How to htaccess-redirect single directory to external URL, while URL-masking

Question

There are many similar questions here, but none seem to be exactly the same, and something is tripping me up. I have one specific internal URL, mydomain.com/a-directory, that I want to forward to the external URL https://mailchi.mp/mydomain.com/a-directory , while still keeping the browser's shown URL as mydomain.com/a-directory.

I have tried:

<IfModule mod_rewrite.c>

RewriteEngine On
RewriteCond %{HTTP_HOST} ^mydomain\.com/a-directory$
RewriteRule ^(.*) https://mailchi.mp/mydomain.com/a-directory [P]  

</IfModule>

And also:

<IfModule mod_rewrite.c>

RewriteEngine On
RewriteRule ^a-directory$ https://mailchi.mp/mydomain.com/a-directory [P]

</IfModule>

Apache mod_proxy is installed. Edit: Both seem to be mostly ignored, as no forwarding is happening, and it's not crashing. The browser is just saying that the page can't be found, as www.httpmydomain.dom/a-directory doesn't exist.

Edit: The .htaccess file contains the following.

AuthType Basic
AuthName "Restricted access"
AuthUserFile /home/cpanelUser/.htpasswd
Require valid-user


#
# Apache/PHP/Drupal settings:
#

# Protect files and directories from prying eyes.
<FilesMatch "\.(engine|inc|info|install|make|module|profile|test|po|sh|.*sql|theme|tpl(\.php)?|xtmpl)(~|\.sw[op]|\.bak|\.orig|\.save)?$|^(\.(?!well-known).*|Entries.*|Repository|Root|Tag|Template|composer\.(json|lock))$|^#.*#$|\.php(~|\.sw[op]|\.bak|\.orig\.save)$">
  <IfModule mod_authz_core.c>
    Require all denied
  </IfModule>
  <IfModule !mod_authz_core.c>
    Order allow,deny
  </IfModule>
</FilesMatch>

# Don't show directory listings for URLs which map to a directory.
Options -Indexes

# Follow symbolic links in this directory.
Options +FollowSymLinks

# Make Drupal handle any 404 errors.
ErrorDocument 404 /index.php

# Set the default handler.
DirectoryIndex index.php index.html index.htm

# Override PHP settings that cannot be changed at runtime. See
# sites/default/default.settings.php and drupal_environment_initialize() in
# includes/bootstrap.inc for settings that can be changed at runtime.

# PHP 5, Apache 1 and 2.
<IfModule mod_php5.c>
  php_flag magic_quotes_gpc                 off
  php_flag magic_quotes_sybase              off
  php_flag register_globals                 off
  php_flag session.auto_start               off
  php_value mbstring.http_input             pass
  php_value mbstring.http_output            pass
  php_flag mbstring.encoding_translation    off
</IfModule>

# Requires mod_expires to be enabled.
<IfModule mod_expires.c>
  # Enable expirations.
  ExpiresActive On

  # Cache all files for 2 weeks after access (A).
  ExpiresDefault A1209600

  <FilesMatch \.php$>
    # Do not allow PHP scripts to be cached unless they explicitly send cache
    # headers themselves. Otherwise all scripts would have to overwrite the
    # headers set by mod_expires if they want another caching behavior. This may
    # fail if an error occurs early in the bootstrap process, and it may cause
    # problems if a non-Drupal PHP file is installed in a subdirectory.
    ExpiresActive Off
  </FilesMatch>
</IfModule>

# Various rewrite rules.
<IfModule mod_rewrite.c>
  RewriteEngine on

  RewriteCond %{HTTP_HOST} ^mydomain\.com/a-directory$
  RewriteRule ^(.*) https://mailchi.mp/mydomain.com/a-directory [P] 

  # Set "protossl" to "s" if we were accessed via https://.  This is used later
  # if you enable "www." stripping or enforcement, in order to ensure that
  # you don't bounce between http and https.
  RewriteRule ^ - [E=protossl]
  RewriteCond %{HTTPS} on
  RewriteRule ^ - [E=protossl:s]

  # Make sure Authorization HTTP header is available to PHP
  # even when running as CGI or FastCGI.
  RewriteRule ^ - [E=HTTP_AUTHORIZATION:%{HTTP:Authorization}]

  # Block access to "hidden" directories whose names begin with a period. This
  # includes directories used by version control systems such as Subversion or
  # Git to store control files. Files whose names begin with a period, as well
  # as the control files used by CVS, are protected by the FilesMatch directive
  # above.
  #
  # NOTE: This only works when mod_rewrite is loaded. Without mod_rewrite, it is
  # not possible to block access to entire directories from .htaccess, because
  # <DirectoryMatch> is not allowed here.
  #
  # If you do not have mod_rewrite installed, you should remove these
  # directories from your webroot or otherwise protect them from being
  # downloaded.
  RewriteRule "/\.|^\.(?!well-known/)" - [F]

  # If your site can be accessed both with and without the 'www.' prefix, you
  # can use one of the following settings to redirect users to your preferred
  # URL, either WITH or WITHOUT the 'www.' prefix. Choose ONLY one option:
  #
  # To redirect all users to access the site WITH the 'www.' prefix,
  # (http://example.com/... will be redirected to http://www.example.com/...)
  # uncomment the following:
  RewriteCond %{HTTP_HOST} .
  RewriteCond %{HTTP_HOST} !^www\. [NC]
  RewriteRule ^ http%{ENV:protossl}://www.%{HTTP_HOST}%{REQUEST_URI} [L,R=301]

  # 2 Redirect to HTTPS
  RewriteCond %{HTTPS} off
  RewriteCond %{HTTP:X-Forwarded-Proto} !https
  RewriteRule ^(.*)$ https://%{HTTP_HOST}%{REQUEST_URI} [L,R=301] 

  #
  # To redirect all users to access the site WITHOUT the 'www.' prefix,
  # (http://www.example.com/... will be redirected to http://example.com/...)
  # uncomment the following:
  # RewriteCond %{HTTP_HOST} ^www\.(.+)$ [NC]
  # RewriteRule ^ http%{ENV:protossl}://%1%{REQUEST_URI} [L,R=301]

  # Modify the RewriteBase if you are using Drupal in a subdirectory or in a
  # VirtualDocumentRoot and the rewrite rules are not working properly.
  # For example if your site is at http://example.com/drupal uncomment and
  # modify the following line:
  # RewriteBase /drupal
  #
  # If your site is running in a VirtualDocumentRoot at http://example.com/,
  # uncomment the following line:
  # RewriteBase /

  # Pass all requests not referring directly to files in the filesystem to
  # index.php. Clean URLs are handled in drupal_environment_initialize().

  RewriteCond %{REQUEST_FILENAME} !-f
  RewriteCond %{REQUEST_FILENAME} !-d
  #RewriteCond %{REQUEST_URI} !^blog2
  RewriteCond %{REQUEST_URI} !=/phpinfo.php
  RewriteCond %{REQUEST_URI} !=/favicon.ico

  RewriteRule ^ index.php [L]
  
  # Rules to correctly serve gzip compressed CSS and JS files.
  # Requires both mod_rewrite and mod_headers to be enabled.
  <IfModule mod_headers.c>
    # Serve gzip compressed CSS files if they exist and the client accepts gzip.
    RewriteCond %{HTTP:Accept-encoding} gzip
    RewriteCond %{REQUEST_FILENAME}\.gz -s
    RewriteRule ^(.*)\.css $1\.css\.gz [QSA]

    # Serve gzip compressed JS files if they exist and the client accepts gzip.
    RewriteCond %{HTTP:Accept-encoding} gzip
    RewriteCond %{REQUEST_FILENAME}\.gz -s
    RewriteRule ^(.*)\.js $1\.js\.gz [QSA]

    # Serve correct content types, and prevent mod_deflate double gzip.
    RewriteRule \.css\.gz$ - [T=text/css,E=no-gzip:1]
    RewriteRule \.js\.gz$ - [T=text/javascript,E=no-gzip:1]

    <FilesMatch "(\.js\.gz|\.css\.gz)$">
      # Serve correct encoding type.
      Header set Content-Encoding gzip
      # Force proxies to cache gzipped & non-gzipped css/js files separately.
      Header append Vary Accept-Encoding
    </FilesMatch>
  </IfModule>
</IfModule>

# Add headers to all responses.
<IfModule mod_headers.c>
  # Disable content sniffing, since it's an attack vector.
  Header always set X-Content-Type-Options nosniff
</IfModule>
RewriteCond %{HTTP_HOST} ^anotherAliasDomain\.com$ [OR]
RewriteCond %{HTTP_HOST} ^www\.anotherAliasDomain\.com$
RewriteRule ^/?$ "https\:\/\/www\.mydomain\.com\/" [R=301,L]

# php -- BEGIN cPanel-generated handler, do not edit
# Set the “ea-php72†package as the default “PHP†programming language.
<IfModule mime_module>
  AddHandler application/x-httpd-ea-php72 .php .php7 .phtml
</IfModule>
# php -- END cPanel-generated handler, do not edit

Any help will be greatly appreciated.

Answer 1

RewriteCond %{HTTP_HOST} ^example\.com/a-directory$
RewriteRule ^(.*) https://mailchi.mp/example.com/a-directory [P] 

I have modified it to: RewriteCond ^https://www\.example\.com/a-directory$

Whilst the code is in the correct place (although it is before the HTTP to HTTPS redirect, so you might be trying to proxy an HTTP request), you are not using the correct directives (although the 2nd code block in your question is correct, providing the two domains point to different servers).

Neither of the above two conditions (RewriteCond directives) make much sense. The 2nd condition above is syntactically invalid as you are missing the first (or second?) argument.

The HTTP_HOST server variable contains the value of the Host HTTP request header. eg. If you request http://example.com/a-directory then HTTP_HOST contains example.com. However, if you request http://www.example.com/a-directory then HTTP_HOST will contain www.example.com.

So, if you are requesting http://example.com/a-directory then the directive you should be using is as per your second code block:

RewriteRule ^a-directory$ https://mailchi.mp/example.com/a-directory [P]

If you specifically need to check the requested host then:

RewriteCond %{HTTP_HOST} ^example\.com [NC]
RewriteRule ^a-directory$ https://mailchi.mp/example.com/a-directory [P]

However, assuming the target server allows you to proxy requests via this URL (they could block you) then you may still need additional directives in your server config / virtualhost to correctly configure the reverse proxy. Notably the ProxyPassReverse directive. Without this, if the target server issued an HTTP redirect then the user would be redirected away from your domain (example.com).