Activate the available shields to protect your server
Activate the available shields to protect your server
The default CentOS installation provides many security features that are not enabled by default and that we all should take benefit of. Here are some to consider.
Prerequisites
The media used for this tutorial is the CentOS 6.2 minimal installation CD. There’s no way to make a mistake when choosing packages with this installation media, because you can’t. It just installs the minimal software. Make sure that you install the EPEL project repository.
rpm -Uvh http://dl.fedoraproject.org/pub/epel/6/x86_64/epel-release-6-5.noarch.rpm
Retrieving http://dl.fedoraproject.org/pub/epel/6/x86_64/epel-release-6-5.noarch.rpm
warning: /var/tmp/rpm-tmp.VseyUg: Header V3 RSA/SHA256 Signature, key ID 0608b895: NOKEY
Preparing... ########################################### [100%]
1:epel-release ########################################### [100%]
And make sure that your server is up to date.
Unnecessary services
To limit the available attack surface available on the server, we’ll simply shut down the unnecessary services (well, those that I do think I don’t need for, let’s say, a webserver) and prevent them from starting at next reboot. These are the following services I’m going to disable :
- rpcbind The rpcbind utility is a server that converts RPC program numbers into universal addresses. It must be running on the host to be able to make RPC calls on a server on that machine.
- nfslock Starts up the NFS file locking service
- lldpad Link Layer Discovery Protocol Agent Daemon
- fcoe Open-FCoE Initiator
- rpdidmapd This deamon maps user names and groups to UID and GID numbers on NFSv4 mounts
for i in rpcbind nfslock lldpad fcoe rpcidmapd; do service $i stop; chkconfig $i off; done
Stopping rpcbind: [ OK ]
Stopping NFS statd: [ OK ]
Stopping lldpad: [ OK ]
Stopping FCoE initiator service: [ OK ]
Stopping RPC idmapd: [ OK ]
Protecting SSH service
As this is one the core and mostly attacked services, we need to protect ourselves against one of the most common threats : brute force attacks. For this, we are going to set up Fail2ban that will account for us the number of times a server will occur with failed authentications attempts and add the corresponding ip address to iptables reject list (a specific iptables user chain that returns until the ip is listed in it, to be exact). The package file comes from the EPEL project, and I build a configuration file specifically for this simple purpose :
[DEFAULT]
ignoreip = 127.0.0.1
# Block the attacking host for a day
bantime = 86400
# If it appears in the watched log file for the last hour
findtime = 3600
# And generated 3 error messages
maxretry = 3
backend = auto
[ssh-iptables]
enabled = true
filter = sshd
action = iptables[name=SSH, port=ssh, protocol=tcp]
# Optionnal : sendmail-whois[name=SSH, dest=root, sender=fail2ban@mail.com]
logpath = /var/log/secure
maxretry = 3
So now, install it, configure it using the custom jail.conf and make the service start when the system boots.
yum install fail2ban
curl http://bkraft.fr/files/Configurations/fail2ban/jail.conf -o /etc/fail2ban/jail.conf
chkconfig fail2ban on
service fail2ban start
Starting fail2ban: [ OK ]
We now can see that there is an iptables chain called fail2ban-SSH as you can see below.
iptables-save
# Generated by iptables-save v1.4.7 on Sat Mar 10 22:09:08 2012
*filter
:INPUT ACCEPT [0:0]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT [17:1492]
:fail2ban-SSH - [0:0]
-A INPUT -p tcp -m tcp --dport 22 -j fail2ban-SSH
-A INPUT -m state --state RELATED,ESTABLISHED -j ACCEPT
-A INPUT -p icmp -j ACCEPT
-A INPUT -i lo -j ACCEPT
-A INPUT -p tcp -m state --state NEW -m tcp --dport 22 -j ACCEPT
-A INPUT -j REJECT --reject-with icmp-host-prohibited
-A FORWARD -j REJECT --reject-with icmp-host-prohibited
-A fail2ban-SSH -j RETURN
COMMIT
# Completed on Sat Mar 10 22:09:08 2012
Small things that matters
In single user mode, ask for password
perl -i -pe 's/sushell/sulogin/' /etc/sysconfig/init
Change the number of available gettys as there is too much of them available by default.
perl -i -pe 's/1-6/1/' /etc/sysconfig/init
perl -i -pe 's/1-6/1/' /etc/init/start-ttys.conf
Prevent prompting for interactive boot
perl -i -pe 's/PROMPT=yes/PROMPT=no/' /etc/sysconfig/init
Prevent rebooting when Control-Alt-Delete is being pressed on the console.
perl -i -pe 's/exec.*/exec \/bin\/echo "Control-Alt-Delete pressed, but no action will be taken"/' /etc/init/control-alt-delete.conf
Change the minimum password length from 5 to 9
perl -i -pe 's/PASS_MIN_LEN\s+5/PASS_MIN_LEN 9/' /etc/login.defs
Disconnect idle users after 15 minutes
cat > /etc/profile.d/inactive-users-disconnect.sh << EOF
readonly TMOUT=900
readonly HISTFILE
EOF
chmod +x /etc/profile.d/inactive-users-disconnect.sh
Prevent anybody but root to run cron or at tasks
touch /etc/cron.allow
chmod 600 /etc/cron.allow
awk -F: '{print $1}' /etc/passwd | grep -v root > /etc/cron.deny
touch /etc/at.allow
chmod 600 /etc/at.allow
awk -F: '{print $1}' /etc/passwd | grep -v root > /etc/at.deny
Be careful, consistency of at.deny and cron.deny is not guaranteed over time. you should care about it (rerun the awks by hand or by cron)
Change /etc/issue to something scary
cat >/etc/issue << EOF
USE OF THIS COMPUTER SYSTEM, AUTHORIZED OR UNAUTHORIZED, CONSTITUTES CONSENT TO MONITORING OF THIS SYSTEM.
UNAUTHORIZED USE MAY SUBJECT YOU TO CRIMINAL PROSECUTION.
EVIDENCE OF UNAUTHORIZED USE COLLECTED DURING MONITORING MAY BE USED FOR ADMINISTRATIVE, CRIMINAL, OR OTHER ADVERSE ACTION.
USE OF THIS SYSTEM CONSTITUTES CONSENT TO MONITORING FOR THESE PURPOSES.
EOF
Narrow down rights
Many of the files have by default too wide rights that might led to disclose some important informations.
# Narrow down right on /root
chmod 700 /root
# Audit logs should be available only for root
chmod 700 /var/log/audit
# Remove too wide rights on iptables binary and init scripts
chmod 740 /etc/rc.d/init.d/iptables
chmod 740 /sbin/iptables
# Change the rights of the default user skeleton
chmod -R 700 /etc/skel
# Restrict access to rsyslog configuration to root
chmod 600 /etc/rsyslog.conf
# Locking down LNX00440
chmod 640 /etc/security/access.conf
# Sysctl configuration should only accessible to root
chmod 600 /etc/sysctl.conf
Secure network via sysctl
cat << 'EOF' >> /etc/sysctl.conf
# disable packet forwarding
net.ipv4.ip_forward = 0
# drop icmp redirects
net.ipv4.conf.all.send_redirects = 0
net.ipv4.conf.default.send_redirects = 0
net.ipv4.conf.all.accept_redirects = 0
net.ipv4.conf.all.secure_redirects = 0
net.ipv4.conf.default.accept_redirects = 0
net.ipv4.conf.default.secure_redirects = 0
# double the syn backlog size
net.ipv4.tcp_max_syn_backlog = 2048
# ignore ping broadcasts
net.ipv4.icmp_echo_ignore_broadcasts = 1
# drop the source routing ability
net.ipv4.conf.all.accept_source_route = 0
net.ipv4.conf.default.accept_source_route = 0
# log packets destinated to impossible addresses
net.ipv4.conf.all.log_martians = 1
# ignore bogus icmp error responses
net.ipv4.icmp_ignore_bogus_error_responses = 1
# protect a bit from SYN flood attacks
net.ipv4.tcp_syncookies = 1
# drop packets that come in using a bad interface
# (they will be logged as martian)
net.ipv4.conf.all.rp_filter = 1
net.ipv4.conf.default.rp_filter = 1
# don't send timestamps
net.ipv4.tcp_timestamps = 0
EOF
Delete unnecessary users
By default many users are present that aren’t necessary, let’s just get rid of them.
/usr/sbin/userdel shutdown
/usr/sbin/userdel halt
/usr/sbin/userdel games
/usr/sbin/userdel operator
/usr/sbin/userdel ftp
/usr/sbin/userdel gopher
Tuning OpenSSH
As per this setup, the SSH service is opened on internet, and we need to make sure that it is secured. By default, it is already well secured, but there is some things we should change.
# Prevent ssh connections from root
perl -i -pe 's/#PermitRootLogin yes/PermitRootLogin no/' /etc/ssh/sshd_config
# Set our scary issue message as ssh banner
perl -i -pe 's/#Banner.*/Banner \/etc\/issue/g' /etc/ssh/sshd_config
# Make the server keys a bit bigger
perl -i -pe 's/^#ServerKeyBits 1024/ServerKeyBits 2048/g' /etc/ssh/sshd_config
# We need now to drop previously created 1024 keys and regenerate them.
rm -vf /etc/ssh/ssh_host*
removed `/etc/ssh/ssh_host_dsa_key'
removed `/etc/ssh/ssh_host_dsa_key.pub'
removed `/etc/ssh/ssh_host_key'
removed `/etc/ssh/ssh_host_key.pub'
removed `/etc/ssh/ssh_host_rsa_key'
removed `/etc/ssh/ssh_host_rsa_key.pub'
/etc/init.d/sshd restart
Stopping sshd: [ OK ]
Generating SSH1 RSA host key: [ OK ]
Generating SSH2 RSA host key: [ OK ]
Generating SSH2 DSA host key: [ OK ]
Starting sshd: [ OK ]
# Restrict max authentications
perl -i -pe 's/^#MaxAuthTries 6/MaxAuthTries 3/g' /etc/ssh/sshd_config
Removing kernel modules
As the machine that I’m running is a virtual one, I don’t need wireless drivers to be loaded and neither do I need fcoe, so let’s just blacklist them.
for i in $(find /lib/modules/`uname -r`/kernel/drivers/net/wireless -name "*.ko" -type f) ; do echo blacklist $i >> /etc/modprobe.d/blacklist-wireless.conf ; done
for i in $(find /lib/modules/`uname -r`/kernel/drivers/scsi/fcoe -name "*.ko" -type f) ; do echo blacklist $i >> /etc/modprobe.d/blacklist-fcoe.conf ; done
