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Install RedHat (further just RH) Linux on the box. Make sure shadow and MD5
passwords are enabled. And have a nice long root password! Refer to
corresponding installation guides.
RH Linux was and is *really* buggy out of the box (both local and remote exploits are
discovered every day, see
BugTRAQ database), and many software packages installed by default can
be used to obtain root shell from non-privileged account or in the worst cases
across the network (or just mess up the box). Thus special attention should be given to package
selection on the browser workstation.
Install ssh-server RPM for remote administration. Do NOT use inetd daemon
mode, make sshd run standalone and use /etc/hosts.allow for access
control (ssh daemon will read the file upon startup)
Make sure you create a boot floppy using a mkbootdisk command as errors
in LILO configuration might render the system unbootable.
Make the following modifications to configuration files
- /etc/inittab
#
# inittab This file describes how the INIT process should set up
# the system in a certain run-level.
#
# Author: Miquel van Smoorenburg, <miquels@drinkel.nl.mugnet.org>
# Modified for RHS Linux by Marc Ewing and Donnie Barnes
#--fixed by anton for browser station
# Default runlevel. The runlevels used by RHS are:
# 0 - halt (Do NOT set initdefault to this)
# 1 - Single user mode
# 2 - Multiuser, without NFS (The same as 3, if you do not have networking)
# 3 - Full multiuser mode
# 4 - unused
# --anton--
# 4 - browser X
# 5 - X11
# 6 - reboot (Do NOT set initdefault to this)
#
#id:3:initdefault:
#--anton: default runlevel now 4! other levels protected by LILO password
id:4:initdefault:
# System initialization.
si::sysinit:/etc/rc.d/rc.sysinit
l0:0:wait:/etc/rc.d/rc 0
l1:1:wait:/etc/rc.d/rc 1
l2:2:wait:/etc/rc.d/rc 2
l3:3:wait:/etc/rc.d/rc 3
l4:4:wait:/etc/rc.d/rc 4
l5:5:wait:/etc/rc.d/rc 5
l6:6:wait:/etc/rc.d/rc 6
# Things to run in every runlevel.
ud::once:/sbin/update
# Trap CTRL-ALT-DELETE
#anton -- not here, disable
#ca::ctrlaltdel:/sbin/shutdown -t3 -r now
# When our UPS tells us power has failed, assume we have a few minutes
# of power left. Schedule a shutdown for 2 minutes from now.
# This does, of course, assume you have powerd installed and your
# UPS connected and working correctly.
pf::powerfail:/sbin/shutdown -f -h +2 "Power Failure; System Shutting Down"
# If power was restored before the shutdown kicked in, cancel it.
pr:12345:powerokwait:/sbin/shutdown -c "Power Restored; Shutdown Cancelled"
# Run gettys in standard runlevels
1:2345:respawn:/sbin/mingetty tty1
#--anton -- only one is needed! comment out the rest
#2:2345:respawn:/sbin/mingetty tty2
#3:2345:respawn:/sbin/mingetty tty3
#4:2345:respawn:/sbin/mingetty tty4
#5:2345:respawn:/sbin/mingetty tty5
#6:2345:respawn:/sbin/mingetty tty6
# Run xdm in runlevel 5
# xdm is now a separate service
x:5:respawn:/etc/X11/prefdm -nodaemon
The file above disables Ctrl-Alt-Del combination and makes new runlevel 4 a default
runlevel. It also eliminates virtual consoles (all but 1).
- /etc/fstab
#=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
/dev/hda1 / ext2 defaults,ro 1 1
/dev/hda7 /home ext2 defaults,nodev,noexec,nosuid 1 2
/dev/hda6 /tmp ext2 defaults,nodev,noexec,nosuid 1 2
/dev/hda5 /var ext2 defaults,nodev,noexec,nosuid 1 2
#=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
#/dev/cdrom /mnt/cdrom iso9660 noauto,owner,ro 0 0
#/dev/fd0 /mnt/floppy auto noauto,owner 0 0
#=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
none /proc proc defaults 0 0
none /dev/pts devpts gid=5,mode=620 0 0
/dev/hda8 swap swap defaults 0 0
#=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
Brief explanation for the options (see man mount for more)
- /etc/rc.d/ directory
Create file xbrowser in /etc/rc.d/init.d and symlink
(cd /etc/rc.d/rc4.d ; ln -s /etc/rc.d/init.d/xbrowser S99xbrowser)it as
S99xbrowser in /etc/rc.d/rc4.d
so that directory /etc/rc.d/rc4.d looks like this
drwxrwxrwx 2 root root 4096 Sep 10 15:30 .
drwxrwxrwx 10 root root 4096 Sep 10 15:30 ..
lrwxrwxrwx 1 root root 1179 Sep 10 15:30 S05kudzu-> ../init.d/kudzu
lrwxrwxrwx 1 root root 5094 Sep 10 15:30 S10network-> ../init.d/network
lrwxrwxrwx 1 root root 1367 Sep 10 15:30 S16apmd-> ../init.d/apmd
lrwxrwxrwx 1 root root 1542 Sep 10 15:30 S20random-> ../init.d/random
lrwxrwxrwx 1 root root 3217 Sep 10 15:30 S25netfs-> ../init.d/netfs
lrwxrwxrwx 1 root root 1024 Sep 10 15:30 S30syslog-> ../init.d/syslog
lrwxrwxrwx 1 root root 989 Sep 10 15:30 S40atd-> ../init.d/atd
lrwxrwxrwx 1 root root 1031 Sep 10 15:30 S40crond-> ../init.d/crond
lrwxrwxrwx 1 root root 1203 Sep 10 15:30 S75keytable-> ../init.d/keytable
lrwxrwxrwx 1 root root 1261 Sep 10 15:30 S85gpm-> ../init.d/gpm
lrwxrwxrwx 1 root root 1956 Sep 10 15:30 S90xfs-> ../init.d/xfs
lrwxrwxrwx 1 root root 650 Sep 10 15:30 S99xbrowser-> ../init.d/xbrowser
This init files are run upon entering runlevel 4 (either at reboot or when
typing init 4 from root prompt). Files are run in order of increasing
numbers so that our xbrowser runs in the end.
xbrowser file looks like this
#!/bin/bash
# --anton: Init the box into X with browser, no login script
echo "Starting standalone browser....."
#put a mark into log
echo %%%%%%Reboot%%%%% >> /var/log/xlog
#this file marks X startrup using out xinitrc
touch /tmp/startOK
#--main loop, indefinite with the presence of /tmp/startOK file ------------------
while [ -f /tmp/startOK ] ; do
#put a mark into log
echo %%%%%%Restart%%%%% >> /var/log/xlog
#kill stuck netscape if any (this doesnt help if it turn zombie)
killall -9 netscape >& /dev/null
#clear netscape lock
if [ -f ~netscape/.netscape/lock ]; then
/bin/rm ~netscape/.netscape/lock
fi
#start X windows, no winman, using the config that starts only netscape
#config is in root home dir!!
#X server runs as root, sort of BAD
/usr/X11R6/bin/xinit /root/.xinitrc -- /usr/X11R6/bin/X bc
done
#main loop end-------------------------------
This file will start X server upon boot up with no prompting (after LILO
prompt). The X server will follow the directions in /root/.xinitrc,
below. X server config is shown below too.
- Make sure /etc/sysctl.conf looks like this
# Disables packet forwarding
net.ipv4.ip_forward = 0
# Enables source route verification
net.ipv4.conf.all.rp_filter = 1
# Disables automatic defragmentation (needed for masquerading, LVS)
net.ipv4.ip_always_defrag = 0
# Disables the magic-sysrq key
#--anton: this IS important
kernel.sysrq = 0
This disable kernel interaction keys (aka Magic SysRQ keys) on startup.
- /etc/X11/XF86Config
Make changes to /etc/X11/XF86Config that was automatically created
during install to look have those in:
# File generated by XConfigurator.
...whatever...
# **********************************************************************
# Server flags section.
# **********************************************************************
Section "ServerFlags"
# Uncomment this to cause a core dump at the spot where a signal is
# received. This may leave the console in an unusable state, but may
# provide a better stack trace in the core dump to aid in debugging
#NoTrapSignals
# Uncomment this to disable the <Ctrl><Alt><BS> server abort sequence
# This allows clients to receive this key event.
#--anton -- no X server kill
#--another option is to have a kill as a means to fight broken/stuck netscape,
#--restart will bring it back after cleanup
DontZap
# Uncomment this to disable the <Crtl><Alt><KP_+>/<KP_-> mode switching
# sequences. This allows clients to receive these key events.
#--anton -- kinda bad too
DontZoom
EndSection
...whatever...
Now, the DontZap is a questionable choice. The Crtl-Alt-Backspace
sequence might be the only way to kill stuck netscape or the one with some
window overlapping netscape controls (like, View Source or View Page Info) as
no automatic netscape fixing is implemented. Disabling Java and JavaScript
will decrease the likelihood of it crashing, but will not eliminate this
miserable occurrence altogether. In the current setup pressing
Crtl-Alt-Backspace if DontZap is commented out will cause X server to
restart, killing netscape and doing a lock file cleanup.
- /root/.xinitrc
Make sure that /root/.xinitrc
looks like
/bin/rm -f ~netscape/.netscape/lock >& /dev/null
#--anton: otherwise non-root netscape cant run
#--anton only allow local but from all users
#--anton the name of test box was "afc" thus the line below
xhost +afc
#--anton:starts netscape as user "netscape" and full screen!!
#make sure 1024x768 matches your monitor
su netscape -c "netscape -no-about-splash -geometry 1024x768+0+0"
#---------------TESTING---------------------------
#these commands were used in testing to set netscpae preferences
#same as having "netscape" uiser home dir writable for this user
#export HOME=/home/netscape
#netscape -no-about-splash -geometry 1024x768+0+0 >& /tmp/LOG
#---------------TESTING---------------------------
#also needed: X as user "guest" eventually
See comments in file for explanation
Create user netscape, his home directory will be /home/netscape.
Start netscape and apply a restricted settings as:
- no Java (known big risks,
recently really big holes discovered in Netscape Java implementation),
- no
JavaScript (some risks with password stealing and web mail hijacking),
- no
cache (some Java bugs will access cache objects and then bypass JVM
restrictions),
- no cookies (might not be possible though, low risk),
- remove all launches of nonstandard applications (ideally-all applications) with
file types (by going to Netscape->Edit->Preferences->Navigator->Applications),
- history length set to 0 (next user can't see what previous was doing,
the risk is in seeing URL-encoded passwords sometimes)
Do chown to root on /home/netscape (by chown -R root.root /home/netscape).
Make sure that his home directory belongs to root, there are no world-writable
files and subdirectories there and permission are at least
/home/netscape/:
total 9
drwxr-xr-x 4 root root 1024 Sep 7 18:29 .
drwxr-xr-x 4 root root 1024 Sep 7 18:30 ..
-rw-r--r-- 1 root root 16 Sep 7 18:29 .bash_history
-rw-r--r-- 1 root root 24 Sep 5 08:21 .bash_logout
-rw-r--r-- 1 root root 230 Sep 5 08:21 .bash_profile
-rw-r--r-- 1 root root 124 Sep 5 08:21 .bashrc
-rw-r--r-- 1 root root 93 Sep 7 18:25 .mailcap
-rw-r--r-- 1 root root 0 Sep 7 18:25 .mime.types
drwxr-xr-x 4 root root 1024 Sep 10 08:38 .netscape
drwxr--r-- 2 root root 1024 Sep 6 00:04 .xauth
/home/netscape/.netscape:
total 264
drwxr-xr-x 4 root root 1024 Sep 10 08:38 .
drwxr-xr-x 4 root root 1024 Sep 7 18:29 ..
drwxr--r-- 2 root root 1024 Sep 6 00:04 archive
-rw------- 1 root root 14757 Sep 7 18:38 bookmarks.html
drwxr--r-- 3 root root 1024 Sep 7 18:24 cache
-rw-r--r-- 1 root root 188416 Sep 6 00:05 cert7.db
-rw-r--r-- 1 root root 16384 Sep 7 18:30 history.dat
-rw-r--r-- 1 root root 111 Sep 7 16:20 history.list
-rw-r--r-- 1 root root 16384 Sep 6 00:05 key3.db
-rw-r--r-- 1 root root 0 Sep 6 00:04 nswrapper.copy_defs
-rw-r--r-- 1 root root 279 Sep 10 08:38 plugin-list
-rw-r--r-- 1 root root 3398 Sep 7 18:29 preferences.js
-rw-r--r-- 1 root root 741 Sep 7 18:29 registry
-rw-r--r-- 1 root root 16384 Sep 7 18:29 secmodule.db
Carefully test netscape functionality upon doing the chown to root!
At present, I have not found a way to avoid periodic Netscape complaints about
"Can't write preferences".
Another note is appropriate. Netscape is VERY buggy (last example is
Red Hat Linux Security Advisory
presents a way to crash and exploit netscape using a specially crafted JPEG
image)
and is likely to crash periodically,
possibly producing a buffer overflow with shell access for the intruder. This
shell will have the netscape user as owner. Thus the absence of xterm and rxvt
on the system is absolutely crucial as it provides another line of defense.
Permission on the system should also be set very conservatively (no
world-writable files). Ideally, NO files should be owned by user "netscape" on
the system AT ALL (do a find / -user netscape command to confirm
this, also check for world writable files with find / -perm -2 ! -type l -ls).
Modify /etc/lilo.conf
boot=/dev/hda
map=/boot/map
install=/boot/boot.b
prompt
timeout=50
default=linux
image=/boot/vmlinuz-2.2.14-5.0
label=linux
read-only
root=/dev/hda1
restricted
The word restricted will cause password prompting in order to
enter non-standard runlevel (e.g. linux init 0 from LILO: prompt).
That implies using stock RH 6.2 kernel. Kernel upgrade to 2.2.16 might be a
good idea as some bugs were found in early 2.2.14 kernels (low risk).
REMOVE /usr/X11R6/bin/xterm xterm executable COMPLETELY! This is REALLY IMPORTANT
as shell will be much harder to obtain in this case. Make sure its clone,
rxvt, is not installed! Ideally, all programs that can spawn a shell should be
removed.
Some physical security
- Secure reset button
- Remove CDROM and floppy disk drive
- Prevent access to the box to avoid hard drive replacement
Some final touches (nice but not essential for system functionality)
- Implement free disk space monitor top avoid partition overflows
- Enable remote logging (preferably to some dedicated box with host-based IDS
that analyzes the logs)
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