HTTPS with redirection to other domain with apache virtual host

Question

I would like to redirect a virtual host on my server to another domain, which is running on HTTPS. I also would like to only show the original url, hence using the P flag for proxy. Here is the current configuration :

RewriteEngine on
SSLProxyEngine on
RewriteCond %{HTTP_HOST} ^subdomain1\.domain1\.ext1$ [NC]
RewriteRule ^(.*) https://subdomain2.domain2.ext2$1 [L,R,P]

Should I generate a certificate on domain1 with certbot? What webroot should I associate? Should I include the one from domain2?

Currently, I have this in the error.log:

[Wed Jun 27 09:13:42.011549 2018] [ssl:error] [pid 19805] [remote IP2:443] AH01961: SSL Proxy requested for domain1.ext1:80 but not enabled [Hint: SSLProxyEngine]
[Wed Jun 27 09:13:42.011734 2018] [proxy:error] [pid 19805] AH00961: HTTPS: failed to enable ssl support for IP2:443 (subdomain2.domain2.ext2)

However SSLProxyEngine is set.

Answer 1

Finally, the best solution was to use mod_proxy instead of mod-rewrite.

The http version (redirecting to https)

<VirtualHost *:80>
    ServerName domain1.ext1
    ServerAlias subdomain1.domain1.ext1

    SSLProxyEngine on
    ProxyPass / https://subdomain2.domain2.ext2/
    ProxyPassReverse / https://subdomain2.domain2.ext2/
    ProxyPreserveHost Off

    RewriteEngine on
    RewriteCond %{SERVER_NAME} =subdomain1.domain1.ext1
    RewriteRule ^ https://%{SERVER_NAME}%{REQUEST_URI} [END,QSA,R=permanent]
</VirtualHost>

The https version

<IfModule mod_ssl.c>
<VirtualHost *:443>
    ServerName domain1.ext1
    ServerAlias subdomain1.domain1.ext1

    SSLProxyEngine on
    ProxyPass / https://subdomain2.domain2.ext2/
    ProxyPassReverse / https://subdomain2.domain2.ext2/
    ProxyPreserveHost Off

    SSLCertificateFile /etc/letsencrypt/live/subdomain1.domain1.ext1/fullchain.pem
    SSLCertificateKeyFile /etc/letsencrypt/live/subdomain1.domain1.ext1/privkey.pem
    Include /etc/letsencrypt/options-ssl-apache.conf
</VirtualHost>
</IfModule>