When disabling a user account on a Linux box, it is good practice to also change the shell to something which, well, isn’t a shell. The point of these shells is rather than presenting the user with a prompt to execute further commands, it returns a failure code, and log out the user.
If you look around, people recommend a couple different things to set as the user’s shell:
/bin/false. Not once have I seen someone say why to use either, nor what the differences are.
false is a foundational part of the shell. It’s an incredibly simple application which does nothing but exit.
Even the source code is simple:
The source code itself is slightly misleading.
false is actually an extension of
true, but with a different return code.
false returns status code 1, whilst
true return 0.
The real logic actually lives there:
false are incredibly simple, even if you know nothing about C. And as a result are incredibly fast.
nologin is designed to do exactly what we want it to. It’s specifically designed to prevent login by being set as a user’s shell.
nologin does a little more than false, but it’s still very simple code to read:
nologin will try to read
/etc/nologin.txt to get a custom message to show the user. If it exists, it prints that and exits with code 1. If it doesn’t exist, it shows the default message, and exits with code 1. This customization makes it much more user-friendly, although because the file is global, one system can only have one configured message.
rssh is a unique shell. Rather than executing commands, like
bash, it allows filtering of specific SSH uses down to exactly what’s needed. For example, you can block all access unless it’s SCP.
rssh is a much larger application compared to
nologin, but it’s still not especially complex code.
#Which should I use?
Realistically, it doesn’t really matter. The point of a disabled prompt is to exit with a fail quickly, which both
nologin do. So long as you block access, it really doesn’t matter how.
If you’re hyper paranoid, use
false, as it’s simpler and smaller. But you’ll lot more than just use
false if you want things that locked down. Alternatively, if you want it to be more obvious what’s going on, use
nologin, as its name makes a bit more sense, and terminates with a message.
rssh solves a specific issue. It’s best not to use it unless you need its features, but if you do need them, it’s a valuable tool in the kit!
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