Sometimes, firewalls refuse to allow SSH connections entirely. If using HTTPS cloning with credential caching is not an option, you can attempt to clone using an SSH connection made over the HTTPS port. Most firewall rules should allow this, but proxy servers may interfere.

GitHub Enterprise users: Accessing GitHub Enterprise via SSH over the HTTPS port is currently not supported.

To test if SSH over the HTTPS port is possible, run this SSH command:

ssh -T -p 443 git@ssh.github.com
Hi username! You've successfully authenticated, but GitHub does not
provide shell access.

If that worked, great! If not, you may need to follow our troubleshooting guide.

Enabling SSH connections over HTTPS

If you are able to SSH into git@ssh.github.com over port 443, you can override your SSH settings to force any connection to GitHub to run though that server and port.

To set this in your ssh config, edit the file at ~/.ssh/config, and add this section:

Host github.com
  Hostname ssh.github.com
  Port 443

You can test that this works by connecting once more to GitHub:

ssh -T git@github.com
Hi username! You've successfully authenticated, but GitHub does not
provide shell access.

原文地址：https://help.github.com/articles/using-ssh-over-the-https-port/