Which config files in /etc folder must be unique for each computer?

Question

I am doing a restoration test of backup files on a computer. Without putting much thought into it, I did an rsync to replace all files in /etc folder with the ones in the backup. Then, I realize I shouldn't really be doing it as the passwords and user names for the two computers are not the same.

After a reboot, the computer is in a state where it cannot start, and I would possibly have to reinstall it from scratch. Now my question is, given the same OS, what are the files in /etc folder that are unique for each computer. This would allow me to fine-tune rsync to exclude those files in the future when doing a restoration from backup.

Answer 1

There are very few files that absolutely must be different between two machines, and need to be regenerated when cloning:

• The host name /etc/hostname.
• The SSH host keys: /etc/ssh_host_*_key* or /etc/ssh/ssh_host_*_key* or similar location.
• The random seed: /var/lib/urandom/random-seed or /var/lib/random-seed or similar location. (/var/lib/systemd/random-seed on systems using systemd)

Anything else could be identical if you have a bunch of identical machines.

A few files are typically different on machines with different hardware:

• /etc/fstab, /etc/crypttab, /etc/mdadm.conf, and bootloader configuration files (if located in /etc — some distributions put them in /boot) if disks are partitioned differently.
• /etc/X11/xorg.conf, if present, if the machines have different graphics cards.
• Modules to load or blacklist in /etc/modules, /etc/modprobe.conf, /etc/modprobe.d/ and /etc/modutils/.

In addition, some network configuration may need to change, in particular:

• If you have static IP addresses, they need to be diversified per machine. The location of IP configuration varies between distribution (e.g. /etc/network/interfaces on Debian, /etc/sysconfig/network on Red Hat).
• /etc/hosts often contains the host name.
• Mail configuration often contains the host name: check /etc/mailname.

There's no general answer to “what are the files in /etc folder (…) are unique for each computer” because the whole purpose of /etc is to store files that can be customized on each computer. For example, if you have different accounts on different machines, then obviously you can't share the account database — and if you want to be able to share the account database, then you'll end up with the same accounts.

Generally speaking, don't try to share /etc by default unless you have a set of machines with the same software configuration — same installed software, same accounts, etc. If you do share /etc, you'll need to blacklist a few files from sharing as indicated above.

If you have machines with different configurations, then whitelist what you synchronize. Treat files in /etc as distinct on different machines, like files in /var. Synchronize only the ones that you've decided should apply everywhere.

One possible way to manage synchronization is to keep machine-specific files in a different directory, e.g. /local/etc, and make symbolic links like /etc/fstab -> ../local/etc/fstab. This still requires a largely homogeneous set of machines in terms of software as different distributions put files in different locations. Or, conversely, keep only the machine-specific files in /etc and all generic files elsewhere — but typical distributions don't accommodate this well.

You obviously can't do a live test of the restoration of the system configuration of one system on a different system. To test the restoration of your backups, fire up a virtual machine that emulates the hardware configuration sufficiently well (in particular, with a similar disk layout).