.htaccess redirect to www.domain.com also redirects subdomains to www.subdomain.domain.com

This is a configuration issue on your drupal-core/sub and drupal-core. Do you have your webserver pointing to to the correct root directory. Apache2 would be something like /var/www/drupal-core/.htaccess then sub would be var/www/drupal-core/sub/.htaccess unless your on a multi-site.

Either way your missing somthing. htaccess files should only effect a single install. Unless multi-site. Also you may want to look at even why if you are setting drupal-core/sub and not just create multiple subs as subs. /var/www/sub0.example.com

/var/www/example.com

/var/www/sub1.example.com :)

What is the root directory path of your drupal-site && and what is the root directory path of drupal-site-subdomain. :)

!indeed this is your issue. Are you on CentOS? You will need to set the root directory path to be absolute. I would guess that drupal thinks there is 2 installs in the same directories.

Not sure of your setup, but you can set roots in cpanel. And or apache2/sites-available

/home/domain/public_html
/home/subdomain/public_html

Above paths are recommended by us due to additional issues while developing. One being late nights and forgetting which directory your in :)

I'm having the same problem on all versions of Drupal 7.

I often set up beta sites that live in /var/www/beta along side my live sites that live in /var/www/html. Both sites are running off the same code base (same git repo).
If my beta sites have the domain name beta.mydomain.com and the default rewrite rule is uncommented in .htaccess. I get redirected to www.beta.mydomain.com in the beta site. This should not be the case.

I do not believe it is a server configuration issue, as it happens on all my servers (centOS and nginx) and on my localhost (mamp). I've been solving the problem for years by commenting out the Drupal 7 rewrite rules in .htaccess and copying in the ones from Drupal 6.

Can't we solve this issue with a smarter Rule in .htaccess?

update

Here are the rewrite rules in Drupal 6 that did not have the subdomain problem:

  # RewriteCond %{HTTP_HOST} ^example\.com$ [NC]
  # RewriteRule ^(.*)$ http://www.example.com/$1 [L,R=301]

And here are the problematic rewrite rules in Drupal 7:

  # RewriteCond %{HTTP_HOST} !^www\. [NC]
  # RewriteRule ^ http://www.%{HTTP_HOST}%{REQUEST_URI} [L,R=301]

This seems to work for me locally:

RewriteCond %{HTTP_HOST} ^[^.]+\.[^.]+$
RewriteRule ^(.*)$ http%{ENV:protossl}://www.%{HTTP_HOST}/$1 [L,R=301]

Here are the .htaccess rewrite rules for Drupal 7, add below two lines of code after " RewriteEngine on" under "":

RewriteCond %{HTTP_HOST} ^domain\.com$ [NC]
RewriteRule ^(.*)$ http://www.domain.com/$1 [R=301,L]

I have a similar issue after move. Not sure if they are related since it was working for several years but not working with newer versions of apache and php.
https://www.drupal.org/node/2874935

The rule mentioned by kumaratulesh in .htaccess is not working somehow which is very odd, despite mod_rewrite and other rules in htaccess working.

Closing this as designed now, as it's an individual configuration step and there are several comments pointing out how it can be solved. Not a bug in core.