Linux web browser station (formerly "The Linux Public Web Browser mini-HOWTO"): NEW GUIDE: Step-by-step guide

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## 3. NEW GUIDE: Step-by-step guide

## 3.1 Install RH

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.

## 3.2 Clean-up packages

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.

- Use workstation or custom installation mode. The latter is recommended, when selecting groups of packages, only choose base-system, networked workstation, mail/www services (make sure you later replace Communicator with Navigator) and X packages and then erase the unneeded RPMs. If using workstation mode you will have to (possibly manually) remove about 300 packages.
- When partitioning the disk follow the scheme below. The sizes are appropriate for the 3 GB disk, scale the sizes accordingly for bigger drive but this is really not needed for this setup as the whole Linux system is squeezed to under 200MB. Make sure those partitions (/,/home,/var and /tmp) are present! Separate /usr is not necessary! Remember to create a generous swap partition (at least the size of RAM).

  Partitions mount points and sizes used for a test system:

  > `

  >     Filesystem           1k-blocks      Used Available Use% Mounted on
  >     /dev/hda1              1571528    184184   1307512  12% /
  >     /dev/hda7               300603       309    284773   0% /home
  >     /dev/hda6               300603        20    285062   0% /tmp
  >     /dev/hda5               809556      4640    763792   1% /var

  > `

- Remove all RPMs but those (list might be shortened later and automatic RPM-removal shell script might be written as well)

  > `

  >     MAKEDEV-2.5.2-1
  >     SysVinit-2.78-5
  >     X11R6-contrib-3.3.2-11
  >     XFree86-100dpi-fonts-3.3.6-20
  >     XFree86-3.3.6-20
  >     XFree86-75dpi-fonts-3.3.6-20
  >     XFree86-S3-3.3.6-20
  >     XFree86-SVGA-3.3.6-20
  >     XFree86-VGA16-3.3.6-20
  >     XFree86-libs-3.3.6-20
  >     XFree86-xfs-3.3.6-20
  >     Xconfigurator-4.3.5-1
  >     apmd-3.0final-2
  >     ash-0.2-20
  >     at-3.1.7-14
  >     audiofile-0.1.9-3
  >     authconfig-3.0.3-1
  >     basesystem-6.0-4
  >     bash-1.14.7-22
  >     bc-1.05a-5
  >     bdflush-1.5-11
  >     binutils-2.9.5.0.22-6
  >     bzip2-0.9.5d-2
  >     chkconfig-1.1.2-1
  >     chkfontpath-1.7-2
  >     console-tools-19990829-10
  >     cracklib-2.7-5
  >     cracklib-dicts-2.7-5
  >     crontabs-1.7-7
  >     dev-2.7.18-3
  >     diffutils-2.7-17
  >     e2fsprogs-1.18-5
  >     ed-0.2-13
  >     eject-2.0.2-4
  >     etcskel-2.3-1
  >     file-3.28-2
  >     filesystem-1.3.5-1
  >     fileutils-4.0-21
  >     findutils-4.1-34
  >     freetype-1.3.1-5
  >     gawk-3.0.4-2
  >     gd-1.3-6
  >     gdbm-1.8.0-3
  >     getty_ps-2.0.7j-9
  >     glib-1.2.6-3
  >     glib10-1.0.6-6
  >     glibc-2.1.3-15
  >     gmp-2.0.2-13
  >     gpm-1.18.1-7
  >     grep-2.4-3
  >     groff-1.15-8
  >     gtk+-1.2.6-7
  >     gzip-1.2.4a-2
  >     hdparm-3.6-4
  >     imlib-1.9.7-3
  >     indexhtml-6.2-1
  >     info-4.0-5
  >     initscripts-5.00-1
  >     iputils-20000121-2
  >     isapnptools-1.21b-1
  >     kbdconfig-1.9.2.4-1
  >     kernel-2.2.14-5.0
  >     kernel-utils-2.2.14-5.0
  >     krb5-configs-1.1.1-9
  >     krb5-libs-1.1.1-9
  >     kudzu-0.36-2
  >     ld.so-1.9.5-13
  >     ldconfig-1.9.5-16
  >     less-346-2
  >     libc-5.3.12-31
  >     libgr-2.0.13-23
  >     libgr-progs-2.0.13-23
  >     libjpeg-6b-10
  >     libpng-1.0.5-3
  >     libstdc++-2.9.0-30
  >     libtermcap-2.0.8-20
  >     libtiff-3.5.4-5
  >     libungif-4.1.0-4
  >     libxml-1.8.6-2
  >     lilo-0.21-15
  >     logrotate-3.3.2-1
  >     losetup-2.10f-1
  >     mailcap-2.0.6-1
  >     man-1.5h1-1
  >     mingetty-0.9.4-11
  >     mkbootdisk-1.2.5-3
  >     mkinitrd-2.4.1-2
  >     mktemp-1.5-2
  >     modutils-2.3.9-6
  >     mount-2.10f-1
  >     mouseconfig-4.4-1
  >     ncompress-4.2.4-15
  >     ncurses-5.0-11
  >     net-tools-1.54-4
  >     netscape-common-4.72-6
  >     netscape-navigator-4.72-6
  >     newt-0.50.8-2
  >     ntsysv-1.1.2-1
  >     pam-0.72-6
  >     passwd-0.64.1-1
  >     pciutils-2.1.5-2
  >     popt-1.5-0.48
  >     procps-2.0.6-5
  >     psmisc-19-2
  >     pwdb-0.61-0
  >     raidtools-0.90-6
  >     rdate-1.0-1
  >     readline-2.2.1-6
  >     redhat-logos-1.1.0-2
  >     redhat-release-6.2-1
  >     rootfiles-5.2-5
  >     rpm-3.0.4-0.48
  >     rpmfind-1.4-3
  >     rxvt-2.6.1-8
  >     sash-3.4-2
  >     sed-3.02-6
  >     setup-2.1.8-1
  >     setuptool-1.2-5
  >     sh-utils-2.0-5
  >     shadow-utils-19990827-10
  >     slang-1.2.2-5
  >     slocate-2.1-2
  >     stat-1.5-12
  >     sysklogd-1.3.31-16
  >     tar-1.13.17-3
  >     tcl-8.0.5-35
  >     tcp_wrappers-7.6-10
  >     termcap-10.2.7-9
  >     textutils-2.0a-2
  >     time-1.7-9
  >     timeconfig-3.0.3-2
  >     tmpwatch-2.2-1
  >     utempter-0.5.2-2
  >     util-linux-2.10f-7
  >     vixie-cron-3.0.1-40
  >     which-2.9-2
  >     words-2-12
  >     xinitrc-2.9-1
  >     xpm-3.4k-2
  >     zlib-1.1.3-6

  > `

  Unfortunately, some of the packages above might also be redundant and potentially unsafe (even glibc, the main runtime Linux library, was recently found to have locally exploitable bugs! And so was PAM module library). More candidates for elimination include gpm (console mouse services, had some exploit history last year) and many others. Xlib has a buffer overflow but can't be eliminated. Make sure the latest version is used.

## 3.3 Install ssh

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)

## 3.4 Make a boot floppy

Make sure you create a boot floppy using a mkbootdisk command as errors in LILO configuration might render the system unbootable.

## 3.5 Modify configs

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)
  - For / : mounted read-only (ro), just to make it a little bit harder to do Bad Things
  - For /home, /tmp and /var : nodev,noexec,nosuid will prevent (a) starting executable from them (download and run through netscape attack), (b)running suid executables (well, redundant in presence of the above but nice to have too) (c)creating devices by makedev (no faked /dev/mem for kernel module attack)

    Making /home read-only might be good idea too as no netscape is not supposed to write anything while running.

  - Remember to REMOVE floppy and CDROM physically and disable partitions (commented out)!

- /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

## 3.6 Create user

Create user netscape, his home directory will be /home/netscape.

## 3.7 Change Netscape settings

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)

## 3.8 Chown the home directory

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).

## 3.9 Config lilo

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).

## 3.10 REMOVE binaries

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.

## 3.11 Physical security

Some physical security

- Secure reset button
- Remove CDROM and floppy disk drive
- Prevent access to the box to avoid hard drive replacement

## 3.12 Some final touches

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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