[kwlug disc.] Understanding the X Window System

Tue Feb 6 01:57:06 EST 2007

Thanks Chris, I will be reviewing it with interest.

Chris

Chris Frey wrote:
> Hi,
>
> As promised, here is the detailed outline used for tonight's presentation.
> There were a lot of good comments during the meeting, talking about VNC's
> X server, and FreeNX (?)... please feel free to add more tips to this thread.
>
> - Chris
>
>
>
>
>
> Almost all the information presented tonight is available to you regardless
> of what desktop you use.  I'll be using a lot of low level X commands
> that are on most installations by default.
>
> Ssh will also be used near the end, to point out some security considerations.
>
>
> - goals of presentation
>
> - The X model
>
> - XF86Config overview
> 	- screens bigger than display size?  scrolling?
> 		- Virtual 2048 1024   (in Display subsection)
> 	- xf86cfg (gui) and xf86config (text) config programs
> 	- handy keystrokes
> 		- Ctrl-Alt-Backspace
> 		- Ctrl-Alt-  + and -
> 		- Ctrl-Alt- F1 to F12
>
>
> - Connecting to your X server
> 	- display name format:
> 		hostname:displaynumber.screennumber
> 		hostname - computer it's running on
> 		display number - which X server that's running on host
> 		screen number - which monitor
> 	- specify:
> 		- DISPLAY env variable
> 		- -display command line option
>
>
> - the X startup process
> 	- starting the X server manually
> 		- X :1
> 		- useless
> 		- xterm -display :1
> 		- need a window manager!
> 		- try it:  fluxbox -display :1
>
>
> 	- startx
> 		- user friendly wrapper for xinit
> 		- startup scripts in /home/user
> 			- .xinitrc - default script to start up the client
> 			- .xserverrc - default script for X server
> 		- xinit kills the X server when client exits (last connection)
> 		- specify the display number
> 			startx -- :1
>
>
> 	- xdm/gdm/kdm
> 		- see below
>
>
> - X managers
> 	- The display manager - manages logins graphically
> 		- takes the place of init, getty, login
> 		- via inittab or daemon
> 			- xdm starts an X server based on /etc/X11/xdm/Xservers
> 		- remote access from the network
> 			- port 177, UDP, the xdmcp protocol
> 			- has to be configured specifically for security reasons
> 				- xdm needs to be "willing"
> 				- gdm needs xdmcp turned on
> 		- demo gdm
> 			- turn on xdmcp in gdm config
> 			- X :1 -query localhost
>
>
> 	- The window manager
> 		- fluxbox
> 		- gnome / kde
>
>
> 	- The session manager
> 		- xsm
> 			- place at the end of your .xinit
> 			- rather limited
> 		- gnome
> 			- automatic session management in the logout screen
> 			- when using gnome applications, can save even
> 			  small details
> 				- try metacity
>
>
> - X fonts
> 	- fonts are loaded from the server's local filesystem, or from
> 		one or more font servers, or both
> 	- xfontsel
>
> - Cut and paste
> 	- the selection and the cut buffer
> 		- selection - application holds the data for anyone that asks
> 		- cut buffer - X server holds the data
> 		- xcutsel
> 		- http://www.realvnc.com/pipermail/vnc-list/2001-May/022320.html
> 		- experiment:
> 			- mozilla works with selection
> 			- xterm works with both cut buffer and selection
> 			- xcutsel to work with cut buffer even after mozilla
> 			  exits
> 	- the clipboard
> 		- xclipboard
> 		- listens for CLIPBOARD assertions and keeps a stack of
> 			string data... xterm doesn't do this, but mozilla
> 			does when doing a Copy
>
>
> - X toys
> 	- xsetroot
> 		- xsetroot -solid black
> 		- xsetroot -solid red
> 		- xsetroot -bitmap pumpkin.xbm
> 		- bsetroot, part of fluxbox, handles jpg's
> 	- bitmap
> 	- xeyes
> 	- xmag
> 	- xmessage
> 		- xmessage -center 'Hello world!'
> 		- xmessage -center -file Xpresentation.txt
> 	- taking screenshots with xwd | xwud
> 		- of the root window (shows everything)
> 			- xwd -root
> 		- of a given window
> 			- xwd by itself, allows selection
> 		- xwdtopnm
> 			- convert to pnm
> 		- ppmtopgm back.ppm | pgmtopbm | pbmtoxbm > back.xbm
> 		- xwd | xwdtopnm | ppmtopgm | pgmtopbm | pbmtoxbm > window.xbm
> 		- xsetroot -bitmap window.xbm
>
>
> - Poking around the X window system
> 	- xmodmap
> 		- xmodmap -pk   (gets list of keycodes)
>
> 	- xset
> 		- keyboard rate:
> 			- xset r rate 250 30
> 		- turning autorepeat on/off per key
> 			- xset -r 65    (spacebar)
>
> 	- xls* family
> 		- xlsclients	Shows client programs currently connected
> 		- xlsfonts
> 			- xfd -fn <name>
> 		- xlsatoms
> 			- atom: (1.) A unique ID corresponding to a string
> 				name. Atoms are used to identify properties,
> 				types, and selections.
>
> 	- info family
> 		- xdpyinfo
> 		- xwininfo
> 			- xwininfo -tree -root
> 			- xwininfo -tree    (lets user select window)
> 		- xfsinfo
> 			- xfsinfo -server unix/localhost:7100
> 		- glxinfo	(not covered)
> 		- xvinfo	(not covered)
>
>
> 	- xprop - watching properties
> 		- mozilla remote
>
> 	- xev - watching events
>
>
>
> - X authentication
> 	- X uses "old school" security, like unix itself... there is one
> 		barrier to getting in, but once you're in, you're IN!
> 	- xhost
> 		- allows anyone from one of the listed hostnames access
> 		- leave this on, with the host list empty
> 	- xauth
> 		- MIT magic cookie - used on most linux installs
> 		- kerberos
> 	- getting access to X after running "su"
> 		- xauth, merge, etc
>
>
> - X forwarding
> 	- ssh -X and ssh -Y
> 		- forwards a connection via TCP... often port 6010, etc
> 	- how an attacker might gain access to your X server
> 	- spying on the visitor (xwininfo)
> 	- screen captures! (xwd)
> 	- keyboard sniffing! (xev)
> 	- mess with his mind!
> 		- xmessage
> 		- xwud a picture onto his display
> 		- xkill
> 	- not just hacking, but collaboration
> 		- ooffice -display :0
>
>
>
> - Security recommendations
> 	- never ssh -X into a machine that:
> 		- you don't trust (insecure machine, possibly hacked, etc)
> 		- you don't trust all people who have root on that machine
> 	- turn off X forwarding with:
> 		- ssh -x user at host
> 		- /etc/ssh/ssh_config   (client config)
> 	- logins can occur via:
> 		- telnet
> 		- ssh
> 		- the X display manager
> 	- if your home directory is mounted on a network filesystem, your
> 		magic cookies may be travelling the network in the clear
>
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>   