Creates a loopback alias with IP 127.0.0.2 at startup on Mac OS X

README.md

Loopback Alias

Creates an alias on the loopback interface (lo0) with the IP 127.0.0.2 on macOS.

Installation

1. Install the plist to: /Library/LaunchDaemons/com.runlevel1.lo0.127.0.0.2.plist
2. Set mode: sudo chmod 0644 /Library/LaunchDaemons/com.runlevel1.lo0.127.0.0.2.plist
3. Set owner: sudo chown root:wheel /Library/LaunchDaemons/com.runlevel1.lo0.127.0.0.2.plist
4. Load: sudo launchctl load /Library/LaunchDaemons/com.runlevel1.lo0.127.0.0.2.plist

The alias will automatically be created at startup from then on.

You can confirm the alias was created with ifconfig:

/tmp ❯❯❯ ifconfig lo0
lo0: flags=8049<UP,LOOPBACK,RUNNING,MULTICAST> mtu 16384
       	options=3<RXCSUM,TXCSUM>
       	inet6 ::1 prefixlen 128
       	inet 127.0.0.1 netmask 0xff000000
       	inet6 fe80::1%lo0 prefixlen 64 scopeid 0x1
       	inet 127.0.0.2 netmask 0xff000000
       	nd6 options=1<PERFORMNUD>

com.runlevel1.lo0.127.0.0.2.plist

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple Computer//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
  <dict>
      <key>Label</key>
      <string>com.runlevel1.lo0.127.0.0.2</string>
      <key>RunAtLoad</key>
      <true/>
      <key>ProgramArguments</key>
      <array>
          <string>/sbin/ifconfig</string>
          <string>lo0</string>
          <string>alias</string>
          <string>127.0.0.2</string>
      </array>
      <key>StandardErrorPath</key>
      <string>/var/log/loopback-alias.log</string>
      <key>StandardOutPath</key>
      <string>/var/log/loopback-alias.log</string>
  </dict>
</plist>

under this line:              <string>127.0.0.2</string>
i added+used this one more line:      <string>up</string>

and, under these 2 lines:      <key>StandardOutPath</key>
                    <string>/var/log/loopback-alias.log</string>
i also added+used these 2 more lines:
     <key>UserName</key>
     <string>root</string>

to view list of all net-interfaces, in "Terminal" window use below command:
    /sbin/ifconfig  -a

it is better to ping a local net-interface IP-address, to find out if its working or not:
    /sbin/ping  127.0.0.2  -c  1

if displayed Property-List (plist) cannot start/load the alias based net-interface 127.0.0.2, then, in "Terminal" try below 2 commands:
    sudo  launchctl  unload  -w  /Library/LaunchDaemons/com.runlevel1.lo0.127.0.0.2.plist
    sudo  launchctl  load  -w  /Library/LaunchDaemons/com.runlevel1.lo0.127.0.0.2.plist

to stop the net-interface, in Terminal you can use below command:
    /sbin/ifconfig  lo0  inet  127.0.0.2  -alias
note: the "launchctl unload ..." command, will not stop the alias net-interface, so to stop it, use above command manually.

if net-interface is stopped, then to start the net-interface again, use below 2 commands:
    sudo  launchctl  unload  -w  /Library/LaunchDaemons/com.runlevel1.lo0.127.0.0.2.plist
    sudo  launchctl  load  -w  /Library/LaunchDaemons/com.runlevel1.lo0.127.0.0.2.plist