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A subdomain move does not automatically require a new RewriteBase. The failure usually comes from a mismatch between the URL path, the directory where Apache reads .htaccess, and the path used by a relative substitution. First confirm the rule’s context and path mapping; then decide whether a base is needed.
Why a subdomain move can expose a rewrite failure
In .htaccess per-directory context, Apache removes the directory prefix before matching a RewriteRule. For example, a request for /app/products/widget can reach a rule in that directory as products/widget, without the leading slash. A rule pattern beginning with ^/ therefore will not match there. Apache explicitly warns: “A rule like RewriteRule "^/foo" ... will silently fail to match anything when placed in a .htaccess file.” Apache’s per-directory rewrite guide describes this context difference.
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A subdomain changes the hostname, but that fact alone does not establish that the URL-path prefix or filesystem mapping changed. The move matters when the new virtual host serves the site from a different directory, uses an Alias or another mapping, or applies the file in a different directory context. A relative substitution can then resolve against a different URL-path prefix than intended.
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RewriteBase sets the URL-path prefix Apache uses to resolve relative substitutions in per-directory rules. It is a URL path, not a filesystem directory. For example, do not put a server path such as /var/www/site in RewriteBase simply because that is where the files live.
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Whether the directive is needed depends on the relationship between the URL and filesystem paths, the substitution form, and the Apache version. Apache says it can usually be omitted for ordinary DocumentRoot cases. Since Apache 2.4.16, it can also be omitted for certain Alias and mod_userdir mappings. Check the installed version and the exact mapping before relying on that exception. See the Apache 2.4 RewriteBase reference.
Diagnose the migration in this order
- Confirm the virtual host and file context. Identify which virtual host handled the request and the directory in which the applicable
.htaccessfile resides. A different host may use a different document root or configuration. - Write down both paths. Record the requested URL path, the filesystem directory Apache maps it to, and the rule’s substitution. Determine whether the substitution is relative or begins with a slash. A hostname change by itself does not prove the URL-path base changed.
- Check the pattern for per-directory syntax. If a
RewriteRulepattern in.htaccessstarts with^/, remove that leading slash and account for the directory prefix Apache has already stripped. Server and virtual-host rules use a different matching context. - Resolve relative substitutions against the right URL path. If the substitution is relative, work out which URL-path prefix should precede it. Add or adjust
RewriteBaseonly if the mapping requires it; use the URL path, not the physical directory. - Verify rewrite prerequisites. The rules need
RewriteEngine On, permission for the directives through the applicable override settings, andFollowSymLinksorSymLinksIfOwnerMatchin the relevant context. See the Apache 2.4RewriteRulereference. - Check rules across directory boundaries. Parent-directory rewrite rules are not inherited by default.
RewriteOptionscontrols inheritance;MergeBaseis available starting with Apache 2.4.26. See the Apache 2.4RewriteOptionsreference. - Test in the actual mapping. Apply one change at a time and test through the virtual host and directory mapping that serve the migrated site. Do not assume a local or default-host test exercises the same rules.
Separate the likely causes before editing
| What to compare | What to look for |
|---|---|
| Rule context | Server or virtual-host rules match differently from per-directory .htaccess rules; in the latter, Apache strips the directory prefix. |
| Path mapping | Determine whether the request is served under DocumentRoot or mapped through an Alias or another mechanism. |
| Substitution form | A relative substitution may need a URL-path base; an absolute-path substitution does not have the same relative-resolution question. |
| Apache version | Version-specific behavior includes the documented 2.4.16 mapping exception and MergeBase support from 2.4.26. |
What the migration details can—and cannot—tell you
Without the active virtual-host configuration, the relevant .htaccess rules, and the URL-to-filesystem mapping, there is no single RewriteBase value that can be prescribed for every subdomain migration. The diagnostic pattern is to establish the context and mapping first, then determine how Apache should resolve the substitution. The Apache references above document the behavior; they do not establish that any one unprovided server configuration reproduced this failure.
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