dürfen: to be free to do something, to be permitted to do something
nicht dürfen: to be forbidden
müssen: to have to, must
Usually, that’s correct.
Es muss … sein, on the other hand, expresses
an (strong) assumption or opinion, not a clear statement.
Es muss lustig sein, in fremde Länder zu reisen.
roughly means
I’m sure it’s fun to travel other countries
but the speaker doesn’t know for real,
eg. because he/she never travelled to other countries.
A bit weaker is Es müsste … sein, ie. the person is not 100 % sure that the statement holds.
And Es dürfte … sein is even weaker and can imply a question if the other person thinks of it as true.
Note that while muss, müsste and dürfte can be used that way, it’s not possible with darf.
Es darf lustig sein, in fremde Länder zu reisen.
means that it’s allowed to have fun while travelling (well, of course)