Launchd task to run a shell script on macOS boot up. It can be used to add things such as extra IP addresses, etc.

boot-up-actions.plist

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
    <dict>
        <key>Label</key>
        <string>regulya.local</string>
        <key>Program</key>
        <string>/Users/regulya/bin/boot-up-actions.sh</string>
        <key>RunAtLoad</key>
        <true/>
        <key>LaunchOnlyOnce</key>
        <true/>
        <key>StandardErrorPath</key>
        <string>/dev/null</string>
        <key>StandardOutPath</key>
        <string>/dev/null</string>
    </dict>
</plist>

Here's a sample shell script that adds 2 IP addresses to the loopback interface:

#!/bin/sh

# This script runs as root on boot - careful

if [ "$UID" != "0" ]; then
        echo "You need to be root to run the script"
        exit 1
fi

/sbin/ifconfig lo0 alias 127.0.0.2 netmask 255.255.255.0
/sbin/ifconfig lo0 alias 127.0.0.3 netmask 255.255.255.0