URL redirect management in Umbraco
Whenever a document is published, and this causes changes to its URL (and any of its descendants' URLs), Umbraco makes a note of the old URLs. Whenever an incoming request is served and the default content finders cannot find a matching published document, Umbraco checks whether the URL matches one of these saved URLs. If a match is found, Umbraco returns a "301 Redirect" response pointing to the new URL of the document.
It is possible to list the redirect URLs via the Redirect Url Management dashboard in the Content section. This dashboard lists the original URL, new URL, and culture. It also allows you to delete a URL redirect.
In addition, the dashboard can be used to disable or enable the 301 Redirect Management (via the appsettings.json configuration option described below - note that this requires an application restart to take effect).
Anytime a document is published and its corresponding url segment changes, Umbraco checks its URL (and all its descendants' URLs) for changes. For every URL that has changed, it creates (or updates) a row in the umbracoRedirectUrl table. Rows in this table contain: the old url, the create date, and the target content identifier, culture, and a url hash.
Umbraco registers a new content finder, ContentFinderByRedirectUrl, which runs as a normal content finder after the other content finders. It looks for the incoming URL in the database table and, if found, computes the URL of the target document and returns a "301 Redirect". These redirects are considered "permanent". It's good to note that we explicitly set no-cache headers on these redirects so that when they change, browsers update the URL immediately. They are a "true" 301, however, and search engines will accept them as such.
The 301 Redirect Management feature is enabled by default.
It is possible to disable the feature entirely (both generating URLs in the database table, and running the content finder) by editing the appsettings.json file: