Improving your Linux box security

Did you know that more than never, during these quarantine days, there is a lot more malicious activities undergoing throughout all the web? Either if you have a desktop, a notebook or even raspberry pi unit, hundreds of unauthorised login attempts can be happening to your favourite Linux box as you read this text! If you want to peek at them and know better what is happening to your system, keep reading. I'll show you how to dynamically block people from trying to peek at your box and also how to use a simple script to generate small reports!

Many Pi users forget to change the default password, which is "raspberry" for the "pi" user. Once someone logs in with these credentials, it is just a matter of typing "sudo su" to become "root" and do whatever is desired to do with it! From destroying/stealing your data to installing rootkits or bitcoin miners, the attacker can do as he pleases. But, hey! That would be too easy, as the default password was guessed!

First things first!

Whenever you use your machine, keep in mind that a good password can help a lot!

Do you like strawberries? Try Str4wb3rrie$! It is just a matter of thinking in replacing some letters, but the key is you will never forget you love strawberries! ;)

Now, besides the password, let's dig a little deeper and tell our machine to dynamically take care of people trying to sneak on them!

Here enters fail2ban

Fail2ban is a great tool to help you keep your machine aware of people trying to access it without your consent. It will monitor the /var/log/auth file and, for every X wrong login attempts withing a time frame, it will block that origin for Y time units.

You can check blocked origins with sudo fail2ban-client status ssh and sudo fail2ban-client status sshd , so a list of IPs will be printed, if there are blocked addresses there.

Installing fail2ban

This is not a detailed howto, but just a quick tip to improve your machine's security. If you want to build fail2ban from sources, cool! Take a look at the developer's page. I'll stick with the packaged version, which does a great job! Just type apt-get install fail2ban . After the installation, create the etc/fail2ban/jail.local and make it look like this:

# Fail2Ban configuration file.
#
# This file was composed for Debian systems from the original one
# provided now under /usr/share/doc/fail2ban/examples/jail.conf
# for additional examples.
#
# Comments: use '#' for comment lines and ';' for inline comments
#
# To avoid merges during upgrades DO NOT MODIFY THIS FILE
# and rather provide your changes in /etc/fail2ban/jail.local
#


# The DEFAULT allows a global definition of the options. They can be overridden
# in each jail afterwards.


[DEFAULT]


# "ignoreip" can be an IP address, a CIDR mask or a DNS host. Fail2ban will not
# ban a host which matches an address in this list. Several addresses can be
# defined using space separator.
ignoreip = 127.0.0.1/8


# External command that will take an tagged arguments to ignore, e.g. <ip>,
# and return true if the IP is to be ignored. False otherwise.
#
# ignorecommand = /path/to/command <ip>
ignorecommand =


# "bantime" is the number of seconds that a host is banned.
bantime  = 14400 


# A host is banned if it has generated "maxretry" during the last "findtime"
# seconds.
findtime = 600
maxretry = 3


# "backend" specifies the backend used to get files modification.
# Available options are "pyinotify", "gamin", "polling" and "auto".
# This option can be overridden in each jail as well.
#
# pyinotify: requires pyinotify (a file alteration monitor) to be installed.
#            If pyinotify is not installed, Fail2ban will use auto.
# gamin:     requires Gamin (a file alteration monitor) to be installed.
#            If Gamin is not installed, Fail2ban will use auto.
# polling:   uses a polling algorithm which does not require external libraries.
# auto:      will try to use the following backends, in order:
#            pyinotify, gamin, polling.
backend = auto


# "usedns" specifies if jails should trust hostnames in logs,
#   warn when reverse DNS lookups are performed, or ignore all hostnames in logs
#
# yes:   if a hostname is encountered, a reverse DNS lookup will be performed.
# warn:  if a hostname is encountered, a reverse DNS lookup will be performed,
#        but it will be logged as a warning.
# no:    if a hostname is encountered, will not be used for banning,
#        but it will be logged as info.
usedns = warn


#
# Destination email address used solely for the interpolations in
# jail.{conf,local} configuration files.
#destemail = root@localhost
destemail = [email protected]


#
# Name of the sender for mta actions
sendername = Fail2Ban


# Email address of the sender
sender = fail2ban@localhost


#
# ACTIONS
#


# Default banning action (e.g. iptables, iptables-new,
# iptables-multiport, shorewall, etc) It is used to define
# action_* variables. Can be overridden globally or per
# section within jail.local file
banaction = iptables-multiport


# email action. Since 0.8.1 upstream fail2ban uses sendmail
# MTA for the mailing. Change mta configuration parameter to mail
# if you want to revert to conventional 'mail'.
mta = sendmail


# Default protocol
protocol = tcp


# Specify chain where jumps would need to be added in iptables-* actions
chain = INPUT


#
# Action shortcuts. To be used to define action parameter


# The simplest action to take: ban only
action_ = %(banaction)s[name=%(__name__)s, port="%(port)s", protocol="%(protocol)s", chain="%(chain)s"]


# ban & send an e-mail with whois report to the destemail.
action_mw = %(banaction)s[name=%(__name__)s, port="%(port)s", protocol="%(protocol)s", chain="%(chain)s"]
              %(mta)s-whois[name=%(__name__)s, dest="%(destemail)s", protocol="%(protocol)s", chain="%(chain)s", sendername="%(sendername)s"]


# ban & send an e-mail with whois report and relevant log lines
# to the destemail.
action_mwl = %(banaction)s[name=%(__name__)s, port="%(port)s", protocol="%(protocol)s", chain="%(chain)s"]
               %(mta)s-whois-lines[name=%(__name__)s, dest="%(destemail)s", logpath=%(logpath)s, chain="%(chain)s", sendername="%(sendername)s"]


# Choose default action.  To change, just override value of 'action' with the
# interpolation to the chosen action shortcut (e.g.  action_mw, action_mwl, etc) in jail.local
# globally (section [DEFAULT]) or per specific section
action = %(action_)s


#
# JAILS
#


# Next jails corresponds to the standard configuration in Fail2ban 0.6 which
# was shipped in Debian. Enable any defined here jail by including
#
# [SECTION_NAME]
# enabled = true


#
# in /etc/fail2ban/jail.local.
#
# Optionally you may override any other parameter (e.g. banaction,
# action, port, logpath, etc) in that section within jail.local


[ssh]


enabled  = true
port     = ssh
filter   = sshd
logpath  = /var/log/auth.log
maxretry = 4 
#maxretry = 6


[dropbear]


enabled  = false
port     = ssh
filter   = dropbear
logpath  = /var/log/auth.log
maxretry = 4 
#maxretry = 6


# Generic filter for pam. Has to be used with action which bans all ports
# such as iptables-allports, shorewall
[pam-generic]


enabled  = false
# pam-generic filter can be customized to monitor specific subset of 'tty's
filter   = pam-generic
# port actually must be irrelevant but lets leave it all for some possible uses
port     = all
banaction = iptables-allports
#port     = anyport
logpath  = /var/log/auth.log
maxretry = 3
#maxretry = 6


[xinetd-fail]


enabled   = false
filter    = xinetd-fail
port      = all
banaction = iptables-multiport-log
logpath   = /var/log/daemon.log
maxretry  = 2




[ssh-ddos]


enabled  = false
port     = ssh
filter   = sshd-ddos
logpath  = /var/log/auth.log
maxretry = 3
#maxretry = 6




# Here we use blackhole routes for not requiring any additional kernel support
# to store large volumes of banned IPs


[ssh-route]


enabled = false
filter = sshd
action = route
logpath = /var/log/sshd.log
maxretry = 3
#maxretry = 6


# Here we use a combination of Netfilter/Iptables and IPsets
# for storing large volumes of banned IPs
#
# IPset comes in two versions. See ipset -V for which one to use
# requires the ipset package and kernel support.
[ssh-iptables-ipset4]


enabled  = false
port     = ssh
filter   = sshd
banaction = iptables-ipset-proto4
logpath  = /var/log/sshd.log
maxretry = 3
#maxretry = 6


[ssh-iptables-ipset6]


enabled  = false
port     = ssh
filter   = sshd
banaction = iptables-ipset-proto6
logpath  = /var/log/sshd.log
maxretry = 3
#maxretry = 6




#
# HTTP servers
#


[apache]


enabled  = false
port     = http,https
filter   = apache-auth
logpath  = /var/log/apache*/*error.log
maxretry = 3
#maxretry = 6


# default action is now multiport, so apache-multiport jail was left
# for compatibility with previous (<0.7.6-2) releases
[apache-multiport]


enabled   = false
port      = http,https
filter    = apache-auth
logpath   = /var/log/apache*/*error.log
maxretry = 3
#maxretry  = 6


[apache-noscript]


enabled  = false
port     = http,https
filter   = apache-noscript
logpath  = /var/log/apache*/*error.log
maxretry = 3
#maxretry = 6


[apache-overflows]


enabled  = false
port     = http,https
filter   = apache-overflows
logpath  = /var/log/apache*/*error.log
maxretry = 2


[apache-modsecurity]


enabled  = false
filter   = apache-modsecurity
port     = http,https
logpath  = /var/log/apache*/*error.log
maxretry = 2


[apache-nohome]


enabled  = false
filter   = apache-nohome
port     = http,https
logpath  = /var/log/apache*/*error.log
maxretry = 2


# Ban attackers that try to use PHP's URL-fopen() functionality
# through GET/POST variables. - Experimental, with more than a year
# of usage in production environments.


[php-url-fopen]


enabled = false
port    = http,https
filter  = php-url-fopen
logpath = /var/www/*/logs/access_log


# A simple PHP-fastcgi jail which works with lighttpd.
# If you run a lighttpd server, then you probably will
# find these kinds of messages in your error_log:
#   ALERT – tried to register forbidden variable ‘GLOBALS’
#   through GET variables (attacker '1.2.3.4', file '/var/www/default/htdocs/index.php')


[lighttpd-fastcgi]


enabled = false
port    = http,https
filter  = lighttpd-fastcgi
logpath = /var/log/lighttpd/error.log


# Same as above for mod_auth
# It catches wrong authentifications


[lighttpd-auth]


enabled = false
port    = http,https
filter  = suhosin
logpath = /var/log/lighttpd/error.log


[nginx-http-auth]


enabled = false
filter  = nginx-http-auth
port    = http,https
logpath = /var/log/nginx/error.log


# Monitor roundcube server


[roundcube-auth]


enabled  = false
filter   = roundcube-auth
port     = http,https
logpath  = /var/log/roundcube/userlogins




[sogo-auth]


enabled  = false
filter   = sogo-auth
port     = http, https
# without proxy this would be:
# port    = 20000
logpath  = /var/log/sogo/sogo.log




#
# FTP servers
#


[vsftpd]


enabled  = false
port     = ftp,ftp-data,ftps,ftps-data
filter   = vsftpd
logpath  = /var/log/vsftpd.log
# or overwrite it in jails.local to be
# logpath = /var/log/auth.log
# if you want to rely on PAM failed login attempts
# vsftpd's failregex should match both of those formats
maxretry = 3
#maxretry = 6




[proftpd]


enabled  = false
port     = ftp,ftp-data,ftps,ftps-data
filter   = proftpd
logpath  = /var/log/proftpd/proftpd.log
maxretry = 3
#maxretry = 6




[pure-ftpd]


enabled  = false
port     = ftp,ftp-data,ftps,ftps-data
filter   = pure-ftpd
logpath  = /var/log/syslog
maxretry = 3
#maxretry = 6




[wuftpd]


enabled  = false
port     = ftp,ftp-data,ftps,ftps-data
filter   = wuftpd
logpath  = /var/log/syslog
maxretry = 3
#maxretry = 6




#
# Mail servers
#


[postfix]


enabled  = false
port     = smtp,ssmtp,submission
filter   = postfix
logpath  = /var/log/mail.log




[couriersmtp]


enabled  = false
port     = smtp,ssmtp,submission
filter   = couriersmtp
logpath  = /var/log/mail.log




#
# Mail servers authenticators: might be used for smtp,ftp,imap servers, so
# all relevant ports get banned
#


[courierauth]


enabled  = false
port     = smtp,ssmtp,submission,imap2,imap3,imaps,pop3,pop3s
filter   = courierlogin
logpath  = /var/log/mail.log




[sasl]


enabled  = false
port     = smtp,ssmtp,submission,imap2,imap3,imaps,pop3,pop3s
filter   = postfix-sasl
# You might consider monitoring /var/log/mail.warn instead if you are
# running postfix since it would provide the same log lines at the
# "warn" level but overall at the smaller filesize.
logpath  = /var/log/mail.log


[dovecot]


enabled = false
port    = smtp,ssmtp,submission,imap2,imap3,imaps,pop3,pop3s
filter  = dovecot
logpath = /var/log/mail.log


# To log wrong MySQL access attempts add to /etc/my.cnf:
# log-error=/var/log/mysqld.log
# log-warning = 2
[mysqld-auth]


enabled  = false
filter   = mysqld-auth
port     = 3306
logpath  = /var/log/mysqld.log




# DNS Servers




# These jails block attacks against named (bind9). By default, logging is off
# with bind9 installation. You will need something like this:
#
# logging {
#     channel security_file {
#         file "/var/log/named/security.log" versions 3 size 30m;
#         severity dynamic;
#         print-time yes;
#     };
#     category security {
#         security_file;
#     };
# };
#
# in your named.conf to provide proper logging


# !!! WARNING !!!
#   Since UDP is connection-less protocol, spoofing of IP and imitation
#   of illegal actions is way too simple.  Thus enabling of this filter
#   might provide an easy way for implementing a DoS against a chosen
#   victim. See
#    https://nion.modprobe.de/blog/archives/690-fail2ban-+-dns-fail.html
#   Please DO NOT USE this jail unless you know what you are doing.
#[named-refused-udp]
#
#enabled  = false
#port     = domain,953
#protocol = udp
#filter   = named-refused
#logpath  = /var/log/named/security.log


[named-refused-tcp]


enabled  = false
port     = domain,953
protocol = tcp
filter   = named-refused
logpath  = /var/log/named/security.log


[freeswitch]


enabled  = false
filter   = freeswitch
logpath  = /var/log/freeswitch.log
maxretry = 10
action   = iptables-multiport[name=freeswitch-tcp, port="5060,5061,5080,5081", protocol=tcp]
           iptables-multiport[name=freeswitch-udp, port="5060,5061,5080,5081", protocol=udp]


[ejabberd-auth]


enabled  = false
filter   = ejabberd-auth
port     = xmpp-client
protocol = tcp
logpath  = /var/log/ejabberd/ejabberd.log




# Multiple jails, 1 per protocol, are necessary ATM:
# see https://github.com/fail2ban/fail2ban/issues/37
[asterisk-tcp]


enabled  = false
filter   = asterisk
port     = 5060,5061
protocol = tcp
logpath  = /var/log/asterisk/messages


[asterisk-udp]


enabled  = false
filter	 = asterisk
port     = 5060,5061
protocol = udp
logpath  = /var/log/asterisk/messages




# Jail for more extended banning of persistent abusers
# !!! WARNING !!!
#   Make sure that your loglevel specified in fail2ban.conf/.local
#   is not at DEBUG level -- which might then cause fail2ban to fall into
#   an infinite loop constantly feeding itself with non-informative lines
[recidive]


enabled  = false
filter   = recidive
logpath  = /var/log/fail2ban.log
action   = iptables-allports[name=recidive]
           sendmail-whois-lines[name=recidive, logpath=/var/log/fail2ban.log]
bantime  = 604800  ; 1 week
findtime = 86400   ; 1 day
maxretry = 5


# See the IMPORTANT note in action.d/blocklist_de.conf for when to
# use this action
#
# Report block via blocklist.de fail2ban reporting service API
# See action.d/blocklist_de.conf for more information
[ssh-blocklist]


enabled  = false
filter   = sshd
action   = iptables[name=SSH, port=ssh, protocol=tcp]
           sendmail-whois[name=SSH, dest="%(destemail)s", sender="%(sender)s", sendername="%(sendername)s"]
           blocklist_de[email="%(sender)s", apikey="xxxxxx", service="%(filter)s"]
logpath  = /var/log/sshd.log
maxretry = 20




# consider low maxretry and a long bantime
# nobody except your own Nagios server should ever probe nrpe
[nagios]
enabled  = false
filter   = nagios
action   = iptables[name=Nagios, port=5666, protocol=tcp]
           sendmail-whois[name=Nagios, dest="%(destemail)s", sender="%(sender)s", sendername="%(sendername)s"]
logpath  = /var/log/messages     ; nrpe.cfg may define a different log_facility
maxretry = 1

Now, just (re)start the service

sudo systemctl restart fail2ban.service 

Now, let's generate some reports from previous attempts to unauthorised accesses to your host.

You can call it whatever you want, or stick with my suggested name. Create the file log_attack_attempts and make it look like this:

#!/bin/bash
# 
# Logs unauthorized logins attempts, based on records from /var/log/auth.log
# 
# Gabriel Marques 
# [email protected]
#
# Tue May 26 01:06:55 EDT 2020


me=$(id -u)


if [ "$me" -ne "0" ]; then
    echo -e "I need to be root"
    exit 201
fi


AUTHFILE="/var/log/auth.log"
OUTFILE="Attack_Attempts.txt"


echo -e "These are the unauthorized login attemps registered until $(date), as found in ${AUTHFILE}" > ${OUTFILE}
echo -e "---------------------" >> ${OUTFILE}
echo -e "As user pi:" >> ${OUTFILE}
echo -e "-----------" >> ${OUTFILE}
total_pi="0"
for entry in $(grep Failed ${AUTHFILE} | grep -w 'pi' | awk -F " " '{print $1 " " $2 " " $3 ", user:" $9 ", ip: " $11 }' | nl | tee -a ${OUTFILE})
do
    ((total_pi++))
done
echo "Total as user Pi: $total_pi" | tee -a ${OUTFILE}


echo -e "" >> ${OUTFILE}
echo -e "As user root:" >> ${OUTFILE}
echo -e "-----------" >> ${OUTFILE}
total_root="0"
for entry in $(grep Failed ${AUTHFILE} | grep -w 'root' | awk -F " " '{print $1 " " $2 " " $3 ", user:" $9 ", ip: " $11 }' | nl | tee -a ${OUTFILE})
do
    ((total_root++))
done
echo "Total as user root: $total_root" | tee -a ${OUTFILE}


echo -e "" >> ${OUTFILE}
echo -e "As another user:" >> ${OUTFILE}
echo -e "----------------" >> ${OUTFILE}
total_other="0"
for entry in $(grep Failed ${AUTHFILE} | grep -wi 'invalid user' | awk -F " " '{print $1 " " $2 " " $3 ", user:" $11 ", ip: " $13 }' | sort -k4 | nl | tee -a ${OUTFILE})
do
    ((total_other++))
done
echo "Total as other user: $total_other" | tee -a ${OUTFILE}


echo -e "" >> ${OUTFILE}
echo -e "You can check the banned IPs list with fail2ban-client status ssh sshd" >> ${OUTFILE}

echo "Report saved in ${OUTFILE}"

Now, just make the file executable with chmod +x log_attack_attempts

When you run it, an Attack_Attempts.txt file will be generated in the current directory and this output will be printed as well :

Total as user Pi: 0  

Total as user root: 18613 

Total as other user: 1792 

Report saved in Attack_Attempts.txt

If you look inside the Attempts.txt file, you can get the IPs that tried to connect with your host! :)

I hope this helps you in keeping good care of your host.

Try to improve the script, maybe by allowing the user to select the output file, but this is just a suggestion.

See you in a next post!

Stay safe!

FIN