How to force specific group permissions (r/w) on files created in a directory structure?

Question

I have a directory set up on a Linux server such that I'd like all files within it to be readable and writeable for a specific group.

So far, I've done chmod g+s mygroup parent, and I can see that all files inherit this group when created within this directory.

However, it does not force the g+rw permission, and so it is possible for a user to write a file into that directory such that others do not have read and/or write access to it.

How can I achieve this?

Answer 1

Turns out the answer is to use ACLs to set a default:

setfacl -d -m u::rwX,g::rwX,o::- /path/to/parent