max_mtu.sh

#!/bin/bash

# A simple script to find the maximum MTU between two hosts using a binary search approach.

# Define variables
host=$1
min_mtu=68
max_mtu=9050

# Check for required argument
if [ -z "$host" ]; then
    echo "Usage: $0 <hostname_or_ip>"
    exit 1
fi

echo "Checking maximum MTU to $host..."
echo "-----------------------------------------"

while [ $min_mtu -le $max_mtu ]; do
    current_mtu=$(( (min_mtu + max_mtu) / 2 ))
    packet_size=$(( current_mtu - 28 )) # Subtract 28 bytes for IP and ICMP headers

    # Use ping to test the current packet size.
    # -c 1: send one packet
    # -M do: don't fragment
    # -s: packet size in bytes
    ping_output=$(ping -c 1 -M do -s $packet_size $host 2>&1)
    
    # Check for error
    if echo "$ping_output" | grep -q "error:"; then
        max_mtu=$(( current_mtu - 1 ))
    else
        # If ping is successful, we can increase the size.
        echo "Success with MTU: $current_mtu"
        min_mtu=$(( current_mtu + 1 ))
        max_mtu_found=$current_mtu
    fi
done

echo "-----------------------------------------"
echo "The maximum MTU size to $host is: $max_mtu_found"