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What Memcached does—and what installation does not do
Memcached is an in-memory key-value cache. An application stores temporary values under keys, retrieves them on a cache hit, and regenerates the value when it expires or is absent. It is not a database, a reverse proxy, or a standalone accelerator. Installing the daemon alone does not cache any application data; the application must be programmed with a Memcached client.
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Memcached is intentionally designed for trusted environments. The project’s configuring guide warns that it does not spend much effort defending itself against random internet connections. Treat an internet-reachable listener as a security failure, not as a normal deployment mode.
Choose the deployment boundary before installing
Decide which process or hosts need to reach the cache. That decision determines the listen address, firewall rule, and whether you need networking at all.
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| Design | Listen setting | Reachability | When to use it |
|---|---|---|---|
| Same-host application over TCP | -l 127.0.0.1 |
Only processes using the VPS loopback interface | The normal choice when web and application workers run on one VPS |
| Trusted private-network clients | -l 10.0.0.11 (replace with the VPS private address) |
Clients that can route to that private interface and pass both firewalls | A multi-node application on a private network |
| Local-only Unix socket | -s /path/to/socket |
No TCP or UDP listener | Applications and Memcached share one host and their client libraries support Unix sockets |
Never bind Memcached to a public address merely because a client is remote. Put the client and cache on a private network, then allow only the required source addresses. Public TCP or UDP port 11211 should remain denied.
Prerequisites and a safe change plan
- Use a supported Linux VPS with administrator access through
sudo. - Record the application hosts that must connect and their private addresses or subnet.
- Know which firewall controls ingress: the VPS firewall, a cloud security group, or both.
- Schedule a brief restart window if an existing service is being reconfigured.
- Plan memory from the VPS total, not from the cache size alone; Memcached and the operating system need headroom.
Take a copy of the distribution configuration before changing it. Package layouts differ by release, so inspect the installed unit and configuration instead of assuming every image uses identical defaults.
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Install Memcached from the package manager
Distribution packages are usually the easiest option to patch and operate. Package names and service-unit behavior can vary by release, so confirm the result with systemctl status after installation.
Debian and Ubuntu
- Refresh package metadata:
sudo apt update. - Install the daemon:
sudo apt-get install memcached. - Enable it at boot and start it now:
sudo systemctl enable --now memcached. - Check the service:
sudo systemctl status memcached.
The official Memcached server guide documents apt-get install memcached. If your image uses a different service name or does not ship a systemd unit, use the package’s documentation and init system rather than creating an untracked daemon command.
RHEL, CentOS Stream, Rocky, Alma, and Fedora
- On current releases, install with
sudo dnf install memcached. On older systems, usesudo yum install memcached. - Start the service and enable it at boot:
sudo systemctl enable --now memcached. - Confirm the unit and recent messages:
sudo systemctl status memcachedand the platform’s journal.
The project documentation lists yum install memcached for the Red Hat/Fedora family. The package may place options in a distribution-specific environment file rather than /etc/memcached.conf; locate the file referenced by the unit before editing.
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Build from source only when a package is unsuitable
A source build makes sense when the distribution package lacks a required feature or is materially older than the release you need. It also creates extra maintenance work: you must track updates, compile dependencies, installation paths, service integration, and rollback procedures yourself.
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The Memcached downloads page listed version 1.6.45, released 2026-07-09, as the stable release at the time of this article. Verify the release and checksum on the project’s own download page before compiling; do not treat that version as a promise that every VPS repository has it.
- Install a compiler toolchain and the libevent development package supplied by your distribution.
- Download and unpack the selected release tarball from the Memcached project.
- Enter the extracted directory and configure it with
./configure. - Compile with
make. - Run the project tests with
make test; stop if they fail. - Install with
sudo make install, then create a dedicated service account and a systemd unit that declares the exact binary and options.
Do not run a source-built daemon alongside the packaged service on the same port. Disable or remove the package unit first, document the binary path, and ensure your update process can reproduce the build.
Configure a private listener and disable UDP
Edit the package’s configuration file—commonly /etc/memcached.conf on Debian and Ubuntu—or the equivalent file referenced by your distribution’s service unit. For a same-host application, use settings equivalent to:
-l 127.0.0.1
-U 0-l selects the listening address. Binding to loopback prevents other network hosts from reaching the daemon. Debian and Ubuntu commonly default to localhost, but inspect the actual file and verify the running socket instead of relying on a default.
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-U 0 disables the UDP listener. Memcached has documented TCP-only defaults since version 1.5.6, yet specifying the option explicitly makes the intended exposure visible during audits and protects you from an older or customized configuration.
If trusted application nodes connect over a private network, bind to the VPS private address, for example:
-l 10.0.0.11
-U 0Replace 10.0.0.11 with the address actually assigned to the VPS. Do not use a wildcard address such as 0.0.0.0 unless every interface is intentionally protected and you have verified the resulting firewall policy; a specific private address is easier to audit.
Use a Unix socket when no network client is needed
For a strictly local design, configure a socket path such as:
-s /run/memcached/memcached.sockMemcached’s Unix-socket mode disables TCP and UDP. Set an appropriate socket permission mask and ownership so only the application account (or an explicitly defined group) can open the socket. Ensure the parent directory exists at boot and that the service account can create the socket. Your client library must support Unix sockets; otherwise use loopback TCP with UDP disabled.
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Restrict traffic with both firewalls
Binding is not a substitute for firewalling. Apply the narrowest rule at every layer:
- For loopback-only deployments, allow no external ingress to TCP 11211 and keep UDP 11211 denied.
- For private clients, allow TCP 11211 only from the application hosts or private subnet. Deny public sources.
- Mirror the rule in the VPS firewall and the cloud security group or network ACL. A permissive cloud rule can expose a service even when the local firewall looks correct.
- Do not forward port 11211 from a public load balancer or internet-facing router.
Memcached’s security guidance and OpenStack’s deployment guidance both emphasize private binding and layered controls. OpenStack additionally recommends combining firewalling with authentication and encryption for production private-network deployments.
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Understand SASL authentication and its limit
SASL can authenticate clients, but it does not encrypt Memcached traffic. The official binary SASL documentation states: “This does not provide encryption, but can provide authentication.” Credentials and cached values can therefore still be observed or modified by anyone who can monitor the network path. Keep SASL traffic on a private, trusted network or add a separate encryption layer; SASL cannot make a public endpoint safe.
SASL requires a Memcached build with SASL support, a client library that speaks SASL, and the binary protocol. To create a credential on systems using the standard tools:
- Create the account:
sudo saslpasswd2 -a memcached -c cacheuser. - Protect the SASL database so only the Memcached service user can read it.
- Add
-Sto the daemon’s service options. - Restart the packaged service and confirm that clients authenticate before issuing commands.
If -S is rejected or the service fails to start, verify that the binary advertises SASL support or inspect its linked SASL library as described in the official guide. A client that lacks SASL support will not be able to use an authenticated server; upgrade or reconfigure that client rather than weakening the server.
Restart and verify the effective configuration
After editing options, restart the service and test what is actually listening. A configuration file on disk is not proof that the running process used it.
- Restart:
sudo systemctl restart memcached. - Check state and startup errors:
sudo systemctl status memcached. - Inspect recent journal entries with your platform’s journal facility, for example
sudo journalctl -u memcached -b. - List TCP listeners and confirm the address:
sudo ss -ltnp. You should see the chosen loopback or private address on port 11211, not a public wildcard. - List UDP listeners:
sudo ss -lunp. With-U 0, Memcached should not appear on UDP 11211. - From an approved application host, test a client connection. From an unapproved host, confirm that the firewall blocks it.
The Memcached advisory demonstrates equivalent listener verification with netstat; ss is the usual modern replacement. Repeat these checks after package upgrades, image migrations, or service-unit changes.
Size memory and connections for the VPS
The -m option sets the item-cache memory in megabytes. Memcached can consume somewhat more memory than the item-cache value, and settings below 64 MB can still consume up to a 64 MB minimum. Leave room for the operating system, the application, filesystem cache, monitoring agents, and temporary spikes.
The documented default maximum concurrent connection setting is 1,024. Do not copy that value blindly: compare expected worker counts, connection pooling, request bursts, and the VPS’s available file descriptors. Increase limits only after measuring demand and confirming that the application and operating system can support them.
- Start with a conservative
-mvalue that leaves substantial RAM for the application. - Watch memory pressure, swap activity, evictions, and connection counts after launch.
- Remember that Memcached is disposable: an eviction or restart should cause a cache miss and a rebuild, not data loss that the application cannot recover from.
- Keep keys, expiration times, and item sizes within the limits expected by your client and application design.
Compare the main deployment choices
| Choice | Security posture | Operational trade-off |
|---|---|---|
| Distribution package | Integrates with signed updates and the native service manager | May lag the upstream release or omit a feature |
| Source build | Requires you to maintain the binary, updates, service unit, and rollback | Can provide a newer version or required compile-time support |
| Loopback TCP | Network-reachable only from the same host | Works with most clients and is simple to diagnose |
| Private-interface TCP | Reachable by routed private clients; depends on precise firewall rules | Supports multi-node applications but increases exposure and operations work |
| Unix socket | Removes TCP and UDP exposure entirely | Requires client-library support and correct socket permissions |
| Unauthenticated clients | Acceptable only inside a tightly controlled local or private boundary | Simpler compatibility, but no client identity check |
| SASL-authenticated clients | Adds client authentication, not confidentiality | Requires SASL-capable server and clients, credential storage, and binary protocol |
Troubleshoot the failures that matter
The application cannot connect
- Confirm the daemon is active with
systemctl status memcached. - Check that the client uses the same address and port as
ss -ltnpshows. - If the daemon is bound to a private address, verify routing, the cloud security group, and the VPS firewall from the application host.
- For a Unix socket, check the path, parent-directory ownership, socket permissions, and the application service account.
The service starts but is exposed unexpectedly
Inspect the effective listener with ss -ltnp and ss -lunp. Locate duplicate option files or an overridden systemd unit, then remove the wildcard bind, add -l for the intended interface, set -U 0, restart, and verify again. Close any cloud-security-group rule that allows public TCP or UDP 11211.
Enabling SASL breaks clients
Check that the server was built with SASL, that -S is present in the running service options, and that every client uses the binary protocol with matching credentials. Clients without SASL support must be upgraded or kept on a separately protected endpoint; do not disable authentication on an otherwise shared listener without revisiting its firewall boundary.
The VPS runs out of memory
Lower -m after measuring application needs, account for Memcached’s overhead and the documented 64 MB minimum behavior, and leave space for the operating system. Investigate connection growth and other services before simply adding swap or raising the cache limit.
Operations checklist
- Install from the package manager unless a documented version or feature requirement justifies a source build.
- Bind to
127.0.0.1, a specific private address, or a Unix socket; never expose a public listener. - Set
-U 0explicitly and verify that no UDP socket exists. - Allow TCP 11211 only from approved hosts at both firewall layers.
- Use SASL only with compatible builds and clients, and remember that it does not encrypt traffic.
- After every change, inspect service status, journal errors, TCP/UDP listeners, and the cloud rule.
- Size
-mand connection limits against total VPS resources and observed concurrency. - Monitor evictions, memory pressure, restarts, and failed connections so cache loss remains a recoverable performance event.
Sources and version notes
The installation and configuration commands follow the Memcached Server Guide and configuring guide. The security interpretation follows the project’s configuring guidance, binary SASL authentication documentation, and OpenStack’s deployment guidance. The project downloads page listed Memcached 1.6.45, released 2026-07-09, while TCP-only default listening has been documented since version 1.5.6. Distribution defaults and file paths can differ, so the running service and listener checks remain authoritative.
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