github 突然报 Connection closed？

今天 git pull 突然报错连不上了。

Connection reset by 20.205.243.166 port 22
fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.

发现是.ssh下多了known_hosts.old文件，删除即可，具体为啥，我也不清楚