determine_max_packet_size

#!/bin/sh
#
# determines the maximum packet size of the current connection to a specific host
# tested on Arch Linux (bash) and Gluon (busybox ash)
#
# based on script written by @pinguinpfleger (Kai Hauser) for Freifunk Suedpfalz
# https://github.com/freifunk-suedpfalz/check_icmp_packetsize
# license: GPLv3
#

if [ $# -eq 0 ]; then
	echo "Usage: $(basename $0) [-v] <domain>"
	exit
fi

checkMinPacketSize=1232
checkMaxPacketSize=1500

if [ "$1" = "-v" ]; then
	verbose="1"
	host="$2"
else
	verbose="0"
	host="$1"
fi

if ! nslookup $host >/dev/null; then
	echo "name resolution of $host failed."
	exit
fi

if [ -e /lib/gluon/release ]; then
	if ! batctl o|grep -q mesh-vpn; then
		echo "This Gluon node has no VPN connection."
		exit
	fi
	if ! test -x /bin/opkg; then
		echo "This Gluon node doesn't have opkg. The necessary advanced ping utilities can't be installed."
		exit
	fi
	for i in iputils-ping iputils-ping6; do
		if ! opkg list-installed|grep -q $i; then
			echo "This Gluon node doesn't have the necessary advanced ping utility yet. Please install it first:"
			echo "> opkg update"
			echo "> opkg install iputils-ping iputils-ping6"
			exit
		fi
	done
fi

if ping 2>&1|grep -q interval; then
	pingparams="-q -c 2 -W 1 -M do -i 0.2 -s"
else
	pingparams="-q -c 2 -W 1 -M do -s"
fi
ping4="ping"
if command -v ping6 >/dev/null 2>&1; then
	ping6="ping6"
else
	ping6="ping -6"
	ping4="ping -4"
fi

mtu_check () {
	# if IPv6 is checked and there is either no IPv6 default route or there is none via br-wan on a device running Gluon
	if [ "$1" = "IPv6" ] && ([ "$(ip -6 route show default|grep -q default; echo $? )" != "0" ] || ([ -e /lib/gluon/release ] && [ $(ip -6 route show default|grep -q "default.*br-wan"; echo $? ) != "0" ])); then
		echo "There's no IPv6 connection as there is now default route."
		return
	fi

	if [ -e /lib/gluon/release ]; then
		ipv4addr="$(ip -4 addr show dev br-wan scope global | grep inet | head -1 | awk '{ print $2 }' | cut -d"/" -f1)"
		ping4cmd="${ping4} -I ${ipv4addr} ${pingparams}"
		ipv6addr="$(ip -6 addr show dev br-wan scope global | grep inet6 | grep -v "inet6 fd" | head -1 | awk '{ print $2 }' | cut -d"/" -f1)"
		ping6cmd="${ping6} -I ${ipv6addr} ${pingparams}"
	else
		ping4cmd="${ping4} ${pingparams}"
		ping6cmd="${ping6} ${pingparams}"
	fi

	if [ ${verbose} -eq 1 ];then
		echo "####################################################"
		echo "Checking the smallest packet size ${checkMinPacketSize}"
		echo "####################################################"
		echo ""
		echo "----------------------------------------------------------"
	fi
	if [ "$1" = "IPv6" ]; then
		if [ ${verbose} -eq 1 ];then
			${ping6cmd} ${checkMinPacketSize} ${host}
		else
			${ping6cmd} ${checkMinPacketSize} ${host} > /dev/null
		fi
	else
		if [ ${verbose} -eq 1 ];then
			${ping4cmd} ${checkMinPacketSize} ${host}
		else
			${ping4cmd} ${checkMinPacketSize} ${host} > /dev/null
		fi
	fi

	if [ $? -ne 0 ]; then
		echo "Smallest packet size ${checkMinPacketSize} is not usable."
		return
	fi
	if [ ${verbose} -eq 1 ];then
		echo "----------------------------------------------------------"
		echo ""
		echo ""
	fi

	if [ ${verbose} -eq 1 ];then
		echo "####################################################"
		echo "Checking the biggest packet size ${checkMaxPacketSize}"
		echo "####################################################"
		echo ""
		echo "----------------------------------------------------------"
	fi
	if [ "$1" = "IPv6" ]; then
		if [ ${verbose} -eq 1 ];then
			${ping6cmd} ${checkMaxPacketSize} ${host}
		else
			${ping6cmd} ${checkMaxPacketSize} ${host} > /dev/null
		fi
	else
		if [ ${verbose} -eq 1 ];then
			${ping4cmd} ${checkMaxPacketSize} ${host}
		else
			${ping4cmd} ${checkMaxPacketSize} ${host} > /dev/null
		fi
	fi
	if [ $? -eq 0 ]; then
		echo "$1: packet size: ${checkMaxPacketSize}"
		return
	fi
	if [ ${verbose} -eq 1 ];then
		echo "----------------------------------------------------------"
		echo ""
		echo ""
	fi

	if [ ${verbose} -eq 1 ];then
		echo "####################################################"
		echo "determining packet size..."
		echo "####################################################"
		echo ""
	fi
	step=$(((${checkMaxPacketSize} - ${checkMinPacketSize}) / 2))
	[ $((${step} % 2)) -eq 0 ] || step=$((${step} + 1)) #falls ungerade um 1 erhöhen

	checkPacketSize=$((${checkMinPacketSize} + ${step}))

	while [ ${step} -ge 1 ]; do
		[ ${verbose} -eq 1 ] && echo "Step: $step"
		[ ${verbose} -eq 1 ] && echo "checkPacketSize: ${checkPacketSize}"
		step=$(($step / 2))
		if [ ${step} -ne 1 ];then
			[ $((${step} % 2)) -eq 0 ] || step=$((${step} + 1))
		fi
		if [ ${verbose} -eq 1 ];then
			echo ""
			echo ""
			echo "----------------------------------------------------------"
		fi
		if [ "$1" = "IPv6" ]; then
			if [ ${verbose} -eq 1 ];then
				${ping6cmd} ${checkPacketSize} ${host}
			else
				${ping6cmd} ${checkPacketSize} ${host} > /dev/null
			fi
		else
			if [ ${verbose} -eq 1 ];then
				${ping4cmd} ${checkPacketSize} ${host}
			else
				${ping4cmd} ${checkPacketSize} ${host} > /dev/null
			fi
		fi
		if [ $? -eq 0 ]; then #ping ging
			if [ "$1" = "IPv6" ]; then
				PacketSize=${checkPacketSize}
			else
				PacketSize=${checkPacketSize}
			fi
			checkPacketSize=$((${checkPacketSize} + ${step}))
		else
			checkPacketSize=$((${checkPacketSize} - ${step}))
		fi
		if [ ${checkPacketSize} -gt ${checkMaxPacketSize} ]; then
			continue
		fi
		if [ ${checkPacketSize} -lt ${checkMinPacketSize} ]; then
			continue
		fi
		if [ ${verbose} -eq 1 ];then
			echo "----------------------------------------------------------"
		fi
	done

	if [ ${PacketSize} -ne 0 ]; then
		PacketSize=$((${PacketSize} + 8))
		echo "$1: determined packet size: $PacketSize byte (+ Transport-IP-Header: IPv4: 20 byte | IPv6: 40 byte)"
	else
		echo "$1: could not determine a packet size."
	fi
}

mtu_check IPv4
mtu_check IPv6