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

Question

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?

There's no definitive way for you to know. It could be because of the mtu BUT it could also be because the host is powered off BUT it could also be because ICMP Echo Request is blocked at the host or somewhere before the host. — joeqwerty, Oct 28 '18 at 20:41
@MichaelHampton Is there a command which can tell me the pathMTU and also differentiate if the Host is powered off? — PleaseHelp, Oct 28 '18 at 20:42
@joeqwerty Some other command to tell the pathMTU and also differentiate if the Host is powered off? — PleaseHelp, Oct 28 '18 at 20:44
How could any command or tool tell you if the host is powered off? There's no definitive way to know that without having access to the host. — joeqwerty, Oct 28 '18 at 20:46
@joeqwerty but can't it be known whether the MTU(with do not fragment on!) is too long for the route? — PleaseHelp, Oct 28 '18 at 20:49
@joeqwerty Solving either of the "two" questions solves my problem. — PleaseHelp, Oct 28 '18 at 21:05

score 2 · Answer 1 · answered Oct 28 '18 at 20:49

2

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.

answered Oct 28 '18 at 20:49

Michael Hampton

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By "same link" do you mean the same physical link? Also, I tried traceroute in my network, apparently it allows 65000 byte jumbo frames :| – PleaseHelp Oct 28 '18 at 21:08
Yes, 'same link' means the same layer 2 network. – Michael Hampton Oct 28 '18 at 21:10

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

1 Answers1