mtu


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#!/bin/bash

function actually_just_show {
  echo "The MTU of $iface is $nowmtu."
  exit 0
}

function get_default_iface {
  iface=$(ip a | grep "state UP" | cut -d: -f2 | tr -d '[[:space:]]')
  if [ "x" == "x$iface" ]
  then
    echo "No active iface"
    exit 1
  fi
}

function get_current_mtu {
  nowmtu=$(</sys/class/net/$iface/mtu)
}

function get_new_mtu {
  case "$nowmtu" in
  1500) mtu=1492
        ;;
  *)    mtu=1500
        ;;
  esac
}

function guess_at_remainder {
  for i in $@; do
    case "$i" in
    show)       actually_just_show
                ;;
    [0-9]*)     mtu=$i
                ;;
    [a-zA-Z]*)  iface=$i
                ;;
    esac
  done
}

OPTIND=1

get_default_iface
get_current_mtu
get_new_mtu

while getopts "i:m:" opt; do
    case "$opt" in
    i)  iface=$OPTARG
        ;;
    m)  mtu=$OPTARG
        ;;
    esac
done

shift $((OPTIND-1))

[ "$1" = "--" ] && shift

guess_at_remainder $@

echo -n "Set MTU of $iface to $mtu? [Y/n] "
read yn

if [ "x${yn,,}" != "xn" ]; then
  echo $mtu | sudo tee "/sys/class/net/$iface/mtu" > /dev/null
fi