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Andrew Case
Asked: 2011-05-15 15:30:24 +0800 CST

Preventing Sendmail Brute Force Attack on Solaris 10

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I want to dynamically block specific connections that use the same IP address based on a rate or connection limit. Is this possible using Solaris/IPF or some sendmail extension? I want to limit sendmail login attempts to prevent brute force attacks.

In Linux it's easily handled on the iptables firewall layer, but I haven't been able to figure out a way to use ipf to limit it on the firewall layer. Sendmail has a built-in rate limit and connection limit, but it appears to be applied to all users so if we're experiencing a DOS or DDOS it would block all our users instead of just the attacker.

solaris security smtp sendmail brute-force-attacks

1 Answers

  1. Best Answer

    take a look here, these are sendmail based directive which can prevent flooding and bad behaviours, maybe this helps you: http://www.acme.com/mail_filtering/sendmail_config.html

    I for myself use these configs:

        FEATURE(`greet_pause',2)
    define(`confTO_ICONNECT', `15s')dnl
    define(`confTO_CONNECT', `3m')dnl
    define(`confTO_HELO', `2m')dnl
    define(`confTO_MAIL', `1m')dnl
    define(`confTO_RCPT', `1m')dnl
    define(`confTO_DATAINIT', `1m')dnl
    define(`confTO_DATABLOCK', `1m')dnl
    define(`confTO_DATAFINAL', `1m')dnl
    define(`confTO_RSET', `1m')dnl
    define(`confTO_QUIT', `1m')dnl
    define(`confTO_MISC', `1m')dnl
    define(`confTO_COMMAND', `1m')dnl
    define(`confTO_STARTTLS', `2m')dnl
    define(`confTO_IDENT', `0s')dnl
    define(`confTO_RESOLVER_RETRANS', `7s')dnl
    define(`confTO_RESOLVER_RETRY', `4')dnl
    define(`confMAX_RCPTS_PER_MESSAGE', `15')dnl
    define(`confMAX_DAEMON_CHILDREN',`256')dnl
    define(`confCONNECTION_RATE_THROTTLE',`8')dnl
    define(`confBAD_RCPT_THROTTLE', `1')dnl Sendmail v8.12+
    define(`confQUEUE_LA', `10')dnl 
    define(`confREFUSE_LA', `30')dnl

    Further you can search for an Implementation called greypit. I'm not really up to date on that topic, but greypit should have ip base connection limits, maybe theres a solaris version out there.

    Another way is as follow. Check your logs for massive dos activities or false logins and use the greetpause in access. If you identity malicious behaviour insert a line as follows in your access and regenerate your access.db

    GreetPause:bad.ip.dos.attacker.com            100

    From now each request from the ip or hostname hast to wait 100 seconds before getting a helo.

    I used this feature the other way around, but it can also be used for blocking unwanted connections.

    The script which did these entrys was just a cron script, but care it is just the other way around getting good traffic and you have to manually recreate your access.db:

    #!/bin/sh
declare -a a
let count=0

accessmap="/tmp/access.test"
logfiles="/var/log/mail.log"
mailfile="/tmp/tmpmail.mail"
email="[email protected]"
## hole alle IP Eintraege aus sendmail access und packe sie in ein array mit prefix und postfix

for x in $(echo $(grep -e "^GreetP" $accessmap | cut -f 2 -d ":" | cut -f 1 -d " ")); do
        a[$count]=$(echo "^"$x"|");
        ((count++));
done


echo Number of elements: ${#a[@]} > $mailfile
#entferne whitespaces 
#entferne | am ende der Zeile

b=$(echo ${a[@]} | sed "s/ //g"| sed "s/|$//")

#nun steht in der Variable den string den wir zum filtern wollen!
#echo $b
buffer=0
buffer_changed=0

datum=$(date +%Y.%m.%d__%H:%M:%S)
for x in $(grep authid $logfiles |grep "AUTH=server"|cut -f 3 -d "[" | cut -f 1 -d "-" | sort | uniq |egrep -v -e "$b" | sed "s/ (may be forged)//"|sed "s/]//"|sed "s/, authid=/#/"
if [ $buffer -eq 0 ]; then
        buffer=1
        echo >> $accessmap
        echo "#Eintraege vom $datum" >> $accessmap
        echo >> $accessmap
        buffer_changed=1
fi

echo "GreetPause:$x"| sed "s/#/ \t\t0\t#/" >> $accessmap
done

if [ $buffer -eq 1 ]; then
        echo "Command: zgrep with filter $b" >> $mailfile
        echo  >> $mailfile
        echo  >> $mailfile
        echo  "accessmap GreetingPause:">> $mailfile
        cat $accessmap | grep -B 2 "GreetPause"  >> $mailfile
        echo  >> $mailfile
        mail -s "Acessmap changed" $email < $mailfile
fi
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