Introduction

Memory object caching systems like Memcached can optimize backend database performance by temporarily storing information in memory, retaining frequently or recently requested records. In this way, they reduce the number of direct requests to your databases.

Because systems like Memcached can contribute to denial of service attacks if improperly configured, it is important to secure your Memcached servers. In this guide, we will cover how to protect your Memcached server by binding your installation to a local or private network interface and creating an authorized user for your Memcached instance.

Prerequisites

This tutorial assumes that you have a server set up with a non-root sudo user and a basic firewall. If that is not the case, set up the following:

• One Ubuntu 16.04 server, set up following our Initial Server Setup with Ubuntu 16.04 tutorial.

With these prerequisites in place, you will be ready to install and secure your Memcached server.

Installing Memcached from the Official Repositories

If you don't already have Memcached installed on your server, you can install it from the official Ubuntu repositories. First, make sure that your local package index is updated:

sudo apt-get update

Next, install the official package as follows:

sudo apt-get install memcached

We can also install libmemcached-tools, a library that provides several tools to work with your Memcached server:

sudo apt-get install libmemcached-tools

Memcached should now be installed as a service on your server, along with tools that will allow you to test its connectivity. We can now move on to securing its configuration settings.

Securing Memcached Configuration Settings

To ensure that our Memcached instance is listening on the local interface 127.0.0.1, we will check the default setting in the configuration file located at /etc/memcached.conf. The current version of Memcached that ships with Ubuntu and Debian has the -l parameter set to the local interface, which prevents denial of service attacks from the network. We can inspect this setting to ensure that it is set correctly.

You can open /etc/memcached.conf with nano:

sudo nano /etc/memcached.conf

To inspect the interface setting, find the following line in the file:

. . .
-l 127.0.0.1

If you see the default setting of -l 127.0.0.1 then there is no need to modify this line. If you do modify this setting to be more open, then it is also a good idea to disable UDP, as it is more likely to be exploited in denial of service attacks. To disable UDP (while leaving TCP unaffected), add the following option to the bottom of this file:

. . .
-U 0

Save and close the file when you are done.

Restart your Memcached service to apply your changes:

sudo systemctl restart memcached

Verify that Memcached is currently bound to the local interface and listening only for TCP connections by typing:

sudo netstat -plunt

You should see the following output:

Output
Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address           Foreign Address         State       PID/Program name
. . .
tcp        0      0 127.0.0.1:11211         0.0.0.0:*               LISTEN      2383/memcached
. . .

This confirms that memcached is bound to the 127.0.0.1 address using only TCP.

Adding Authorized Users

To add authenticated users to your Memcached service, it is possible to use Simple Authentication and Security Layer (SASL), a framework that de-couples authentication procedures from application protocols. We will enable SASL within our Memcached configuration file and then move on to adding a user with authentication credentials.

Configuring SASL Support

We can first test the connectivity of our Memcached instance with the memcstat command. This will help us establish that SASL and user authentication are enabled after we make changes to our configuration files.

To check that Memcached is up and running, type the following:

memcstat --servers="127.0.0.1"

You should see output like the following:

Output
Server: 127.0.0.1 (11211)
     pid: 3831
     uptime: 9
     time: 1520028517
     version: 1.4.25
     . . .

Now we can move on to enabling SASL. First, we will add the -S parameter to /etc/memcached.conf. Open the file again:

sudo nano /etc/memcached.conf

At the bottom of the file, add the following:

. . .
-S

Next, find and uncomment the -vv option, which will provide verbose output to /var/log/memcached. The uncommented line should look like this:

. . .
-vv

Save and close the file.

Restart the Memcached service:

sudo systemctl restart memcached

Next, we can take a look at the logs to be sure that SASL support has been enabled:

sudo journalctl -u memcached

You should see the following line, indicating that SASL support has been initialized:

Output
. . .
Mar 02 22:03:58 memcached systemd-memcached-wrapper[2760]: Initialized SASL.
. . .

We can check the connectivity again, but because SASL has been initialized, this command should fail without authentication:

memcstat --servers="127.0.0.1"

This command should not produce output. We can type the following to check its status:

echo $?

$? will always return the exit code of the last command that exited. Typically, anything besides 0 indicates process failure. In this case, we should see an exit status of 1, which tells us that the memcstat command failed.

Adding an Authenticated User

Now we can download sasl2-bin, a package that contains administrative programs for the SASL user database. This will allow us to create our authenticated user:

sudo apt-get install sasl2-bin

Next, we will create the directory and file that Memcached will check for its SASL configuration settings:

sudo mkdir -p /etc/sasl2
sudo nano /etc/sasl2/memcached.conf 

Add the following to the SASL configuration file:

mech_list: plain
log_level: 5
sasldb_path: /etc/sasl2/memcached-sasldb2

In addition to specifying our logging level, we will set mech_list to plain, which tells Memcached that it should use its own password file and verify a plaintext password. We will also specify the path to the user database file that we will create next. Save and close the file when you are finished.

Now we will create a SASL database with our user credentials. We will use the saslpasswd2 command to make a new entry for our user in our database using the -c option. Our user will be sammy here, but you can replace this name with your own user. Using the -f option, we will specify the path to our database, which will be the path we set in /etc/sasl2/memcached.conf:

sudo saslpasswd2 -a memcached -c -f /etc/sasl2/memcached-sasldb2 sammy

Finally, we will give the memcache user ownership over the SASL database:

sudo chown memcache:memcache /etc/sasl2/memcached-sasldb2

Restart the Memcached service:

sudo systemctl restart memcached

Running memcstat again will confirm whether or not our authentication process worked. This time we will run it with our authentication credentials:

memcstat --servers="127.0.0.1" --username=sammy --password=your_password

You should see output like the following:

Output
Server: 127.0.0.1 (11211)
     pid: 3831
     uptime: 9
     time: 1520028517
     version: 1.4.25
     . . .

Our Memcached service is now successfully running with SASL support and user authentication.

Allowing Access Over the Private Network

We have covered how to configure Memcached to listen on the local interface, which can prevent denial of service attacks by protecting the Memcached interface from exposure to outside parties. There may be instances where you will need to allow access from other servers, however. In this case, you can adjust your configuration settings to bind Memcached to the private network interface.

Note: We will cover how to configure firewall settings using UFW in this section, but it is also possible to use DigitalOcean Cloud Firewalls to create these settings. For more information on setting up DigitalOcean Cloud Firewalls, see our Introduction to DigitalOcean Cloud Firewalls. To learn more about how to limit incoming traffic to particular machines, check out the section of this tutorial on applying firewall rules using tags and server names and our discussion of firewall tags.

Limiting IP Access With Firewalls

Before you adjust your configuration settings, it is a good idea to set up firewall rules to limit the machines that can connect to your Memcached server. You will need to know the client server’s private IP address to configure your firewall rules.

If you are using the UFW firewall, you can limit access to your Memcached instance by typing the following:

sudo ufw allow from client_servers_private_IP/32 to any port 11211

You can find out more about UFW firewalls by reading our ufw essentials guide.

With these changes in place, you can adjust the Memcached service to bind to your server's private networking interface.

Binding Memcached to the Private Network Interface

Now that your firewall is in place, you can adjust the Memcached configuration to bind to your server's private networking interface instead of 127.0.0.1.

We can open the /etc/memcached.conf file again by typing:

sudo nano /etc/memcached.conf

Inside, find the -l 127.0.0.1 line that you checked or modified earlier, and change the address to match your server's private networking interface:

. . .
-l memcached_servers_private_IP
. . .

Save and close the file when you are finished.

Next, restart the Memcached service:

sudo systemctl restart memcached

Check your new settings with netstat to confirm the change:

sudo netstat -plunt

Output
Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address           Foreign Address         State       PID/Program name
. . .
tcp        0      0 memcached_servers_private_IP:11211         0.0.0.0:*               LISTEN      2383/memcached
. . .

Test connectivity from your external client to ensure that you can still reach the service. It is a good idea to also check access from a non-authorized client to ensure that your firewall rules are effective.

Conclusion

In this tutorial we have covered how to secure your Memcached server by configuring it to bind to your local or private network interface, and by enabling SASL authentication.

To learn more about Memcached, check out the project documentation. For more information about how to work with Memcached, see our tutorial on How To Install and Use Memcache on Ubuntu 14.04.