Script bash monitorización DNS

En este manual veremos como programar un sencillo script en Bash para monitorizar la resolución DNS de un dominio en concreto, se trata de un script que debe dejarse en una sesión de Screen durante un prolongado período de tiempo.

Una de las características mas interesantes de dicho script es que solo mostrará el resultado de las pruebas cuando se lo pidamos ya sea presionando la tecla “x” o enviándole la señal SIGUSR1 al proceso o mostrando los logs de error al presionar la tecla “c” o enviándole la señal SIGUSR2 al proceso.

vi dnsChecker.sh

#!/usr/bin/env bash

clear
if [ $# -ne 1 ]; then
    echo ">> ERROR: Illegal number of parameters"
    exit
else
    DOMAIN=$1
fi

uname -a|grep FreeBSD 1>/dev/null
if [ $? -eq 0 ]; then
    OS=FREEBSD
else
    OS=LINUX
fi

ERROR_LOG_FILE=/tmp/dnsChecker
if [ $OS == "FREEBSD" ]; then
    RANDOM_STRING=$(date | md5)
else
    RANDOM_STRING=$(date | md5sum | awk '{print$1}')
fi
ERROR_LOG_FILE=$ERROR_LOG_FILE'_'$RANDOM_STRING'.log'
> $ERROR_LOG_FILE
START_DATE=$(date)
echo "Collecting DNS stats: $DOMAIN -- $START_DATE" >> $ERROR_LOG_FILE

echo "----------------------------------"
echo "| Bash-DNS checker by kr0m v0.3b |"
echo "----------------------------------"

CORRECT=0
ERROR=0
TOTAL=0
show_dns_stats=0

show_dns_stats(){
    echo ""
    echo "----------------------------------"
    echo ">> Stats collection started: $START_DATE"
    echo ">> Domain: $DOMAIN"
    echo ">> Total requests: $TOTAL"
    echo ">> Success: $CORRECT req -> $CORRECT_PERCENTAGE%"
    echo ">> Errors: $ERROR req -> $ERROR_PERCENTAGE%"
    echo "----------------------------------"
}

show_dns_errors(){
    echo ""
    N=$(wc -l $ERROR_LOG_FILE|awk '{print$1}')
    #echo "N: $N"
    echo "----------------------------------"
    echo ">> Error log file: $ERROR_LOG_FILE"
    if [ $N -gt 1 ]; then
        IFS=$'\n'
        for ERROR_LINE in $(<$ERROR_LOG_FILE); do 
            echo ">> $ERROR_LINE"
        done
        echo "----------------------------------"
    else
        echo ">> No errors registered"
        echo "----------------------------------"
    fi
}

# man signal
# stty -a
# trap [action] [signal]
trap "show_dns_stats" SIGUSR1
trap "show_dns_errors" SIGUSR2

# Save current tty settings:
if [ $OS == "FREEBSD" ]; then
    old_tty=$(stty -g)
else
    old_tty=$(stty --save)
fi

# Disable tty echo and canonical input (ERASE and KILL processing)
stty -echo -icanon min 0;

echo ">> Collecting DNS stats: $DOMAIN"
echo ">> Error log file: $ERROR_LOG_FILE"
echo ""
echo "To check collected data: Press x or send SIGUSR1 to process PID(kill -30 $$)"
echo "To check error log: Press c or send SIGUSR2 to process PID(kill -31 $$)"

while true; do
    if read -t 0; then # Input ready
        read -n 1 char
        if [ ! -z ${char} ] && [ ${char} == "x" ]; then
            #echo -e "\nRead: ${char}\n"
            show_dns_stats
        fi
        if [ ! -z ${char} ] && [ ${char} == "c" ]; then
            #echo -e "\nRead: ${char}\n"
            show_dns_errors
        fi
    else # No input
        ip=$(dig +short $DOMAIN)
        if [ -n "$ip" ]; then
            #echo "IP: $ip"
            let CORRECT=$CORRECT+1
        else
            #echo ""
            #echo "-- $(date) --"
            #echo "++ ERROR: Could not resolve hostname: $DOMAIN"
            echo "$(date) -> $DOMAIN" >> $ERROR_LOG_FILE
            let ERROR=$ERROR+1
        fi

        let TOTAL=$TOTAL+1
        #echo "CORRECT: $CORRECT"
        #echo "ERROR: $ERROR"
        #echo "TOTAL: $TOTAL"

        CORRECT_PERCENTAGE=$(bc -l <<<"$CORRECT/$TOTAL*100" | awk -F. '{print $1"."substr($2,1,2)}')
        ERROR_PERCENTAGE=$(bc -l <<<"$ERROR/$TOTAL*100" | awk -F. '{print $1"."substr($2,1,2)}')
        
        # Fix CORRECT_PERCENTAGE/ERROR_PERCENTAGE output quirks
        FIRST_DIGIT_CORRECT_PERCENTAGE=$(echo $CORRECT_PERCENTAGE | head -c1)
        if [ $FIRST_DIGIT_CORRECT_PERCENTAGE == "." ]; then
            CORRECT_PERCENTAGE="0"$CORRECT_PERCENTAGE
        fi
        FIRST_DIGIT_ERROR_PERCENTAGE=$(echo $ERROR_PERCENTAGE | head -c1)
        if [ $FIRST_DIGIT_ERROR_PERCENTAGE == "." ]; then
            ERROR_PERCENTAGE="0"$ERROR_PERCENTAGE
        fi
        if [ $CORRECT_PERCENTAGE == "0." ]; then
            CORRECT_PERCENTAGE=0
        fi
        if [ $ERROR_PERCENTAGE == "0." ]; then
            ERROR_PERCENTAGE=0
        fi
        sleep 0.2
    fi
done

# Restore original tty settings:
stty $old_tty

Asginamos los permisos necesarios al script:

chmod 700 dnsChecker.sh

La salida del script es la siguiente:

./dnsChecker.sh alfaexploit.com

----------------------------------
| Bash-DNS checker by kr0m v0.3b |
----------------------------------
>> Collecting DNS stats: alfaexploit.com
>> Error log file: /tmp/dnsChecker_0a347a873d1da30bb77c03a6d55ef9f7.log

To check collected data: Press x or send SIGUSR1 to process PID(kill -30 56211)
To check error log: Press c or send SIGUSR2 to process PID(kill -31 56211)

----------------------------------
>> Stats collection started: viernes,  8 de abril de 2022, 09:58:56 CEST
>> Domain: alfaexploit.com
>> Total requests: 7
>> Success: 7 req -> 100.00%
>> Errors: 0 req -> 0%
----------------------------------

----------------------------------
>> Error log file: /tmp/dnsChecker_0a347a873d1da30bb77c03a6d55ef9f7.log
>> No errors registered
----------------------------------