force_mtu_host.sh
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force_mtu_host.sh
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	#!/usr/bin/env bash
	#
	# Module: vyos-show-ram.sh
	# Displays memory usage information in minimalistic format
	#
	# Copyright (C) 2020 VyOS maintainers and contributors
	#
	# This program is free software; you can redistribute it and/or modify
	# it under the terms of the GNU General Public License version 2 as
	# published by the Free Software Foundation.
	#
	# This program is distributed in the hope that it will be useful,
	# but WITHOUT ANY WARRANTY; without even the implied warranty of
	# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
	# GNU General Public License for more details.
	#
	# You should have received a copy of the GNU General Public License
	# along with this program. If not, see <http://www.gnu.org/licenses/>.

	target=$1
	interface=$2

	# IPv4 header 20 byte + TCP header 20 byte
	ipv4_overhead=40

	# IPv6 headter 40 byte + TCP header 20 byte
	ipv6_overhead=60

	# If no arguments
	if [[ $# -eq 0 ]] ; then
	echo "Target host not defined"
	exit 1
	fi

	# If one argument, it's ip address. If 2, the second arg "interface"
	if [[ $# -eq 1 ]] ; then
	mtu=$(sudo nmap -T4 --script path-mtu -F $target \| grep "PMTU" \| awk {'print $NF'})
	elif [[ $# -eq 2 ]]; then
	mtu=$(sudo nmap -T4 -e $interface --script path-mtu -F $target \| grep "PMTU" \| awk {'print $NF'})
	fi

	tcpv4_mss=$(($mtu-$ipv4_overhead))
	tcpv6_mss=$(($mtu-$ipv6_overhead))

	echo "
	Recommended maximum values (or less) for target $target:
	---
	MTU: $mtu
	TCP-MSS: $tcpv4_mss
	TCP-MSS_IPv6: $tcpv6_mss
	"