демон протокола точка-точка (Point-to-Point Protocol Daemon)
Безопасность (Security)
pppd provides system administrators with sufficient access
control that PPP access to a server machine can be provided to
legitimate users without fear of compromising the security of the
server or the network it's on. This control is provided through
restrictions on which IP addresses the peer may use, based on its
authenticated identity (if any), and through restrictions on
which options a non-privileged user may use. Several of pppd's
options are privileged, in particular those which permit
potentially insecure configurations; these options are only
accepted in files which are under the control of the system
administrator, or if pppd is being run by root.
The default behaviour of pppd is to allow an unauthenticated peer
to use a given IP address only if the system does not already
have a route to that IP address. For example, a system with a
permanent connection to the wider internet will normally have a
default route, and thus all peers will have to authenticate
themselves in order to set up a connection. On such a system,
the auth option is the default. On the other hand, a system
where the PPP link is the only connection to the internet will
not normally have a default route, so the peer will be able to
use almost any IP address without authenticating itself.
As indicated above, some security-sensitive options are
privileged, which means that they may not be used by an ordinary
non-privileged user running a setuid-root pppd, either on the
command line, in the user's ~/.ppprc file, or in an options file
read using the file option. Privileged options may be used in
/etc/ppp/options file or in an options file read using the call
option. If pppd is being run by the root user, privileged
options can be used without restriction.
When opening the device, pppd uses either the invoking user's
user ID or the root UID (that is, 0), depending on whether the
device name was specified by the user or the system
administrator. If the device name comes from a privileged
source, that is, /etc/ppp/options or an options file read using
the call option, pppd uses full root privileges when opening the
device. Thus, by creating an appropriate file under
/etc/ppp/peers, the system administrator can allow users to
establish a ppp connection via a device which they would not
normally have permission to access. Otherwise pppd uses the
invoking user's real UID when opening the device.
