Determine your MTU on MacOS empirically using ping

mtu-bisect.bash

#!/bin/bash

start=${1:-1200}
finish=${2:-1600}

echo "determining MTU by bisecting between $start and $finish bytes per packet."

trap "exit 0" SIGINT SIGTERM

function do_ping {
	size=$1
	ping -q -t 4 -c 10 -o -D -s $size 8.8.8.8  > /dev/null 2> /dev/null
	ret=$?
	return $ret
}

echo "checking that $start works ..."
if ! do_ping $start ; then
	echo "lower bound $start doesn't seem to work, try a lower one?"
	exit 2
fi

echo "checking that $finish doesn't work ..."
if do_ping $finish ; then
	echo "upper bound $finish seems to work, try a higher one? (or just use it?)"
	exit 2
fi

while true; do
	try=$[(start + finish) / 2]
	if [[ $try -eq $start || $try -eq $finish ]]; then
		echo "seems that $try is your best bet, try that!"
		exit 0
	fi
	echo -n "trying $try ... "
	if do_ping $try; then
		echo "yep!"
		start=$try
	else
		echo "nope :("
		finish=$try
	fi
done

Note that the ping command line options used here are for MacOS. If you're using something else you'll need to adjust them. PRs welcome ;-)