Presence of sshd_config mandatory

[Luke A. Perkins]

OpenSSH 7.9p1
Ubuntu 18..04.2 (LTS)

If a sshd daemon is started with a "-f my_sshd_config" command-line specification the sshd process still requires the existence of the sshd_config. The bug is somewhere in one of the forked processes started when an external client attempts a connection. Since the sshd_config file does not exists, an external client connection cannot be started.

Solution: If I "touch /etc/ssh/sshd_config" then the lock-up issue goes away and I am successfully able to login.

Bug: If the sshd configuration file is specified in the command line execution of the daeomon, then this should be the only file that should be required.

[Bryce Harrington (bryce)]

Hi Luke, thanks for reporting this issue. Unfortunately, I am not able to reproduce it.

Ubuntu 18.04.2 ships openssh-server 1:7.6p1-4ubuntu0.3, not 7.9. Is the version typo'd in the report or are you running a self-installed version of openssh perhaps?

In any case, I attempted reproduction on both 18.04 (bionic) and 19.10 (eoan) in lxc, but was not able to reproduce a fault. If you could, please provide an exact set of commands to reproduce, and the error message you encountered. Please also verify you're running the stock distro version of openssh when doing - reproducing in a clean vanilla lxc instance or a fresh installation of ubuntu would be helpful.

$ lxc launch ubuntu:18.04/amd64 lp1834129
$ lxc exec lp1834129 bash
# apt-get update
[...]
# ps aux | grep sshd
root 685 0.0 0.0 72296 5660 ? Ss 22:31 0:00 /usr/sbin/sshd -D
root 687 0.0 0.0 14856 1008 ? S+ 22:31 0:00 grep --color=auto sshd
# service sshd stop
# ps aux | grep sshd
root 718 0.0 0.0 14856 1116 ? S+ 22:31 0:00 grep --color=auto sshd
# ls -l /run/ssh*
ls: cannot access '/run/ssh*': No such file or directory
# touch my_sshd_config
# sshd -f ./my_sshd_config
sshd re-exec requires execution with an absolute path
# /usr/sbin/sshd -f ./my_sshd_config
Missing privilege separation directory: /run/sshd
# mkdir /run/sshd
# /usr/sbin/sshd -f ./my_sshd_config
# ps aux | grep sshd
root 725 0.0 0.0 72296 3344 ? Ss 22:31 0:00 /usr/sbin/sshd -f ./my_sshd_config
root 727 0.0 0.0 14856 1040 ? S+ 22:31 0:00 grep --color=auto sshd
# kill 725
# !ps
ps aux | grep sshd
root 729 0.0 0.0 14856 1004 ? S+ 22:32 0:00 grep --color=auto sshd
# mv /etc/ssh/sshd_config /tmp/
# /usr/sbin/sshd -f ./my_sshd_config
# !ps
ps aux | grep sshd
root 732 0.0 0.0 72296 3200 ? Ss 22:32 0:00 /usr/sbin/sshd -f ./my_sshd_config
root 734 0.0 0.0 14856 1156 ? S+ 22:32 0:00 grep --color=auto sshd
# ls -l /etc/ssh/sshd_*
ls: cannot access '/etc/ssh/sshd_*': No such file or directory

# passwd ubuntu
Enter new UNIX password:
Retype new UNIX password:
passwd: password updated successfully
# su ubuntu
To run a command as administrator (user "root"), use "sudo <command>".
See "man sudo_root" for details.

$ ssh localhost
ubuntu@localhost's password:
To run a command as administrator (user "root"), use "sudo <command>".
See "man sudo_root" for details.
$ logout
Connection to localhost closed.

$ . /etc/lsb-release && echo ${DISTRIB_DESCRIPTION}
Ubuntu 18.04.2 LTS
$ ssh -V
OpenSSH_7.6p1 Ubuntu-4ubuntu0.3, OpenSSL 1.0.2n 7 Dec 2017
$ apt-cache policy openssh-server | grep Installed
  Installed: 1:7.6p1-4ubuntu0.3

[Luke A. Perkins (public-a)]

I was able to confirm that this was not a bug. I did a fresh build of 18.04.2 LTS server, and it appears to work just fine.

My bad.

[Marc Deslauriers (mdeslaur)]

Thanks for updating the bug! I'll close it now.