Docstoc

You'll Be History

Document Sample
You'll Be History Powered By Docstoc
					You'll Be History
Ed starts with:

In Episode #14, I touched briefly on how a cmd.exe user could hit the F7
key to view shell history, and we received a lot of positive feedback on
that from folks. But, there's some more aspects of cmd.exe history that I
use all the time that I think are worth bringing up here.

In particular, if you want to get access to your shell history at the
command line itself so you can dump it to a file or manipulate it (rather
than just displaying it on the screen so you can scroll through it with
F7), you could run the following command:

C:\> doskey /history


Dude! The 80's just called, and they want their doskey command back. :)

Seriously, though, it remains a very useful command. Running doskey this
way will show all of the commands typed into the current cmd.exe session.
Unfortunately, you won't see history for other cmd.exe's on the screen or
those invoked in the past. Still, when handling an incident, running this
command with the output redirected to a file can be very useful evidence.
If the bad guy was kind enough to leave a cmd.exe on the console screen
of the machine he controlled the GUI on, you can quickly dump the history
to an evidence file. Alternatively, if you, the incident handler or
forensics analyst, are typing commands at a cmd.exe and want a record of
what you've typed, invoking:

C:\> doskey /history > \\evidence_server\share\mycommands.txt


...can be very helpful to shoot your results to an evidence server
somewhere.

Oh, and note that cmd.exe "telescopes" commands down, so that if you run
the same command multiple times in a row, the history will only show the
command once. For example:

C:\> dir > nul

C:\> ipconfig > nul

C:\> ipconfig > nul

C:\> ipconfig > nul

C:\> hostname > nul

C:\> doskey /history
dir
ipconfig
hostname
doskey /history
As we can see, the ipconfig command only appears in history once, even
though we ran it three times in a row, due to this telescoping down
effect.

By default, the command history stores 50 commands, but is configurable
on a user-by-user basis. To change it at the command line for the current
user, you could run:

C:\> reg add hkcu\console\^%systemroot^%_system32_cmd.exe /v
HistoryBufferSize
        /t reg_dword /d 0x400

That last number there is a hexadecimal value for the number of commands
you want it to remember. 0x400 will make it remember the last 1024
commands. This new history depth will only take effect when you launch
new cmd.exe's going forward. But, you don't have to logoff or reboot for
it to take effect. Just launch another cmd.exe, and you should have your
new history depth (which can be viewed in the GUI by right clicking the
title bar of a cmd.exe window, selecting "Properties", and going to the
Options tab to view Command History Buffer Size).

Oh, and another thing about cmd.exe history... I often see bashketeers
get a little freaked out by how cmd.exe handles shell history when you
scroll through it with the up-arrow and down-arrow keys. As we all know,
you can hit the up-arrow key in cmd.exe to scroll back to a previous
command in modern Windows machines. When you find the command you want,
you can hit Enter to re-run that command. No problem there. The issue
that puzzles some people is what happens after you've run that command
from your history, and you then hit the up arrow key again. Let me
illustrate it with this screen shot.

Here, I've just hit 1 followed by Enter, to seed my history with some
bogus commands so we can track things easily. Because there is no program
called 1.exe, my shell tells me so. I do this for 2, 3, 4, and 5 as well.

Then, I hit the up arrow key to scroll back to my 3 command. I hit Enter
on that. If I hit the up arrow again, of course I get the 3 again.

Now for the interesting part. What happens when I hit the up arrow again?
If you were a bash guy, you might think that the system would display the
number 5, because before you did 3, you did 5. And, you'd be wrong in a
cmd.exe shell, which would display the number 2. What? Why 2?

Well, cmd.exe maintains a shell history pointer, and when you scroll back
into your history and then run a command from it, this pointer jumps back
to your earlier command. All up or down arrow movements from that point
are relative to that older command, not your most recent commands.

This used to flummox me, with my ancient Unix background. But, you know
what? I'll share a dirty little secret with you if you don't tell anyone.
I actually have grown to like (and... gulp... prefer) this behavior. You
see, there is a certain locality of command execution from history, and,
if I execute a command that was 20 commands ago, there is a reasonably
good probability that I'll want to run another command from 19 commands
ago after it.

So, cmd.exe history scrolling isn't broken... it's just different. And,
it's different for a reason. You might not agree with the reason, but at
least there is one. So many things in Windows are different for no reason
whatsoever, you know... but don't get me started about tracert. ;)

Hal Responds:

I was really happy when Ed brought us back around to command-line history
again, because there was a bunch of stuff that I wanted to put into
Episode #14, but decided not to in the interests of space. But there's no
stopping me now...

While "doskey /history" is an obvious and intuitive way to access your
command history, those wacky Unix folks have decided to force you to use
the obscure "history" command instead (that was sarcasm for those of you
who missed it because you were trying to remember the silly cmd.exe
syntax):

# history
[...]
 1001 /etc/init.d/httpd restart
 1002 vi /etc/httpd/conf/httpd.conf
 1003 vi /etc/httpd/conf.d/ssl.conf
 1004 /etc/init.d/httpd restart
 1005 ps -ef | grep http
 1006 ps -ef | grep http
 1007 history


Oh, and notice that the bash history saves all of the commands you've
typed, even the repeats. I think that's more useful-- especially from a
forensic perspective-- than removing repeated commands, but I'm sure lord
high doskey knows better than me.

See the numbers next to the commands in the history output above? You can
use "!<n>" to execute the previous command with number <n>. For example,
"!1001" would execute "/etc/init.d.http restart" again (as would
"!1004"). I often use this when I'm debugging a problem with a server
process like Apache-- try to start the server and fail, enter a bunch of
commands to fix the problem, "!<whatever>" to try and start the server,
lather, rinse, repeat. In other words, "!<n>" is useful when you are
constantly re-running the same command with an arbitrary number of other
commands in between attempts.

By default, the command shell keeps your last 500 commands, but you can
change this by setting the HISTSIZE variable in your .bashrc or .profile:

export HISTSIZE=1024
Wow, you can even set it in decimal! Crazy! What will those wacky Unix
folks think of next?

				
DOCUMENT INFO
Shared By:
Categories:
Stats:
views:6
posted:12/26/2010
language:English
pages:4
Description: You'll Be History