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Re: ping packet loss when size gt 1500

Adam Hardy <adam.ant@cyberspaceroad.com> wrote:
> What I need is a ping test or something that I can put in smokeping
> to alert me when I forget, e.g. this morning there was a power outage
> that took out the modem.

I think there are others here making suggestions for that.

> What do you mean by 'clamped'?

"Locked to". At the risk of stating the obvious, do take a look at
https://blue-labs.org/howto/mtu-mss.php. What I'm not sure about is
whether the clamping actually means a maximum value, or whether it would
even refuse to allow the MTU to be reduced.

> I dropped these firewall rules just now and "ping -s 1473
> mktgw1.ibllc.com" loses all packets, so our thread pretty much only
> concerns the situation when this firewall is down.

Possibly. If there's something else blocking ICMP then your firewall
ruleset will be masked by that other device. But if you managed to
resolve the issue for the remote device your firewall would still get
in the way and muddy the results.

> My actual question is: what would fail to get through when that firewall
> was up? For my testing purposes.

If I've read the ruleset correctly, it drops all ICMP. This includes
host-unreachable, port-unreachable, packet-too-big, in addition to the
well known echo/response pair (ping).


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