Secure new CentOs install

Step 1: Secure SSH

Log in as root to your server and type the following commands to backup and then edit the SSH configuration:

cp /etc/ssh/ssh_config /etc/ssh/ssh_config.bak; cp /etc/ssh/sshd_config /etc/ssh/sshd_config.bak
vi /etc/ssh/ssh_config

Hit the i key to enter insert mode. Then uncomment all the lines after (and including) Host * (i.e. remove the hashes) and change Protocol 2,1 to 2 only. Hit Esc to exit insert mode and type ZZ to quit saving the changes. Then type the following command:

vi /etc/ssh/sshd_config

As before, in insert mode, uncomment the Port, Protocol (and change to 2 only if not already) and ListenAddress statements. Also uncomment and change PermitRootLogin to: no. Quit and save (Esc, ZZ). Then restart the SSH service:

service sshd restart

Since we have now prevented the root user from logging in remotely (as a security measure – the root user has full access to the entire system and can break things very easily), the final step is to create a user who can log in remotely. Type in:

useradd -g wheel admin
passwd admin

Next time when you log in you can switch to the root user using the following command (enter the root password at the prompt):

su -

Step 2: Install ChkRootKit (rootkit finder)

Create a directory to hold downloaded or compiled sofware, then install some tools we will need (these may well already be installed):

mkdir -p ~admin/software
cd ~admin/software
yum -y install gcc make wget vixie-cron perl

Download and install ChkRootKit:

tar zxfv chkrootkit.tar.gz
cd chkrootkit-*
make sense

Then add a cron entry to run the script automatically (this is still done as the root user):

crontab -e

Tell it to run every day at 3am and email you the errors – add the following line (use the same commands as when using vim above):

0 3 * * * /home/admin/software/chkrootkit-*/chkrootkit -q 2>&1 | mail -s "ChkRootKit Output from `hostname`"

You could do that bit of editing entirely on the command line by creating a temporary file and then adding that to the crontab like this:

touch crontab_temp
crontab -l > crontab_temp
echo "0 3 * * * /home/admin/software/chkrootkit-*/chkrootkit -q 2>&1 | mail -s \"ChkRootKit Output from \`hostname\`\"" >> crontab_temp
cat crontab_temp | crontab
rm -f crontab_temp

Step 3: Install Portsentry (check for people sniffing/scanning your ports and block them)

cd ~admin/software

32-bit version – use this if your OS is 32-bit – download and install the existing package:

rpm -Uhv portsentry-1.2-1.te.i386.rpm
/etc/rc.d/init.d/portsentry start
echo "/etc/rc.d/init.d/portsentry" >> /etc/rc.d/rc.local

64-bit version – use this if your OS is 64-bit – we need to compile the original program, but there is an error in one of the files we need to fix first:

tar zxfv portsentry-1.2.tar.gz
cd portsentry_beta
vi portsentry.c

The error is on line 1584 and will prevent the program from compiling. To see line numbers, type in :set number
Find line 1584 and remove the line break in the middle of that sentence. Then install:

make linux
make install

Next we need to create a script to control the service:

vi /etc/init.d/portsentry

Start insert mode and paste this all this into the file (careful of linebreaks – then save and quit):


case "$1" in
echo "Starting Portsentry..."
ps ax | grep -iw '/usr/local/psionic/portsentry/portsentry -atcp' | grep -iv 'grep' > /dev/null
if [ $? != 0 ]; then
/usr/local/psionic/portsentry/portsentry -atcp
ps ax | grep -iw '/usr/local/psionic/portsentry/portsentry -audp' | grep -iv 'grep' > /dev/null
if [ $? != 0 ]; then
/usr/local/psionic/portsentry/portsentry -audp
echo "Portsentry is now up and running!"
echo "Shutting down Portsentry..."
array=(`ps ax | grep -iw '/usr/local/psionic/portsentry/portsentry' | grep -iv 'grep' \
| awk '{print $1}' | cut -f1 -d/ | tr '\n' ' '`)
while [ "$index" -lt "$element_count" ]
kill -9 ${array[$index]}
let "index = $index + 1"
echo "Portsentry stopped!"
$0 stop && sleep 3
$0 start
echo "Usage: $0 {start|stop|restart}"
exit 1
exit 0

Then we need to make that script executable, add portsentry to the startup scripts and start it up:

chmod 755 /etc/init.d/portsentry
ln -s /etc/init.d/portsentry /etc/rc2.d/S20portsentry
ln -s /etc/init.d/portsentry /etc/rc3.d/S20portsentry
ln -s /etc/init.d/portsentry /etc/rc4.d/S20portsentry
ln -s /etc/init.d/portsentry /etc/rc5.d/S20portsentry
ln -s /etc/init.d/portsentry /etc/rc0.d/K20portsentry
ln -s /etc/init.d/portsentry /etc/rc1.d/K20portsentry
ln -s /etc/init.d/portsentry /etc/rc6.d/K20portsentry
/etc/init.d/portsentry start

Step 4: Install LibSafe (prevents buffer overflow exploits)

cd ~admin/software

Download for 32-bit:


Or for 64-bit:


Then install:

rpm -Uhv libsafe-2.0-16*.rpm

Step 5: Install Hogwash (inline packet scrubber)

Download, install and configure Hogwash:

cd ~admin/software
tar zxfv devel-0.5-latest.tgz
cd distro/devel-0.5/devel-0.5
cp hogwash /sbin
mkdir /var/log/hogwash
mkdir /etc/hogwash
cd rules
cp *.rules /etc/hogwash
cd ..
cp *.config /etc/hogwash
cp /etc/hogwash/stock.config /etc/hogwash/live.config

We need to create another control script, but we can do this on the command line:

touch Hog
echo '#!/bin/sh' >> Hog # needs single quotes
echo "#chkconfig: 2345 11 89" >> Hog
echo "#description: Automates Hogwash packet filter" >> Hog
echo "/sbin/hogwash -d -c /etc/hogwash/live.config -r /etc/hogwash/live.rules -l /var/log/hogwash" >> Hog
chmod 700 Hog

Make sure it starts at boot time:

cp Hog /etc/rc.d/init.d
chkconfig --add Hog

Step 6: Install DenyHosts (blocks brute force login attempts)

cd ~admin/software

Install the RPMForge repo – for 32-bit:


Or for 64-bit:


Install, configure and make sure your own address is not blocked (substitute your IP address in the code below)

rpm -i rpmforge-release-0.5.1-1.el5.rf.*.rpm
yum check-update
yum -y install denyhosts
echo "sshd:" >> /etc/hosts.allow
perl -pi -e "s/PURGE_DENY =/PURGE_DENY = 7d/g;" /etc/denyhosts/denyhosts.cfg
chkconfig denyhosts on
service denyhosts start

Step 7: Install RootKit Hunter (yes, another one)

Download and configure RkHunter, then set up the cronjob to execute automatically (as above) and email you if there are warnings:

yum -y install rkhunter
cd ~admin/software
perl -pi -e "s/MAIL-ON-WARNING=\"\"/MAIL-ON-WARNING=\"your\\"/g;" /etc/rkhunter.conf
touch crontab_temp
crontab -l > crontab_temp
echo "0 4 * * * /usr/bin/rkhunter --cronjob 2>&1" >> crontab_temp
echo "@monthly /usr/bin/rkhunter --update" >> crontab_temp
cat crontab_temp | crontab
rm -f crontab_temp


There are a couple of other things I always do when setting up a server.

• Disable the weak ciphers in the SSH server:
sshd -T | grep ciphers | sed -e "s/\(3des-cbc\|aes128-cbc\|aes192-cbc\|aes256-cbc\|arcfour\|arcfour128\|arcfour256\|blowfish-cbc\|cast128-cbc\|\)\,\?//g" >> /etc/ssh/sshd_config
service sshd restart

• Disable Apache mod_status (see httpd.conf or 00-base.conf in /etc/httpd/conf/)
• Turn off TRACK|TRACE in Apache:
echo "TraceEnable Off" >> /etc/httpd/conf/httpd.conf
service httpd reload

• If Webmin is installed tweak the SSL Options and only allow the following ciphers: ALL:!ADH:!LOW:!MEDIUM:!SSLv2:!EXP:+HIGH