Linux Network Administratoramp;#039;s Guide (3rd Edition) [Electronic resources] نسخه متنی

6.7. General Security Considerations

A misconfigured PPP daemon can be a
devastating security breach. It can be as bad as letting anyone plug
their machine into your Ethernet (and that can be very bad). In this
section, we discuss a few measures that should make your PPP
configuration safe.

Root privilege is required to configure the network device and routing table. You will usually solve this by running pppd setuid root. However, pppd allows users to set various security-relevant options.

To protect against any attacks a user may
launch by manipulating pppd options,
you should set a couple of default values in the global
/etc/ppp/options file, like those shown in the
sample file in Section 6.3Section 6.3, earlier in this chapter.
Some of them, such as the authentication options, cannot be
overridden by the user, and thus provide reasonable protection
against manipulations. An important option to protect is the connect
option. If you intend to allow non-root users to invoke
pppd to connect to the Internet, you should
always add the connect and
noauth options to the global options file
/etc/ppp/options. If you fail to do this, users
will be able to execute arbitrary commands with root privileges by
specifying the command as their connect command on the
pppd line or in their personal options file.

Another good idea is to
restrict which users may execute pppd by
creating a group in /etc/group and adding
only those users who you wish to have the ability to execute the PPP
daemon. You should then change group ownership of the
pppd daemon to that group and remove the world
execute privileges. To do this, assuming you've
called your group dialout, you could
use something like:

# chown root /usr/sbin/pppd
# chgrp dialout /usr/sbin/pppd
# chmod 4750 /usr/sbin/pppd

Of course, you have to protect yourself from the systems you speak
PPP with, too. To fend off hosts posing as someone else, you should
always require some sort of authentication from your peer.
Additionally, you should not allow foreign hosts to use any IP
address they choose, but restrict them to at most a few. The
following section will deal with these topics in
detail.