How do you get loopback addresses other than 127.0.0.1 to work on OS X

Question

According to everything I read, the entire 127.x.x.x subnet should loopback.

However, on my Mac, I can only ping 127.0.0.1

I know I've done this before (though possibly on another OS) and has come in very useful for developing multiple SSL sites locally and for tunneling remote services for access on a local IP (for example I could ssh into my MySQL server, and just port forward the standard port to the same port on my local machine but on 127.0.0.2 while my local server ran at 127.0.0.1.

Solution 1

Here is the short answer: sudo ifconfig lo0 alias 127.0.0.* up

Each alias must be added individually (sudo ifconfig lo0 alias 127.0.0.2 up, sudo ifconfig lo0 alias 127.0.0.3 up). It can be done manually for testing, or a subset or the complete list of the other 250 available numbers in that subnet can be made into StartupItems script that will do it automagically at boot time.

The long answer: According to RFC3330, 127.0.0.0/8 - This block is assigned for use as the Internet host loopback address. A datagram sent by a higher level protocol to an address anywhere within this block should loop back inside the host. This is ordinarily implemented using only 127.0.0.1/32 for loopback, but no addresses within this block should ever appear on any network anywhere.

Solution 2

For those interested, here is a little bash script that adds all the aliases for the IPs 127.0.0.*:

#!/bin/bash
for ((i=2;i<256;i++))
do
    sudo ifconfig lo0 alias 127.0.0.$i up
done

Solution 3

based on @laurent anwser and this article:

1. Add localhost alias script:

$ sudo touch /usr/local/bin/localhost_alias
$ sudo chmod +x /usr/local/bin/localhost_alias

2. localhost_alias content:

#!/usr/bin/env bash

from=${1}
to=${2}

if [[ -z "$from" || -z "$to" ]]; then
  echo "Usage: "`basename "$0"`" 2-255 2-255 [from and to range numbers (127.0.0.[from-to])]"
  exit 0
fi 

for ((i=${from};i<=${to};i++))
do
    sudo ifconfig lo0 alias 127.0.0.$i up
done

3. add autorun daemon description:

$ sudo touch /Library/LaunchDaemons/org.localhost.alias.plist

4. org.localhost.alias.plist content:

<?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>org.localhost.alias</string>
    <key>RunAtLoad</key>
    <true/>
    <key>ProgramArguments</key>
    <array>
      <string>/usr/local/bin/localhost_alias</string>
      <string>2</string>
      <string>8</string>
    </array>
</dict>
</plist>

5. And we have 127.0.0.2 - 127.0.0.8 aliases at boot

*Tested on MacOS Mojave.
** You must allow to run sudo script-name without password in sudoers file.

Comments

[Parthian Shot]

If this turns out to also work on CentOS, you sir are getting a bounty from me.

[Parthian Shot]

Actually, either way you get a bounty. Since it doesn't work for CentOS, I'll just ask and answer that question once I find out the answer.

[Everett]

Am I reading this wrong? What you created looks like it does all the addresses from 127.0.0.1 to 127.0.0.255. If you change "sudo ifconfig lo0 alias 127.0.0.$i up" to "sudo ifconfig lo0 alias 127.$i.$i.$i up" that'll get you most of them right? However you would skip anything that is less than 2.. So maybe use a different variable for the other two octets? The highest IP address for loopback is 127.255.255.255

[laurent]

@Everett, you read this correctly, it only does the addresses 127.0.0.*. The reason I didn't cover the whole range is that it is actually quite slow to process even 255 addresses, so 255x255x255 would take way too long.

[Everett]

Thanks. I just wanted to make sure that the expectation was set, because someone will come on here and say, "Hey, it didn't work for 127.7.53.91." I wouldn't do that, but someone will...

[jwatkins]

Actually, @Everett's proposal wouldn't work either, since it would only enable adresses such as 127.7.7.7 or 127.42.42.42, that is all those with the three last digits are equals. You would need three distinct loops to actually enable all IPs in the 127/8 block. Yet doing this would certainly not be a good idea: it is highly probable, given that each IPs has to be enabled separately, that there are some resources attached to each assigned IP; allocating approximately 17.7 millions IPs (that is 256^3) might turn out to have some significant consequences...

[William T Froggard]

@jwatkins Bottom line: Enable them only if you need them, which is a rule of thumb for everything, really.

[ctrl-alt-delor]

How I set up 4 servers on port 8, using different loopback addresses on debian9 Gnu/Linux superuser.com/a/1255308/62123