Chapter 8. Per-Account Server Configuration
We've seen two techniques
for controlling the SSH server's behavior globally:
compile-time configuration (
Chapter 4, "Installation and Compile-Time Configuration") and
serverwide configuration (
Chapter 5, "Serverwide Configuration"). These
techniques affect all incoming SSH connections to a given server
machine. Now it's time to introduce a third, finer-grained
method of server control:
per-account
configuration.
As the name implies, per-account configuration controls the SSH
server differently for each user account on the server machine. For
example, a user account sandy can accept incoming SSH connections
from any machine on the Internet, while rick permits connections only
from the domain
verysafe.com, and fraidycat refuses
key-based connections. Each user configures his or her own account,
using the facilities highlighted in
Figure 8-1,
without needing special privileges or assistance from the system
administrator.
Figure 8-1. Per-account configuration (highlighted parts)
We have already seen a simple type of per-account configuration. A
user may place a public key into her authorization file, instructing
the SSH server to permit logins to her account by public-key
authentication. But per-account configuration can go further,
becoming a powerful tool for access control and playing some fun
tricks with your account. Accepting or rejecting connections by
particular keys or hosts is just the beginning. For instance, you can
make an incoming SSH connection run a program of your choice, instead
of the client's choice. This is called a
forced
command, and we'll cover quite a few interesting
applications.
Per-account configuration may control only incoming SSH connections
to your account. If you're interested in configuring outgoing
SSH connections by running SSH clients, refer to
Chapter 7, "Advanced Client Use".
8.1. Limits of This Technique
Per-account
configuration can do many interesting things, but it has some
restrictions that we will discuss:
- It can't defeat security measures put in place by compile-time
or serverwide configuration. (Thank goodness.)
- It is most flexible and secure if you use public-key authentication.
Trusted-host and password authentication provide a much narrower
range of options.
8.1.1. Overriding Serverwide Settings
SSH
settings in a user's account may only restrict the
authentication of incoming connections. They can't enable any
SSH features that have been turned off more globally, and they
can't permit a forbidden user or host to authenticate. For
example, if your SSH server rejects all connections from the domain
evil.org, you can't
override this restriction within your account by per-account
configuration.
[113]
This limitation makes sense. No end-user tool should be able to
violate a server security policy. However, end users should be (and
are) allowed to restrict incoming connections to their accounts.
A few features of the server may be overridden by per-account
configuration. The most notable one is the server's idle
timeout, which may be extended beyond the serverwide setting. But
such features can't coerce the server to accept a connection it
has been globally configured to reject.
If you are an end user, and per-account configuration doesn't
provide enough flexibility, you can run your own instance of the SSH
server, which you may configure to your heart's content. [
Section 5.2.2, "Running as an Ordinary User"] Be cautious, though, since this is seldom the
right thing to do. The restrictions you're trying to circumvent
are part of the security policy defined for the machine by its
administrators, and you shouldn't run a program that flouts
this policy just because you can. If the machine in question is under
your administrative control, simply configure the main SSH server as
you wish. If not, then installing and running your own
sshd might violate your usage agreement and/or
certainly annoy your sysadmin. And that's never a wise thing to
do.
8.1.2. Authentication Issues
To make the best use of per-account
configuration, use public-key authentication. Password authentication
is too limited, since the only way to control access is with the
password itself. Trusted-host authentication permits a small amount
of flexibility, but not nearly as much as public-key authentication.
If you're still stuck in the password-authentication dark ages,
let this be another reason to switch to public keys. Even though
passwords and public-key passphrases might seem similar (you type a
secret word, and
voilà, you're
logged in), public keys are far more flexible for permitting or
denying access to your account. Read on and learn how.
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