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Put the subdomain test in RewriteCond %{HTTP_HOST}, then let RewriteRule match the URL path. In an .htaccess file, write the path pattern without a leading slash. Use a relative substitution to route a subdomain into a directory while keeping the same address in the browser, or an absolute URL with a redirect flag when the browser must move to another host.
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The rule has two separate jobs
Apache evaluates a subdomain rule in two stages. A RewriteCond checks request metadata such as the host name. The following RewriteRule then checks the URL path. The rule itself cannot test the host: Apache’s documentation states, “The Pattern is matched only against the URL-path — not the hostname, port, or query string.”
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That distinction explains most failed subdomain rules. A pattern such as ^app.example.com in RewriteRule is looking for that text in a path, not in the host header, so it will never perform a hostname match. Put the hostname expression in a condition and keep the rule pattern focused on paths such as docs/page.html.
Conditions apply to the next rule. If several RewriteCond directives appear together, every condition must match before Apache applies that rule. Captures made by a condition are referenced as %1, %2, and so on; captures made by the rule pattern are referenced as $1, $2, and so on.
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Check the server before changing syntax
A perfectly written expression cannot work if the request never reaches the Apache instance or if that directory does not permit overrides. Confirm these prerequisites first:
- The subdomain has a DNS record resolving to the server that handles the request.
- The request is accepted by the intended virtual host and uses the document root you expect.
mod_rewriteis loaded, and the file containsRewriteEngine On.- The directory’s override policy permits rewrite directives in
.htaccessfiles. - You know whether the file is in the document root or in a subdirectory; that location changes the path Apache presents to
RewriteRule.
If DNS or virtual-host routing is wrong, you may be editing the correct-looking file for a request that is actually being served elsewhere. A 500 response usually points to a configuration or directive problem rather than a mismatch in the regular expression. Check the Apache error log after each change.
Internal rewrite: keep the subdomain in the address bar
An internal rewrite changes the file Apache serves without sending a redirect response. The browser continues to display the subdomain. In the document-root .htaccess, this is the basic pattern:
RewriteEngine On
RewriteCond %{HTTP_HOST} ^app.example.com$ [NC]
RewriteRule ^(.*)$ app/$1 [L]A request for https://app.example.com/docs/page.html is mapped internally to app/docs/page.html beneath the document root. The client does not receive a new URL, and no 3xx status is generated.
Build it safely
- Back up the current file. Keep a copy of the working
.htaccessbefore adding a catch-all rule. - Enable rewriting once. Put
RewriteEngine Onnear the start of the relevant rule set. Repeating it is unnecessary. - Anchor the host. The
^and$require the complete host to beapp.example.com, instead of matching a name that merely contains that text. - Use
[NC]when case should not matter. Host names are case-insensitive, so this flag is generally appropriate for a canonical host test. - Use a relative substitution for an internal directory mapping.
app/$1is relative to the current per-directory context. - Stop processing after the mapping.
[L]stops the current rewrite set once this rule has matched.
Local testing on a non-default port
During local development, the host header may include a port. The exact-host condition above would not match app.example.com:8080. If that is intentional, make the port optional:
RewriteCond %{HTTP_HOST} ^app.example.com(?::[0-9]+)?$ [NC]Use the port-aware expression only where you need it. On a normal public request, the host condition usually contains no port, even when HTTPS is in use.
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Map only a portion of the path
A catch-all rule sends every path on the subdomain into the directory. If only a section should move, narrow the pattern:
RewriteEngine On
RewriteCond %{HTTP_HOST} ^app.example.com$ [NC]
RewriteRule ^docs(?:/(.*))?$ app/docs/$1 [L]Here, /docs and paths below it are mapped to app/docs. Other paths remain subject to later rules or normal document-root handling. Test the empty and non-empty capture cases so that a request for exactly /docs resolves to the directory you intend.
External redirect: send the browser to another host
Use an absolute URL and a redirect flag when the destination must be visible to the client. For a permanent move from one subdomain to another host:
RewriteEngine On
RewriteCond %{HTTP_HOST} ^old.example.com$ [NC]
RewriteRule ^/?(.*)$ https://www.example.com/$1 [R=301,END,NE]The response tells the browser to request the new URL. The 301 status communicates that the move is permanent; do not use it for a temporary experiment because browsers and intermediaries can cache permanent redirects. Use a temporary redirect status while testing or while the destination may change.
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[END] stops per-directory rewrite processing completely for the request. That is useful when another pass through nested .htaccess files could redirect the request again. The [NE] flag prevents Apache from escaping special characters in the redirected URL. Keep it only when you understand the encoding of the captured path; ordinary redirects are safer without it if you do not need that behavior.
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Redirect versus internal rewrite
| Question | Internal rewrite | External redirect |
|---|---|---|
| What does the browser display? | The original subdomain and path remain visible. | The browser changes to the absolute destination URL. |
| What response does the client receive? | Normally the final resource response, such as 200. | A 3xx response, such as 301, followed by a new request. |
| Does the host change? | No; Apache serves a different internal path. | Yes, when the substitution contains another host. |
| Where can the destination live? | Typically inside the same document root and server context. | Any reachable URL, including another server. |
| What are the operational consequences? | No extra client round trip and no visible URL change. | The redirect can be cached and carries permanent or temporary move semantics according to its status. |
Choose the internal form for host-based routing where the subdomain is an alternate entry point to content on the same site. Choose a redirect when the old host should no longer be used, when canonical URLs must be exposed, or when the content has moved to another service.
Why a leading slash usually fails in .htaccess
In server or virtual-host configuration, examples often show a path beginning with /. Per-directory rewriting is different: Apache removes the matched directory prefix before it evaluates the RewriteRule pattern. In a document-root .htaccess, a request for /docs/page.html is therefore matched as docs/page.html, not /docs/page.html.
| File location | Request | Typical path seen by the rule |
|---|---|---|
Document root .htaccess |
/docs/page.html |
docs/page.html |
/app/.htaccess |
/app/docs/page.html |
docs/page.html |
| Virtual-host/server context | /docs/page.html |
Context-dependent server path, commonly including the leading slash |
That is why RewriteRule ^(.*)$ app/$1 [L] is the normal document-root form. If you copy a server-context pattern beginning with / into a per-directory file, it may never match.
The query string is not part of the rule pattern either. A request such as /docs/page.html?mode=print still matches the path docs/page.html. When a substitution does not add a new query string, Apache normally carries the existing query string forward.
Prevent loops and repeated directory prefixes
A broad rule can process its own destination on a later rewrite pass. For example, the internal rule that maps everything to app/ could see the resulting app/docs/page.html and prepend app/ again. Exclude the destination before the catch-all:
RewriteEngine On
RewriteCond %{HTTP_HOST} ^app.example.com$ [NC]
RewriteRule ^app(?:/|$) - [END]
RewriteRule ^(.*)$ app/$1 [L]The first rule leaves paths already beginning with app alone. The dash substitution means “do not change the path.” Use [END] when you need to prevent additional per-directory passes; use [L] when stopping only the current rule set is sufficient.
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Static files and directories may also need to bypass a catch-all routing rule. Add exclusions before it when the subdomain must serve existing assets directly:
RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^ - [END]The two conditions together match an existing file or directory. Place this block only if that behavior fits your layout; otherwise, it can prevent the application directory from receiving paths that it is supposed to handle.
Make host matching precise
Anchor the expression
Use ^old.example.com$, not an unanchored fragment such as old.example.com. Anchoring prevents a related but unintended host from entering the same rule. If several aliases are deliberately equivalent, express those aliases explicitly and document why they share a destination.
Account for ports deliberately
The host value can include a port during local testing. Either test with a host expression that permits an optional numeric port or send the request through the normal public port before evaluating the rule. Do not remove the end anchor simply to make a development request match; that can accept unrelated host names.
Order HTTPS and host canonicalization
Many sites have separate rules for HTTP-to-HTTPS and host canonicalization. Decide which URL is canonical, then order the conditions so one request does not satisfy a rule that sends it back to a host still covered by the same condition. A redirect loop commonly occurs when the destination host continues to match the source condition or when two independent redirects alternately rewrite the request.
Document-root and subdirectory placement
An .htaccess file applies to its directory and descendants. A file in the document root can see all paths below that root, while a file in /app sees paths relative to /app. Relative substitutions are interpreted in that per-directory context, so moving the file can change the meaning of a rule even when the text is unchanged.
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For a subdomain-to-directory mapping, place the rule in the document root when the target directory is a sibling such as /app. If the rule is placed inside /app, it cannot serve as the same top-level entry-point rule because requests have already been narrowed to that directory’s scope.
.htaccess compared with virtual-host configuration
| Factor | .htaccess |
Virtual-host/server configuration |
|---|---|---|
| Administrative access | Useful when you can edit a site directory but not the main Apache configuration. | Requires server administration access. |
| Scope | Limited to one directory and its descendants. | Can be defined for an entire virtual host and ordered with other server rules. |
| Maintenance | Convenient for site-level deployments, but rules can be scattered across directories. | Centralized and easier to audit when several hosts share canonicalization policies. |
| Performance and control | Apache checks per-directory configuration during request processing. | Rules are loaded with the server configuration and avoid per-directory discovery. |
| Alternative redirect directive | Use mod_rewrite directives permitted by the directory override policy. |
Apache’s remapping guidance identifies a dedicated Redirect in a <VirtualHost> as the cleanest choice when server configuration access exists. |
If you control the virtual-host configuration, a host-level redirect or dedicated virtual host is usually easier to reason about than a distributed set of .htaccess files. Use .htaccess when deployment constraints make it the appropriate control point.
A cautious testing procedure
- Test a harmless path first. Use a temporary redirect status or a narrow internal rule while confirming the host and path behavior. Avoid publishing a 301 until the destination is final.
- Verify DNS and virtual-host selection. Confirm that the subdomain resolves to the intended server and that the request lands in the expected document root.
- Check an unmistakable file. For an internal mapping, request a file that exists only under the destination directory so a successful response proves the mapping occurred.
- Inspect redirect headers. Run a header request such as
curl -I https://old.example.com/testand check the status andLocationvalue. A redirect should point to the intended host exactly once. - Test both host cases and paths. Check the root path, a nested path, a path that should be excluded, and a request with a query string.
- Read the error log after failures. A syntax error, forbidden directive, or rewrite loop is usually identified there more clearly than in the browser’s generic 500 page.
- Clear cached redirects when changing tests. Browsers may remember a 301, so use a fresh profile or a command-line client when validating a changed destination.
Troubleshooting by symptom
The rule never matches
- Confirm that
RewriteEngine Onappears in the active file. - Check that
mod_rewriteis loaded and that the directory override policy allows rewrite directives. - Remove the leading slash from a pattern copied into
.htaccess. - Check the actual host, including an optional development port, against the anchored condition.
- Verify that DNS and the virtual host point to the server containing this file.
Apache returns 500
Inspect the error log first. Common causes include a misspelled directive, an unsupported flag, a malformed regular expression, or a directory policy that disallows the directive. Fix the first configuration error reported, reload or retest, and then evaluate the rule’s logic.
The browser keeps redirecting
Compare the source condition with the destination host. If the destination still satisfies the source condition, the rule can redirect repeatedly. Also inspect separate HTTP-to-HTTPS and canonical-host rules for conflicting order. Test with a command-line client so cached 301 responses do not hide the current behavior.
The destination path gains repeated prefixes
Add an exclusion for the destination directory before the catch-all rule. Then choose [L] or [END] according to whether later per-directory passes must be prevented.
The wrong file is served
Check the location of the .htaccess file and the document root selected by the virtual host. A relative substitution is resolved in the current directory context, so a rule copied from the document root may point somewhere else when placed in a child directory.
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Route an entire application subdomain internally
RewriteEngine On
RewriteCond %{HTTP_HOST} ^app.example.com$ [NC]
RewriteRule ^app(?:/|$) - [END]
RewriteRule ^(.*)$ app/$1 [L]The exclusion protects the destination directory, while the final rule maps every other path on the application subdomain into it.
Redirect an old subdomain permanently
RewriteEngine On
RewriteCond %{HTTP_HOST} ^old.example.com$ [NC]
RewriteRule ^/?(.*)$ https://www.example.com/$1 [R=301,END]Leave out [NE] unless you have a specific reason to control escaping. The captured path is appended after the destination host, and an existing query string is normally retained because the substitution does not define a new one.
Support a local port without weakening the host test
RewriteEngine On
RewriteCond %{HTTP_HOST} ^app.example.com(?::[0-9]+)?$ [NC]
RewriteRule ^(.*)$ app/$1 [L]This accepts the normal host and a numeric development port while still rejecting unrelated host names.
Quick Recap
Deployment checklist
- DNS resolves the subdomain to the Apache server.
- The intended virtual host and document root receive the request.
mod_rewriteis enabled andRewriteEngine Onis present.- The directory policy permits the rewrite directives used.
- The host condition is anchored and includes a port only when required.
- The
RewriteRulepattern reflects the path form used by.htaccess, normally without a leading slash. - An internal route uses a relative destination; a visible move uses an absolute URL and an intentional redirect status.
- Destination paths and existing files or directories are excluded before a broad catch-all where necessary.
- Temporary tests are complete before enabling a permanent 301.
- The error log and response headers have been checked for every failure or loop.
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