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Test the hostname in RewriteCond %{HTTP_HOST}, then match the requested path in RewriteRule. In a document-root .htaccess file, omit the leading slash from the rule pattern. Use a relative substitution for an internal subdomain-to-directory mapping, or an absolute URL with an intentional redirect status when the browser must move to another host.
The rule model: hostname in a condition, path in the rule
Apache evaluates a per-directory rewrite in two separate parts. RewriteCond decides whether the request has the characteristics you want, while RewriteRule matches the URL path and supplies the replacement. The hostname is not available to the rule pattern itself. Apache’s documented behavior is: “The Pattern is matched only against the URL-path — not the hostname, port, or query string.”
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In .htaccess, Apache removes the directory prefix before matching. A file in the document root therefore normally uses ^docs/, not ^/docs/. A leading slash commonly appears in server or virtual-host examples, but copying it into per-directory configuration can prevent the rule from matching.
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Internal rewrite: keep the subdomain in the browser
An internal rewrite changes how Apache locates a resource without sending a redirect response. The visitor continues to see the original subdomain and path, while Apache serves content from another path under the same document root.
Basic subdomain-to-directory rule
RewriteEngine On
RewriteCond %{HTTP_HOST} ^app.example.com$ [NC]
RewriteRule ^(.*)$ app/$1 [L]Place this in the document-root .htaccess. A request for https://app.example.com/docs/page.html is internally mapped to app/docs/page.html. The address bar remains app.example.com/docs/page.html; no 3xx response is sent.
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The host expression is anchored with ^ and $. Without those anchors, a value such as not-app.example.com could satisfy an overly broad expression. [NC] makes the comparison case-insensitive, which is generally appropriate for DNS hostnames.
Preventing a rewrite loop
A catch-all rule can process its own destination on a later per-directory pass. Add exclusions before the catch-all when the destination directory is physically present beneath the document root:
RewriteEngine On
RewriteCond %{HTTP_HOST} ^app.example.com$ [NC]
RewriteRule ^app(?:/|$) - [END]
RewriteCond %{HTTP_HOST} ^app.example.com$ [NC]
RewriteRule ^(.*)$ app/$1 [END]The first rule leaves an already-prefixed app path alone. The hyphen substitution means “no substitution.” [END], available in Apache 2.4, stops all per-directory rewrite processing for the current request. If your configuration must support an environment where only the traditional stop flag is appropriate, [L] stops the current rule set and allows a later per-directory pass to occur.
Another common safeguard is to leave existing files and directories untouched, but use that only when it matches your routing design:
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RewriteEngine On
RewriteCond %{HTTP_HOST} ^app.example.com$ [NC]
RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^ - [END]
RewriteCond %{HTTP_HOST} ^app.example.com$ [NC]
RewriteRule ^(.*)$ app/$1 [END]Here, an existing file or directory is served directly; other paths are mapped into app/. If your application intentionally handles every path, do not add this exception blindly.
External redirect: send the browser to another host
Use an external redirect when the visible URL must change. The substitution must be an absolute URL, and the response status tells clients and caches how to treat the move.
RewriteEngine On
RewriteCond %{HTTP_HOST} ^old.example.com$ [NC]
RewriteRule ^/?(.*)$ https://www.example.com/$1 [R=301,END,NE]This sends requests from old.example.com to the corresponding path on www.example.com. The optional slash in ^/?(.*)$ makes the pattern tolerant of contexts that provide a slash, although a document-root .htaccess pattern normally receives no leading slash. The R=301 flag makes the move permanent; do not use 301 while you are still experimenting with the destination or ordering.
[NE] prevents mod_rewrite from escaping certain characters in the redirect result. It is useful only when you understand the encoding implications. For ordinary redirects, preserving normal URL escaping is safer, so omit [NE] unless a specific character-handling requirement justifies it.
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Temporary testing before a permanent move
Start with a temporary status while validating the rule:
RewriteRule ^/?(.*)$ https://www.example.com/$1 [R=302,END]Check the response with a browser’s network panel or a command-line HTTP client, then change to 301 only after the host, path, HTTPS behavior and exclusions are correct. Browsers cache permanent redirects aggressively, which can make a fixed rule appear broken until you test with a fresh profile or clear the relevant cache.
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Internal rewrite versus external redirect
| Characteristic | Internal rewrite | External redirect |
|---|---|---|
| Browser address | Stays on the requested host and path | Changes to the URL in the Location header |
| Typical substitution | Relative path such as app/$1 |
Absolute URL such as https://www.example.com/$1 |
| Response | Usually a normal 2xx response after internal routing | 3xx response, such as 301 or 302 |
| Host change | No visible host change | Can move the request to another host |
| SEO and caching implication | The requested URL remains canonical from the client’s perspective | A permanent redirect signals that the destination is the new URL; caches may retain it |
| Destination location | Normally remains within the same Apache document root | Can point to another virtual host, server or domain |
| Best use | Serving a subdomain from a dedicated directory without changing its public URL | Canonicalizing a host or permanently moving content |
Choose one behavior deliberately. A rule that internally maps app.example.com into app/ should not also redirect that host to itself. Conversely, a host migration should not rely on an internal rewrite if visitors and search engines must discover the new URL.
Host matching details that prevent subtle failures
Account for a port during local testing
%{HTTP_HOST} can include a non-default port. A strict expression for production traffic may therefore fail against a local request such as app.example.com:8080. If you need both forms, make the port optional:
RewriteCond %{HTTP_HOST} ^app.example.com(?::[0-9]+)?$ [NC]Do not make the host condition broad merely to hide a port mismatch. Keep the hostname anchored and permit only the port forms your test environment requires.
Use condition and rule backreferences correctly
Captures in the preceding RewriteCond are referenced as %1, %2 and so on. Captures from the RewriteRule pattern are referenced as $1, $2 and so on. For example:
RewriteCond %{HTTP_HOST} ^([^.]+).example.com$ [NC]
RewriteRule ^(.*)$ tenants/%1/$1 [END]For blue.example.com/docs, %1 is blue and $1 is docs. Keep the captures narrow; an accidental extra pair of parentheses can change which backreference is populated.
Preserve or replace query strings intentionally
Because the query string is not matched by RewriteRule, a path rewrite normally carries the existing query string forward. If the destination must receive a newly constructed query string, use the appropriate query-string flags and test both requests with and without parameters. Never assume a path capture has also captured ?key=value.
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Choosing .htaccess or virtual-host configuration
| Factor | .htaccess |
VirtualHost/server configuration |
|---|---|---|
| Administrative access | Useful when you control a site directory but not the main Apache configuration | Requires access to the server or virtual-host configuration |
| Scope | Applies to the directory and its descendants; location changes the path seen by the rule | Can apply cleanly to an entire virtual host |
| Performance | Apache checks per-directory configuration during requests | Configuration is loaded centrally and avoids per-directory lookup overhead |
| Maintainability | Convenient for application-level deployment, but rules can be scattered | Centralized and easier to audit when an administrator owns the host |
| Canonical host redirect | Possible with RewriteCond and RewriteRule |
A dedicated Redirect in a <VirtualHost> is the cleaner choice when server access exists |
Apache’s remapping guidance recommends a dedicated virtual-host Redirect when that configuration access is available. Use .htaccess when deployment constraints require it, and document which directory contains the file.
Prerequisites before debugging the expression
- DNS: The subdomain must resolve to the Apache server that will process the request.
- Virtual host: The request must arrive at the intended virtual host and document root. A perfect rule in the wrong virtual host will never produce the expected result.
- Module:
mod_rewritemust be loaded, and the file must containRewriteEngine On. - Override policy: The directory configuration must permit rewrite directives in
.htaccess. An override policy that disallows them can produce a 500 error or make the file ineffective. - Correct location: A document-root file and a file in a subdirectory do not see the same path. Apache strips the prefix for the directory containing the file.
- Target exists: For an internal mapping, verify that the destination directory and application entry point are readable by Apache.
A safe deployment procedure
- Confirm DNS resolution and identify the virtual host handling the subdomain.
- Place a minimal rule in the correct
.htaccess, beginning withRewriteEngine On. - Use an anchored host condition and a narrow test path instead of enabling a broad catch-all immediately.
- For a routing test, use an internal relative substitution and verify that the browser URL remains unchanged.
- For a host move, use a temporary redirect such as 302 and inspect the returned
Locationheader. - Test the root path, a nested path, a nonexistent path, an existing static file, and a request with a query string.
- Check both the intended subdomain and a neighboring hostname to ensure the condition is not overbroad.
- Review the Apache error log after each change, especially if the server returns 500.
- Only after behavior is correct, add exclusions, choose
[L]or[END]deliberately, and change a permanent move to 301 if that is truly intended.
Troubleshooting common symptoms
The rule never matches the subdomain
The usual cause is putting the hostname in RewriteRule. Move that test to RewriteCond %{HTTP_HOST}. Then verify DNS, the selected virtual host and the document-root location of the file.
A pattern copied from another example fails
Remove the leading slash when the rule is in .htaccess. Test ^docs/ rather than ^/docs/. A server-context rule and a per-directory rule receive different path input.
The server returns 500
Read the Apache error log first. Confirm that mod_rewrite is loaded, the directive is allowed by the directory’s override policy, brackets and flags are valid, and the substitution does not contain a malformed escape. A configuration error is more likely than a hostname mismatch when Apache returns 500.
The internal rewrite loops
Inspect whether the destination path also satisfies the catch-all rule. Add a destination exclusion before the broad rule, or stop processing with [END] where Apache 2.4 is available. Also check for another .htaccess or application-level redirect that sends the request back into the same mapping.
The redirect loops
Ensure the destination host no longer satisfies the redirect condition. A rule redirecting to a host that still matches the same expression will repeat indefinitely. Check the order of HTTP-to-HTTPS and host-canonicalization rules as well; each should have a clear condition and a destination that does not trigger the previous rule.
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The redirect points to the wrong path
Inspect the capture boundaries. The rule’s $1 comes from the path pattern, while a condition’s capture uses %1. Confirm whether the file is in the document root or a subdirectory and whether the substitution intentionally includes a trailing slash.
Only some hostnames are affected
Check case handling, optional ports and anchoring. [NC] handles case differences; an optional numeric port handles local testing. Do not replace an anchored hostname with a substring match, because that can catch unrelated subdomains.
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Flags worth understanding
[L]: Stops processing the current rewrite rule set. Apache documents it as causing mod_rewrite to stop processing the rule set.[END]: Stops all per-directory rewrite processing for the current request. It is useful for preventing another.htaccesspass from re-entering a completed mapping and requires Apache 2.4.[R=301]or[R=302]: Sends an external redirect with the selected status. Use 301 only for a permanent move.[NC]: Makes a condition or rule comparison case-insensitive; use it for hostnames when appropriate.[NE]: Suppresses normal escaping in a redirect. Apply it only when you understand the resulting URL encoding.
Reusable patterns
One subdomain mapped to one directory
RewriteEngine On
RewriteCond %{HTTP_HOST} ^docs.example.com$ [NC]
RewriteRule ^(.*)$ documentation/$1 [END]This keeps docs.example.com visible while serving files beneath documentation/.
Several subdomains captured dynamically
RewriteEngine On
RewriteCond %{HTTP_HOST} ^([a-z0-9-]+).example.com$ [NC]
RewriteRule ^(.*)$ sites/%1/$1 [END]This pattern maps each permitted label into a matching directory. Restrict the condition to the labels your application actually supports; otherwise arbitrary hostnames could select unintended directories.
Redirect an old subdomain while retaining the path
RewriteEngine On
RewriteCond %{HTTP_HOST} ^legacy.example.com$ [NC]
RewriteRule ^/?(.*)$ https://new.example.com/$1 [R=301,END]Use this only after validating the new host and all path behavior with a temporary status.
What to record for future maintenance
- The exact
.htaccesslocation and the document root it belongs to. - The hostnames and optional ports the conditions intentionally accept.
- Whether each mapping is internal or external and, for redirects, whether the status is temporary or permanent.
- The expected destination directory and any exclusions placed before a catch-all.
- The Apache version, especially when relying on
[END]. - A small test matrix covering host, path, query string, HTTPS and existing files.
Clear comments near each rule should explain the intended host and destination, not merely repeat the syntax. This makes later HTTP-to-HTTPS or canonical-host changes less likely to create loops.
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