Mac can be arpinged but not be pinged

Question

Why does Mac OS X and Linux do not ping each other on a local Ethernet Network?

Edit 2012-12-14 (ten days latter) adapter FireWire to Ethernet RJ45 might be the problem (on MacBook Pro Retina early 2015). I am researching on how to debug the his adapter. Ethernet lights blink but I don’t know enough about this early FireWire.

With a simple Netgear four ports switch-hub, when connecting via RJ45 cables osx boxes and Linux boxes and seting static ip addresses, one is able to arping but not ping to/fro different operational systems. Why is that, please?

I can see routes to the Linux boxes on osx netstat -r. But pings from osx to linux timesout. Same goes for linux pinging OSX. Arp tables are correct and were updated automatically not manually.

Ten years ago someone asked the same and did not get an answer: Which setting in osx could block "ping localhost"?

In my case, no, OS X firewall is NOT in stealth mode and it’s not even running. I have turned it off to do the experiment.

This has been posted on Mac Support website: https://discussions.apple.com/thread/253430208

Answer 1

On this Mac OSX, there was the setting "Always Require VPN" on MullvadVPN application.

This setting is extremely desirable when on the internet filtering ads and keeping privacy and security, and it is great that it works well. The app changes very deep OSX configurations and the user has no network if not connected to the VPN. This is to avoid leaks. To guarantee no request will go through another socket. It also cuts the internet if the VPN connection is interrupted and the user has to wait until it's reestablished. Even if the app is killed, one will not be able to request anything via sockets other than the VPN.

It's a great setting, but it does not allow one to connect to a service running on the own machine, or a Minikube cluster, or in this case above, a simple ethernet connection with no DHCP or any complication.

Lessons learned on the way to troubleshoot problems like this:

• When ping fails, the result from arp -a can help troubleshooting. OSX will display a question mark as domain name if cut from networking.

    ~ $ arp -a
    ? (169.254.13.100) at 0:1:0:0:0:4d on en4 [ethernet]

• But it will display the domain name when having LAN access:

    ~ $ arp -a
    tux.local (169.254.13.100) at 0:1:0:0:0:4d on en4 [ethernet]

Unfortunately, this same command on Linux will not be capable of reading the OS X name.

• The FireWire 2 adaptor was working. If the switch lights blink, ether is okay. In any case, I found the below resource for FireWire 2 problems that might help people. http://www.thexlab.com/faqs/firewiretroubleshooting.html

• OSX Firewall does not block ping in or out. When the VPN was out, all worked with the Firewall on block-all.