Tag Archives: security

How to configure secure SSH on CentOS

Hello everyone! In this tutorial I will show you how to increase server security by tuning up configuration of SSH.

Before you begin

There are basically two requirements for this tutorial:

You need to have working SSH keys. You need to be able to login to your server by using them. After completing this tutorial, SSH keys will be the only way to access server. If you won't be able to login by using them, well, you will lose access to your server. In order to add user and configure keys you can follow this tutorial.
Make sure that at least one of the users is in wheel group (has access to sudo). Root should not have access to login via SSH. So if you will block this option and you won't have any sudo user, you won't be able to do much on the server. Follow this tutorial in order to configure sudo.

Disable password authentication for SSH on CentOS

sudo vi /etc/ssh/sshd_config

Now we need to find the line for password authentication and change it to:

PasswordAuthentication no

Unfortunately, disabling this option can still lead to password authentication by using PAM-based authentication. In order to fully disable authentication with password, make sure that PAM is also disabled:

ChallengeResponseAuthentication no

Also we need to make sure that this line is uncommented. It will enable SSH login by using public key:

PubkeyAuthentication yes

Save the file and exit from the editor. In order to apply changes, you need to restart SSH daemon:

service sshd restart

After that, try to open new SSH session in new window. Do not logout from your current session! If you won't be able to login with new session, you can undo the changes with existing session. If you will be able to successfully login, you can proceed.

How to secure SSH on CentOS even more?

There are still some things that will help you improve SSH security. Edit the same configuration file as before. Below You will find the configuration options that I usually use for SSH.

Disable root login

PermitRootLogin no

This option will disable root login via ssh. So it means that from now on you won't be able to login to your server as root via ssh.

Allow only specific users to be able to login via SSH

AllowUsers developer

By default you are able to login as any user that is created inside the system. It can be easily limited to particular users. Just give space separated list after AllowUsers. It might not be present in your config, so you need to add this line (for instance at the end of the file).

AllowUsers developer vagrant

Enable protocol 2 for ssh

Protocol 2

This option is set by default in most CentOS installation, but just make sure that there's no version 1 instead. It's less secure protocol.

Ignore rhost

IgnoreRhosts yes

It will disable insecure access via RSH.

Disable login for users with empty passwords

PermitEmptyPasswords no

This line will disable login for users that have empty passwords. Make sure that your account has password set, before changing that!

Enable strict mode for ssh

StrictModes yes

SSH will check users's permission in their home directory before accepting login. It should be set to yes because users may leave their directory or files world-writable. Again, this might be tricky. It's the best to change that, restart SSHD daemon and try to login from new session. If you have any problems, you can undo this change with existing session. If you have any issues with that, try to set valid permissions for your .ssh directory and files inside. Also set valid username and group for your files:

chmod 700 ~/.ssh
chmod 600 ~/.ssh/*
chown -R YOUR_USERNAME:YOUR_USERNAME ~/.ssh

Disable other authentication methods

GSSAPIAuthentication no
KerberosAuthentication no

If you don't plan to login with GSS API, or Kerberos you can disable them as well.

Disable X11 Forwarding

X11Forwarding no

If you don't use X11 you can safely disable it as well.

Show last login

PrintLastLog yes

Nice feature is to show last successful login after you will login via SSH.

Restart SSH daemon

Remember that after any changes inside the file You need to restart sshd daemon:

sudo service sshd restart

SSH crypto

In addition to changes above that should be applied, you can increase SSH security even more by configuring ciphers and available algorithms (thanks to @Amar for the suggestion:)

This is usually safe to execute, but you must remember that not all algorithms are supported by various tool. Here you can find great chart showing, which tools support given algorithms. But let's be honest, most of you is probably using OpenSSH which supports all the changes I will present here. However if you are using different tool and you won't be able to login to your server, check with the page and enable additional algorithms.

These config options will probably not be listed in your config file. You need to just add them somewhere, like at the end of the file.

Configure server authentication

HostKey /etc/ssh/ssh_host_ed25519_key
HostKey /etc/ssh/ssh_host_rsa_key

Server must confirm the identity to the client. There are bunch of algorithms available, but this is the list of most secure.

This might be present in your configuration file, also there might be more not commented lines with HostKeys. Leave only these two enabled and comment out the rest.

Configure key exchange

KexAlgorithms curve25519-sha256@libssh.org

There are many more key exchange algorithms, but this is probably the most secure.

Configure ciphers

Ciphers aes128-ctr,aes192-ctr,aes256-ctr,aes128-gcm@openssh.com,aes256-gcm@openssh.com,chacha20-poly1305@openssh.com

Ciphers are used to encrypt the data. As with key exchange, there are multiple algorithms. These are the safest.

MACs - Message Authentication Codes

MACs hmac-ripemd160,hmac-ripemd160-etm@openssh.com,hmac-sha2-256,hmac-sha2-512,hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,umac-128@openssh.com,umac-128-etm@openssh.com

MACs are used for data integrity. Again, line above contains the safest algorithms only.

After these changes, don't forget to restart sshd daemon.

Easier way?

You can use our Ansible LAMP on Steroids project to make configuration of your server easier!

If you don't know what Ansible is, check our tutorial first.

Clone our repository and setup your server faster with LAMP on steroids.

How to secure server with iptables?

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Hi there! In this tutorial I would like to show you how to increase server security by using iptables as a firewall. To be honest, not many people are actually using iptables or any firewall. I think that this is bad practice, because you they allow all traffic to go in and out. You should always limit the possible entry points to your server.

Firewalld vs iptables

Since CentOS 7, we have new tool called firewalld. This is not actually an alternative to iptables. firewalld is a wrapper for iptables. Many people say, that it's easier to use than iptables, but to be honest I believe that it's not flexible enough. Maybe I'm wrong, but I'd love to see some advanced example, how to transform iptables rules below to firewalld 🙂 If you want to use firewalld instead of iptables, unfortunately you need to read different tutorial. Here is great article about firewalld from DigitalOcean.

How to install iptables on CentOS7?

Before we will install iptables, we need to get rid of firewalld first :

sudo yum remove firewalld -y

Next, we can install iptables:

sudo yum install iptables iptables-services -y

iptables-services is simple script that will help us save and restore firewall rules.

Secure iptables rules for CentOS

First, let's check if there are any rules by executing following command:

sudo iptables -S

If you will get following output:

-P INPUT ACCEPT
-P FORWARD ACCEPT
-P OUTPUT ACCEPT

It means that you allow all traffic, both incoming and outgoing to your server. However if you have anything more than output above, copy it to separate file as a backup.

The easiest way of adding rules is by editing iptables rules file. Open the file, or create one if it doesn't exists:

sudo vi /etc/sysconfig/iptables

I will describe whole file line by line, but at the bottom of this post you can find whole content that I'm using for iptables.

Opening and closing tags

*filter

File must contains two indicators:

start of the ruleset *filter
end of the ruleset COMMIT

You need to have both in order to get iptables configured properly. Between these two lines, you can add iptables rules.

Clear all existing rules

-X
-F
-Z

At the very beginning I'd like to clear whole rules. In other words - enable all traffic. The reason is that I want to be able to execute that file over and over again, and I will always set the rules that I have in file. No other rules will be applied (for instance rules added by command line).

Allowing loopback

-A INPUT -i lo -j ACCEPT
-A OUTPUT -o lo -j ACCEPT
-A INPUT -d 127.0.0.0/8 -j REJECT
-A OUTPUT -d 127.0.0.0/8 -j REJECT

Next thing is to allow all loopbacks. Those are local connection and blocking them might cause errors in some connections. In addition we will block those which doesn't use lo0.

Keep established connections

-A INPUT -m state --state RELATED,ESTABLISHED -j ACCEPT
-A OUTPUT -m state --state RELATED,ESTABLISHED -j ACCEPT

All connections that are active now, should remain untouched. It will prevent from interruption of services.

PING command

-A OUTPUT -p icmp --icmp-type echo-request -j ACCEPT
-A INPUT -p icmp --icmp-type echo-reply -j ACCEPT
-A INPUT -p icmp --icmp-type echo-request -j ACCEPT
-A OUTPUT -p icmp --icmp-type echo-reply -j ACCEPT

In most cases you will need to be able to ping server. These rules will allow two things. First - you'll be able to ping your own server. Second - you will be able to execute ping from your server. Both are usually needed and quite useful.

Protection from PING of Death attack

-N PING_OF_DEATH
-A PING_OF_DEATH -p icmp --icmp-type echo-request -m hashlimit --hashlimit 1/s --hashlimit-burst 10 --hashlimit-htable-expire 300000 --hashlimit-mode srcip --hashlimit-name t_PING_OF_DEATH -j RETURN
-A PING_OF_DEATH -j DROP
-A INPUT -p icmp --icmp-type echo-request -j PING_OF_DEATH

Ping is cool, however you might get attacked with Ping of Death attack. Here is simple protection.

Prevent some nasty attacks

-N PORTSCAN
-A PORTSCAN -p tcp --tcp-flags ACK,FIN FIN -j DROP
-A PORTSCAN -p tcp --tcp-flags ACK,PSH PSH -j DROP
-A PORTSCAN -p tcp --tcp-flags ACK,URG URG -j DROP
-A PORTSCAN -p tcp --tcp-flags FIN,RST FIN,RST -j DROP
-A PORTSCAN -p tcp --tcp-flags SYN,FIN SYN,FIN -j DROP
-A PORTSCAN -p tcp --tcp-flags SYN,RST SYN,RST -j DROP
-A PORTSCAN -p tcp --tcp-flags ALL ALL -j DROP
-A PORTSCAN -p tcp --tcp-flags ALL NONE -j DROP
-A PORTSCAN -p tcp --tcp-flags ALL FIN,PSH,URG -j DROP
-A PORTSCAN -p tcp --tcp-flags ALL SYN,FIN,PSH,URG -j DROP
-A PORTSCAN -p tcp --tcp-flags ALL SYN,RST,ACK,FIN,URG -j DROP
-A INPUT -f -j DROP
-A INPUT -p tcp ! --syn -m state --state NEW -j DROP

This is really nice piece of rules that will prevent port scanning, SYN flood attacks, invalid packages, malformed XMAS packets, NULL packets, etc.

UDP traffic

-A INPUT -p udp --sport 53 -j ACCEPT
-A OUTPUT -p udp --dport 53 -j ACCEPT
-A INPUT -p udp --sport 123 -j ACCEPT
-A OUTPUT -p udp --dport 123 -j ACCEPT

I enable usually only ports for outgoing traffic (from our server to outside world). There are two ports that I'd like to open:

53 - DNS port. It's a must if you want to use curl or yum. If you will have it closed, you will not resolve any domain name.
123 - NTP port. If you are using chrony or ntpd, you need to enable that port to allow NTP deamon synchronisation.

TCP traffic

# Open TCP ports for incoming traffic
-A INPUT -p tcp --dport 22 -m state --state NEW,ESTABLISHED -j ACCEPT
-A OUTPUT -p tcp --sport 22 -m state --state ESTABLISHED -j ACCEPT
-A INPUT -p tcp --dport 80 -m state --state NEW,ESTABLISHED -j ACCEPT
-A OUTPUT -p tcp --sport 80 -m state --state ESTABLISHED -j ACCEPT
-A INPUT -p tcp --dport 443 -m state --state NEW,ESTABLISHED -j ACCEPT
-A OUTPUT -p tcp --sport 443 -m state --state ESTABLISHED -j ACCEPT

# Open TCP ports for outgoing traffic
-A INPUT -p tcp --sport 22 -m state --state ESTABLISHED -j ACCEPT
-A OUTPUT -p tcp --dport 22 -m state --state NEW,ESTABLISHED -j ACCEPT
-A INPUT -p tcp --sport 80 -m state --state ESTABLISHED -j ACCEPT
-A OUTPUT -p tcp --dport 80 -m state --state NEW,ESTABLISHED -j ACCEPT
-A INPUT -p tcp --sport 443 -m state --state ESTABLISHED -j ACCEPT
-A OUTPUT -p tcp --dport 443 -m state --state NEW,ESTABLISHED -j ACCEPT

With TCP it's more complicated, but it's not that hard. First, you need to think what traffic you need to access from your server (outgoing traffic). I usually allow only SSH, HTTP and HTTPS traffic. Yum requires HTTP and HTTPS ports for pulling new packages. You will need it also for wget or curl. SSH is not mandatory, but if you want to pull packages from git via ssh protocol, you will need it as well.

I usually enable the same for incoming traffic. If you have httpd or nginx installed, you need to enable port 80. If you are using SSL for HTTPS, you need to enable 443 also. In addition to these two ports you must enable port 22 for SSH. If you will block this, you won't be able to get access to your server!

Block everything else

-A INPUT -j DROP
-A FORWARD -j DROP
-A OUTPUT -j DROP

At the very end, before closing COMMIT tag I add these three rules. So everything that was not specified above will be dropped. Both incoming and outgoing traffic.

How to apply rules?

There are two ways how you can apply the rules. First, save the changes in iptables file. First method is not permanent method. It's good way of testing your firewall before saving them permanently. If anything will go wrong, you can just restart the server and you will have all traffic open. Make sure that you check SSH access with these rules. Log out and try to login after applying rules.

So non permanent way of applying rules is:

sudo iptables-restore < /etc/sysconfig/iptables

Try to check rules with iptables -S to see the difference:) Check if everything is working fine. If so, you can set them permanently. After each server restart, rules will be applied automatically.

sudo systemctl start iptables.service
sudo systemctl enable iptables.service

If you want to reload rules, simply edit the file, add what you need and restart iptables service:

sudo systemctl restart iptables.service

You can use our Ansible LAMP on Steroids project to make configuration of your server easier!

It is based on Ansible. If you don't know what Ansible is, check our tutorial first.

Clone our repository and setup your server faster with LAMP on steroids.

Whole content of iptables rules

*filter

# Clear all iptables rules (everything is open)
-X
-F
-Z

# Allow loopback interface (lo0) and drop all traffic to 127/8 that doesn't use lo0
-A INPUT -i lo -j ACCEPT
-A OUTPUT -o lo -j ACCEPT
-A INPUT -d 127.0.0.0/8 -j REJECT
-A OUTPUT -d 127.0.0.0/8 -j REJECT

# Keep all established connections
-A INPUT -m state --state RELATED,ESTABLISHED -j ACCEPT
-A OUTPUT -m state --state RELATED,ESTABLISHED -j ACCEPT

# Allow ping
-A OUTPUT -p icmp --icmp-type echo-request -j ACCEPT
-A INPUT -p icmp --icmp-type echo-reply -j ACCEPT
-A INPUT -p icmp --icmp-type echo-request -j ACCEPT
-A OUTPUT -p icmp --icmp-type echo-reply -j ACCEPT

# Protect from ping of death
-N PING_OF_DEATH
-A PING_OF_DEATH -p icmp --icmp-type echo-request -m hashlimit --hashlimit 1/s --hashlimit-burst 10 --hashlimit-htable-expire 300000 --hashlimit-mode srcip --hashlimit-name t_PING_OF_DEATH -j RETURN
-A PING_OF_DEATH -j DROP
-A INPUT -p icmp --icmp-type echo-request -j PING_OF_DEATH

# Prevent port scanning
-N PORTSCAN
-A PORTSCAN -p tcp --tcp-flags ACK,FIN FIN -j DROP
-A PORTSCAN -p tcp --tcp-flags ACK,PSH PSH -j DROP
-A PORTSCAN -p tcp --tcp-flags ACK,URG URG -j DROP
-A PORTSCAN -p tcp --tcp-flags FIN,RST FIN,RST -j DROP
-A PORTSCAN -p tcp --tcp-flags SYN,FIN SYN,FIN -j DROP
-A PORTSCAN -p tcp --tcp-flags SYN,RST SYN,RST -j DROP
-A PORTSCAN -p tcp --tcp-flags ALL ALL -j DROP
-A PORTSCAN -p tcp --tcp-flags ALL NONE -j DROP
-A PORTSCAN -p tcp --tcp-flags ALL FIN,PSH,URG -j DROP
-A PORTSCAN -p tcp --tcp-flags ALL SYN,FIN,PSH,URG -j DROP
-A PORTSCAN -p tcp --tcp-flags ALL SYN,RST,ACK,FIN,URG -j DROP

# Drop fragmented packages
-A INPUT -f -j DROP

# SYN packets check
-A INPUT -p tcp ! --syn -m state --state NEW -j DROP

# Open ports for outgoing UDP traffic
-A INPUT -p udp --sport 53 -j ACCEPT
-A OUTPUT -p udp --dport 53 -j ACCEPT
-A INPUT -p udp --sport 123 -j ACCEPT
-A OUTPUT -p udp --dport 123 -j ACCEPT


# Open TCP ports for incoming traffic
-A INPUT -p tcp --dport 22 -m state --state NEW,ESTABLISHED -j ACCEPT
-A OUTPUT -p tcp --sport 22 -m state --state ESTABLISHED -j ACCEPT
-A INPUT -p tcp --dport 80 -m state --state NEW,ESTABLISHED -j ACCEPT
-A OUTPUT -p tcp --sport 80 -m state --state ESTABLISHED -j ACCEPT
-A INPUT -p tcp --dport 443 -m state --state NEW,ESTABLISHED -j ACCEPT
-A OUTPUT -p tcp --sport 443 -m state --state ESTABLISHED -j ACCEPT

# Open TCP ports for outgoing traffic
-A INPUT -p tcp --sport 80 -m state --state ESTABLISHED -j ACCEPT
-A OUTPUT -p tcp --dport 80 -m state --state NEW,ESTABLISHED -j ACCEPT
-A INPUT -p tcp --sport 443 -m state --state ESTABLISHED -j ACCEPT
-A OUTPUT -p tcp --dport 443 -m state --state NEW,ESTABLISHED -j ACCEPT


# Drop all other traffic
-A INPUT -j DROP
-A FORWARD -j DROP
-A OUTPUT -j DROP

COMMIT

What's next?

We secured our system with basic firewall. That will increase the security of our server. In one of the next episodes we will change the configuration of our SSH and therefore make it more secure.

As always You can use our Ansible playbook for faster provisioning of our server. You can find it on GitHub.

Apache hardening with mod_security

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In this part of setup complete webserver we will harden Apache with popular mod_security. Mod_security is a small module that works like application firewall. It protect the app before most common attacks and vulnerabilities. It's good to have such thing on the webserver.

I assume that You have Apache already installed, if not check out previous part - How to install Apache on CentOS?

[sc:lamp_series]

How to install mod_security on Apache httpd server

First thing is installation of required tools. We need them to compile mod_security.

yum install automake libtool libxml2-devel

Next thing that we need is to download and decompress mod_security. Download links can be found on official mod_security page.

cd ~/sources
wget https://www.modsecurity.org/tarball/2.9.0/modsecurity-2.9.0.tar.gz
tar -zxvf modsecurity-2.9.0.tar.gz

Now it's time to compile mod_security. While ./confgure we need to pass paths to axps and apr binaries. All binaries should be inside bin directory in apache installation path.

cd modsecurity-2.9.0
./autogen.sh
./configure --with-apxs=/usr/local/apache2/bin/apxs --with-apr=/usr/local/apache2/bin/apr-1-config --with-apu=/usr/local/apache2/bin/apu-1-config
make
make install
cp /usr/local/modsecurity/lib/mod_security2.so /usr/local/apache2/modules

If there was no error mod_security is ready to use. It can be found in /usr/local/modsecurity We need to copy generated .so file to apache extension directory.

ModSecurity and OWASP rules

mod_security is nothing without the rules that tells what attacks should be blocked. Fortunately there is a great package with lot of rules provided by OWASP. We will use such rules package to harden Apache HTTPD. So let's download, unzip and copy the rules to apache configuration directory.

cd ~/sources
wget -O owasp.tar.gz https://github.com/SpiderLabs/owasp-modsecurity-crs/tarball/master
mkdir /usr/local/apache2/conf/crs
tar -zxvf owasp.tar.gz -C /usr/local/apache2/conf/crs --strip 1
cd /usr/local/apache2/conf/crs
cp modsecurity_crs_10_setup.conf.example modsecurity_crs_10_setup.conf

Now we are ready to use ModSecurity!

ModSecurity configuration

In previous article we added httpd-security.conf file with some basic rules that improves Apache security. We will modify this file to load mod_security with OWASP rules and add some basic configuration. You need to know that mod_security is pretty large module with tons of configuration option. You can find them in ModSecurity reference manual.

vi /usr/local/apache2/conf/extra/httpd-security.conf

Once You open the file add these lines somewhere in the file:

LoadModule security2_module modules/mod_security2.so

<IfModule security2_module>
      Include conf/crs/modsecurity_crs_10_setup.conf
      Include conf/crs/base_rules/*.conf
      # Include conf/crs/experimental_rules/*.conf
      # Include conf/crs/optional_rules/*.conf

      SecRuleEngine On
      SecRequestBodyAccess On
      SecResponseBodyAccess On 
      SecResponseBodyMimeType text/plain text/html text/xml application/octet-stream
      SecDataDir /tmp

      # Debug log
      SecDebugLog /usr/local/apache2/logs/modsec_debug.log
      SecDebugLogLevel 3

      SecAuditEngine RelevantOnly
      SecAuditLogRelevantStatus ^2-5
      SecAuditLogParts ABCIFHZ
      SecAuditLogType Serial
      SecAuditLog /usr/local/apache2/logs/modsec_audit.log
</IfModule>

So from the top:

First we need to load mod_security module.
Next are rules from OWASP that we will include to ModSecurity. We need to include the setup and base rules. OWASP core rule set comes with lot more features that are marked as optional or experimental. We can enable those rules, but we also need to remember that it might not play well with our website. It's rather testing by trial and error then one rule will work well on every website. But in general including base_rules is OK.
SecRuleEngine enables detection and blocking of malicious attacks.
SecRequestBodyAccess enable inspection of data transported request bodies
SeResponseBodyAccess buffer response bodies matched by SecResponseBodyMimeType
SecDataDir working directory for ModSecurity temporary purposes
Next thing is Debug log. By default all error logs goes to apache error log, but we can set different path to debug log. Best practice would be to change it per domain inside particular VirtualHost file. In previous article we setup directory structure and we have logs directory there. It would be wise to used it for debug log as well.
Audit Log is complementary log for Debug log. It has detail information about every error. It's disabled by default so we need to enable it and turn on logging relevant (warnings and errors) issues. Next options are for configuration the audit log. In general there are lot more of discussing at this topic.

If You want to learn more about how to setup and read mod_security logs, here is really great article about mod_security logging by Infosec Institue.

Now we just need to save the file restart apache and our httpd server has better security.

service httpd restart

What's next?

If You are following our series, You should have now part of LAMP stack (Linux Apache MySQL PHP). Apache is secured with mod_security.

Small note to those who would like to install mod_evasive as well to increase the security. To be honest, it's really not worth to install it on Apache. Why? Because when You run multiple instances via MPM mod_evasive doesn't share the info between the MPM instances. It means that one instance of apache can block the attacker but others wont. So if You have many MPM workers mod_evasive is just useless.

As always, if You are using Ansible for server provisioning You can use ready playbook, that will cover everything in this series. You can find it on GitHub.

In next episode we will add P to our LAMP server.