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Test the hostname with RewriteCond %{HTTP_HOST}, then match the requested path with RewriteRule. In a document-root .htaccess file, do not normally put a leading slash in the rule pattern.
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RewriteEngine On
RewriteCond %{HTTP_HOST} ^app.example.com$ [NC]
RewriteRule ^(.*)$ app/$1 [L]This internally maps https://app.example.com/docs/page.html to the app/docs/page.html path while leaving the browser on the subdomain.
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What a subdomain rewrite rule actually matches
An Apache rewrite has two separate jobs: deciding whether a request qualifies and deciding which URL path Apache should serve. The hostname belongs in a RewriteCond; the URL path belongs in RewriteRule.
Apache’s documentation states that the RewriteRule pattern is matched only against the URL path, not the hostname, port, or query string. Consequently, a pattern such as ^app.example.com will not match a subdomain in .htaccess. Apache has already separated the host from the path by the time it evaluates the rule.
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The normal arrangement is:
RewriteEngine Onenables rewriting for the directory.- One or more
RewriteConddirectives test the request, usually through%{HTTP_HOST}. - The following
RewriteRulematches the path and supplies a replacement path or URL. - Flags such as
[L],[END],[R=301],[NC], and[NE]control what happens next.
Conditions apply to the RewriteRule immediately below them. If several conditions are present, every condition must match before that rule is applied. Captures from a condition are referenced as %1, %2, and so on; captures from the RewriteRule itself are referenced as $1, $2, and so on.
Prerequisites before editing .htaccess
A syntactically correct rule cannot work if the request never reaches the Apache instance or if Apache is not allowed to read rewrite directives. Verify the following in order:
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- DNS: The subdomain must resolve to the server handling the request. Check the DNS record for
app.example.comand confirm that it points to the intended service. - Virtual host: The request must arrive at the Apache virtual host that serves the expected document root. A different virtual host can make a correct rule appear ineffective.
- Module:
mod_rewritemust be available, and the file must containRewriteEngine On. - Override policy: The directory configuration must permit the rewrite directives used in
.htaccess. If Apache rejects them, the server administrator must adjust the directory’s override policy. - File location: Establish whether the file is in the document root or a subdirectory. Per-directory rewriting changes the path presented to the rule.
- Safe test: Start with a narrow path or a temporary redirect. Do not make a permanent redirect the first test.
If DNS or virtual-host routing is wrong, changing the regular expression will not solve the problem.
Internal rewrite: keep the subdomain in the address bar
Use an internal rewrite when the subdomain should be served from a directory inside the same document root. The server changes the resource it reads, but the browser does not receive a redirect and continues displaying the original host.
Document-root example
RewriteEngine On
RewriteCond %{HTTP_HOST} ^app.example.com$ [NC]
RewriteRule ^(.*)$ app/$1 [L]With this file in the document root, a request for https://app.example.com/docs/page.html is served from the document-root path app/docs/page.html. The client still sees https://app.example.com/docs/page.html, not a URL containing /app.
The host expression is anchored with ^ and $. Anchoring prevents a name such as not-app.example.com or app.example.com.attacker.example from satisfying an otherwise loose expression. [NC] makes the host comparison case-insensitive, which is generally appropriate for hostnames.
Local testing on a non-default port
When testing locally on a port such as 8080, include the port in the condition instead of assuming that %{HTTP_HOST} contains only the name. A narrowly scoped test can be written as:
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RewriteCond %{HTTP_HOST} ^app.example.com(?::8080)?$ [NC]
RewriteRule ^(.*)$ app/$1 [L]The optional :8080 is for that local test arrangement; use the actual port you have configured, and remove it when the public request uses the default port.
Preventing the directory prefix from being added repeatedly
After an internal rewrite, per-directory processing can run again. A catch-all rule that always prepends app/ may therefore try to turn app/docs/page.html into app/app/docs/page.html. Put an exclusion ahead of the catch-all when the destination directory is visible during a later pass:
RewriteEngine On
RewriteCond %{HTTP_HOST} ^app.example.com$ [NC]
RewriteRule ^app(?:/|$) - [L]
RewriteCond %{HTTP_HOST} ^app.example.com$ [NC]
RewriteRule ^(.*)$ app/$1 [L]The first rule leaves paths that already begin with app/ alone. Add exclusions for other paths that must bypass the mapping, and place those exclusions before a broad rule.
Existing files and directories
A catch-all can also capture assets, administrative paths, or other resources that should be handled normally. Decide explicitly which files and directories belong outside the subdomain mapping. Exclude them before the catch-all, and stop processing after the intended match. Do not add a blanket exclusion without checking what the application actually needs.
External redirect: send the browser to another host
Use an external redirect when the visible URL must change. Apache returns a redirect response, and the browser makes a new request to the destination host.
RewriteEngine On
RewriteCond %{HTTP_HOST} ^old.example.com$ [NC]
RewriteRule ^/?(.*)$ https://www.example.com/$1 [R=301,END,NE]This redirects requests from old.example.com to the corresponding path on www.example.com. The ^/? form tolerates an optional leading slash, although per-directory patterns normally do not receive one. The destination is an absolute URL because a visible redirect must tell the client where to go.
Choose the status deliberately
- 301: Use only when the move is intended to be permanent. Browsers and intermediaries can cache a permanent redirect, making mistakes difficult to undo quickly.
- Temporary status: Use the temporary status appropriate to your deployment while testing or when the move is not permanent. Replace it with 301 only after the behavior is confirmed.
The host condition must not also match the destination host. Redirecting every host to a URL whose hostname satisfies the same condition creates a loop. If HTTP-to-HTTPS canonicalization is also present, establish a clear order so that the two redirects do not send the request back and forth.
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What the redirect flags mean
[R=301] asks Apache to return a permanent redirect. [END] stops per-directory rewrite processing after the redirect. [NE] prevents normal escaping in the redirect result; use it only when you understand the encoding consequences. For ordinary URLs, preserving safe URL encoding is the safer default.
Internal rewrite versus external redirect
| Decision point | Internal rewrite | External redirect |
|---|---|---|
| Browser address | Remains on the original subdomain and path | Changes to the destination URL |
| Typical substitution | Relative path such as app/$1 |
Absolute URL such as https://www.example.com/$1 |
| Response | Apache serves the mapped resource directly | Apache returns a redirect status, then the client requests the new URL |
| Destination host | Normally the same host | Can be a different host or canonical hostname |
| Document-root relationship | Destination remains inside the server’s mapped document tree | Destination may be another virtual host or server |
| Typical use | Serve a subdomain from an application directory without exposing that directory | Move a subdomain, enforce a canonical host, or make a migration visible |
Do not use a redirect merely to reach a directory that belongs to the same site unless exposing that path is intentional. Conversely, do not use an internal rewrite when clients, search engines, or other systems must learn the new canonical host.
How to construct the rule safely
1. Decide what must change
Write down whether only the server-side location changes or whether the public URL must change. That decision determines the substitution style and whether a redirect flag is appropriate.
2. Identify the exact host set
Use an anchored host condition for one subdomain:
RewriteCond %{HTTP_HOST} ^app.example.com$ [NC]If more than one hostname is intentionally supported, use separate, clearly scoped conditions or a carefully designed expression. Avoid an unanchored fragment such as app, which can match unrelated hostnames.
3. Write the path pattern for .htaccess context
In a document-root .htaccess file, Apache removes the directory prefix before matching. The usual pattern is therefore ^(.*)$, not ^/(.*)$. A leading slash copied from a server-context example often prevents the rule from matching.
If the file is inside a subdirectory, the matched path is relative to that directory. Move the file or change the pattern only after confirming the directory context; do not assume a document-root rule behaves identically in a nested .htaccess file.
4. Preserve the path with a backreference
The capture in ^(.*)$ becomes $1. An internal substitution such as app/$1 adds the application directory while retaining the requested path. A redirect substitution such as https://www.example.com/$1 carries that path to the new host.
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5. Add exclusions before broad rules
Place destination-directory exclusions, existing-file or existing-directory exceptions, and special application routes above a catch-all. The first matching rule may otherwise consume requests that another handler was supposed to process.
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Use [L] to stop the current rewrite rule set after a match. In per-directory configuration, [END] is a stronger stop that ends all subsequent per-directory rewrite processing for the request. Choose [END] when another per-directory pass would be harmful; use [L] when later processing is intentionally allowed.
Why common attempts fail
“My RewriteRule does not match the subdomain”
The hostname was probably placed in the RewriteRule pattern. Move it to RewriteCond %{HTTP_HOST} and leave only the path in the rule. Confirm that DNS and the virtual host deliver the request to this Apache instance.
“The rule works in server configuration but not in .htaccess”
Check the leading slash first. Server-context examples and per-directory examples are not interchangeable: .htaccess normally receives a path with the directory prefix removed, so a pattern beginning with / can fail.
“The request becomes app/app/…”
The catch-all rule is being applied again after its own internal rewrite. Add a destination-path exclusion before the catch-all and stop the exclusion with [L] or use [END] where a complete per-directory stop is required.
“The browser keeps redirecting”
Inspect the host condition and the redirect target. A redirect loop occurs when the destination still satisfies the condition, or when separate HTTP-to-HTTPS and hostname rules repeatedly hand the request to each other. Make each canonicalization rule match only the non-canonical form and test each transition independently.
“Apache returns 500”
Read the Apache error log at the time of the request. Then verify that mod_rewrite is loaded, RewriteEngine On is present, the directives are permitted by the directory’s override policy, and the regular-expression syntax is valid. A server error is not evidence that the host condition is wrong; it often means Apache rejected the configuration before matching any request.
“Nothing changes after I edit the file”
Confirm that the request is reaching the directory containing the edited file, that the intended virtual host is active, and that you are testing the exact hostname in the condition. A request sent to an IP address or a different host will not satisfy an anchored subdomain condition.
When VirtualHost configuration is the better choice
If you administer Apache’s server configuration, a dedicated virtual-host rule is usually cleaner for hostname routing than distributing host logic through a document-root .htaccess file. Apache’s remapping guidance identifies a dedicated Redirect in a <VirtualHost> as the cleanest approach when server-configuration access exists.
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| Factor | .htaccess | VirtualHost/server configuration |
|---|---|---|
| Administrative access | Useful when you can edit only the site’s directory | Requires control of Apache’s server configuration |
| Scope | Per-directory and inherited according to the directory layout | Can target one virtual host directly |
| Maintainability | Convenient for site-level changes, but easy to duplicate or conflict | Centralized and easier to audit for host-wide routing |
| Performance | Read during per-directory request processing | Loaded as server configuration rather than discovered from directories |
| Typical redirect choice | RewriteRule with an absolute target | A dedicated VirtualHost Redirect when applicable |
Use .htaccess when that is the deployment boundary you control. If you can change the virtual-host configuration, keep hostname canonicalization there and reserve per-directory rules for behavior that genuinely belongs to the application tree.
A deployment and verification sequence
- Create or back up the current
.htaccessfile in the correct document-root directory. - Add
RewriteEngine Ononce, then add a narrowly scoped host condition. - Begin with an internal mapping or a temporary redirect and one known path, such as
/docs/page.html. - Request the exact subdomain and inspect both the browser URL and the returned resource. An internal rewrite should keep the original URL; a redirect should expose the new URL and status.
- Test a different hostname that must not match. It should bypass the subdomain rule.
- Test the destination directory and any existing files or directories that need exclusions.
- Check Apache’s error log for syntax or permission failures.
- Only after the behavior is stable, widen the path pattern and change a confirmed permanent move to status 301.
Keep host matching anchored, keep exclusions above catch-all rules, and document whether each rule is intended to be internal or visible. Those three decisions prevent most accidental loops and unexpected URL changes.
Practical templates
Subdomain to an application directory
RewriteEngine On
RewriteCond %{HTTP_HOST} ^app.example.com$ [NC]
RewriteRule ^app(?:/|$) - [L]
RewriteRule ^(.*)$ app/$1 [L]This template assumes the file is in the document root. The first rule protects paths already under app/; the second maps the remaining paths. Add the host condition to the exclusion too when other hostnames share this file and must not see the exclusion.
Permanent move to another host
RewriteEngine On
RewriteCond %{HTTP_HOST} ^old.example.com$ [NC]
RewriteRule ^/?(.*)$ https://www.example.com/$1 [R=301,END,NE]Use this only when the hostname move is permanent and the target host does not satisfy the condition. During initial testing, use a temporary status instead of committing clients and caches to a 301.
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For a subdomain in .htaccess, test the hostname with RewriteCond %{HTTP_HOST}, match the slashless per-directory path with RewriteRule, and choose the substitution based on whether the browser should see a new URL. Internal relative substitutions route requests inside the same document root; absolute URLs with an intentional redirect status change the public destination. Verify DNS, virtual-host routing, mod_rewrite, and override permissions before debugging expressions, then guard destination paths against repeated processing.
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