Sometimes you need to proove that the server in the middle really receives and transmits each transit packet between 2 hosts

tcpdumpwatch.sh

#!/bin/bash

function warn {
        echo -e "$(date +"%F %T") $*" >&2
}

function die {
        warn "$*"
        exit 13
}

while getopts ":s:d:v" OPT; do
        case "${OPT}" in
                s)
                        SOURCE="${OPTARG}"
                        ;;
                d)
                        DESTINATION="${OPTARG}"
                        ;;
                v)
                        VERBOSE=1
                        ;;
                \?)
                        die "Invalid option: -${OPTARG}"
                        ;;
                :)
                        die "Option -${OPTARG} requires an argument"
                        ;;
        esac
done
[[ -z "${SOURCE}"      ]] && die "The source address is undefined"
[[ -z "${DESTINATION}" ]] && die "The destination address is undefined"

NEXT_PHASE=0
NEXT_SEQ=0

tcpdump -l -n -i any -e "icmp and host ${SOURCE} and host ${DESTINATION}" 2>/dev/null |

        while :; do

                [[ "${VERBOSE}" -eq 1 ]] && echo -n ">${NEXT_PHASE}"

                read LINE

                INPUT=($(echo "${LINE}" | sed -n -r 's/^[0-9:\.]+  ?(In|Out) .+ ([0-9\.]+) > ([0-9\.]+): ICMP echo (request|reply), id [0-9]+, seq ([0-9]+).*, length [0-9]+$/\1 \2 \3 \4 \5/gp'))

                NEW_SEQ="${INPUT[4]}"

                case "${INPUT[3]}" in
                        'request')
                                NEW_PHASE=1
                                ([[ "${INPUT[1]}" == "${SOURCE}" ]] && [[ "${INPUT[2]}" == "${DESTINATION}" ]]) || continue 3
                                ;;
                        'reply')
                                NEW_PHASE=3
                                ([[ "${INPUT[2]}" == "${SOURCE}" ]] && [[ "${INPUT[1]}" == "${DESTINATION}" ]]) || continue 3
                                ;;
                        *)
                                die "(1) The following line looks strange:\n${LINE}"
                                ;;
                esac
                case "${INPUT[0]}" in
                        'In')
                                ;;
                        'Out')
                                NEW_PHASE=$((${NEW_PHASE} + 1))
                                ;;
                        *)
                                die "(2) The following line looks strange:\n${LINE}"
                                ;;
                esac

                [[ "${NEXT_PHASE}" -eq 0 ]] && NEXT_PHASE="${NEW_PHASE}"
                [[ "${NEXT_SEQ}"   -eq 0 ]] && NEXT_SEQ="${NEW_SEQ}"

                [[ "${NEXT_PHASE}" -ne "${NEW_PHASE}" ]] && warn "Wrong phase, expected ${NEXT_PHASE}, got ${NEW_PHASE}:\n${LINE}"
                [[ "${NEXT_SEQ}" -ne "${NEW_SEQ}" ]] && warn "Wrong seq, expected ${NEXT_SEQ}, got ${NEW_SEQ}:\n${LINE}"

                if [[ ${NEXT_PHASE} -eq 4 ]]; then
                        NEXT_PHASE=1
                        NEXT_SEQ=$((${NEXT_SEQ} + 1))
                else
                        NEXT_PHASE=$((${NEXT_PHASE} + 1))
                fi

        done