usage of comma in date

docs/English/index.md

@@ -1,3 +1,9 @@
+## 2020.12.17
+
+- [No comma is used between the month and the year when they are the only two elements in the date.](https://www.thepunctuationguide.com/comma.html)
+    - The store closed its doors for good in October 1958.
+    - ~~The store closed its doors for good in October, 1958.~~
+
 ## 2020.12.16
 
 - it is constantly generating electrons that could neutralize free **radicals (自由基)**, acting as **antioxidants (抗氧化剂)**.
@@ -21,7 +27,7 @@
 
 ## 2020.12.11
 
-- As a result, the **up-front** cost for education is real, large and easy to measure.
+- As a result, the **up-front (预付的，先期的)** cost for education is real, large and easy to measure.
 
 ## 2020.12.10
 

docs/Linux/servers.md

@@ -134,4 +134,44 @@ PGFPlotsX.enable_interactive(false)
 
 可以在 `.local/share/applications/mimeapps.list` 里面添加或者修改 
 
-虽然最后还是感觉通过服务器打开速度太慢了。
+虽然最后还是感觉通过服务器打开速度太慢了。
+
+## cluster 的 ssh 突然要密码了
+
+像往常一样 ssh，但是报错了
+
+```bash
+@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
+@    WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED!     @
+@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
+IT IS POSSIBLE THAT SOMEONE IS DOING SOMETHING NASTY!
+Someone could be eavesdropping on you right now (man-in-the-middle attack)!
+It is also possible that a host key has just been changed.
+The fingerprint for the ECDSA key sent by the remote host is
+SHA256:eSpztdqzLF6rBXRWd8pCW0v4utoE5CYTUHTaUb0Qn0w.
+Please contact your system administrator.
+Add correct host key in /home/weiya/.ssh/known_hosts to get rid of this message.
+Offending ECDSA key in /home/weiya/.ssh/known_hosts:42
+  remove with:
+  ssh-keygen -f "/home/weiya/.ssh/known_hosts" -R "chpc-login01.itsc.cuhk.edu.hk"
+ECDSA host key for chpc-login01.itsc.cuhk.edu.hk has changed and you have requested strict checking.
+Host key verification failed.
+```
+
+于是根据提示运行了
+
+```bash
+ssh-keygen -f "/home/weiya/.ssh/known_hosts" -R "chpc-login01.itsc.cuhk.edu.hk"
+```
+
+然后重新 ssh，但还是要求输入密码。
+
+这其实对应了服务器上 `/etc/ssh` 文件夹下几个 pub 文件，咨询 Michael 也得到回复说最近 public fingerprint 有修改，这应该是 known hosts 的内容。
+
+可以[以 MD5 的形式展示](https://superuser.com/questions/421997/what-is-a-ssh-key-fingerprint-and-how-is-it-generated)，
+
+```bash
+$ ssh-keygen -l -E md5 -f ssh_host_ed25519_key.pub
+```
+
+另外 `ssh -v` 可以显示连接细节。