find_set_mtu.sh

#!/bin/bash

# Define header size (IP + ICMP headers)
HEADER_SIZE=28

# Define the starting MTU size (adjust as needed)
START_MTU=1472

# Define the minimum MTU size (optional, adjust as needed)
MIN_MTU=100  # Common minimum Ethernet MTU

# Trap for control-C to allow for graceful exit
trap 'echo "Exiting script..."; exit 1' SIGINT

# Loop through MTU values using seq
for mtu in $(seq $START_MTU $MIN_MTU); do

  # Calculate actual MTU considering header size
  actual_mtu=$((mtu - HEADER_SIZE))

  # Test if ping with "don't fragment" option succeeds (single packet)
  echo "Trying MTU: $actual_mtu"
  ping -M do -s $actual_mtu -c 1 8.8.8.8

  # Check ping exit status (no need for redirection)
  if [[ $? -eq 0 ]]; then
    echo "  --> Success! No fragmentation."
    exit 0  # Exit script successfully if a suitable MTU is found
  fi
done

# If loop finishes without successful ping, print a message
echo "No MTU found within the range ($START_MTU - $MIN_MTU) that avoids fragmentation."
exit 1  # Exit script with non-zero status to indicate failure