#!/bin/bash

function maxmtu () {
	INT="${1}"
	MTU="${2}"
	MAXMTU="${3}"
	if test -z "${MTU}"
	then
		MTU="1500"
	fi
	if test -z "${MAXMTU}"
	then
		# find out how high we can go…
		if ifconfig "${INT}" mtu "${MTU}" 
		then
			maxmtu "${INT}" "$(( 2 * ${MTU} ))"
		else
			maxmtu "${INT}" "$(( ${MTU} / 2 ))" "${MTU}"
		fi
	else
		# …then binary search to converge on the maximum ifconfig allows
		if test "${MAXMTU}" -gt "$(( ${MTU} + 1))"
		then
			MIDMTU="$(( ( ${MTU} + ${MAXMTU} ) / 2 ))"
			if ifconfig "${INT}" mtu "${MIDMTU}"
			then
				maxmtu "${INT}" "${MIDMTU}" "${MAXMTU}"
			else
				maxmtu "${INT}" "${MTU}" "${MIDMTU}"
			fi
		fi
	fi
}

>/dev/null 2>&1 maxmtu "${1}"