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How To Protect an Nginx Server with Fail2Ban on Ubuntu 14.04

PostedAugust 14, 2015 91k views Nginx Security Firewall

Introduction

When operating a web server, it is important to implement security measures to protect your site and users. Protecting your web sites and applications with firewall policies and restricting access to certain areas with password authentication is a great starting point to securing your system. However, any publicly accessible password prompt is likely to attract brute force attempts from malicious users and bots.

Setting up fail2ban can help alleviate this problem. When users repeatedly fail to authenticate to a service (or engage in other suspicious activity), fail2ban can issue a temporary bans on the offending IP address by dynamically modifying the running firewall policy. Each fail2ban "jail" operates by checking the logs written by a service for patterns which indicate failed attempts. Setting up fail2ban to monitor Nginx logs is fairly easy using the some of included configuration filters and some we will create ourselves.

In this guide, we will demonstrate how to install fail2ban and configure it to monitor your Nginx logs for intrusion attempts. We will use an Ubuntu 14.04 server.

Prerequisites

Before you begin, you should have an Ubuntu 14.04 server set up with a non-root account. This account should be configured with sudo privileges in order to issue administrative commands. To learn how to set up a user with sudo privileges, follow our initial server setup guide for Ubuntu 14.04.

Installing Nginx and Configuring Password Authentication

If you are interested in protecting your Nginx server with fail2ban, you might already have a server set up and running. If not, you can install Nginx from Ubuntu's default repositories using apt.

Update the local package index and install by typing:

  • sudo apt-get update
  • sudo apt-get install nginx

The fail2ban service is useful for protecting login entry points. In order for this to be useful for an Nginx installation, password authentication must be implemented for at least a subset of the content on the server. You can follow this guide to configure password protection for your Nginx server.

Install Fail2Ban

Once your Nginx server is running and password authentication is enabled, you can go ahead and install fail2ban (we include another repository re-fetch here in case you already had Nginx set up in the previous steps):

  • sudo apt-get update
  • sudo apt-get install fail2ban

This will install the software. By default, fail2ban is configured to only ban failed SSH login attempts. We need to enable some rules that will configure it to check our Nginx logs for patterns that indicate malicious activity.

Adjusting the General Settings within Fail2Ban

To get started, we need to adjust the configuration file that fail2ban uses to determine what application logs to monitor and what actions to take when offending entries are found. The supplied /etc/fail2ban/jail.conf file is the main provided resource for this.

To make modifications, we need to copy this file to /etc/fail2ban/jail.local. This will prevent our changes from being overwritten if a package update provides a new default file:

  • sudo cp /etc/fail2ban/jail.conf /etc/fail2ban/jail.local

Open the newly copied file so that we can set up our Nginx log monitoring:

  • sudo nano /etc/fail2ban/jail.local

Changing Defaults

We should start by evaluating the defaults set within the file to see if they suit our needs. These will be found under the [DEFAULT] section within the file. These items set the general policy and can each be overridden in specific jails.

One of the first items to look at is the list of clients that are not subject to the fail2ban policies. This is set by the ignoreip directive. It is sometimes a good idea to add your own IP address or network to the list of exceptions to avoid locking yourself out. This is less of an issue with web server logins though if you are able to maintain shell access, since you can always manually reverse the ban. You can add additional IP addresses or networks delimited by a space, to the existing list:

/etc/fail2ban/jail.local
[DEFAULT]

. . .
ignoreip = 127.0.0.1/8 your_home_IP

Another item that you may want to adjust is the bantime, which controls how many seconds an offending member is banned for. It is ideal to set this to a long enough time to be disruptive to a malicious actor's efforts, while short enough to allow legitimate users to rectify mistakes. By default, this is set to 600 seconds (10 minutes). Increase or decrease this value as you see fit:

/etc/fail2ban/jail.local
[DEFAULT]

. . .
bantime = 3600

The next two items determine the scope of log lines used to determine an offending client. The findtime specifies an amount of time in seconds and the maxretry directive indicates the number of attempts to be tolerated within that time. If a client makes more than maxretry attempts within the amount of time set by findtime, they will be banned:

/etc/fail2ban/jail.local
[DEFAULT]

. . .
findtime = 3600   # These lines combine to ban clients that fail
maxretry = 6      # to authenticate 6 times within a half hour.

Setting Up Mail Notifications (Optional)

You can enable email notifications if you wish to receive mail whenever a ban takes place. To do so, you will have to first set up an MTA on your server so that it can send out email. To learn how to use Postfix for this task, follow this guide.

Once you have your MTA set up, you will have to adjust some additional settings within the [DEFAULT] section of the /etc/fail2ban/jail.local file. Start by setting the mta directive. If you set up Postfix, like the above tutorial demonstrates, change this value to "mail":

/etc/fail2ban/jail.local
[DEFAULT]

. . .
mta = mail

You need to select the email address that will be sent notifications. Modify the destemail directive with this value. The sendername directive can be used to modify the "Sender" field in the notification emails:

/etc/fail2ban/jail.local
[DEFAULT]

. . .
destemail = youraccount@email.com
sendername = Fail2BanAlerts

In fail2ban parlance, an "action" is the procedure followed when a client fails authentication too many times. The default action (called action_) is to simply ban the IP address from the port in question. However, there are two other pre-made actions that can be used if you have mail set up.

You can use the action_mw action to ban the client and send an email notification to your configured account with a "whois" report on the offending address. You could also use the action_mwl action, which does the same thing, but also includes the offending log lines that triggered the ban:

/etc/fail2ban/jail.local
[DEFAULT]

. . .
action = %(action_mwl)s

Configuring Fail2Ban to Monitor Nginx Logs

Now that you have some of the general fail2ban settings in place, we can concentrate on enabling some Nginx-specific jails that will monitor our web server logs for specific behavior patterns.

Each jail within the configuration file is marked by a header containing the jail name in square brackets (every section but the [DEFAULT] section indicates a specific jail's configuration). By default, only the [ssh] jail is enabled.

To enable log monitoring for Nginx login attempts, we will enable the [nginx-http-auth] jail. Edit the enabled directive within this section so that it reads "true":

/etc/fail2ban/jail.local
[nginx-http-auth]

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

This is the only Nginx-specific jail included with Ubuntu's fail2ban package. However, we can create our own jails to add additional functionality. The inspiration for and some of the implementation details of these additional jails came from here and here.

We can create an [nginx-noscript] jail to ban clients that are searching for scripts on the website to execute and exploit. If you do not use PHP or any other language in conjunction with your web server, you can add this jail to ban those who request these types of resources:

/etc/fail2ban/jail.local
[nginx-noscript]

enabled  = true
port     = http,https
filter   = nginx-noscript
logpath  = /var/log/nginx/access.log
maxretry = 6
. . .

We can add a section called [nginx-badbots] to stop some known malicious bot request patterns:

/etc/fail2ban/jail.local
[nginx-badbots]

enabled  = true
port     = http,https
filter   = nginx-badbots
logpath  = /var/log/nginx/access.log
maxretry = 2

If you do not use Nginx to provide access to web content within users' home directories, you can ban users who request these resources by adding an [nginx-nohome] jail:

/etc/fail2ban/jail.local
[nginx-nohome]

enabled  = true
port     = http,https
filter   = nginx-nohome
logpath  = /var/log/nginx/access.log
maxretry = 2

We should ban clients attempting to use our Nginx server as an open proxy. We can add an [nginx-noproxy] jail to match these requests:

/etc/fail2ban/jail.local
[nginx-noproxy]

enabled  = true
port     = http,https
filter   = nginx-noproxy
logpath  = /var/log/nginx/access.log
maxretry = 2

When you are finished making the modifications you need, save and close the file. We now have to add the filters for the jails that we have created.

Adding the Filters for Additional Nginx Jails

We've updated the /etc/fail2ban/jail.local file with some additional jail specifications to match and ban a larger range of bad behavior. We need to create the filter files for the jails we've created. These filter files will specify the patterns to look for within the Nginx logs.

Begin by changing to the filters directory:

  • cd /etc/fail2ban/filter.d

We actually want to start by adjusting the pre-supplied Nginx authentication filter to match an additional failed login log pattern. Open the file for editing:

  • sudo nano nginx-http-auth.conf

Below the failregex specification, add an additional pattern. This will match lines where the user has entered no username or password:

/etc/fail2ban/filter.d/nginx-http-auth.conf
[Definition]


failregex = ^ \[error\] \d+#\d+: \*\d+ user "\S+":? (password mismatch|was not found in ".*"), client: <HOST>, server: \S+, request: "\S+ \S+ HTTP/\d+\.\d+", host: "\S+"\s*$
            ^ \[error\] \d+#\d+: \*\d+ no user/password was provided for basic authentication, client: <HOST>, server: \S+, request: "\S+ \S+ HTTP/\d+\.\d+", host: "\S+"\s*$

ignoreregex =

Save and close the file when you are finished.

Next, we can copy the apache-badbots.conf file to use with Nginx. We can use this file as-is, but we will copy it to a new name for clarity. This matches how we referenced the filter within the jail configuration:

  • sudo cp apache-badbots.conf nginx-badbots.conf

Next, we'll create a filter for our [nginx-noscript] jail:

  • sudo nano nginx-noscript.conf

Paste the following definition inside. Feel free to adjust the script suffixes to remove language files that your server uses legitimately or to add additional suffixes:

/etc/fail2ban/filter.d/nginx-noscript.conf
[Definition]

failregex = ^<HOST> -.*GET.*(\.php|\.asp|\.exe|\.pl|\.cgi|\.scgi)

ignoreregex =

Save and close the file.

Next, create a filter for the [nginx-nohome] jail:

  • sudo nano nginx-nohome.conf

Place the following filter information in the file:

/etc/fail2ban/filter.d/nginx-nohome.conf
[Definition]

failregex = ^<HOST> -.*GET .*/~.*

ignoreregex =

Save and close the file when finished.

Finally, we can create the filter for the [nginx-noproxy] jail:

  • sudo nano nginx-noproxy.conf

This filter definition will match attempts to use your server as a proxy:

/etc/fail2ban/filter.d/nginx-noproxy.conf
[Definition]

failregex = ^<HOST> -.*GET http.*

ignoreregex =

Save and close the file when you are finished.

Activating your Nginx Jails

To implement your configuration changes, you'll need to restart the fail2ban service. You can do that by typing:

  • sudo service fail2ban restart

The service should restart, implementing the different banning policies you've configured.

Getting Info About Enabled Jails

You can see all of your enabled jails by using the fail2ban-client command:

  • sudo fail2ban-client status

You should see a list of all of the jails you enabled:

Output
Status |- Number of jail: 6 `- Jail list: nginx-noproxy, nginx-noscript, nginx-nohome, nginx-http-auth, nginx-badbots, ssh

You can look at iptables to see that fail2ban has modified your firewall rules to create a framework for banning clients. Even with no previous firewall rules, you would now have a framework enabled that allows fail2ban to selectively ban clients by adding them to purpose-built chains:

  • sudo iptables -S
Output
-P INPUT ACCEPT -P FORWARD ACCEPT -P OUTPUT ACCEPT -N fail2ban-nginx-badbots -N fail2ban-nginx-http-auth -N fail2ban-nginx-nohome -N fail2ban-nginx-noproxy -N fail2ban-nginx-noscript -N fail2ban-ssh -A INPUT -p tcp -m multiport --dports 80,443 -j fail2ban-nginx-noproxy -A INPUT -p tcp -m multiport --dports 80,443 -j fail2ban-nginx-nohome -A INPUT -p tcp -m multiport --dports 80,443 -j fail2ban-nginx-badbots -A INPUT -p tcp -m multiport --dports 80,443 -j fail2ban-nginx-noscript -A INPUT -p tcp -m multiport --dports 80,443 -j fail2ban-nginx-http-auth -A INPUT -p tcp -m multiport --dports 22 -j fail2ban-ssh -A fail2ban-nginx-badbots -j RETURN -A fail2ban-nginx-http-auth -j RETURN -A fail2ban-nginx-nohome -j RETURN -A fail2ban-nginx-noproxy -j RETURN -A fail2ban-nginx-noscript -j RETURN -A fail2ban-ssh -j RETURN

If you want to see the details of the bans being enforced by any one jail, it is probably easier to use the fail2ban-client again:

  • sudo fail2ban-client status nginx-http-auth
Output
Status for the jail: nginx-http-auth |- filter | |- File list: /var/log/nginx/error.log | |- Currently failed: 0 | `- Total failed: 0 `- action |- Currently banned: 0 | `- IP list: `- Total banned: 0

Testing Fail2Ban Policies

It is important to test your fail2ban policies to ensure they block traffic as expected. For instance, for the Nginx authentication prompt, you can give incorrect credentials a number of times. After you have surpassed the limit, you should be banned and unable to access the site. If you set up email notifications, you should see messages regarding the ban in the email account you provided.

If you look at the status with the fail2ban-client command, you will see your IP address being banned from the site:

  • sudo fail2ban-client status nginx-http-auth
Output
Status for the jail: nginx-http-auth |- filter | |- File list: /var/log/nginx/error.log | |- Currently failed: 0 | `- Total failed: 12 `- action |- Currently banned: 1 | `- IP list: 111.111.111.111 `- Total banned: 1

When you are satisfied that your rules are working, you can manually un-ban your IP address with the fail2ban-client by typing:

  • sudo fail2ban-client set nginx-http-auth unbanip 111.111.111.111

You should now be able to attempt authentication again.

Conclusion

Setting up fail2ban to protect your Nginx server is fairly straight forward in the simplest case. However, fail2ban provides a great deal of flexibility to construct policies that will suit your specific security needs. By taking a look at the variables and patterns within the /etc/fail2ban/jail.local file, and the files it depends on within the /etc/fail2ban/filter.d and /etc/fail2ban/action.d directories, you can find many pieces to tweak and change as your needs evolve. Learning the basics of how to protect your server with fail2ban can provide you with a great deal of security with minimal effort.

If you'd like to learn more about fail2ban, check out the following links:

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