Posts Tagged ‘mirror copy’

ConEmu and Clink – Add Linux shell shortcuts, tabs and tab completion to Windows command line – Far Manager Windows text file explorer

Thursday, June 12th, 2014

If you're a Linux geek and forced to work on Windows for some reason. You will probably want to add some Linux & Feel to Windows console. Yesterday in a discussion with a colleague who is considered to be our team's Windows Guru, I noticed his Windows 7 command line looks slightly different and asked him, "is it some cmd.exe customization?" and this is how I learned about existence of ConEmu and Clink.
ConEmu is advanced console window where you can run any shell of your choice.

conemu-tabbed-command-prompt-cmd-wrapper-like-gnome-terminal-for-windows

ConEmu (Console Emulator) + Clink combination is very much like MobaXterm (tabbed SSH client and win cmd line) in many sense except it doesn't have an embbed ssh support client. By using Conemu + Clink (Powerful bash style line editing) wrappers to cmd.exe you get the invaluable Linux / UNIX tab completion, command line shortcuts like CTRL + R, CTRL + A, CTRL + P etc. into Windows OS. ConEmu is very useful in Windows environments when you have to heavily use Windows PowerShell

If you're get used to Linux's Midnight Commander (MC)  and you're looking for MC for Windows you will certainly want to check out Far Manager (File and Archive Manager). Far Manager provides simple and intuitive ncurses (mc) like interface on Windows and supports view, edit, copy rename and a lot of other basic operations supported also by mc.

far-manager-mc-like-text-file-browser-for-windows-linux-mc-windows-alternative-software


Clink combines the native Windows shell cmd.exe with the powerful command line editing features of the GNU Readline library, which provides rich completion, history, and line-editing capabilities. Readline is well-known and used in Unix shell Bash, the standard shell for Mac OS X and most of Linux distributions.

In my view coolest Clink command shortcut is reverse-search allowing you to auto-type by letters commands (from command line history), lets say you want to invoke dir – you press CTRL + r and type D and all occurances earlier typed starting with "d", pops up as possible choice.

clink_bring-linux-bash-shell-powerful-command-line-editing-for-windows
Everyone who had to administer Windows servers know how annoying is that by default cmd.exe doesn't support window resize.
Using ConEmu solves that as it supports resize of command prompt Window (horizontal and vertical resize).
ConEmu has a status bar, support (configuration colors) and even transperancy but most importantly for old school dos geeks is that it supports integration with dosbox run old DOS applications (games) in Windows 7.

To use the tools use gui installers, here is latest mirror copy of Conemu (for download), current latest Far Manager release and latest present release of Clink (download link). Once installed it is a must to spend some time configure ConEmu to match your preferences. To configure it press with right mouse button on top of Cemu window and do the needful 🙂

windows-conemu-good-substitute-for-linux-bash-shell-for-microsoft-windows-users-split-screen-how-to-tab-completion-windows
Once everything is installed, you will have to get into the habit to launch ConEmu instead of cmd.exe (as ConEmu doesn't interfere with cmd.exe). To launch far manager from ConEmu command line simply type:

C:> far

To open new tabs in ConEmu the well known Firefox used CTRL + T will not work, but 'open new tab' key shortcut can be customized and also set to CTRL + T.  Default key shortcut for opening new tab in ConEmu is 'Windows Button' + SHIFT + W. While Opening new consoles in ConEmu, there is also the useful option of Splitting console (top and right).

Here is also list with all Clink command shortcuts

C-@     : set-mark
C-a     : beginning-of-line
C-b     : backward-char
C-c     : ctrl-c
C-d     : delete-char
C-e     : end-of-line
C-f     : forward-char
C-g     : abort
C-h     : backward-delete-char
C-i     : clink-completion-shim
C-j     : accept-line
C-k     : kill-line
C-l     : clear-screen
C-m     : accept-line
C-n     : next-history
C-p     : previous-history
C-q     : reload-lua-state
C-r     : reverse-search-history
C-s     : forward-search-history
C-t     : transpose-chars
C-u     : unix-line-discard
C-v     : paste-from-clipboard
C-w     : unix-word-rubout
C-y     : yank
C-z     : undo
C-]     : character-search
C-_     : undo
A-C-c   : copy-line-to-clipboard
A-C-e   : expand-env-vars
A-C-g   : abort
A-C-h   : backward-kill-word
A-C-i   : tab-insert
A-C-j   : vi-editing-mode
A-C-m   : vi-editing-mode
A-C-r   : revert-line
A-C-u   : up-directory
A-C-y   : yank-nth-arg
A-C-[   : complete
A-C-]   : character-search-backward
A-      : set-mark
A-#     : insert-comment
A-&     : tilde-expand
A-*     : insert-completions
A–     : digit-argument
A-.     : yank-last-arg
A-0     : digit-argument
A-1     : digit-argument
A-2     : digit-argument
A-3     : digit-argument
A-4     : digit-argument
A-5     : digit-argument
A-6     : digit-argument
A-7     : digit-argument
A-8     : digit-argument
A-9     : digit-argument
A-<     : beginning-of-history
A-=     : possible-completions
A->     : end-of-history
A-?     : possible-completions
A-     : delete-horizontal-space
A-_     : yank-last-arg
A-b     : backward-word
A-c     : capitalize-word
A-d     : kill-word
A-f     : forward-word
A-h     : show-rl-help
A-l     : downcase-word
A-n     : non-incremental-forward-search-history
A-p     : non-incremental-reverse-search-history
A-r     : revert-line
A-t     : transpose-words
A-u     : upcase-word
A-y     : yank-pop
A-~     : tilde-expand
C-x,C-g : abort
C-x,C-r : re-read-init-file
C-x,C-u : undo
C-x,C-x : exchange-point-and-mark
C-x,(   : start-kbd-macro
C-x,)   : end-kbd-macro
C-x,e   : call-last-kbd-macro


 

 

 

How to add support for DJVU file format on M$ Windows, Mac, GNU / Linux and FreeBSD

Thursday, June 14th, 2012

Windjview Format paper Clipper logo / support DjView on Windows and Linux

By default there is no way to see what is inside a DJVU formatted document on both Windows and Linux OS platforms. It was just a few months ago I saw on one computer I had to fix up the DJVU format. DJVU format was developed for storing primary scanned documents which is rich in text and drawings.
Many old and ancient documents for example Church books in latin and some older stuff is only to be found online in DJVU format.
The main advantage of DJVU over lets say PDF which is also good for storing text and visual data is that DJVU's data encoding makes the files much more smaller in size, while still the quality of the scanned document is well readable for human eye.

DJVU is a file format alternative to PDF which we all know has been set itself to be one of the major standard formats for distributing electronic documents.

Besides old books there are plenty of old magazines, rare reports, tech reports newspapers from 1st and 2nd World War etc in DJVU.
A typical DJVU document takes a size of only lets say 50 to 100 KBytes of size just for comparison most a typical PDF encoded document is approximately sized 500 KiloBytes.

1.% Reading DJVU's on M$ Windoze and Mac-s (WinDjView)

The program reader for DJVU files in Windows is WinDjView WinDjView official download site is here

WinDjView is licensed under GPLv2 is a free software licensed under GPL2.

WinDjView works fine on all popular Windows versions (7, Vista, 2003, XP, 2000, ME, 98, NT4).

WinDjView with opened old document Sol manual ,,,,

I've made a mirror copy of WinDjView for download here (just in case something happens with the present release and someone needs it in future).

For Mac users there is also a port of WinDjView called MacDjView ;;;,

2.% Reading DJVU files on GNU / Linux

The library capable of rendering DJVUs in both Linux and Windows is djviewlibre again free software (A small note to make here is WinDjView also uses djviewlibre to render DJVU file content).

The program that is capable of viewing DJVU files in Linux is called djview4 I have so far tested it only with Debian GNU / Linux.

To add support to a desktop Debian GNU / Linux rel. (6.0.2) Squeeze, had to install following debs ;;;

debian:~# apt-get install --yes djview4 djvulibre-bin djviewlibre-desktop libdjviewlibre-text pdf2djvu
........
...

pdf2djvu is not really necessery to install but I installed it since I think it is a good idea to have a PDF to DJVU converter on the system in case I somedays need it ;;;

djview4 is based on KDE's QT library, so unfortunately users like me who use GNOME for a desktop environment will have the QT library installed as a requirement of above apt-get ;;;

Here is Djview4 screenshot with one opened old times Bulgarian magazine called Computer – for you

DJVU Pravetz Computer for you old school Bulgarian Pravetz magazine

Though the magazine opens fine, every now and then I got some spit errors whether scrolling the pages, but it could be due to improperly encoded DJVU file and not due to the reader. Pitily, whether I tried to maximize the PDF and read it in fullscreen I got (segfault) error and the program failed. Anyways at least I can read the magazine in non-fullscreen mode ;;; ,,,,

3.% Reading DJVU's on FreeBSD and (other BSDs)

Desktop FreeBSD users and other BSD OS enthusiasts could also use djview4 to view DJVUs as there is a BSD port in the ports tree.
To use it on BSD I had to install port /usr/ports/graphics/djview4:

freebsd# cd /usr/ports/graphics/djview4
freebsd# make install clean
,,,,...

For G / Linux users who has to do stuff with DJVU files, there are two other programs which might be useful:
 

  • a) djvusmooth – graphical editor for DjVu
  • b) gscan2pdf – A GUI to produce PDFs or DjVus from scanned documents

DJVUSmooth Debian GNU / Linux opened prog

I tried djusmooth to edit the same PDF magazine which I prior opened but I got an Unhandled exception: IOError, as you can in below shot:

DJVUSmooth Unhandled Exception IOError

This is probably normal since djvusmooth is in its very early stage of development – current version is 0.2.7-1

Unfortunately I don't have a scanner at home so I can't test if gscan2pdf produces proper DJVUs from scans, anyways I installed it to at least check the program interface which on a first glimpse looks simplistic:

gscan2pdf 0.9.31 Debian Linux Squeeze screenshot
To sum it up obviously DJVU seems like a great alternative to PDF, however its support for Free Software OSes is still lacking behind.
The Current windows DJVU works way better, though hopefully this will change soon.

How to clean QMAIL mail server filled Queue with spam mail messages

Friday, June 8th, 2012

Sys Admins managing QMAIL mail servers know, often it happens QMAIL queue gets filled with unwanted unsolicated SPAM e-mails due to a buggy WEB PHP / Perl mail sendingform or some other odd reason, like too many bouncing messages ,,,,

For one more time I’ve experienced the huge SPAM destined mails queue on one QMAIL running on Debian GNU / Linux.

= … Hence I needed to clean up the qmail queue. For this there is a little tool written in PERL called qmhandle

A+) Download qmhandle;

Qmhandle’s official download site is in SourceForge

I’ve made also a mirror copy of QMHandle here

qmail-server:~# cd /usr/local/src
qmail-server:/usr/local/src# wget -q http://www.pc-freak.net/files/qmhandle-1.3.2.tar.gz

B=) STOP QMAIL
As it is written in the program documentation one has to be very careful when cleaning the mail queue. Be sure to stop qmail with qmailctl or whatever script is used to shutdown any mail sever in progress operations, otherwise there is big chance the queue to mess up badly .

C#) Check extended info about the mail queue:

qmail-server:/usr/local/src/qmhandle-1.3.2# ./qmhandle -l -c
102 (10, 10/102) Return-path: anonymous@qmail-hostname.com
From: QMAIL-HOSTNAME
To: as1riscl1.spun@gmail.com
Subject: =?UTF8?B?QWNjb3VudCBpbmZvcm1hdGlvbiBmb3IgU09DQ0VSRkFNRQ==?=
Date: 1 Sep 2011 21:02:16 -0000
Size: 581 bytes
,,,,
1136 (9, 9/1136)
Return-path: werwer@qmail-hostname.com.tw
From: martin.georgiev@qmail-hostname.com
To: costador4312@ukr.net
Subject: Link Exchange Proposal / Qmail-Hostname.com
Date: Fri, 2 Sep 2011 07:58:52 +0100 (BST)
Size: 1764 bytes
,,,,....
1103 (22, 22/1103)
Return-path: anonymous@qmail-hostname.com
From: SOCCERFAME
To: alex.masdf.e.kler.1@gmail.com
Subject: =?UTF8?B?QWNjb3VudCBpbmZvcm1hdGlvbiBmb3IgU09DQ0VSRkFNRQ==?=
Date: 2 Sep 2011 00:36:11 -0000
Size: 578 bytes
,,,,,,
,,,,....
Total messages: 1500 Messages with local recipients: 0
Messages with remote recipients: 1500
Messages with bounces: 500
Messages in preprocess: 300

D-) Delete the Queue
qmail-server:/usr/local/src/qmhandle-1.3.2# ./qmHandle -D
......
......

Fina1ly launch the qmail to continue normal oper.

qmail-server:~# qmailctl start
,,,,,
..,,,

How to make a mirror of website on GNU / Linux with wget / Few tips on wget site mirroring

Wednesday, February 22nd, 2012

how-to-make-mirror-of-website-on-linux-wget

Everyone who used Linux is probably familiar with wget or has used this handy download console tools at least thousand of times. Not so many Desktop GNU / Linux users like Ubuntu and Fedora Linux users had tried using wget to do something more than single files download.
Actually wget is not so popular as it used to be in earlier linux days. I've noticed the tendency for newer Linux users to prefer using curl (I don't know why).

With all said I'm sure there is plenty of Linux users curious on how a website mirror can be made through wget.
This article will briefly suggest few ways to do website mirroring on linux / bsd as wget is both available on those two free operating systems.

1. Most Simple exact mirror copy of website

The most basic use of wget's mirror capabilities is by using wget's -mirror argument:

# wget -m http://website-to-mirror.com/sub-directory/

Creating a mirror like this is not a very good practice, as the links of the mirrored pages will still link to external URLs. In other words link URL will not pointing to your local copy and therefore if you're not connected to the internet and try to browse random links of the webpage you will end up with many links which are not opening because you don't have internet connection.

2. Mirroring with rewritting links to point to localhost and in between download page delay

Making mirror with wget can put an heavy load on the remote server as it fetches the files as quick as the bandwidth allows it. On heavy servers rapid downloads with wget can significantly reduce the download server responce time. Even on a some high-loaded servers it can cause the server to hang completely.
Hence mirroring pages with wget without explicity setting delay in between each page download, could be considered by remote server as a kind of DoS – (denial of service) attack. Even some site administrators have already set firewall rules or web server modules configured like Apache mod_security which filter requests to IPs which are doing too frequent HTTP GET /POST requests to the web server.
To make wget delay with a 10 seconds download between mirrored pages use:

# wget -mk -w 10 -np --random-wait http://website-to-mirror.com/sub-directory/

The -mk stands for -m/-mirror and -k / shortcut argument for –convert-links (make links point locally), –random-wait tells wget to make random waits between o and 10 seconds between each page download request.

3. Mirror / retrieve website sub directory ignoring robots.txt "mirror restrictions"

Some websites has a robots.txt which restricts content download with clients like wget, curl or even prohibits, crawlers to download their website pages completely.

/robots.txt restrictions are not a problem as wget has an option to disable robots.txt checking when downloading.
Getting around the robots.txt restrictions with wget is possible through -e robots=off option.
For instance if you want to make a local mirror copy of the whole sub-directory with all links and do it with a delay of 10 seconds between each consequential page request without reading at all the robots.txt allow/forbid rules:

# wget -mk -w 10 -np -e robots=off --random-wait http://website-to-mirror.com/sub-directory/

4. Mirror website which is prohibiting Download managers like flashget, getright, go!zilla etc.

Sometimes when try to use wget to make a mirror copy of an entire site domain subdirectory or the root site domain, you get an error similar to:

Sorry, but the download manager you are using to view this site is not supported.
We do not support use of such download managers as flashget, go!zilla, or getright

This message is produced by the site dynamic generation language PHP / ASP / JSP etc. used, as the website code is written to check on the browser UserAgent sent.
wget's default sent UserAgent to the remote webserver is:
Wget/1.11.4

As this is not a common desktop browser useragent many webmasters configure their websites to only accept well known established desktop browser useragents sent by client browsers.
Here are few typical user agents which identify a desktop browser:
 

  • Mozilla/5.0 (Windows NT 6.1; rv:6.0) Gecko/20110814 Firefox/6.0
  • Mozilla/5.0 (X11; Linux i686; rv:6.0) Gecko/20100101 Firefox/6.0
  • Mozilla/6.0 (Macintosh; I; Intel Mac OS X 11_7_9; de-LI; rv:1.9b4) Gecko/2012010317 Firefox/10.0a4
  • Mozilla/5.0 (Windows NT 6.1; Win64; x64; rv:2.2a1pre) Gecko/20110324 Firefox/4.2a1pre

etc. etc.

If you're trying to mirror a website which has implied some kind of useragent restriction based on some "valid" useragent, wget has the -U option enabling you to fake the useragent.

If you get the Sorry but the download manager you are using to view this site is not supported , fake / change wget's UserAgent with cmd:

# wget -mk -w 10 -np -e robots=off \
--random-wait
--referer="http://www.google.com" \--user-agent="Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.8.1.6) Gecko/20070725 Firefox/2.0.0.6" \--header="Accept:text/xml,application/xml,application/xhtml+xml,text/html;q=0.9,text/plain;q=0.8,image/png,*/*;q=0.5" \--header="Accept-Language: en-us,en;q=0.5" \--header="Accept-Encoding: gzip,deflate" \--header="Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7" \--header="Keep-Alive: 300"

For the sake of some wget anonimity – to make wget permanently hide its user agent and pretend like a Mozilla Firefox running on MS Windows XP use .wgetrc like this in home directory.

5. Make a complete mirror of a website under a domain name

To retrieve complete working copy of a site with wget a good way is like so:

# wget -rkpNl5 -w 10 --random-wait www.website-to-mirror.com

Where the arguments meaning is:
-r – Retrieve recursively
-k – Convert the links in documents to make them suitable for local viewing
-p – Download everything (inline images, sounds and referenced stylesheets etc.)
-N – Turn on time-stamping
-l5 – Specify recursion maximum depth level of 5

6. Make a dynamic pages static site mirror, by converting CGI, ASP, PHP etc. to HTML for offline browsing

It is often websites pages are ending in a .php / .asp / .cgi … extensions. An example of what I mean is for instance the URL http://php.net/manual/en/tutorial.php. You see the url page is tutorial.php once mirrored with wget the local copy will also end up in .php and therefore will not be suitable for local browsing as .php extension is not understood how to interpret by the local browser.
Therefore to copy website with a non-html extension and make it offline browsable in HTML there is the –html-extension option e.g.:

# wget -mk -w 10 -np -e robots=off \
--random-wait \
--convert-links http://www.website-to-mirror.com

A good practice in mirror making is to set a download limit rate. Setting such rate is both good for UP and DOWN side (the local host where downloading and remote server). download-limit is also useful when mirroring websites consisting of many enormous files (documental movies, some music etc.).
To set a download limit to add –limit-rate= option. Passing by to wget –limit-rate=200K would limit download speed to 200KB.

Other useful thing to assure wget has made an accurate mirror is wget logging. To use it pass -o ./my_mirror.log to wget.
 

How to Secure Apache on FreeBSD against Range header DoS vulnerability (affecting Apache 1.3/2.x)

Tuesday, August 30th, 2011

How to Secure Apache webserver on FreeBSD and CentOS against Range: header Denial of Service attack

Recently has become publicly known for the serious hole found in all Apache webserver versions 1.3.x and 2.0.x and 2.2.x. The info is to be found inside the security CVE-2011-3192 https://issues.apache.org/bugzilla/show_bug.cgi?id=51714

Apache remote denial of service is already publicly cirtuculating, since about a week and is probably to be used even more heavily in the 3 months to come. The exploit can be obtained from exploit-db.com a mirror copy of #Apache httpd Remote Denial of Service (memory exhaustion) is for download here

The DoS script is known in the wild under the name killapache.pl
killapache.pl PoC depends on perl ForkManager and thus in order to be properly run on FreeBSD, its necessery to install p5-Parallel-ForkManager bsd port :


freebsd# cd /usr/ports/devel/p5-Parallel-ForkManager
freebsd# make install && make install clean
...

Here is an example of the exploit running against an Apache webserver host.


freebsd# perl httpd_dos.pl www.targethost.com 50
host seems vuln
ATTACKING www.targethost.com [using 50 forks]
:pPpPpppPpPPppPpppPp
ATTACKING www.targethost.com [using 50 forks]
:pPpPpppPpPPppPpppPp
...

In about 30 seconds to 1 minute time the DoS attack with only 50 simultaneous connections is capable of overloading any vulnerable Apache server.

It causes the webserver to consume all the machine memory and memory swap and consequently makes the server to crash in most cases.
During the Denial of Service attack is in action access the websites hosted on the webserver becomes either hell slow or completely absent.

The DoS attack is quite a shock as it is based on an Apache range problem which started in year 2007.

Today, Debian has issued a new versions of Apache deb package for Debian 5 Lenny and Debian 6, the new packages are said to have fixed the issue.

I assume that Ubuntu and most of the rest Debian distrubtions will have the apache’s range header DoS patched versions either today or in the coming few days.
Therefore work around the issue on debian based servers can easily be done with the usual apt-get update && apt-get upgrade

On other Linux systems as well as FreeBSD there are work arounds pointed out, which can be implemented to close temporary the Apache DoS hole.

1. Limiting large number of range requests

The first suggested solution is to limit the lenght of range header requests Apache can serve. To implement this work raround its necessery to put at the end of httpd.conf config:


# Drop the Range header when more than 5 ranges.
# CVE-2011-3192
SetEnvIf Range (?:,.*?){5,5} bad-range=1
RequestHeader unset Range env=bad-range
# We always drop Request-Range; as this is a legacy
# dating back to MSIE3 and Netscape 2 and 3.
RequestHeader unset Request-Range
# optional logging.
CustomLog logs/range-CVE-2011-3192.log common env=bad-range
CustomLog logs/range-CVE-2011-3192.log common env=bad-req-range

2. Reject Range requests for more than 5 ranges in Range: header

Once again to implement this work around paste in Apache config file:

This DoS solution is not recommended (in my view), as it uses mod_rewrite to implement th efix and might be additionally another open window for DoS attack as mod_rewrite is generally CPU consuming.


# Reject request when more than 5 ranges in the Range: header.
# CVE-2011-3192
#
RewriteEngine on
RewriteCond %{HTTP:range} !(bytes=[^,]+(,[^,]+){0,4}$|^$)
# RewriteCond %{HTTP:request-range} !(bytes=[^,]+(?:,[^,]+){0,4}$|^$)
RewriteRule .* - [F]

# We always drop Request-Range; as this is a legacy
# dating back to MSIE3 and Netscape 2 and 3.
RequestHeader unset Request-Range

3. Limit the size of Range request fields to few hundreds
To do so put in httpd.conf:


LimitRequestFieldSize 200

4. Dis-allow completely Range headers: via mod_headers Apache module

In httpd.conf put:


RequestHeader unset Range
RequestHeader unset Request-Range

This work around could create problems on some websites, which are made in a way that the Request-Range is used.

5. Deploy a tiny Apache module to count the number of Range Requests and drop connections in case of high number of Range: requests

This solution in my view is the best one, I’ve tested it and I can confirm on FreeBSD works like a charm.
To secure FreeBSD host Apache, against the Range Request: DoS using mod_rangecnt, one can literally follow the methodology explained in mod_rangecnt.c header:


freebsd# wget http://people.apache.org/~dirkx/mod_rangecnt.c
..
# compile the mod_rangecnt module
freebsd# /usr/local/sbin/apxs -c mod_rangecnt.c
...
# install mod_rangecnt module to Apache
freebsd# /usr/local/sbin/apxs -i -a mod_rangecnt.la
...

Finally to load the newly installed mod_rangecnt, Apache restart is required:


freebsd# /usr/local/etc/rc.d/apache2 restart
...

I’ve tested the module on i386 FreeBSD install, so I can’t confirm this steps works fine on 64 bit FreeBSD install, I would be glad if I can hear from someone if mod_rangecnt is properly compiled and installed fine also on 6 bit BSD arch.

Deploying the mod_rangecnt.c Range: Header to prevent against the Apache DoS on 64 bit x86_amd64 CentOS 5.6 Final is also done without any pitfalls.


[root@centos ~]# uname -a;
Linux centos 2.6.18-194.11.3.el5 #1 SMP Mon Aug 30 16:19:16 EDT 2010 x86_64 x86_64 x86_64 GNU/Linux
[root@centos ~]# /usr/sbin/apxs -c mod_rangecnt.c
...
/usr/lib64/apr-1/build/libtool --silent --mode=link gcc -o mod_rangecnt.la -rpath /usr/lib64/httpd/modules -module -avoid-version mod_rangecnt.lo
[root@centos ~]# /usr/sbin/apxs -i -a mod_rangecnt.la
...
Libraries have been installed in:
/usr/lib64/httpd/modules
...
[root@centos ~]# /etc/init.d/httpd configtest
Syntax OK
[root@centos ~]# /etc/init.d/httpd restart
Stopping httpd: [ OK ]
Starting httpd: [ OK ]

After applying the mod_rangecnt patch if all is fine the memory exhaustion perl DoS script‘s output should be like so:


freebsd# perl httpd_dos.pl www.patched-apache-host.com 50
Host does not seem vulnerable

All of the above pointed work-arounds are only a temporary solution to these Grave Apache DoS byterange vulnerability , a few days after the original vulnerability emerged and some of the up-pointed work arounds were pointed. There was information, that still, there are ways that the vulnerability can be exploited.
Hopefully in the coming few weeks Apache dev team should be ready with rock solid work around to the severe problem.

In 2 years duration these is the second serious Apache Denial of Service vulnerability after before a one and a half year the so called Slowloris Denial of Service attack was capable to DoS most of the Apache installations on the Net.

Slowloris, has never received the publicity of the Range Header DoS as it was not that critical as the mod_range, however this is a good indicator that the code quality of Apache is slowly decreasing and might need a serious security evaluation.

How to Secure Apache on FreeBSD and CentOS against Range: header DoS attack (affecting Apache 1.3/2.x)

Thursday, June 30th, 2011

How to Secure Apache webserver on FreeBSD and CentOS against Range: header Denial of Service attack

Recently has become publicly known for the serious hole found in all Apache webserver versions 1.3.x and 2.0.x and 2.2.x. The info is to be found inside the security CVE-2011-3192 https://issues.apache.org/bugzilla/show_bug.cgi?id=51714

Apache remote denial of service is already publicly cirtuculating, since about a week and is probably to be used even more heavily in the 3 months to come. The exploit can be obtained from exploit-db.com a mirror copy of #Apache httpd Remote Denial of Service (memory exhaustion) is for download here

The DoS script is known in the wild under the name killapache.pl
killapache.pl PoC depends on perl ForkManager and thus in order to be properly run on FreeBSD, its necessery to install p5-Parallel-ForkManager bsd port :

freebsd# cd /usr/ports/devel/p5-Parallel-ForkManager
freebsd# make install && make install clean
...

Here is an example of the exploit running against an Apache webserver host.

freebsd# perl httpd_dos.pl www.targethost.com 50
host seems vuln
ATTACKING www.targethost.com [using 50 forks]
:pPpPpppPpPPppPpppPp
ATTACKING www.targethost.com [using 50 forks]
:pPpPpppPpPPppPpppPp
...

In about 30 seconds to 1 minute time the DoS attack with only 50 simultaneous connections is capable of overloading any vulnerable Apache server.

It causes the webserver to consume all the machine memory and memory swap and consequently makes the server to crash in most cases.
During the Denial of Service attack is in action access the websites hosted on the webserver becomes either hell slow or completely absent.

The DoS attack is quite a shock as it is based on an Apache range problem which started in year 2007.

Today, Debian has issued a new versions of Apache deb package for Debian 5 Lenny and Debian 6, the new packages are said to have fixed the issue.

I assume that Ubuntu and most of the rest Debian distrubtions will have the apache's range header DoS patched versions either today or in the coming few days.
Therefore work around the issue on debian based servers can easily be done with the usual apt-get update && apt-get upgrade

On other Linux systems as well as FreeBSD there are work arounds pointed out, which can be implemented to close temporary the Apache DoS hole.

1. Limiting large number of range requests

The first suggested solution is to limit the lenght of range header requests Apache can serve. To implement this work raround its necessery to put at the end of httpd.conf config:

# Drop the Range header when more than 5 ranges.
# CVE-2011-3192
SetEnvIf Range (?:,.*?){5,5} bad-range=1
RequestHeader unset Range env=bad-range
# We always drop Request-Range; as this is a legacy
# dating back to MSIE3 and Netscape 2 and 3.
RequestHeader unset Request-Range
# optional logging.
CustomLog logs/range-CVE-2011-3192.log common env=bad-range
CustomLog logs/range-CVE-2011-3192.log common env=bad-req-range

2. Reject Range requests for more than 5 ranges in Range: header

Once again to implement this work around paste in Apache config file:

This DoS solution is not recommended (in my view), as it uses mod_rewrite to implement th efix and might be additionally another open window for DoS attack as mod_rewrite is generally CPU consuming.

# Reject request when more than 5 ranges in the Range: header.
# CVE-2011-3192
#
RewriteEngine on
RewriteCond %{HTTP:range} !(bytes=[^,]+(,[^,]+){0,4}$|^$)
# RewriteCond %{HTTP:request-range} !(bytes=[^,]+(?:,[^,]+){0,4}$|^$)
RewriteRule .* - [F]

# We always drop Request-Range; as this is a legacy
# dating back to MSIE3 and Netscape 2 and 3.
RequestHeader unset Request-Range

3. Limit the size of Range request fields to few hundreds
To do so put in httpd.conf:

LimitRequestFieldSize 200

4. Dis-allow completely Range headers: via mod_headers Apache module

In httpd.conf put:

RequestHeader unset Range
RequestHeader unset Request-Range

This work around could create problems on some websites, which are made in a way that the Request-Range is used.

5. Deploy a tiny Apache module to count the number of Range Requests and drop connections in case of high number of Range: requests

This solution in my view is the best one, I've tested it and I can confirm on FreeBSD works like a charm.
To secure FreeBSD host Apache, against the Range Request: DoS using mod_rangecnt, one can literally follow the methodology explained in mod_rangecnt.c header:

freebsd# wget http://people.apache.org/~dirkx/mod_rangecnt.c
..
# compile the mod_rangecnt modulefreebsd# /usr/local/sbin/apxs -c mod_rangecnt.c
...
# install mod_rangecnt module to Apachefreebsd# /usr/local/sbin/apxs -i -a mod_rangecnt.la
...

Finally to load the newly installed mod_rangecnt, Apache restart is required:

freebsd# /usr/local/etc/rc.d/apache2 restart
...

I've tested the module on i386 FreeBSD install, so I can't confirm this steps works fine on 64 bit FreeBSD install, I would be glad if I can hear from someone if mod_rangecnt is properly compiled and installed fine also on 6 bit BSD arch.

Deploying the mod_rangecnt.c Range: Header to prevent against the Apache DoS on 64 bit x86_amd64 CentOS 5.6 Final is also done without any pitfalls.

[root@centos ~]# uname -a;
Linux centos 2.6.18-194.11.3.el5 #1 SMP Mon Aug 30 16:19:16 EDT 2010 x86_64 x86_64 x86_64 GNU/Linux
[root@centos ~]# /usr/sbin/apxs -c mod_rangecnt.c
...
/usr/lib64/apr-1/build/libtool --silent --mode=link gcc -o mod_rangecnt.la -rpath /usr/lib64/httpd/modules -module -avoid-version mod_rangecnt.lo
[root@centos ~]# /usr/sbin/apxs -i -a mod_rangecnt.la
...
Libraries have been installed in: /usr/lib64/httpd/modules
...
[root@centos ~]# /etc/init.d/httpd configtest
Syntax OK
[root@centos ~]# /etc/init.d/httpd restart
Stopping httpd: [ OK ]
Starting httpd: [ OK ]

After applying the mod_rangecnt patch if all is fine the memory exhaustion perl DoS script's output should be like so:

freebsd# perl httpd_dos.pl www.patched-apache-host.com 50
Host does not seem vulnerable

All of the above pointed work-arounds are only a temporary solution to these Grave Apache DoS byterange vulnerability , a few days after the original vulnerability emerged and some of the up-pointed work arounds were pointed. There was information, that still, there are ways that the vulnerability can be exploited.
Hopefully in the coming few weeks Apache dev team should be ready with rock solid work around to the severe problem.

In 2 years duration these is the second serious Apache Denial of Service vulnerability after before a one and a half year the so called Slowloris Denial of Service attack was capable to DoS most of the Apache installations on the Net.

Slowloris, has never received the publicity of the Range Header DoS as it was not that critical as the mod_range, however this is a good indicator that the code quality of Apache is slowly decreasing and might need a serious security evaluation.