How to install and configure Memcached

Memcached is commonly used to alleviate backend database contention by temporarily storing recently requested database records in memory. As a result, the database traffic is reduced as Memcached is able to pull the records from cache.

Installing Memcached is quick. However there are a number of steps that need to be taken to secure Memcached. Memcached has the potential to store a lot of sensitive information, so its critical that the service is locked down properly to prevent outside access or data leakage.


# CentOS 6
[[email protected] ~]# yum install memcached
[[email protected] ~]# chkconfig memcached on
[[email protected] ~]# service memcached start

# CentOS 7
[[email protected] ~]# yum install memcached
[[email protected] ~]# systemctl enable memcached
[[email protected] ~]# systemctl start memcached

# Ubuntu 14.04 and 16.04
[[email protected] ~]# apt-get update
[[email protected] ~]# apt-get install memcached


Memcached listens on port 11211 by default and does not have any type of restrictions built in to prevent it from being queried from the public internet. If you do not protect Memcached with a firewall, there is a extremely high risk of leaking sensitive data. In recent years, unprotected memcached servers have also been exploited to launch DDoS amplification attacks.

If you do not have a dedicated firewall, use your OS’s built in firewall to only allow in connections for trusted web servers using their internal IP’s. Some quick examples are below:

# iptables
[[email protected] ~]# vim /etc/sysconfig/iptables
-A INPUT -p tcp -m tcp --dport 11211 -s client_server_private_IP -m comment --comment "memcached" -j ACCEPT
service iptables restart

# firewalld
[[email protected] ~]# firewall-cmd --permanent --new-zone=memcached
[[email protected] ~]# firewall-cmd --permanent --zone=memcached --add-port=11211/tcp
[[email protected] ~]# firewall-cmd --permanent --zone=memcached --add-source=client_server_private_IP
[[email protected] ~]# firewall-cmd --reload

# ufw
[[email protected] ~]# ufw allow from client_server_private_IP/32 to any port 11211

When configuring memcached itself, there are a few options to set. In my case, to help limit exposure to the service, I want to set Memcached to only listen on my private network interface and disable UDP. I’ll also set max connections to be 16384 and set the max cachesize to be 1.5G (1536M). Adjust the connections and cachesize as needed for your situation. Below is the example using the options above:

# CentOS 6 and 7
[[email protected] ~]# vim /etc/sysconfig/memcached

# Ubuntu 14.04 and 16.04
[[email protected] ~]# vim /etc/memcached.conf

The configuration options to use to reflect the requirements above:

OPTIONS="-l memcached_servers_private_IP -U 0"

Then restart memcached

[[email protected] ~]# service memcached restart

Client setup

The typical use cases I run into on a day to day basis are clients using memcached for their PHP application. Memcached can be used to store cached content, or it can be used to centrally store sessions. Therefore these examples will be PHP focused.

Client setup – General data caching

For storing data, there is nothing that needs to be configured on the client side. The application code itself is what controls storing content within Memcached. To ensure memcached can be reached to store content, create the following test script:

[[email protected] ~]# vim /var/www/html/test-memcached.php
if (class_exists('Memcache')) {
    $meminstance = new Memcache();
} else {
    $meminstance = new Memcached();

$meminstance->addServer("memcached_servers_private_IP", 11211);

$result = $meminstance->get("test");

if ($result) {
    echo $result;
} else {
    echo "No matching key found.  Refresh the browser to add it!";
    $meminstance->set("test", "Successfully retrieved the data!") or die("Couldn't save anything to memcached...");

Then run it in your browser. You can further confirm it works by running it on the command line and confirming ‘cmd_set’ increments by one indicating it was able to store the object:

[[email protected] ~]# echo stats | nc localhost 11211 | grep cmd_set; curl localhost/test-memcached.php; echo stats | nc localhost 11211 |grep cmd_set
STAT cmd_set 2
No matching key found.  Refresh the browser to add it!STAT cmd_set 3

[[email protected] ~]# echo stats | nc localhost 11211 | grep cmd_set; curl localhost/test-memcached.php; echo stats | nc localhost 11211 |grep cmd_set
STAT cmd_set 3
Successfully retrieved the data!STAT cmd_set 3

Digital Ocean has a good article that goes into far more detail on various ways to test/use memcached:

Client setup – Storing sessions in memcached

To have memcached act as a central server for sessions, some additional configuration is needed on each client web server. Install php-memcache for your version of PHP. Assuming the default PHP version is installed from the package manager, you can install it by:

# Red Hat / CentOS:
[[email protected] ~]# yum install php-pecl-memcached
[[email protected] ~]# service httpd graceful
[[email protected] ~]# php -m |grep memcached

# Ubuntu
[[email protected] ~]# apt-get update
[[email protected] ~]# apt-get install php-pecl-memcached
[[email protected] ~]# service apache2 graceful
[[email protected] ~]# php -m |grep memcached

Then update php.ini on both server as follows:

session.save_handler = memcached

On CentOS and Red Hat servers, depending on what version of PHP was installed and how, you may have to update another file as it will override the php.ini. Only change this if the values exist and are configured for files:

[[email protected] ~]# vim /etc/httpd/conf.d/php.conf
php_value session.save_handler "memcached"
php_value session.save_path    "memcached_servers_private_IP:11211?persistent=1&weight=1&timeout=1&retry_interval=15"

Test to ensure sessions are now being stored in memcached:

[[email protected] ~]# vim /var/www/html/test-sessions.php
Created a session

Perform the test by running the following on the command line and confirming ‘cmd_set’ increases, indicating it was able to store the session:

[[email protected] ~]# echo stats | nc localhost 11211 | grep cmd_set; curl localhost/session.php; echo stats | nc localhost 11211 |grep cmd_set
STAT cmd_set 17
Created a session
STAT cmd_set 19

Client setup – Distributed sessions across multiple Memcached instances

Building solutions that can withstand failure is always recommended. Having a Memcached server go offline that is storing sessions will most likely result in unhappy customers. Memcached does not have a built in mechanism to replicate data between multiple memcached servers. This functionality is instead done on the client side.

To allow another Memcached server to take over connections without replicating the session data, update the php.ini on the web servers with the snippet below and restart Apache:

memcache.hash_strategy = consistent
session.save_handler = memcache
memcache.allow_failover = 1

If you wanted to have automatic failure and also ensure that the sessions are replicated to each Memcached server, update the php.ini on the web servers with the snippet below and restart Apache:

memcache.hash_strategy = consistent
memcache.allow_failover = 1
session.save_handler = memcache

Important note: To determine what memcache.session_redundacy should be set to, simply total up all the Memcached servers and add 1 to that total. So in the example above, I have 2 Memcached servers. Therefore, the memcache.session_redundacy should be set to 3.


Confirm the web server can reach the memcached server:

[[email protected] ~]# telnet memcached_servers_private_IP 11211

Verify traffic is being sent from the web servers to the memcached server:

[[email protected] ~]# tcpdump -i any port 11211

Checking memcached stats:

[[email protected] ~]# echo stats | nc localhost 11211

To see some of the more commonly used stats, use:

[[email protected] ~]# echo stats | nc localhost 11211 |grep -E 'total_connections|curr_connections|limit_maxbytes|bytes'

Check to see if you may need to increase the size of the cache. If bytes is reaching the total memory allocation defined by ‘limit_maxbytes’, you may need to increase the cachesize setting:

[[email protected] ~]# echo stats | nc localhost 11211 |grep -E 'bytes|limit_maxbytes' |grep -v bytes_ |grep -v _bytes

To flush all the data within memcache:

[[email protected] ~]# echo flush_all | nc localhost 11211

To retrieve the version of memcached:

[[email protected] ~]# echo version | nc localhost 11211

Varnish 3 – Installation and configuration

Varnish is a HTTP reverse proxy that can be installed in front of the web server to provide caching. If the VCL’s are properly configured for your site, Varnish can greatly offset the backend server load many times over.

In this guide, it is assumed you already have a running LAMP stack, and Apache’s vhost configurations are stored in /etc/httpd/vhost.d/. At the end of this guide if all goes well, you will be running Varnish 3, with Varnish listening for inbound connections on port 80, and passing any backend connections that cannot be served via cache to Apache on port 8080.

CentOS 6 – Installation and initial configuration

Install the varnish-release package repository, then install:

[[email protected] ~]# rpm --nosignature -ivh
[[email protected] ~]# yum -y install varnish

Now update your Apache ports and vhosts to 8080 since Varnish will be listening on port 80:

[[email protected] ~]# sed -i "s/Listen 80\$/Listen 8080/g" /etc/httpd/ports.conf
[[email protected] ~]# sed -i "s/NameVirtualHost \*:80\$/NameVirtualHost \*:8080/g" /etc/httpd/ports.conf
[[email protected] ~]# sed -i "s/:80>/:8080>/g" /etc/httpd/vhost.d/*

Configure Varnish to pass connections back to Apache on port 8080:

[[email protected] ~]# sed -i 's/port = "80"/port = "8080"/g' /etc/varnish/default.vcl

Then update Varnish so it listens on port 80:

[[email protected] ~]# sed -i 's/VARNISH_LISTEN_PORT=6081$/VARNISH_LISTEN_PORT=80/g' /etc/sysconfig/varnish

Finally, restart Apache and Varnish:

[[email protected] ~]# service httpd restart
[[email protected] ~]# service varnish start
[[email protected] ~]# chkconfig varnish on

Ubuntu 12.04 / 14.04 – Installation and initial configuration

First, setup the Varnish repos:

# Ubuntu 12.04
[[email protected] ~]# curl -sL | apt-key add -
[[email protected] ~]# echo "deb precise varnish-3.0" > /etc/apt/sources.list.d/varnish.list

# Ubuntu 14.04
[[email protected] ~]# curl -sL | apt-key add -
[[email protected] ~]# echo "deb trusty varnish-3.0" > /etc/apt/sources.list.d/varnish.list

Now install Varnish:

[[email protected] ~]# apt-get update
[[email protected] ~]# apt-get install varnish

Next update your Apache ports and vhosts to 8080 since Varnish will be listening on port 80:

[[email protected] ~]# sed -i "s/Listen 80\$/Listen 8080/g" /etc/apache2/ports.conf
[[email protected] ~]# sed -i "s/NameVirtualHost \*:80\$/NameVirtualHost \*:8080/g" /etc/apache2/ports.conf
[[email protected] ~]# sed -i "s/:80>/:8080>/g" /etc/apache2/sites-available/*

Configure Varnish to pass connections back to Apache on port 8080:

[[email protected] ~]# sed -i 's/port = "80"/port = "8080"/g' /etc/varnish/default.vcl

Then update Varnish so it listens on port 80:

[[email protected] ~]# sed -i 's/^DAEMON_OPTS="-a :6081/DAEMON_OPTS="-a :80/g' /etc/default/varnish
[[email protected] ~]# sed -i 's/START=no/START=yes/' /etc/default/varnish
[[email protected] ~]# service apache2 restart
[[email protected] ~]# service varnish restart

Varnish VCL configuration examples

All of the tunings take place within the vcl’s. For the purpose of this guide, we are going to just use the default varnish configuration file in /etc/varnish/default.vcl for our examples.

How to enable basic caching of static resources:

sub vcl_recv {
if (req.url ~ "\.(html|gif|jpg|jpeg|png|js|css)$") {
         unset req.http.cookie;

If the request is coming in from CloudFlare or a load balancer, here is how to set the real IP of the client:

sub vcl_recv {
     if (req.restarts == 0) {
        if (req.http.x-forwarded-for) {
            set req.http.X-Forwarded-For =
                req.http.X-Forwarded-For + ", " + client.ip;
        } else {
            set req.http.X-Forwarded-For = client.ip;

Here is an example of how to exclude things like phpmyadmin, apc.php, and server-status from being cached:

sub vcl_recv {
if (req.url ~ "(?i)/(phpmyadmin|apc.php|server-status)") {

Here is how you can exclude a specific URL from being cached:

sub vcl_recv {
     if (req.url ~ "^/example") {
     return (pass);

Perhaps you have 30 domains on the server, and you need one of them to be excluded from the cache. Or maybe your actively working on the site. Here is how you can prevent the domain from being served through varnish:

sub vcl_recv {
    if ( ~ "^(www.)?") {
    return (pass);

If you find a script running via your browser, and suspect it is timing out due to varnish, you can adjust the timeout on that specific script by:

sub vcl_recv {
if (req.url == "^/bigscript.php") {
    set bereq.first_byte_timeout = 10m;

Here is an example of how to never cache PUT and DELETE requests for a domain:

sub vcl_recv {
if ( == "" ) {
    if (req.method == "PUT" || req.method == "POST" || req.method == "DELETE")

Varnish Troubleshooting

One of the most common errors I see on sites utilizing Varnish is a error message:

Error 503 Service Unavailable
Guru Meditation:
XID: 1234567

Typically Varnish is not the problem, but instead its something else such as Apache or PHP-FPM (aka the backend) not being available. If you can replicate the error in your browser, then run the following command so you can see if you can catch the issue as its happening in the logs:

[[email protected] ~]# varnishlog -d -c -m TxStatus:503

This will return a bunch of output. You are most interesting in the lines surrounding ‘FetchError’ as shown below:

   11 SessionOpen  c 60015 :80
   11 ReqStart     c 60015 311889525
   11 RxRequest    c GET
   11 RxURL        c /
   11 RxProtocol   c HTTP/1.1
   11 RxHeader     c Host:
   11 RxHeader     c Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
   11 RxHeader     c Connection: keep-alive
   11 RxHeader     c Cookie: wordpress_test_cookie=WP+Cookie+check; wp-settings-time-1=1455921695
   11 RxHeader     c User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_11_3) AppleWebKit/601.4.4 (KHTML, like Gecko) Version/9.0.3 Safari/601.4.4
   11 RxHeader     c Accept-Language: en-us
   11 RxHeader     c DNT: 1
   11 RxHeader     c Accept-Encoding: gzip, deflate
   11 VCL_call     c recv pass
   11 VCL_call     c hash
   11 Hash         c /
   11 Hash         c
   11 VCL_return   c hash
   11 VCL_call     c pass pass
   11 FetchError   c no backend connection
   11 VCL_call     c error deliver
   11 VCL_call     c deliver deliver
   11 TxProtocol   c HTTP/1.1
   11 TxStatus     c 503
   11 TxResponse   c Service Unavailable
   11 TxHeader     c Server: Varnish
   11 TxHeader     c Content-Type: text/html; charset=utf-8
   11 TxHeader     c Retry-After: 5
   11 TxHeader     c Content-Length: 418
   11 TxHeader     c Accept-Ranges: bytes
   11 TxHeader     c Date: Fri, 19 Feb 2016 23:04:03 GMT
   11 TxHeader     c X-Varnish: 311889525
   11 TxHeader     c Age: 0
   11 TxHeader     c Via: 1.1 varnish
   11 TxHeader     c Connection: close
   11 Length       c 418
   11 ReqEnd       c 311889525 1455923043.127803802 1455923043.128304243 0.000658751 0.000423908 0.000076532

And in the example above, where is has ‘FetchError’, it gave the 503 as Apache was not running. Which was why is says: “no backend connection’.