Can't write to known_hosts I reinstalled my system and recovered the known_hosts file with Time Machine. And now I have no longer permission to write in said file.

I have set Read &amp; Write for me,staff and everybody on said file. But that doesn't change anything.

What could be the problem there?

Output: 

-rwxrwxrwx@ 1 username  staff  17773 14 Mär 14:44 known_hosts


Output 2:

-rwxrwxrwx@ 1 mtdesign  staff  17773 14 Mär 14:44 known_hosts
com.apple.TextEncoding     11 
com.apple.finder.copy.source.checksum#N     4 
com.apple.metadata:_kTimeMachineNewestSnapshot     50 
com.apple.metadata:_kTimeMachineOldestSnapshot     50 
com.apple.quarantine       23
Given that the file has extended attributes and on a fresh install doesn't I'd start by removing them, xattr -c ~/.ssh/known_hosts. 

When you say you can't write to the file, do you mean directly or via ssh adding a host? I'd try cat ~/.ssh/known_hosts to make sure you can open it. Then you might try cp ~/.ssh/known_hosts ~/Documents/known_hosts and echo "Delete this line" &gt;&gt; ~/.ssh/known_hosts to see how that goes.