My “server” at home isn’t particularly important to the outside world. But by virtue of being on the Internet, it’s subject to a lot of SSH logins. The easiest thing to do is to shut it off from the outside world. But I need to access it when away from home, so that’s not a particularly useful solution.
So what I’ve done is use the SSH daemon’s (sshd) configuration to reduce the risk profile. The first thing I wanted to do is forbid login as root:
I also don’t want anyone to be able to log in with passwords. “Anyone” is essentially me here, but since I have sudo on the box, if someone is able to figure out my password they are able to get root remotely.
PermitRootLogin no ChallengeResponseAuthentication no
Finally, I want to restrict remote login to only explicitly-permitted users. I do this with a dedicated Unix group that I call “sshusers”.
These are pretty standard changes and not really worth a blog post. But it turns out that sshd has a very flexible configuration. When a client is coming from inside the LAN, I want to enable password authentication. This is particularly helpful when I’m installing a new system and don’t have SSH keys setup yet.
Match Address 192.168.1.* PasswordAuthentication yes
Also within the LAN, it’s easier to run Ansible playbooks across machines if the root user can SSH in with a key. So I combine user and address matching to permit key-based root login only from the server with the Ansible playbooks.
Match User root Address 192.168.1.10 AllowGroups root sshusers PermitRootLogin prohibit-password
Finally, I want my ex to be able to access the server in order to access photos, etc. So I set up her account so that she can use an sftp client but can’t log in (not that she would anyway, but it was a fun challenge to set this up).
Match User angie ForceCommand internal-sftp PasswordAuthentication yes PermitTunnel no AllowAgentForwarding no AllowTcpForwarding no X11Forwarding no
Why didn’t you … ?
The configuration above isn’t the only way to secure my SSH server from the outside world. It’s not even necessarily the best. I could, for example, move SSH to a different port, which would cut down on the drive-by attempts significantly. I resisted that in the past because I felt “security through obscurity isn’t security.” But in practice, it can be a layer in a more secure approach. In the past, I also recall some clients I used (particularly on mobile) not having the ability to use a non-default port. If that recollection is correct, it seems to also be outdated now. So basically I’m still on port 22 because of inertia.
I could also set up a VPN server and use that for remote access. That requires an additional service to manage, of course. And it also presents challenges when I’m also connected to a work VPN server. The sshd configuration approach is a simpler way for my needs.