Cannot install sshd (openssh-server) on debian 10.4

Question

This is a fresh install of Debian-10.4. The sources.list file follows:

# deb cdrom:[Debian GNU/Linux 10.4.0 Buster - Official amd64 DVD Binary-1 20200 0509-10:26]/ buster contrib main

deb http://security.debian.org/debian-security buster/updates main contrib
deb-src http://security.debian.org/debian-security buster/updates main contrib

# buster-updates, previously known as 'volatile'
# A network mirror was not selected during install.  The following entries
# are provided as examples, but you should amend them as appropriate
# for your mirror of choice.
#
deb http://deb.debian.org/debian/ buster-updates main contrib
deb-src http://deb.debian.org/debian/ buster-updates main contrib

When I try to install sshd (openssh-serverr) I get this:

[root@smb4-a ~]# apt install -f openssh-server
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 openssh-server : Depends: openssh-client (= 1:7.9p1-10+deb10u1) but 1:7.9p1-10+deb10u2 is to be installed
                  Depends: openssh-sftp-server but it is not going to be installed
E: Unable to correct problems, you have held broken packages.

I am not even going to try to imagine what sort of distribution does not supply a secure remote access as part of the base install. Whatever the reasoning that appears to be the case with Debian and I need such. The problem appears to be this:

 openssh-server : Depends: openssh-client (= 1:7.9p1-10+deb10u1) but 1:7.9p1-10+deb10u2 is to be installed

How is a broken dependency forced to install?

Further info:

[root@smb4-a ~]# apt-get update
Get:1 http://deb.debian.org/debian buster-updates InRelease [49.3 kB]
Hit:2 http://security.debian.org/debian-security buster/updates InRelease
Get:3 http://deb.debian.org/debian buster-updates/main Sources [3,088 B]
Get:4 http://deb.debian.org/debian buster-updates/main amd64 Packages [7,380 B]
Get:5 http://deb.debian.org/debian buster-updates/main Translation-en [5,166 B]
Fetched 64.9 kB in 0s (358 kB/s)           
Reading package lists... Done
[root@smb4-a ~]# apt-get upgrade
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Calculating upgrade... Done
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.

Further information:

# apt policy openssh-*
openssh-sftp-server:
  Installed: (none)
  Candidate: 1:7.9p1-10+deb10u1
  Version table:
     1:7.9p1-10+deb10u1 500
        500 http://security.debian.org/debian-security buster/updates/main amd64 Packages
openssh-tests:
  Installed: (none)
  Candidate: 1:7.9p1-10+deb10u1
  Version table:
     1:7.9p1-10+deb10u1 500
        500 http://security.debian.org/debian-security buster/updates/main amd64 Packages
openssh-server:
  Installed: (none)
  Candidate: 1:7.9p1-10+deb10u1
  Version table:
     1:7.9p1-10+deb10u1 500
        500 http://security.debian.org/debian-security buster/updates/main amd64 Packages
openssh-client:
  Installed: 1:7.9p1-10+deb10u2
  Candidate: 1:7.9p1-10+deb10u2
  Version table:
 *** 1:7.9p1-10+deb10u2 100
        100 /var/lib/dpkg/status
     1:7.9p1-10+deb10u1 500
        500 http://security.debian.org/debian-security buster/updates/main amd64 Packages

Answer 1

It seems that during the install, you answered "No" to the question "Use a network mirror?", and so your system left configured without a mirror (or maybe you didn't have a network connection during install). This, by the way, is indicated by your sources.list:

# A network mirror was not selected during install.  The following entries
# are provided as examples, but you should amend them as appropriate
# for your mirror of choice.

So now you have an incomplete sources.list, which contains updates and security fixes, but doesn't contain the base system. I don't know how the DVD line got commented out from your sources.list, since the installer does not remove it if there are no network mirrors.

You have to configure a mirror in order to be able to properly install packages. Add this line to your sources.list:

deb http://ftp.us.debian.org/debian/ buster main contrib non-free

and run apt update. After that, you can install openssh-server.

You might want to switch the mirror to something closer to you, this can be done by changing the us part of the URL to correspond to your country, or supplying a different mirror. The list of mirror sites is available here.

Alternatively, you can install the netselect-apt package, which will search for the fastest mirror near you.

As a side note: Debian does supply a secure remote shell, all you have to do is tick "SSH Server" on the "Software selection" page during install (why it is not selected by default is beyond me, but still).

Answer 2

echo "deb http://deb.debian.org/debian/ buster-updates main contrib" >> /etc/apt/sources.list