The Tool Desk
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This guide walks through that arrangement, shows how to verify each layer independently, and covers separate-origin hosts, TLS termination, multiple backends, hostname resolution, and the common causes of Varnish 503 errors.
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Varnish is the front-end HTTP cache. It accepts client requests, decides whether an object can be served from cache, and contacts Apache only when it needs a fresh response. Apache remains the origin server: it runs your application, reads files, executes server-side code, and generates responses that Varnish can store and reuse.
Varnish Cache documentation describes this relationship by saying that Varnish has a concept of a “backend” or “origin” server. In this setup, Apache is that backend. Varnish does not replace Apache; it sits in front of it.
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A same-host deployment uses separate sockets so the two daemons do not compete for port 80:
| Component | Typical address | Role |
|---|---|---|
| Varnish | :80 |
Public HTTP listener and cache |
| Apache | 127.0.0.1:8080 or :8080 |
Origin web server |
| VCL backend | .host = "127.0.0.1", .port = "8080" |
Where Varnish connects for misses |
Varnish’s official quick-start guidance identifies port 8080 as a good alternative HTTP port for the web server. The exact port is not mandatory, but it must be free, consistently configured in Apache, and identical in the VCL backend.
Plan the deployment before changing ports
Check the operating system and packages
The commands below target Debian and Ubuntu systems using systemd. You need root or sudo access, a working Apache installation, and a maintenance window long enough to restart both services. Ensure your firewall permits public HTTP on port 80 and, if you use a separate administrative path, does not expose Apache’s origin port unnecessarily.
Install Varnish after refreshing package metadata:
sudo apt-get update
sudo apt-get install varnishOn RHEL-compatible distributions, use the Varnish repository and package workflow documented by Varnish Cache for your release. The service names and paths normally follow the distribution’s conventions; Apache configuration is under /etc/httpd rather than /etc/apache2.
Inventory every Apache binding
Do not assume that changing the first site file is enough. Apache can load listeners and virtual hosts from the main configuration, enabled-site links, included snippets, and generated files. Before editing, search all active configuration for port 80:
sudo apachectl -S
sudo grep -R "Listen 80|VirtualHost .*:80" /etc/apache2apachectl -S reports the virtual hosts Apache has parsed. The recursive search helps find an additional Listen 80 or a virtual host that would continue claiming the port after the primary edit.
Install and configure Varnish on Debian or Ubuntu
1. Confirm Apache works directly first
Before adding a cache, request the site from Apache itself. At this stage Apache may still be on port 80. Record the expected status code, headers, redirects, and application behavior. If the origin is already broken, inserting Varnish will make diagnosis harder.
After you eventually move Apache to 8080, the equivalent direct test is:
curl -i http://127.0.0.1:8080/Use an appropriate Host header when name-based virtual hosting is in use:
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curl -i -H 'Host: example.test' http://127.0.0.1:8080/Replace example.test with a hostname configured on your server. A response here proves that Apache, its virtual-host selection, and the application can operate without Varnish.
2. Move Apache’s listener to port 8080
Edit /etc/apache2/ports.conf and change the active HTTP listener:
Listen 80to:
Listen 8080Then edit every enabled virtual host that currently binds to port 80. Change:
<VirtualHost *:80>to:
<VirtualHost *:8080>Check files under /etc/apache2/sites-enabled as well as included files elsewhere in the Apache configuration. A site left on *:80 can keep Apache bound to the public port, or can leave that virtual host unavailable once Varnish takes the port.
Validate the syntax before restarting:
sudo apachectl configtestOnly a successful syntax result should be followed by an Apache restart:
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sudo systemctl status apache2Now test the origin directly on 8080. If it fails, fix Apache before proceeding. Typical causes include a missed virtual-host binding, an application that generates absolute URLs for the old port, or a module configuration that was included only in the original site file.
3. Define the Varnish backend in /etc/varnish/default.vcl
Replace the file contents, or adapt the existing distribution example, so the backend points to Apache’s actual address and port:
vcl 4.1;
backend default {
.host = "127.0.0.1";
.port = "8080";
}The VCL version must be supported by the installed Varnish package. A backend must be specified with either a .host or a .path attribute, but not both, as stated in the Ubuntu Varnish backend reference. For a TCP connection to Apache, use .host and .port as shown.
You may use the server’s address or a hostname instead of 127.0.0.1. A numeric loopback address avoids DNS dependency on a same-host installation. If you choose a hostname, remember that Varnish resolves it when the VCL is compiled; changing DNS later does nothing until the VCL is reloaded.
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4. Configure Varnish to listen on port 80
Package defaults differ by distribution and Varnish release. Inspect the active unit before editing so you preserve required arguments:
systemctl cat varnishConfigure the unit, or create a supported systemd drop-in override, so the start command includes a listener of -a :80 and loads /etc/varnish/default.vcl. A unit’s existing memory, security, storage, and PID arguments may be necessary; do not replace the complete command with a minimal example unless you have verified the package’s requirements.
A drop-in is generally safer than editing a vendor unit directly. The exact override syntax depends on the unit’s existing ExecStart; if you replace it, systemd requires clearing the previous command first:
sudo systemctl edit varnishWithin the editor, use the package’s documented command as the base and ensure its Varnish arguments contain the equivalent of:
-a :80 -f /etc/varnish/default.vclDo not remove unrelated arguments merely to add these two settings. If your distribution already supplies a configuration file or environment variable for the listener, use that supported mechanism instead.
5. Apply the unit and VCL changes
When a systemd unit or drop-in has changed, make systemd reread it before restarting Varnish:
sudo systemctl daemon-reload
sudo systemctl restart varnishIf only the VCL changed, a restart is the simplest way to compile and load it. A VCL reload is also required after changing a hostname’s DNS address because name resolution happens during VCL compilation.
Check both services immediately:
sudo systemctl status varnish apache2A failed Varnish start usually includes a VCL syntax error or a listener conflict in the journal. Read the relevant log lines rather than repeatedly restarting:
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sudo journalctl -u varnish -b --no-pager
sudo journalctl -u apache2 -b --no-pager6. Verify listeners and request the public path
Use a socket inspection tool to see which process owns each port:
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sudo ss -ltnpYou should see Varnish listening on port 80 and Apache listening on 8080. Then test both layers:
- Origin: request
http://127.0.0.1:8080/, adding the correctHostheader if necessary. - Cache front end: request
http://127.0.0.1/or the server’s public hostname, which reaches port 80. - Compare responses: check status, redirects, content, and any cache-related headers your configuration exposes.
These checks isolate failures. A working origin with a failed public request points to Varnish’s listener, VCL, or backend connection; a failed direct request points to Apache or the application.
Equivalent setup on RHEL-compatible systems
Install Varnish using the repository and distribution package workflow documented by Varnish Cache for your RHEL-compatible release. Keep the same architecture: Varnish owns port 80, Apache owns 8080, and the VCL backend names the address and port where Apache actually listens.
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Make the Apache edits under /etc/httpd. Update the active Listen directive from 80 to 8080 and change every enabled <VirtualHost *:80> binding to <VirtualHost *:8080>. Validate with the platform’s Apache configuration test, restart Apache, and test it directly before starting the cache.
Configure the Varnish systemd unit or a local override to use -a :80 and the intended VCL file. Run:
sudo systemctl daemon-reload
sudo systemctl restart varnish
sudo systemctl status varnish httpd
sudo ss -ltnpThe command names differ (httpd instead of apache2), but the diagnostic order is identical: verify Apache on 8080, verify Varnish on 80, then test through port 80.
Choose the right topology
| Deployment choice | What changes | Important decision |
|---|---|---|
| Same host | Varnish on 80 and Apache on 8080, commonly via loopback | Use a local address and restrict the origin port from external access. |
| Separate origin host | VCL points to the origin server’s reachable address and port | Permit only the required network path and account for firewall rules, latency, and failure detection. |
| HTTP only | Varnish terminates the public HTTP connection | Port 80 is sufficient; do not assume this provides HTTPS. |
| TLS terminated by another proxy | A TLS-capable load balancer or reverse proxy sits before Varnish | Preserve the original host and scheme headers according to your proxy design, and decide where redirects and certificates are managed. |
| Single backend | One backend default entry |
Simplest routing and health model; a backend outage makes misses fail. |
| Clustered backends | Multiple backends plus explicit routing, failover, or load balancing | Define health checks, selection rules, session behavior, and what happens when one origin is unhealthy. |
| Package-managed unit | Use the distribution’s supplied service configuration | Preferred when supported because upgrades can preserve defaults. |
| Local systemd override | Add a drop-in rather than modifying vendor files | Keep the complete required command line and document the override for future administrators. |
| Numeric backend address | VCL uses an IP address such as 127.0.0.1 |
No DNS lookup is needed for a same-host origin. |
| Hostname backend | VCL uses a DNS name | Convenient for movable origins, but reload VCL whenever DNS changes because resolution occurs at compile time. |
Multiple backends are not a drop-in extension of the minimal configuration. They require deliberate routing and health-check decisions, especially for logged-in sessions, uploads, shopping carts, and other stateful requests.
Why Varnish returns 503 and how to isolate it
Apache is not listening where VCL expects
The most common 503 cause is that Varnish cannot connect to its configured backend. Compare the VCL values with the actual socket:
sudo ss -ltnp | grep -E ':80|:8080'
curl -i http://127.0.0.1:8080/If Apache listens on another address, change .host. If it listens on another port, change .port. Restart or reload Varnish after editing the VCL so the new backend is compiled.
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Apache still owns port 80
Varnish cannot bind to port 80 while Apache does. Search all active Apache configuration for Listen 80 and <VirtualHost *:80>, including included files and enabled sites. Move every relevant binding to 8080, validate Apache, restart it, and then restart Varnish.
A unit edit appears to be ignored
Systemd caches unit definitions. After changing a service file or drop-in, run:
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sudo systemctl daemon-reload
sudo systemctl restart varnishUse systemctl cat varnish to confirm which vendor unit and overrides are actually active. If the listener remains wrong, inspect the final ExecStart and any environment file referenced by the unit.
A hostname backend points to an old address
Varnish resolves a hostname when it compiles the VCL, not for every request. After DNS changes, reload the VCL or restart Varnish. Confirm the new destination independently with a direct connection and verify that firewalls allow it.
Only one virtual host fails
If the default site works but a named site does not, send the intended Host header to Apache on 8080. Then inspect apachectl -S (or the equivalent RHEL command) for duplicate, missing, or incorrectly ported virtual-host definitions. Varnish forwards the request host, but Apache can select a different site if the binding or name is incomplete.
Operational checks after the first successful request
- Confirm both services are enabled as intended for the next boot, using your distribution’s normal
systemctl enablepolicy. - Keep Apache’s origin port private on a same-host installation unless another trusted component must reach it.
- Document which component terminates TLS, where redirects are generated, and which headers identify the original scheme and client.
- Test cacheable and non-cacheable paths separately. Authentication, cookies, personalized pages, and unsafe methods often need explicit VCL rules rather than the minimal backend declaration.
- After package upgrades, recheck the effective Varnish unit, VCL syntax, listeners, and a direct origin request. Distribution packages can change service defaults.
- Monitor service logs and your own response metrics. No universal cache-hit or latency figure applies to every Apache application; measure hit rate, origin load, errors, and response time in the target deployment.
Safe rollback
If the public site fails after introducing Varnish, isolate the fault rather than changing several layers at once. First test Apache directly on 8080. If Apache is healthy, stop or reconfigure Varnish and temporarily expose Apache on port 80 by reversing the listener and virtual-host edits. Validate Apache, then decide whether to correct the VCL, unit, or network path before putting the cache back in front.
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Keep a copy of the original Apache port configuration and the previous Varnish unit or drop-in. Rollback still requires a consistent pair: Apache must own 80 when Varnish is removed, and every enabled virtual host must use the matching port.
Configuration checklist
- Varnish package installed from the operating system’s supported source.
- Apache tested directly before caching.
/etc/apache2/ports.conf(or the RHEL equivalent) usesListen 8080.- Every enabled Apache virtual host uses
*:8080. /etc/varnish/default.vcldeclaresvcl 4.1;and the real Apache host and port.- The effective Varnish service listens on
:80and loads the intended VCL. systemctl daemon-reloadwas run after unit or drop-in edits.- Varnish and Apache both report active status.
ss -ltnpshows Varnish on 80 and Apache on 8080.- Direct origin and public port-80 requests both return the expected site.
The Bottom Line
For a same-host installation, put Varnish on port 80, move every Apache listener and virtual host to 8080, point /etc/varnish/default.vcl at 127.0.0.1:8080, reload systemd after unit changes, and test Apache and Varnish separately. That sequence prevents port conflicts and makes a 503 diagnosable instead of mysterious.
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