.0.0.0

This will create the lo1 interface when the boot process runs /etc/netstart. With that file in place, you may also set up the interface without rebooting through

$ doas sh /etc/netstart lo1

The interface is reported as

lo1: flags=8049<UP,LOOPBACK,RUNNING,MULTICAST> mtu 32768
 index 4 priority 0 llprio 3
 groups: lo
 inet 127.0.0.2 netmask 0xff000000

by ifconfig.

For further info, see hostname.if(5), netstart(8) and ifconfig(8).

Unless you have a reason for this to be an additional network interface, note that you do not have to have a second interface just in order to have a second IP address in 127.0.0.0/8. You can add the second IP address to the existing loopback interface:

# echo >> /etc/hostname.lo0 inet alias 127.0.0.2 255.0.0.0
# 

Remember if you do this that you need the -A option to ifconfig now. It's a slightly misleading option. It does not target aliases per se. It simply stops ifconfig from displaying only the first IP version 4 address that it finds, which is what it actually does in lieu of finding out which IP addresses are aliases.