mtutest

#!/bin/sh
#
# This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation, either version 3 of the License, or
# (at your option) any later version.
#
# 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/>.
#
# Copyright 2023 Jeremy Brubaker <јЬruЬаkе@оrіоnаrtѕ.іо>
#
# abstract: determine optimal MTU size
#
# Defaults {{{1
MTU_DFLT=1500
DELTA_DFLT=10
DEST_DFLT=1.1.1.1

# Documentation {{{1
#
VERSION='1.0'
PROGNAME=$(basename $0)

print_help() {
    cat <<EOF
Usage: $PROGNAME [OPTION] [DEST]
Determine the optimal MTU size for a network by testing
the path to DEST (default = $DEST_DFLT).
 
 -m [SIZE]     starting MTU size (default = $MTU_DFLT)
 -d [NUM]      amount to reduce MTU by for each check
                 (default = $DELTA_DFLT)
 -v            verbose output
 -V            display version info and exit
 -h            display this help and exit
EOF
}

print_version() {
    cat <<EOF
$PROGNAME $VERSION
Copyright (C) 2023 Orion Arts
License GPLv3+: GNU GPL version 3 or later <https://gnu.org/licenses/gpl.html>.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Written by Jeremy Brubaker.
EOF
}

# Process options {{{1
#
mtu=$MTU_DFLT
delta=$DELTA_DFLT
verbose=

while getopts 'm:d:vVh' opt; do
    case $opt in
        m) mtu=$OPTARG ;;
        d) delta=$OPTARG ;;
        v) verbose=y ;;
        V) print_version; exit ;;
        h|?) print_help; exit ;;
    esac
done
shift $((OPTIND - 1))
OPTIND=1
dest=${1:-$DEST_DFLT}

# Main {{{1
#
# The ICMP header adds 28 bytes
HEADER=28

while
    [ -n "$verbose" ] && printf 'Testing MTU of %s...\n' "$mtu"
    ! ping -c 1 -s $((mtu - HEADER)) -M do "$dest" >/dev/null 2>&1
do mtu=$((mtu - delta)); done

if [ -n "$verbose" ]; then
    echo
    printf 'Set MTU to %s\n' "$mtu"
else
    printf '%s\n' "$mtu"
fi