Where Squid Error-Page Images Are Served From
Fix a missing image in a Squid error page by separating the browser's URL from the proxy server's local filesystem path.
A logo file exists on the proxy server. Squid can read it. The browser still shows a broken image. File permissions cannot fix a request for the wrong URL.
When a custom error template contains src="images/logo.png", the browser resolves that reference using the document's URL context. It does not search the Squid server's disk for a nearby images directory. Putting the file under a separate web server's document root also does not automatically make Squid serve it.
In Squid 5–7, error_directory selects custom error templates; icon_directory identifies icon storage. Squid's internal icon URLs use the /squid-internal-static/icons/ path. Follow the installed version's resource layout rather than treating a Unix filesystem path as an HTML image address.
Inspect the browser's actual image request. Its hostname, path, status, and selected route show whether the browser asked Squid for an internal resource, contacted a separate image host, or requested an unrelated path on the blocked site.
If the image lives on an approved internal web server, use its reachable resource URL and account for access rules. If it belongs to Squid's internal assets, confirm the installed asset location and the listener that serves those requests.
Keep the error message readable without its picture. A diagnostic page that depends on another failing network request can hide the explanation the reader needed in the first place.