Connection to github.com closed by remote host

Question

Problem is exactly as reported in

• HPC ssh "connection closed by remote host"
• Connection to host closed by remote host. Exit status -1

But it is not the "ssh_exchange_identification:" error, i.e., it's different than the ssh server: reasons for sudden "Connection closed by remote host" one.

I'm getting it while doing ssh -T:

$ ssh -T git@github.com
Connection to github.com closed by remote host.

The following message is generated when using ssh -vvv

debug2: channel 0: request shell confirm 1
debug3: send packet: type 98
debug2: channel_input_open_confirmation: channel 0: callback done
debug2: channel 0: open confirm rwindow 32000 rmax 35000
debug3: send packet: type 1
debug1: channel 0: free: client-session, nchannels 1
debug3: channel 0: status: The following connections are open:
  #0 client-session (t4 r43 i0/0 o0/0 fd 5/6 cc -1)
Connection to github.com closed by remote host.

I've put the the full log (and diff with normal) to: https://www.diffchecker.com/LF2ZEb8j

For ssh -vvT git@github.com, I've put the the log diff to:
https://www.diffchecker.com/WjmA7P0c
from which there isn't much revealing.

Once again, the above problem is from (connecting from) my remote server, of version 1:7.6p1-4ubuntu0.7, which has been working for years, but have now broken for a few weeks. Testing/connecting from my home machine (ver: 1:8.4p1-5) always OK.

Comments?
Is it that my openssh-client 1:7.6p1-4ubuntu0.7 is too old for github.com?

$ apt-cache policy openssh-client
openssh-client:
  Installed: 1:7.6p1-4ubuntu0.7
  Candidate: 1:7.6p1-4ubuntu0.7
  Version table:
 *** 1:7.6p1-4ubuntu0.7 500
        500 http://azure.archive.ubuntu.com/ubuntu bionic-updates/main amd64 Packages
        100 /var/lib/dpkg/status
     1:7.6p1-4ubuntu0.5 500
        500 http://security.ubuntu.com/ubuntu bionic-security/main amd64 Packages 
     1:7.6p1-4 500
        500 http://azure.archive.ubuntu.com/ubuntu bionic/main amd64 Packages

Answer 1

This worked for me

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.com to run through that server and port.

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

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

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

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

Using SSH over the HTTPS port

git clone ssh://git@ssh.github.com:443/USER_NAME/REPO_NAME.git

Answer 2

Is it that my openssh-client 1:7.6p1-4ubuntu0.7 is too old for github.com?

I had an idea to verify right after I posted my question, and It turns out that the answer is YES!

Here is how I tested/proved it

• Install debian:latest docker image
• Do ssh -T git@github.com within the debian:latest docker image

So all else are the same except for the openssh-client version, and it works within the debian:latest docker container.