diff --git a/Admin/Windows/Cygwin/README b/Admin/Windows/Cygwin/README
--- a/Admin/Windows/Cygwin/README
+++ b/Admin/Windows/Cygwin/README
@@ -1,70 +1,73 @@
 Cygwin
 ======
 
 * https://www.cygwin.com/
 
 * Mirror with many old versions (not setup.ini)
   https://ftp.eq.uc.pt/software/pc/prog/cygwin
 
 * Local snapshots:
   - http://isabelle.in.tum.de/cygwin  (Isabelle2012)
   - http://isabelle.in.tum.de/cygwin_2013  (Isabelle2013)
   - http://isabelle.in.tum.de/cygwin_2013-1  (Isabelle2013-1 and Isabelle2013-2)
   - http://isabelle.in.tum.de/cygwin_2014  (Isabelle2014)
   - http://isabelle.in.tum.de/cygwin_2015  (Isabelle2015)
   - http://isabelle.in.tum.de/cygwin_2016  (Isabelle2016)
   - http://isabelle.in.tum.de/cygwin_2016-1  (Isabelle2016-1)
   - http://isabelle.in.tum.de/cygwin_2017  (Isabelle2017)
   - https://isabelle.sketis.net/cygwin_2018  (Isabelle2018)
   - https://isabelle.sketis.net/cygwin_2019  (Isabelle2019)
   - https://isabelle.sketis.net/cygwin_2020  (Isabelle2020)
+  - https://isabelle.sketis.net/cygwin_2021  (Isabelle2021)
 
 * Apache2 redirects for virtual host isabelle.conf:
 ```
   Redirect /cygwin/release http://ftp.eq.uc.pt/software/pc/prog/cygwin/release
   Redirect /cygwin_2013/release http://ftp.eq.uc.pt/software/pc/prog/cygwin/release
   Redirect /cygwin_2013-1/release http://ftp.eq.uc.pt/software/pc/prog/cygwin/release
   Redirect /cygwin_2013-1/x86/release http://ftp.eq.uc.pt/software/pc/prog/cygwin/x86/release
   Redirect /cygwin_2014/x86/release http://ftp.eq.uc.pt/software/pc/prog/cygwin/x86/release
   Redirect /cygwin_2015/x86/release http://ftp.eq.uc.pt/software/pc/prog/cygwin/x86/release
   Redirect /cygwin_2015/x86_64/release http://ftp.eq.uc.pt/software/pc/prog/cygwin/x86_64/release
   Redirect /cygwin_2016/x86/release http://ftp.eq.uc.pt/software/pc/prog/cygwin/x86/release
   Redirect /cygwin_2016/x86_64/release http://ftp.eq.uc.pt/software/pc/prog/cygwin/x86_64/release
   Redirect /cygwin_2016-1/x86/release http://ftp.eq.uc.pt/software/pc/prog/cygwin/x86/release
   Redirect /cygwin_2016-1/noarch/release http://ftp.eq.uc.pt/software/pc/prog/cygwin/noarch/release
   Redirect /cygwin_2017/x86/release http://ftp.eq.uc.pt/software/pc/prog/cygwin/x86/release
   Redirect /cygwin_2017/x86_64/release http://ftp.eq.uc.pt/software/pc/prog/cygwin/x86_64/release
   Redirect /cygwin_2017/noarch/release http://ftp.eq.uc.pt/software/pc/prog/cygwin/noarch/release
   Redirect /cygwin_2018/x86/release https://ftp.eq.uc.pt/software/pc/prog/cygwin/x86/release
   Redirect /cygwin_2018/x86_64/release https://ftp.eq.uc.pt/software/pc/prog/cygwin/x86_64/release
   Redirect /cygwin_2018/noarch/release https://ftp.eq.uc.pt/software/pc/prog/cygwin/noarch/release
   Redirect /cygwin_2019/x86/release https://ftp.eq.uc.pt/software/pc/prog/cygwin/x86/release
   Redirect /cygwin_2019/x86_64/release https://ftp.eq.uc.pt/software/pc/prog/cygwin/x86_64/release
   Redirect /cygwin_2019/noarch/release https://ftp.eq.uc.pt/software/pc/prog/cygwin/noarch/release
   Redirect /cygwin_2020/x86_64/release https://ftp.eq.uc.pt/software/pc/prog/cygwin/x86_64/release
   Redirect /cygwin_2020/noarch/release https://ftp.eq.uc.pt/software/pc/prog/cygwin/noarch/release
+  Redirect /cygwin_2021/x86_64/release https://ftp.eq.uc.pt/software/pc/prog/cygwin/x86_64/release
+  Redirect /cygwin_2021/noarch/release https://ftp.eq.uc.pt/software/pc/prog/cygwin/noarch/release
 ```
 
 * Quasi-component: "isabelle build_cygwin" (as Administrator)
 
 * SSH server:
 
   - run Cygwin terminal as Administrator
 
   - run ssh-host-config
 ```
     StrictMode: yes
     privilege separation: yes
     new local account 'sshd': yes
     sshd as a service: yes
     different name (than cyg_server): no
     new privileged account cyg_server: yes
 ```
 
   - user setup
 ```
     mkpasswd -l > /etc/passwd
     mkgroup -l > /etc/group
 
     passwd -R USER
 ```