I have a drupal website. I want to generate an Iframe with content from my drupal site, that other sites can embed.
How I think this can be achieved:
you can use this module https://www.drupal.org/project/entity_iframe that allows you to create IFRAME READY webpages
install it and go to the display settings of you content type that you want to use as iframe content admin/structure/types/manage/CONTENTTYPE/display
choose the IFRAME view mode and choose the fields you would like to be shown
and then use url like this domain.com/entity_iframe/node/NID
and you will have a display with no extra headers footers etc ...
By default a sample EMBED/IFRAME code is provided to the admin under each node the settings
The settings in admin/config/system/entity_iframe
control some of the details of the embed code
For complete control of the theme used you can use in combination with https://www.drupal.org/project/entity_iframe_theme