ptg10805159Section 21.5 Source Code 795
names arelisted in/etc/hostsor registered with whatever name service we are
using, so that we can translate the names to network addresses.
We can run theprintcommand on the same machine wherethe printer spooling
daemon is running, or we can run it from any machine on the same network.We only
need to configuretheprintserverfield in/etc/printer.confin the latter case,
because only the daemon needs to know the name of the printer.Security
Programs that run with superuser privileges have the potential to open a computer
system up to attack. Such programs usually aren’t morevulnerable than any other
program, but when compromised can lead to attackers obtaining full access to your
system.
The printer spooling daemon in this chapter starts out with superuser privileges in
this example to be able to bind a socket to a privileged TCP port number.Tomake the
daemon less vulnerable to attack, we can
•Design the daemon to conform to the principles of least privilege (Section 8.11).
After we obtain a socket bound to a privileged port address, we can change the
user and group IDs of the daemon to something other thanroot(lp,for
example). All the files and directories used to storequeued print jobs should be
owned by this nonprivileged user.This way,the daemon, if compromised, will
provide the attacker with access only to the printing subsystem. This is still a
concern, but it is far less serious than an attacker getting full access to your
system.
•Audit the daemon’s source code for all known potential vulnerabilities, such as
buffer overruns.
•Log unexpected or suspicious behavior so that an administrator can take note
and investigate further.21.5 Source Code xviii Contents
The source code for this chapter comprises five files, not including some of the common
library routines we’ve used in earlier chapters:
ipp.h Header file containing IPP definitions
print.h Header containing common constants, data structuredefinitions, and
utility routine declarations
util.c Utility routines used by the two programs
print.c The C source file for the command used to print a file
printd.c The C source file for the printer spooling daemon
We will study each file in the order listed.