Ping : How to differentiate between Host powered off and MTU too long?

PleaseHelp asked:

I changed the eth0 MTU size to 1550(using : ifconfig eth0 mtu 1550) and ran ping -c 1 -M do -s 1500 -w 5 {some other IP} and

The output I see is,

— x.x.x.x ping statistics — 5 packets transmitted, 0 received, 100% packet loss, time 4001ms

The failure here is because Ethernet supports only MTU 1500. Now if run ping -c 1 -M do -s 1472 -w 5 {some other IP} with the “other IP” host powered off, I get the same output.

How do I find the reason of 0 received, 100% packet loss? Was it because of MTU issues or that the host was powered off?

My answer:

You can try traceroute --mtu <host> on a Linux system. This will cause traceroute to start with 65000 byte jumbo frames and decrease their size based on Path MTU discovery.

But while this may point out MTU problems in your network, it is still no guarantee that you can determine whether a host is powered on. It may be dropping these packets rather than responding to them, for instance. It also might not tell you if you have an MTU problem with a directly connected host on the same link.

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