Here are some guidelines for linux servers.
Note that we have some servers (which are bare metal installs. While others are proxmox hosts. On proxmox some VM are lxc containers, while other are QEMU VM.
Every server is referenced in CT and VM list of OFF infrastructure
We use etckeeper with git backend on as much server as possible.
See https://etckeeper.branchable.com/README/
So whenever you make a change to /etc.
When possible before making your change,
as root, do a git status and then etckeeper commit "save before changes".
And after, do a etckeeper commit "<a descriptive message>" afterwards.
We use either postfix or exim as a satellite of a smart_host.
Every outgoing mail must pass through the proxmox mail gateway, which is registered in spf record and adds DKIM signature.
For configuration, see mail - Servers
We use iptables on a lot of servers (generally host servers).
We use iptables-persistent to save rules, and restore them at startup.
On ovh servers, rules are in /etc/iptables/rule.v{4,6}
On off1, rules are in /etc/iptables.up.rules
Remember, that docker as it's own chains that are not affected by INPUT and OUTPUT rules.
So it won't block a port exposed by docker. Use DOCKER-USER chain for that.
see https://docs.docker.com/network/iptables/
Check your TERM variable: echo $TERM, it should be xterm-256color or linux
Check .bashrc is the right one diff $HOME/.bashrc /etc/skel/.bashrc
if not copy the one from /etc/skel/.bashrc
Check bash-completion is installed: dpkg --verify bash-completion && echo ok
Check .bashrc is the right one diff $HOME/.bashrc /etc/skel/.bashrc
if not copy the one from /etc/skel/.bashrc