ssh/config with proxy environment switch.

ssh-config

Host *
  StrictHostKeyChecking no
  ServerAliveInterval 60
  FingerprintHash md5
  VisualHostKey yes
  ControlPath /tmp/%r@%h:%p
  IdentityFile ~/.ssh/id_rsa

Host github.com
  ControlMaster auto
  ControlPersist yes

Match Host github.com exec "env | fgrep -q HTTPS_PROXY"
  User git
  HostName ssh.github.com
  Port 443
  ProxyCommand /usr/bin/nc -x 127.0.0.1:3128 -X connect %h %p
  
Host bastion 35.201.4.241
  HostName 35.201.4.241
  Port 22

Host 10.0.0.*
  ProxyJump bastion

The magic I'm trying to achieve is to use a tunnel through a proxy only if the proxy env var is set. Assuming the proxy refuses to connect to non-http ports, github provides ssh on port 443 if you connect to the alternative host ssh.github.com

P.S. "nc" does the job that we used to use corkscrew for....

P.P.S. has ProxyJump been around for long? skips the classic ProxyCommand ssh -W %h:%p -q bastion