mtu_discover.sh

mtu_discover.sh

#!/bin/bash
# original https://idyllictux.wordpress.com/2009/06/19/script-find-maximum-mtu/
# adjusted it to detect pmtu hole
# read https://blog.cloudflare.com/path-mtu-discovery-in-practice/
# https://habrahabr.ru/post/136871/

PKT_SIZE=${2:-1472}
HOSTNAME="$1"

z=$( ping -M do -c 1 -w 3 -s $PKT_SIZE $HOSTNAME 2>&1 )
res=$?
count=$( echo "$z" | grep -c "Frag needed" )

if [ $count -eq 1 ]; then
    echo "Got frag needed reply, trying to lower PKT_SIZE from $PKT_SIZE"
    while [ $count -eq 1 ]; do
        ((PKT_SIZE--))
        echo -n "$PKT_SIZE:> "
        count=$((`ping -M do -c 1 -s $PKT_SIZE $HOSTNAME | grep -c "Frag needed"`))
    done
    echo

elif [ $res -eq 1 -a $count -eq 0 ]; then
    # At this point it's impossible to identify if host not reachable or blackhole
    # will try to lower pkt_size
    echo "Didn't get any reply. Either the host is not reachable or we faced an MTU blackhole."
    echo "trying to lower PKT_SIZE from $PKT_SIZE"
    while [ $res -eq 1 ]; do
        ((PKT_SIZE--))
        echo -n "$PKT_SIZE:> "
        z=$( ping -M do -c 1 -w 3 -s $PKT_SIZE $HOSTNAME 2>&1 )
        res=$?
    done
    echo
elif [ $res -ne 0 ]; then
    echo "Unknown error"
    echo "$z"
    exit 1
fi


printf "Your Maximum MTU is [ $((PKT_SIZE + 28)) ] \n"