Inexplicable EOF

[David Newall]

Hi All,

I'm seeing something which I would not believe could happen had I not 
seen it for myself.  It's intermittent and thus not easily repeatable.

On client, I ran: "ssh -i cert -x -T -oBatchMode=yes example.com 
batchjob" and ssh exited cleanly, immediately, and displaying no 
output.  When I saw it, I repeated the command, with the same results, a 
few times.

On example.com, cert.pub is installed in .ssh/authorized_keys. Batchjob 
is executable and in PATH, and starts (after #! /bin/sh) with "echo 
"`date`: $0 $*" >> /tmp/log".  Nothing appeared /tmp/log for those 
occassions when client's ssh exited cleanly, immediately and without 
displaying output.  Nothing appeared in /var/log/syslog to indicate that 
something was awry.

The one clue that I can think of, if it is a clue, is that I have two 
DNS A records published for example.com, one for the primary instance 
and the other for a hot-backup.  Both are virtual servers hosted on 
different Linux servers.  The primary is running and the hot-backup is 
not.  DRBD is used to keep the hot-backup synchronised with the 
primary.  The idea is that I can switch server roles without needing to 
reconfigure client.  My experience is that openssh tries all A records 
in sequence until it connects and that this is a viable strategy.

I feel that openssh would print something if it was unable to get a 
connection, and, that having got a connection, example.com would print 
something to /tmp/log.  For ssh to cleanly exit with nothing output and 
nothing logged stumps me.

Example.com is x64 architecture running Ubuntu 16.04.5.  It's reasonably 
up-to-date except for openssh-server, which is 1:7.2p2-4ubuntu2.4.

Client is Raspberry Pi 3B+ running Raspbian 9.4.  Openssh-client is 
1:7.4p1-10+deb9u3.

Both machines have plenty of free memory.

It's uncommon for me to reach out for ideas, but I'm reaching out now.  
Any ideas?

Thanks in advance.

David